EP0887440B1 - Chloride based aqueous electrodeposition bath for the preparation of a coating based on zinc or a zinc alloy - Google Patents

Abstract

Aqueous chloride based bath for preparation of coating based on zinc or one of its alloys in which pH > 4; concentration of zinc ions > 1mol.l<-1>. The solution does not contain compounds with free electron pair such as those in group comprising sodium thiosulphate, nicotinic acid, (thio)urea, nicotinamide, and thioglycolic acid. The bath does contain, in solution, at least one polyethylene - glycol polymer with general formula R1-O-CH2-CH2-O)n-R2 in which n ≤ 13; R1, R2 = H and polymer has molecular mass < 500gmol.<-1>, or R1, R2 = groups substituted at end of chain such as m-Calkyl, alkene, alkyne. m is sufficiently small to allow solution of polymer in bath; or ether, ester, carboxylic acid salt, sulphonyl, sulphonic acid salt, ketone, amine or nitrile. The polymer is adapted for incorporating into coating > 0.1% organic compound. Also claimed is the method for depositing the coating and the steel sheet which has been coated.

Description

The present invention relates to an electro-zinc bath based on chlorides, a process of electrodeposition in this bath of a coating of corrosion protection based on zinc or zinc alloy on a metallic surface, in particular on a steel sheet, as well as a substrate, especially in steel, protected against corrosion by a coating made of from this process.

The invention seeks to simultaneously solve two problems: decrease the roughness of the coated sheets while improving their resistance to corrosion.

The first problem therefore relates to roughness: indeed, after a electrolytic coating of zinc or zinc alloy on a substrate metallic, especially on a steel sheet, we see that the roughness of the coating may be different from the initial roughness of the substrate.

The roughness of a surface can be evaluated in the following conventional manner: several profilometric (or "profile") readings of the surface are made, each profile being filtered during recording, by means of an electronic high-pass filter. reducing the amplitude of the undulations exceeding a predetermined filtering threshold for example to 75% of its value in the profile after filtering (the filtering threshold is for example 0.8 mm); the vertical spread of this profile is then represented, that is to say the distribution of the depth recorded with respect to a given reference line (Ox); according to French standardization (AFNOR EO5.015 / 017/052), this reference line (Ox) is the straight line run parallel to the general direction of the profile and passing through its upper points; on the ordinate axis (Oz), drawn perpendicularly to Ox, the depths of the profile are plotted; the deviation of the roughness profile from the reference line Ox can be considered as a random variable and the set of deviations or depths then forms a statistical distribution from which the position of the mean line of the profile is calculated and l 'arithmetic mean deviation of the depth from the mean line; this arithmetic mean deviation is called arithmetic roughness R _a .

The roughness measurements R _a generally reveal that the roughness of the coating is greater than that of the initial substrate, in particular when electrolysis baths based on chlorides are used and in particular when the coating is carried out under current densities " high ”.

The term "high" current density means a current density greater than 0.25 x J _lim .

J _lim is the limit current density, which corresponds to the level of current density on the "intensity-potential" curve characteristic of an electrogalvanizing bath for a given relative speed of the bath with respect to the surface to be electrogalvanized.

J _lim also corresponds to the current density for which the local concentration of zinc ions in the bath becomes zero in the immediate vicinity of the sheet to be coated.

J _lim also corresponds to the current density from which electrochemical phenomena other than the reduction of zinc ions take place on the surface to be electrogalvanized, in particular the evolution of hydrogen.

J _lim therefore also corresponds to the current density from which the electrochemical (or faradic) yield of zinc deposition drops appreciably.

Thus, in the field of low current densities (less than 0.25 × J _lim ), it is known that the roughness of the coating depends essentially on the size of the grains of the electrodeposited layer.

Conversely, in the field of high current densities (greater than 0.25 × J _lim ), which corresponds to the most common industrial conditions and to those of the invention, it is known that the roughness of the coating depends essentially on the roughness of the substrate.

The border between these two behavioral domains depends on the value of J _lim ., That is to say on the composition of the bath and on the hydrodynamic conditions of use.

In industrial installations, the search for productivity leads to electrodeposition at the highest possible current densities; as the maximum current density, practically usable, depends on J _lim , it is then necessary to adapt the composition of the bath to increase the value of J _lim ; this is why, inter alia, baths with a high ionic concentration are used, in particular the concentration of Zn2 + ions (in the case of electrogalvanizing).

Thus, a current density greater than 50 A / dm2 is considered as "high" for a conventional electrogalvanizing bath based on chlorides containing more than 1 mole / liter of Zn2 + ions used under conditions classic hydrodynamics.

In a bath of this type, the concentration of CI- ions can exceed 5 moles / liter.

The increase in roughness obtained after electrodeposition, called "roughness setting" ΔR _a , may for example be of the order of 0.5 μm for an initial roughness of the substrate R _a of the order of 1.3 .mu.m.

In the prior art, it has been proposed to add to the bath electrogalvanizing additives intended to reduce this roughness, additives which we have therefore called "leveling agents".

Among the known additives that are added in electroplating baths, a classic distinction is made between "leveling agents" and "agents brighteners "," refiners and "wetting agents" (called also "surfactants").

The effect of these additives often depends on the conditions of use of the bath, in particular the current density applied during coating.

A “leveled” or “glossy” electrodeposited coating has generally a fine grain structure; but, conversely, a coating electrodeposited whose structure is "refined" is not "brilliant" nor "leveled".

A "leveling agent" or a "brightening agent" is therefore generally also a "refining agent".

A "brightening agent" is not necessarily "leveling" nor necessarily "Wetting".

A "leveling agent" is not necessarily "brightener" nor necessarily "Wetting".

As an example of the use of these different agents, document US 4 229 268 describes electrogalvanizing baths, based on chlorides, usable on a wide range of current density values.

These baths contain leveling and brightening agents, corresponding to the general formula RS- (R'-O) _n H or S [(R'O) _n H] ₂ .

The additional addition of brightening agents, chosen for example from acetophenones, can also enhance the leveling effect.

The properties of the coating obtained can be further improved by adding polyoxyalkylated naphthols to the bath.

• n # 20 à 24, de masse moléculaire moyenne comprise entre 950 et 1050 g ;
• n # 68 à 85, de masse moléculaire moyenne comprise entre 3000 et 3700 g.

Finally, wetting agents (or surfactants) such as copolymers of ethylene oxide and propylene oxide can be added to the bath; in particular, good results have been found with polyethylene glycol condensates of general formula: HO- (CH2-CH2-O) _n -H, in particular for which:

• n # 20 to 24, of average molecular weight between 950 and 1050 g;
• n # 68 to 85, with an average molecular weight of between 3000 and 3700 g.

The document FR 2 597 118 describes electroplating baths of zinc alloy coatings (Zn-Ni), based on chlorides or sulphates, usable under current densities up to 215 A / dm2.

• soit R¹ = CH3-(CH2)_x-CH3 et R² = H - avec x compris entre 9 et 15,
• soit R¹ = H-(CH2)_x-Ar- et R² = -CH2-CH2-OH - avec x compris entre 6 et 15, Ar désignant un noyau benzénique.

As the brightening agent, polyoxyalkylenated compounds are used such as copolymers of alkylene oxides and of groups R ¹ and R ² of general formula R ¹ -O- (CH2-CH2-O) _n -R ² where n = 10 to 50, where:

• either R ¹ = CH3- (CH2) _x -CH3 and R ² = H - with x between 9 and 15,
• or R ¹ = H- (CH2) _x -Ar- and R ² = -CH2-CH2-OH - with x between 6 and 15, Ar denoting a benzene nucleus.

These brighteners therefore have a structural refining effect on the coating; they are used in baths at fairly low concentrations generally between 0.02 and 5 g / l; no leveling effect is described.

The document EP 0 285 931 describes electrodeposition baths, generally sulphate-based, for coating zinc alloys (Zn-Cr) which also contain polyoxyalkylenated compounds; these additives are here "incorporation agents" intended to promote homogeneous incorporation chromium (between 5 and 40%) in the coating and to improve the appearance "Color" of the coating, so as to avoid gray-black or gray-white colors.

Similarly, document EP 0 342 585 also describes baths zinc alloy coating (Zn-Cr) plating, based on sulfates, which contain polymers on the grounds of which are grafted "quaternary amines" functions with the primary aim of promoting the incorporation of chromium (between 5 and 30%) in the coating.

These cationic polymers are dissolved in the bath at weight concentrations of between 0.005 and 5%.

The effect of these polymers in solution is to promote the precipitation of chromium during plating, with which, moreover, they co-precipitate in very low proportion, which improves the powder resistance of coating.

The content of cationic polymer in the coating obtained can thus reach 5%.

Document US 4 146 442 describes electro-zinc baths, based on cyanides (basic) or sulfates or chlorides (acid), usable under relatively low current densities (up to 15 A / dm2).

These baths contain polyglycol wetting agents at concentrations between 3 and 5 g / l.

Thus, in electroplating baths, in particular electrogalvanizing, can use as "wetting agents" or "surfactants" polymers polyethylene glycol unsubstituted or substituted at one end.

In these same plating baths, it is possible to use as "Brightening agents" of polyethylene glycol compounds substituted for minus one end.

The polyethylene glycol used here as wetting agents or brighteners have a high number of ethylene oxide per molecule, at less than 10, generally greater than 20.

These polymers can also be used in baths Zn-Cr alloy plating to facilitate the incorporation of chromium in the deposit.

• du polyéthylène glycol en tant qu'agent tensio-actif non-ionique (donc agent mouillant), possédant un poids moléculaire plus faible que précédemment, ici compris entre 400 et 800 g/Mole, en une quantité comprise entre 0,01 et 1 g/l ;
• au moins un composé possédant un doublet électronique libre choisi dans le groupe comprenant l'acide nicotinique, l'urée, la thio-urée, la nicotinamide, l'acide thioglycolique, le thiosulfate de sodium, en une quantité comprise entre 0,001 et 1 g/l.

The document US 5,575,899 (corresponding to FR 2,723,966) describes an aqueous electroplating bath based on chlorides for the preparation of a coating of zinc and nickel alloy, containing in solution:

• polyethylene glycol as a non-ionic surfactant (therefore wetting agent), having a lower molecular weight than previously, here between 400 and 800 g / mol, in an amount between 0.01 and 1 g / l;
• at least one compound having a free electronic doublet chosen from the group comprising nicotinic acid, urea, thiourea, nicotinamide, thioglycolic acid, sodium thiosulfate, in an amount of between 0.001 and 1 g / l.

Preferably, the molar concentration of zinc or zinc alloy ions is such that: 1≤ (| Zn ²⁺ | + | Ni ²⁺ |) ≤4 Mole / l, | Cl ^- |> 4 M / I

Preferably, one proceeds to the Zn-Ni electrodeposition using this bath under a current density between 50 and 150 A / dm2.

According to this document, the function of polyethylene glycol is to improve the wettability of the surface to be coated; the choice of molecular weight is determinant: below 400, there are burning problems on the edges - beyond 800, the incorporation of nickel drops significantly in the Zn-Ni deposit.

• l'apparition, sur la surface revêtue, de taches de forme aciculaire résultant d'hétérogénéités d'écoulement du bain sur la surface de la tôle,
• le brûlage sur les bords de la tôle revêtue.

This document mainly describes a way to prevent:

• the appearance, on the coated surface, of acicular-shaped spots resulting from heterogeneities of flow of the bath on the surface of the sheet,
• burning on the edges of the coated sheet.

Thus, the “compound having a free electronic doublet” is intended to prevent the growth of crystals that settle in parts of the surface where the flow of the bath is disturbed, "since the free pair of electrons is adsorbed on the surface ”; preferably a compound is used bearing a bond C = C (double).

• sels de zinc (10 à 30 g/l),
• sels conducteurs : NH₄Cl, (NH₄)₂SO₄, NH₄BF₄, KCl, H₃BO₃ (20 à 350 g/l),
• agents de lissage : polyethylèneoxydiacide de formule générale HOOC-CH2-(CH2CH2O-)_n-CH2COOH et/ou polyéthylèneoxydiamine, avec n compris entre 1 et 1000, (0,01 à 30 g/l) , et des additifs optionnels tels que
• agents de freinage de polymérisation : acide benzoïque, acide phtalique, acide salycilique (0,1 à 20 g/l)
• agents de brillance : cétone ou aldéhyde aromatique (0,01 à 3 g/l) Naphtalènesulfonate de sodium ou urée phényléthylique (0,1 à 20 g/l)

The object of document JP 58136793 relates to an electrolytic bath containing new smoothing agents. This document describes electrolytic acid baths for depositing a layer of zinc on a steel strip, and containing the following components:

• zinc salts (10 to 30 g / l),
• conductive salts: NH ₄ Cl, (NH ₄ ) ₂ SO ₄ , NH ₄ BF ₄ , KCl, H ₃ BO ₃ (20 to 350 g / l),
• smoothing agents: polyethyleneoxydiacide of general formula HOOC-CH2- (CH2CH2O-) _n -CH2COOH and / or polyethyleneoxydiamine, with n between 1 and 1000, (0.01 to 30 g / l), and optional additives such as
• polymerization braking agents: benzoic acid, phthalic acid, salycilic acid (0.1 to 20 g / l)
• brightening agents: ketone or aromatic aldehyde (0.01 to 3 g / l) Sodium naphthalenesulfonate or phenylethyl urea (0.1 to 20 g / l)

None of the other documents cited above describes the use of polyethylene glycol as a leveling agent; as leveling agent, the document US Pat. _No. 4,229,268 already cited describes a compound of type: RS- (R'-O) _n H or S [(R'O) _n H] ₂ , R 'being an alkylene radical and n being especially equal to 2; the disadvantage of such a leveling agent lies on the one hand in its foul smells and its toxicity, on the other hand in its low solubility in electrodeposition baths heavily loaded with salts, such as electrozincing baths based on chlorides usable at high current densities.

The invention therefore aims to provide a leveling agent for bath electrogalvanizing does not have these drawbacks.

The second problem that the invention seeks to solve therefore concerns improving the corrosion resistance of steel sheets: it is classic to apply to them for this purpose, a metallic coating electrodeposited with protection, in particular a coating based on zinc or zinc alloy.

For determined plating conditions, the efficiency of protection is generally proportional to the thickness of the coating.

Conversely, at a given coating thickness, we are looking for electroplating conditions to achieve maximum efficiency of protection.

To assess the effectiveness of the protection provided by a coating protector of known thickness E on a given substrate, we can proceed as following :

An electrochemical cell is created having as anode the sample to be tested (substrate + coating), as cathode a plate of the same nature as the substrate (without coating), and as electrolyte an aqueous solution of sodium chloride at a concentration of 0, 03 mole / liter, and the electric discharge current of said cell is measured as a function of time; the time Tp is identified at the end of which the discharge current suddenly drops to a much lower level (this sudden drop in current corresponds to the appearance of corrosion on the sample, in the form of red rust); this time Tp therefore corresponds to the corrosion protection time which the coating gives the substrate; the duration of the protection being proportional to the thickness of the coating, the time Tp is related to the thickness E; the value obtained T _ps = T _p / E is specific to the nature of the coating and can be considered as an indicator of the specific protection efficiency of the coating.

Document EP 0 472 204 describes a coating for protection against corrosion, based on zinc or on zinc alloy, containing 0.001 to 10% (expressed as carbon) of an acrylic or metacrylic polymer compound of formula: -CH2- CRR'-CO-X- (CH2) _n -NR "R '", where X = NH or O.

The polymeric compound preferably has a molecular weight high, greater than 1000, to avoid formatting problems; the weight molecular must nevertheless remain below 1,000,000 to allow a sufficient solubilization of the compound in the electrogalvanizing bath.

This “composite” coating offers good formability and especially when painting (adhesion and effectiveness of the protection of the layer of paint) but does not have any specific protection effectiveness remarkable.

To prepare such a coating, aqueous baths are used electrogalvanizing with a pH below 4.

The advantage of this organic compound in solution in the bath of electrodeposition is that it allows to control the localization of the current due to the roughness of the substrate surface and can thus contribute to the preparation of coatings with a smooth and uniform surface, or even with a surface uniformly shiny.

The concentration of this organic compound should not however be too high in the bath, to avoid excessively increasing the viscosity; the increase in viscosity would prevent creating the conditions hydrodynamics allowing the use of high current densities.

The examples given in this document indicate that, according to the concentration of solubilized polymer in the electroplating bath, the carbon content in the coating that is obtained varies for example as follows: 0.2 g / l, 7 g / l and 10 g / l in the bath give respectively 0.01%, 0.6% and 0.7-0.8% carbon in the coating.

In addition to the fact that the addition of acrylic polymer compounds or methacrylics in an electrogalvanizing bath does not increase efficiency here specific protection of the coating obtained by this bath, the use of these compounds is not possible in baths based on chlorides whose pH must remain above 4 to offer good faradic yields.

The use of these compounds is also not possible because, to be able to coat zinc or zinc alloys under high current densities, the bath should contain strong concentrations of soluble salts (KCl, ZnCl2, ...) and that, in such baths, the acrylic or metacrylic polymer compounds are no longer sufficient soluble so that they can be incorporated into the coating in progress electrodeposition.

In the case of deposits of zinc and nickel alloys as described in document US 5,575,899 using a chloride-based bath under a high current density (50 to 150 A / dm2), there is no improvement of the specific protection efficiency (see results presented in Example 10), due, in particular, to the presence of "compound with a free electronic doublet "in the bath and despite the presence of polyethylene glycol in the bath.

Furthermore, in this document, only four examples (4, comp. 2, 13, comp. 6) use a lower average molecular weight polyethylene glycol at 600; the other fourteen examples and the other six comparative examples typically use polyethylene glycol of equal average molecular weight at 600, sometimes greater than 600 (ex. 14: 750).

However, Comparative Example 1 below clearly shows that the addition, in the electroplating bath, of a polyethylene glycol mass polymer molecular weight on the order of 600 has no significant effect on the specific effectiveness of the protection provided by the coating (here, zinc pure), even in the absence of "compound having an electronic doublet free ”.

The object of the invention is to provide a leveling agent for baths. electrogalvanizing based on chlorides, making it possible to obtain, with a good faradic efficiency and under high current densities, coatings with specific protection against significantly improved corrosion.

• dont le pH est supérieur à 4,
• dont la concentration molaire en ions de zinc ou d'alliage de zinc est supérieure à 1 mole/litre,
• ne contenant pas de « composé possédant un doublet électronique libre » choisi dans le groupe comprenant le thiosulfate de sodium, l'acide nicotinique, l'urée, la thio-urée, la nicotinamide et l'acide thioglycolique,
• contenant en solution au moins un polymère polyéthylène-glycol répondant à la formule générale R¹-O-(CH2-CH2-O)_n-R²,

   caractérisé en ce que :

• en premier lieu, n ≤ 13,
• en deuxième lieu, la concentration dudit polymère dans le bain est adaptée pour incorporer dans ledit revêtement un composé organique à une teneur pondérale supérieure à 0,1%, exprimée en poids de carbone par rapport au poids dudit revêtement,
• en troisième lieu :

soit R¹ et/ou R² sont des atomes d'hydrogène et ledit polymère présente une masse moléculaire moyenne M inférieure à 500 g/mole,

soit R¹ et R² sont des groupements substituants d'extrémité de chaíne, différents ou identiques, choisis parmi :

- des groupements non substitués, alkyls (-C_mH_2m+1) ,alkènes (-C_mH_2m-1), ou alkynes (-C_mH_2m-3).

- des groupements substitués en position terminale alkyl (C_mH_2m-R³), alkènes (C_mH_2m-2-R³), ou alkynes (C_mH_2m-4-R³),

   où -R³ = -Q-R⁴ (éthers ou "oxy"), -COO-R⁴ (esters ou "carboxy") ou -COOM (sel d'acide carboxylique), -SO3-R⁴ ("sulfonyl") ou -SO3-M (sel d'acide sulfonique), -CO-R⁴ (cétone), -N=R⁴ ou

(amine), -S-R⁴ ("thio"), ou -C≡N (nitrile),
   où R⁴, R⁵ est choisi parmi H, un groupement alkyl (-C_mH_2m+1), alkène (-C_mH_2m-1), ou alkyne (-C_mH_2m-3),
   la valeur de m étant suffisamment faible pour que ledit polymère soit soluble à ladite concentration dans le bain.The subject of the invention is an aqueous electroplating bath based on chlorides for the preparation of a coating based on zinc or on zinc alloy,

• whose pH is higher than 4,
• whose molar concentration of zinc ions or zinc alloy is greater than 1 mole / liter,
• not containing a “compound having a free electronic doublet” chosen from the group comprising sodium thiosulfate, nicotinic acid, urea, thiourea, nicotinamide and thioglycolic acid,
• containing in solution at least one polyethylene glycol polymer corresponding to the general formula R ¹ -O- (CH2-CH2-O) _n -R ² ,

characterized in that:

• first, n ≤ 13,
• secondly, the concentration of said polymer in the bath is adapted to incorporate into said coating an organic compound with a weight content greater than 0.1%, expressed by weight of carbon relative to the weight of said coating,
• In third place :

either R ¹ and / or R ² are hydrogen atoms and said polymer has an average molecular mass M of less than 500 g / mole,

or R ¹ and R ² are substituent groups at the end of the chain, different or identical, chosen from:

- unsubstituted groups, alkyls (-C _m H _{2m + 1} ), alkenes (-C _m H _2m-1 ), or alkynes (-C _m H _2m-3 ).

- groups substituted in the terminal position alkyl (C _m H _2m -R ³ ), alkenes (C _m H _2m-2 -R ³ ), or alkynes (C _m H _2m-4 -R ³ ),

where -R ³ = -QR ⁴ (ethers or "oxy"), -COO-R ⁴ (esters or "carboxy") or -COOM (carboxylic acid salt), -SO3-R ⁴ ("sulfonyl") or -SO3-M (sulfonic acid salt), -CO-R ⁴ (ketone), -N = R ⁴ or

(amine), -SR ⁴ ("thio"), or -C≡N (nitrile),
where R ⁴ , R ⁵ is chosen from H, an alkyl (-C _m H _{2m + 1} ), alkene (-C _m H _2m-1 ), or alkyne (-C _m H _2m-3 ) group,
the value of m being sufficiently low for said polymer to be soluble at said concentration in the bath.

• la masse moléculaire moyenne M dudit polymère est supérieure à 150 g/mole.
• la concentration dudit polymère dans le bain est comprise entre 10^-4 et 10^-1 mole/litre.
• R¹ et R² sont des atomes d'hydrogène.
• R¹ = R² = -CH2-COOH.

The bath according to the invention can also have one or more of the following characteristics:

• the average molecular mass M of said polymer is greater than 150 g / mole.
• the concentration of said polymer in the bath is between 10 ^-4 and 10 ^-1 mole / liter.
• R ¹ and R ² are hydrogen atoms.
• R ¹ = R ² = -CH2-COOH.

It is considered that the bath does not contain "compound having a free electronic doublet "as soon as the measured concentration is less than 0.001 g / l.

• on fait défiler ladite bande dans un bain d'électrodéposition selon l'invention,
• et on fait passer un courant électrique d'électrodéposition entre ladite bande servant de cathode et au moins une anode disposée dans ledit bain face à ladite bande,

   caractérisée en ce que la densité moyenne de courant, mesurée sur la portion de ladite bande faisant face à l'au moins une anode, est supérieure à 0,25 x J_lim , où J_lim est la densité de courant limite, qui correspond au palier de densité de courant sur la courbe "intensité-potentiel" caractéristique dudit bain d'électrozingage pour une vitesse de défilement donnée de ladite bande par rapport à ce bain.The subject of the invention is also a method of electrodeposition of a coating based on zinc or on a zinc alloy on a strip of steel sheet in which:

• said strip is run in an electroplating bath according to the invention,
• and an electric current of electroplating is passed between said strip serving as a cathode and at least one anode placed in said bath facing said strip,

characterized in that the average current density, measured on the portion of said strip facing the at least one anode, is greater than 0.25 x J _lim , where J _lim is the limit current density, which corresponds to plateau of current density on the "intensity-potential" curve characteristic of said electrogalvanizing bath for a given running speed of said strip relative to this bath.

The invention also relates to a steel sheet coated with a layer corrosion protection based on zinc or prepared zinc alloy by the method according to the invention, characterized in that said layer contains more than 0.1%, preferably more than 0.65%, by weight (expressed as carbon) of an organic compound.

Preferably, in the thickness of said layer and outside the zone of steel-layer interface, the carbon content is greater than or equal to 0.5% in weight.

Carbon content can be measured by discharge spectroscopy luminescent so as to obtain a “C” curve of evolution of the content carbon in the thickness of said layer, as illustrated in FIG. 4 which relates to Example 6; we call "carbon content in the thickness of the layer outside the steel-layer interface area "carbon content measured on this curve "C" without taking into account the interface peak "Ci" described in Example 6.

• - aux figures 1 à 3 qui concernent l'exemple 8 et représentent en ordonnée le coefficient de frottement plan-plan d'échantillons de tôle électrozinguées (figures 2 et 3 : selon l'invention) sur une échelle croissante de 0 à 0,27 par incréments de 0,03 et en abscisse la pression de serrage au niveau du frottement sur une échelle croissante de 0 à 800 10⁵ Pa par incréments de 100.10⁵ Pa.
• à la figure 4 qui concerne l'exemple 6 et représente un spectre « SDL » (Spectroscopie de Décharge Luminescente) d'un échantillon d'acier revêtu de zinc selon l'invention, les courbes Zn, C, Fe représentant respectivement la teneur (ordonnée) en Zn, C, Fe dans la profondeur du revêtement (abscisse).
• aux figures 5 à 15 qui concernent l'exemple 7 et représentent des spectres « SDL » d'échantillons d'acier revêtus de zinc, chaque spectre comprenant trois courbes comme à la figure 4.

The invention will be better understood using the following description, given by way of nonlimiting example, and with reference:

• - Figures 1 to 3 which relate to Example 8 and show on the ordinate the coefficient of plane-plane friction of electro-galvanized sheet samples (Figures 2 and 3: according to the invention) on an increasing scale from 0 to 0.27 in increments of 0.03 and on the abscissa the clamping pressure at the friction level on an increasing scale from 0 to 800 10 ⁵ Pa in increments of 100.10 ⁵ Pa.
• in FIG. 4 which relates to Example 6 and represents an “SDL” spectrum (Glow Discharge Spectroscopy) of a sample of zinc-coated steel according to the invention, the curves Zn, C, Fe representing respectively the content ( ordinate) in Zn, C, Fe in the depth of the coating (abscissa).
• in FIGS. 5 to 15 which relate to Example 7 and represent "SDL" spectra of samples of steel coated with zinc, each spectrum comprising three curves as in FIG. 4.

We are trying to achieve, continuously and under high current density, a zinc-based electrolytic coating on a steel strip for the effectively protect against corrosion while limiting roughness.

The plating installation is known per se and will not be described here in detail; it includes a succession of electrolysis cells.

Each electrolysis cell includes a tank, a conductive roller tape support and soluble anodes of zinc or zinc alloy facing said roller.

For the purpose of coating the steel strip, a electroplating bath containing zinc ions in solution.

The electroplating bath is a conventional bath based on chlorides, known in itself, allowing high-yield plating under high current densities, especially above 50 A / dm2, say for example with a pH greater than 4 and an ion concentration Zn2 + greater than 1 mole / liter.

• soit R¹ et/ou R² sont des atomes d'hydrogène et ledit polymère présente une masse moléculaire moyenne M inférieure à 500 g/mole,
• soit R¹ et R² sont des groupements substituants d'extrémité de chaíne, différents ou identiques, choisis parmi :
• des groupements non substitués, alkyls (-C_mH_2m+1) ,alkènes (-C_mH_2m-1), ou alkynes (-C_mH_2m-3).
• des groupements substitués en position terminale alkyl (C_mH_2m-R³), alkènes (C_mH_2m-2-R³), ou alkynes (C_mH_2m-4-R³),

   où -R³ = -O-R⁴ (éthers ou "oxy"), -COO-R⁴ (esters ou "carboxy") ou -COOM (sel d'acide carboxylique), -SO3-R⁴ ("sulfonyl") ou -SO3-M (sel d'acide sulfonique), -CO-R⁴ (cétone), -N=R⁴ ou

(amine), -S-R⁴ ("thio"), ou -C≡N (nitrile),
   où R⁴, R⁵ est choisi parmi H, un groupement alkyl (-C_mH_2m+1), alkène (-C_mH_2m-1), ou alkyne (-C_mH_2m-3). In this bath, according to the invention, at least one polyethylene glycol polymer corresponding to the general formula R ¹ -O- (CH2-CH2-O) _n -R ² is dissolved with n ≤ 13 and where:

• either R ¹ and / or R ² are hydrogen atoms and said polymer has an average molecular mass M of less than 500 g / mole,
• or R ¹ and R ² are substituent groups at the end of the chain, different or identical, chosen from:
• unsubstituted groups, alkyls (-C _m H _{2m + 1} ), alkenes (-C _m H _2m-1 ), or alkynes (-C _m H _2m-3 ).
• groups substituted in the terminal position alkyl (C _m H _2m -R ³ ), alkenes (C _m H _2m-2 -R ³ ), or alkynes (C _m H _2m-4 -R ³ ),

where -R ³ = -OR ⁴ (ethers or "oxy"), -COO-R ⁴ (esters or "carboxy") or -COOM (carboxylic acid salt), -SO3-R ⁴ ("sulfonyl") or -SO3-M (sulfonic acid salt), -CO-R ⁴ (ketone), -N = R ⁴ or

(amine), -SR ⁴ ("thio"), or -C≡N (nitrile),
where R ⁴ , R ⁵ is chosen from H, an alkyl (-C _m H _{2m + 1} ), alkene (-C _m H _2m-1 ), or alkyne (-C _m H _2m-3 ) group.

According to the invention, the bath does not contain organic compounds sulfur like those described as leveling agents and brighteners in document US 4,229,268.

We circulate the electroplating bath in the cells electrodeposition so that the relative speed of the bath in the vicinity of the strip is greater than 30 m / minute, generally between 80 and 160 m / minute, which corresponds to hydrodynamic conditions classics in industrial production.

Preferably, the temperature of the electrolysis bath is maintained between 55 ° C and 65 ° C.

The molar concentration of polyethylene glycol dissolved in the bath must be suitable for obtaining, under the conditions of use of the bath, a coating based on zinc or zinc alloy incorporating an organic compound into a content greater than 0.1% (expressed as carbon); the examples illustrate the adaptation of this concentration in the bath.

Preferably, the molar concentration of polyethylene glycol dissolved in the bath is between 10 ^-4 and 10 ^-1 mole / liter.

In addition, the value m concerning the substitution groups of the polyethylene glycol must be low enough for the polymer to be soluble in sufficient concentrations.

To coat the steel strip, while scrolling the strip in each cell of the installation on the conductive rollers, passes an electric current between said cathode strip and the anodes.

A current density greater than 0.25 x J _lim is applied _. , where J _lim. is the previously defined current limit density which depends on the nature of the electrogalvanizing bath but also on the circulation speed of the bath in the vicinity of the strip.

For a chloride bath according to the invention circulating at a speed of between 100 and 150 m / min. with respect to the band, the value of J _lim. is generally close to 140-150 A / dm2.

Thus, the electric current density must be greater than 35 A / dm2; in practice it is generally between 50 A / dm ² and 140 A / dm ² .

We also adapt the electrodeposition conditions, for example the running speed of the steel strip in the installation, to obtain a sufficient coating thickness for effective tape protection against corrosion; this thickness is generally between 3 and 15 micrometers.

A steel strip is thus obtained coated with a protective layer to zinc base containing an organic compound of the same nature or derived from polyethylene glycol contained in the bath according to the invention.

The electrodeposition bath according to the invention makes it possible to obtain coatings of very good quality, that is to say in particular both little rough and highly resistant to corrosion.

This simultaneous effect of leveling and improving efficiency specific corrosion protection is achieved thanks to the product polymer added to the electrogalvanizing bath and to the organic compound incorporated into the coating; to achieve this effect, it is important that the content of carbon in the coating is greater than 0.1%.

The coating according to the invention has a roughness less than that that a coating produced under the same conditions would have on the same substrate, but with a conventional electroplating bath not containing not of this polymer product.

The leveling effect provided by the polymer product under conditions of high current density is accompanied by a refining effect providing particularly homogeneous coating texture.

The coating according to the invention provides resistance to the steel strip corrosion significantly improved compared to that which a coating of the same thickness in pure zinc or pure zinc alloy according to art anterior, prepared under the same conditions from a bath conventional plating not containing this polymer product.

When looking for a predetermined level of protection against corrosion, so we can be satisfied with thin coatings than in the prior art, which has a certain economic advantage.

Advantageously, this reduction in thickness is accompanied by a reduced risk of cracking of the coating (in the event of deformation of prison).

Besides this leveling effect and specific efficiency improvement corrosion protection, it was found that the coatings according to the invention had interesting tribological properties and offered, if painted, very strong adhesion to the paint layer.

Subject to friction after oiling, these coatings offer less grazing than identical coatings prepared using baths conventional electrogalvanizing.

This advantage in terms of tribological properties allows in particular to facilitate shaping operations, in particular stamping, galvanized sheets.

If we apply a layer of paint by electrophoresis on these coatings, we see that the paint layer adheres much better only on identical coatings prepared using baths conventional electrogalvanizing.

The following examples illustrate the invention:

Example 1 :

The purpose of this example is to illustrate the high level of protection against corrosion and the low degree of roughness obtained by using baths electrodepostion according to the invention containing non-polyethylene glycols substituted.

We use samples of bare steel sheet, cut in the shape of disk.

• Zn 2+ (sous forme ZnCl2)   : 1,6 moles/litre
• KCI   : 5,3 moles/litre
• Polyéthylène-Glycol dit "PEG 300"   : concentration c variable.

   représenté par la formule générale H-O-(CH2-CH2-O)_n-H , où n est compris entre 6 et 7, dont la masse moléculaire moyenne vaut environ 300 g.In a laboratory electrodeposition cell, of the rotating electrode type, an electrolysis bath is prepared according to the following composition:

• Zn 2+ (as ZnCl2): 1.6 moles / liter
• KCI: 5.3 moles / liter
• Polyethylene-Glycol called "PEG 300": variable concentration c.

represented by the general formula HO- (CH2-CH2-O) _n -H, where n is between 6 and 7, whose average molecular weight is approximately 300 g.

The pH of the bath is 5 and its temperature maintained at approximately 63 ° C.

The sample immersed in the bath is then rotated at a rotation speed of V = 2500 rpm.

A layer of zinc is then deposited on this steel sample in passing between the steel sheet forming cathode (here rotating) and anodes immersed in the bath, an electric current such as the density of current per unit area of the sheet is approximately J = 80 A / dm2, which represents a high current density.

This current density is maintained until the coating reaches a thickness of approximately 10 µm.

• l'efficacité spécifique de protection contre la corrosion que le revêtement apporte à la tôle ; cette efficacité est évaluée ici par le terme T_ps tel que défini précédemment; on considère que la précision de mesure de T_ps est de l'ordre de ± 0,5 Heure/µm.
• la prise de rugosité ΔR_a telle que définie précédemment ; on considère que la précision de mesure de ΔR_a est de l'ordre de ± 0,01 µm.

Table 1 shows, for different concentrations c of PEG 300 in the bath:

• the specific corrosion protection efficiency that the coating brings to the sheet; this efficiency is evaluated here by the term T _ps as defined above; it is considered that the measurement accuracy of T _ps is of the order of ± 0.5 Hour / µm.
• the roughness setting ΔR _a as defined above; it is considered that the measurement accuracy of ΔR _a is of the order of ± 0.01 µm.

By way of comparison, the results obtained on a coating prepared in the same way have been reported, under the title "reference", except that the bath does not contain PEG 300 but a commercial additive called USSP from the Company. US Steel. baths containing PEG 300 Electrolysis bath Concentration c (Mole / liter) Protection time T _ps (Hour / µm) Roughness setting ΔR _a (µm) Reference (USSP) - 10 0.13 PEG 300 10 ^-3 14 0.06 PEG 300 10 ^-2 14 0.10 PEG 300 3.10 ^-2 15 PEG 300 5.10 ^-2 15

It can therefore be seen that the electrogalvanizing bath makes it possible to limit the roughness setting when the “PEG 300” concentration remains less than or equal to 10 ^-2 molar and that the coating obtained is more resistant to corrosion than the reference coating.

The coating obtained according to the invention is in the form of grains very homogeneous in size, about 0.2 µm, and contains an organic compound of the same nature or derived from the polymer product introduced according to the invention in the electrogalvanizing bath.

Comparative example 1 :

The purpose of this example is to illustrate the importance of the number n of “ethoxy” radicals of the polyethylene glycol which is used in the electrodeposition bath (according to the formula HO- (CH2-CH2-O) _n -H) .

We proceed as in Example 1 except that we use a polyethylene glycol called "PEG 600", whose average molecular weight is close to 600 and for which n is worth approximately 14, i.e. a value greater than the limit provided by the invention.

The results reported in Table II are then obtained, presented in a manner similar to those of Table I.

It is then found that the addition of such a polymer in the bath has no significant effect on the specific effectiveness of the protection provided by the coating.

Example 2 :

The purpose of this example is to illustrate the high level of protection against corrosion and the low degree of roughness obtained by using baths of electrodepostion according to the invention containing polyethylene glycols substituted at both ends of the chain.

Prepared samples are prepared under the same conditions as in Example 1 with the difference that the PEG 300 is replaced by potyethylene glycolbiscarboxymethyl ether (or PEGbiCOOH 250) of general formula R ¹ -O- (CH2-CH2-O) _n -R ² , where R ¹ = R ² = -CH2-COOH and where n = 3 approximately, the average molecular mass M being here equivalent to approximately 250 g / mole.

The results obtained in terms of corrosion resistance (T _ps ) and roughness setting ΔR _a are reported in Table III below. baths containing PEGbiCOOH 250 Electrolysis bath Concentration c (Mole / liter) Protection time T _ps (Hour / µm) Roughness setting ΔR _a (µm) Reference (USSP) - 10 0.13 PEGbiCOOH 250 10 ^-2 13 0.08 PEGbiCOOH 250 5.10 ^-2 15 0.06

As in Example 1, it can therefore be seen that the electrogalvanizing bath makes it possible to limit the roughness, at least for concentrations of “PEGbiCOOH 250” of between 1 and 5 10 ^-2 molar, and that the coating obtained resists better corrosion than the reference coating.

Example 3 :

The purpose of this example is to illustrate the high level of protection against corrosion and the low degree of roughness obtained by using baths of electrodepostion according to the invention containing polyethylene glycols substituted at both ends of the chain, but with a degree of polymerization higher than that of Example 2.

Coated samples are prepared under the same conditions as in Example 2, with the difference that PEGbiCOOH 250 is replaced by a product with the same general formula but more polymerized, called "PEGbiCOOH 600", for which n = 11 approximately, the molecular mass average M worth around 600 g / mole.

The results obtained in terms of corrosion resistance (T _ps ) and roughness setting (ΔR _a ) are recorded in Table IV below. baths containing PEGbiCOOH 600 Electrolysis bath Concentration c (Mole / liter) Protection time T _ps (Hour / µm) Roughness setting ΔR _a (µm) Reference (USSP) - 10 0.13 PEGbiCOOH 600 0.5.10 ^-3 12.5 - 0.15 PEGbiCOOH 600 10 ^-3 12.5 - 0.05

It can therefore be seen that, under the conditions of use of the bath electroplating and in the case of polyethylene glycol polymers of the type "PEGbiCOOH", we manage, even for very low concentrations, to a decrease in roughness after deposition, even though the specific efficiency coating protection against corrosion is increased.

Example 4 :

The purpose of this example is to illustrate the level of incorporation of compound organic in the protective coatings according to the invention.

On a sample of aluminum which is a substrate deemed "no adherent ”, a“ pre-deposit ”of zinc with a thickness of 3 μm is prepared, under the same conditions as those of Example 3, with the difference do not add polyethylene glycol in the electrogalvanizing bath.

Over this pre-deposit, we continue the electrodeposition in baths identical to those of Example 3 until a thickness of 100 μm is obtained about.

The coating obtained is detached from its aluminum substrate, which allows the total carbon contained in the coating to be measured; thanks to pre-deposit, the polymer possibly contained in this is isolated from the substrate coating.

The carbon contained in the coating is then metered using a conventional carbon metering device including an induction furnace (commercial reference LECO HF-100) coupled to an infrared analyzer.

The results obtained are shown in Table V. PEGbiCOOH 600 baths - carbon content in the coating. Electrolysis bath Concentration c (Mole / liter) Weight content of C in the coating in%. Reference (USSP) - - PEGbiCOOH 600 0.5.10 ^-3 0.1 to 0.4% PEGbiCOOH 600 10 ^-3 0.5 to 1%

The coating obtained according to the invention therefore contains carbon - therefore an organic compound - in significant quantity, proportional to the concentration of polyethylene glycol polymer added to the bath electrogalvanizing according to the invention.

This evaluation of the incorporation of an organic compound in the coating according to the invention does not make it possible to account for phenomena specific incorporation of the substrate-coating interface (unlike Example 7).

Example 5 :

The purpose of this example is to illustrate the importance of the conditions of use of the electrogalvanizing bath according to the invention on the properties of the coating, in terms of resistance to corrosion and increase in roughness (ΔR _a ).

Baths identical to those of Example 2 are prepared, containing, according to the invention, PEGbiCOOH 250 at different concentrations.

This time we operate on steel samples in the form of plates of approximately 300 cm2 of surface.

To prepare coatings from these baths, we use this time an electroplating cell where the sample (plate) is placed in a fixed position opposite the anode and where the electrogalvanizing bath is circulated between the sample serving as the cathode and the anode at a constant flow velocity of 150 m / min.

The samples are thus coated with a zinc-based layer. thickness of the order of 10 µm using current densities included between 50 and 140 A / dm2.

For these electrogalvanizing baths and their flow velocity conditions with respect to the surface to be coated, it is estimated that the value of the limit current density J _lim. is close to 140 A / dm2.

The current density range 50 to 140 A / dm2 therefore corresponds well to an area where the roughness of the coating basically depends on the roughness of the substrate ("spike effect"), not the grain size.

On each coated sample, the resistance to cosmetic corrosion is evaluated, in addition to the resistance to perforating corrosion (T _ps ) and the roughness setting (ΔR _a ) as previously.

To carry out cosmetic corrosion resistance tests on these coated samples, they must first be painted in a standard; this painting includes, in a conventional manner, a phosphating treatment, applying a first coat of paint by cataphoresis, then a second layer called primer, finally a third coat of lacquer.

After this standardized painting, a scratch is made on the coated and painted sheet metal, using a standardized device suitable for forming a scratch about 0.5 mm wide up to the level of the sheet metal.

In order to assess the resistance to cosmetic corrosion, we then submit samples coated, painted and scratched at climatic cycles.

• 24 heures sous brouillard salin en enceinte climatique (selon la norme NF 41 002), puis rinçage à l'eau bi-permutée et essuyage,
• 4 jours en chambre climatique décomposés comme suit :
  • de 9 H à 17 H : 40°C et 95 à 100% d'humidité relative,
  • de 17 H à 9 H : 20°C et 70 à 75 % d'humidité relative,
• 2 jours en chambre de séchage : 20°C et 60 à 65% d'humidité.

Each elementary cycle lasts one week and breaks down as follows:

• 24 hours under salt spray in a climatic chamber (according to standard NF 41 002), then rinsing with bi-permuted water and wiping,
• 4 days in a climatic chamber broken down as follows:
  • 9 a.m. to 5 p.m .: 40 ° C and 95 to 100% relative humidity,
  • from 5 p.m. to 9 a.m .: 20 ° C and 70 to 75% relative humidity,
• 2 days in a drying chamber: 20 ° C and 60 to 65% humidity.

We observe and measure the average width of degradation of the scratch, called "blister width", on each sample, after, in the present case, 14 climatic cycles.

This blistering width makes it possible to evaluate the corrosion resistance cosmetic: the narrower the width, the better the resistance to corrosion.

The results obtained in terms of corrosion resistance (protection time T _ps and blistering width) and roughness setting (ΔR _a ) are given in Tables VI, VII, VIII and IX below, respectively for densities of current J = 50, 80, 110 and 140 A / dm2. baths containing PEGbiCOOH 250 - J = 50 A / dm2 Electrolysis bath Concentration (Mole / liter) Protection T _ps (H / µm) Blister width (mm) Roughness plug ΔR _a (µm) Reference (USSP) - 10.2 1.5 0.10 PEGbiCOOH 250 10 ^-2 12.4 1.5 0.00 PEGbiCOOH 250 5.10 ^-2 14.5 1 0.10 baths containing PEGbiCOOH 250 - J = 80 A / dm2 Electrolysis bath Concentration (Mole / liter) Protection T _ps (H / µm) Blister width (mm) Roughness plug ΔR _a (µm) Reference (USSP) - 10.5 1.5 0.18 PEGbiCOOH 250 10 ^-2 12.4 1.75 0.08 PEGbiCOOH 250 5.10 ² 14.0 1.25 0.18 baths containing PEGbiCOOH 250 - J = 110 A / dm2 Electrolysis bath Concentration (Mole / liter) Protection T _ps (H / µm) Blister width (mm) Roughness plug ΔR _a (µm) Reference (USSP) - 10.6 1.5 0.27 PEGbiCOOH 250 10 ^-2 12.0 1.75 0.15 PEGbiCOOH 250 5.10 ^-2 13.0 1.0 0.27 baths containing PEGbiCOOH 250 - J = 140 A / dm2 Electrolysis bath Concentration (Mole / liter) Protection T _ps (H / µm) Blister width (mm) Roughness plug ΔR _a (µm) Reference (USSP) - 10.7 1.5 0.28 PEGbiCOOH 250 10 ^-2 12.4 1.75 0.20 PEGbiCOOH 250 5.10 ^-2 13.0 0.75 0.28

On the "reference" line, we see that the roughness increase increases with the current density, going from 0.1 µm at 50 A / dm2 to 0.27-0.28 µm under 110 A / dm2 and above.

These results indicate that, compared to the reference under the same current density conditions, the roughness setting significantly decreases at a concentration of 10 ^-2 molar of "PEGbiCOOH 250" in the bath, but remains comparable to the reference for a higher concentration (5 10 ^-2 molar); it is therefore necessary to adapt the polymer concentration in the bath to optimize the leveling effect.

For the same additive "PEGbiCOOH 250", Example 2 indicated that the roughness setting decreased at a concentration of 10 ^-2 molar, and above all decreased even more sharply (without however reaching 0.1 μm) at a higher concentration (5 10 ^-2 molar).

We therefore deduce that the optimum leveling agent concentration according to the invention (here “PEGbiCOOH 250”) depends on its conditions of use.

The examples illustrating the invention show that there is a "threshold" effect for the polymer concentration in the electrogalvanizing bath according to the invention, beyond which the roughness taken driven by the coating does decreases more depending on the concentration; this threshold effect has also been already described for leveling agents other than those of the invention, as for tetrabutyl ammonium chloride (M. Sanchez Cruz: F. Alonso; J.M. Palacios in J. of Applied Electrochemistry, Vol. 20 - 1990 - p 611: "The effect of the concentration of TBACI on the electrodeposition of zinc from chloride and perchlorate electrolytes ”).

• dans les conditions d'utilisation du bain de cet exemple, l'efficacité spécifique de protection contre la corrosion perforante des revêtements (T_ps) dépend peu des densités de courant sous lesquelles ils ont été réalisés (à condition de rester dans le domaine J > 0,25 x J_lim.), mais essentiellement de la concentration en polymère dans le bain d'électrozingage, qui est liée à la concentration en composé organique incorporé dans le revêtement lui-même (selon les exemples 4 et 6).
• la résistance à la corrosion cosmétique (inversement proportionnelle à la largeur de cloquage) est identique ou se dégrade par rapport à celle de la référence pour une concentration de 10^-2 molaire de « PEGbiCOOH 250 » dans le bain, mais s'améliore significativement pour une concentration supérieure (5 10^-2 molaire) ; le résultat de ce test étant également significatif de l'aptitude à la mise en peinture des échantillons revêtus par cataphorèse (cf. tests d'adhérence : exemple 9), il reste difficile d'en tirer des conclusions spécifiques sur la résistance à la corrosion.

In terms of corrosion resistance, the result tables VI to IX show that:

• under the conditions of use of the bath of this example, the specific effectiveness of protection against perforating corrosion of the coatings (T _ps ) depends little on the current densities under which they were produced (provided that they remain in the field J> 0.25 x J _lim. ), But essentially the concentration of polymer in the electrogalvanizing bath, which is linked to the concentration of organic compound incorporated in the coating itself (according to Examples 4 and 6).
• the resistance to cosmetic corrosion (inversely proportional to the width of blistering) is identical or degraded compared to that of the reference for a concentration of 10 ^-2 molar of “PEGbiCOOH 250” in the bath, but significantly improves for a higher concentration (5 10 ^-2 molar); the result of this test also being significant of the ability to paint samples coated by cataphoresis (cf. adhesion tests: example 9), it remains difficult to draw specific conclusions on corrosion resistance .

Example 6 :

• température du bain : environ 63°C.
• vitesse d'écoulement de l'électrolyte contre la tôle à revêtir : 100 m/min.
• densité de courant : 80 A/dm2.
• épaisseur des dépôts : 8 à 10 µm.

The purpose of this example is to illustrate the impact of the concentration of polymer of polyethylene glycol type in the electro-zinc bath on the roughness setting (ΔR _a ), on the specific protection efficiency (T _ps ) and on the level of incorporation of organic compound in the coating obtained (level of C), this time using the electrodeposition device of Example 5 under the following conditions:

• bath temperature: around 63 ° C.
• flow rate of the electrolyte against the sheet to be coated: 100 m / min.
• current density: 80 A / dm2.
• thickness of deposits: 8 to 10 µm.

The reference bath contains 5.3 moles / liter of potassium chloride (Kcl), 1.6 mole / liter of zinc chloride (ZnCI2) and 0.7 to 1 ml / liter of additive referenced USSP of the US Steel Company.

This USSP additive contains mainly polyethylene glycol average molecular weight close to 600, sodium benzoate and boric acid.

Apart from the reference bath, the electrogalvanizing baths used according to the invention are prepared by addition of polymer polyethylene glycol in this reference bath.

The amount of polyethylene glycol provided by the USSP additive in these baths is very much lower than that which is added to baths according to the invention.

To assess the carbon content in the coating, we proceed here by luminescent discharge spectroscopy ("SDL"); then we get a spectrum as shown in Figure 4.

This spectrum represents on the ordinate the concentration of element and abscissa the depth of erosion.

Referring to Figure 4, the glow discharge spectroscopy then gives the evolution, in the depth of the sample from its coated surface, zinc content (signal "Zn", decreasing at coating-substrate interface), iron content (“Fe” signal, increasing at level of the coating-substrate interface) and of the carbon content (signal "VS").

To assess the carbon content in the coating, the area was measured included under the curve formed by the signal "C".

Note that the curve "C" forms a peak in "Ci" positioned approximately at the interface between the zinc coating and the steel sheet; this “Ci” peak reflects an over-concentration of organic compound at the interface.

To assess the carbon content in the thickness of the coating excluding the steel-zinc interface zone, we would measure the area under the curve formed by the signal "C" without taking into account the peak "Ci".

The results obtained are presented as a function of the nature and of the concentration c of polymer in the electrolysis bath in Tables X for coatings of 10 μm thickness and in Table XI for coatings of 8 μm thickness. PEGbiCOOH baths - 10 µm coatings Electrolysis bath Dose. c (Mole / liter) C rate (in%) T. protection T _ps (H / µm) P. roughness ΔR _a (µm) Reference (USSP) - 0.05% 11 0.28 PEGbiCOOH 600 0.5 10 ^-3 0.5% 11 0.10 PEGbiCOOH 600 1.5 10 ^-3 0.65% 11.5 0.04 PEGbiCOOH 600 2.0 10 ^-3 0.8% 13 0.02 PEGbiCOOH 600 3.0 10 ^-3 1% 13.5 0.08 PEGbiCOOH 250 5.10 ^-2 0.3% 14 0.16 PEGbiCOOH baths - 8 µm coatings Electrolysis bath Dose. c (Mole / liter) C rate (in%) T. protection T _ps (H / µm) P. roughness ΔR _a (µm) Reference (USSP) - - 9 0.23 PEGbiCOOH 600 0.5 10 ^-3 - 11.8 0.12 PEGbiCOOH 600 2.0 10 ^-3 - - 0.08 PEGbiCOOH 600 3.0 10 ^-3 - 13.0 0.11 PEGbiCOOH 250 5.10 ^-2 - 13.8 0.13

• que les résultats concernant le « PEGbiCOOH 250 » diffèrent de ceux de l'exemple 5 ; cette différence pourrait être attribuée à l'additif commercial USSP présent dans les bains de l'exemple 6 mais absent dans les bains de l'exemple 5.
• que les résultats concernant le « PEGbiCOOH 600 » comparés à ceux de l'exemple 3, montrent que - comme pour le « PEGbiCOOH 250 » : cf. conclusions de l'exemple 5 - il existe un « optimum » de concentration de polymère dans le bain (« effet seuil ») et que la valeur de cet optimum dépend ici non seulement de ses conditions d'utilisation mais aussi des autres constituants du bain (additif USSP par exemple).

Regarding the roughness, we note:

• that the results concerning “PEGbiCOOH 250” differ from those of Example 5; this difference could be attributed to the USSP commercial additive present in the baths of Example 6 but absent in the baths of Example 5.
• that the results concerning “PEGbiCOOH 600” compared to those of Example 3, show that - as for “PEGbiCOOH 250”: cf. conclusions of example 5 - there is an "optimum" of polymer concentration in the bath ("threshold effect") and that the value of this optimum depends here not only on its conditions of use but also on the other constituents of the bath (USSP additive for example).

Regarding the carbon content in the coating, we observe that the product "PEGbiCOOH 600" gives coatings richer in carbon than the product "PEGbiCOOH 250"; this difference is less large if the incorporation in the coating is expressed in "number of polymer molecules or chains ”.

Example 7 :

The purpose of this example is to illustrate the influence of the current density electrogalvanizing and polyethylene glycol concentration in baths according to the invention on the level and place of incorporation of compound organic in the zinc coatings obtained.

Samples are prepared under the same conditions as in Example 6 (electrogalvanizing baths additive with "PEGbiCOOH 600", thickness 10 µm) at different current densities (20 to 140 A / dm2) and assesses, as in Example 6, the level of carbon incorporated in the coatings obtained.

• aux figures 5, 6, 7 pour le bain de référence à, respectivement, 20, 80 et 140 A/dm2.
• aux figures 8 à 12 pour des bains contenant 1 10^-3 mole/litre de « PEGbiCOOH 600 » à, respectivement, 20, 50, 80, 110 et 140 A/dm2.
• aux figures 13 à 15 pour des bains contenant 2 10^-3 mole/litre de « PEGbiCOOH 600 » à, respectivement, 20, 50 et 80 A/dm2.

The “SDL” spectra of the coatings obtained are represented in a manner analogous to that of FIG. 4:

• in Figures 5, 6, 7 for the reference bath at, respectively, 20, 80 and 140 A / dm2.
• in Figures 8 to 12 for baths containing 1 10 ^-3 mole / liter of "PEGbiCOOH 600" at, respectively, 20, 50, 80, 110 and 140 A / dm2.
• in Figures 13 to 15 for baths containing 2 10 ^-3 mole / liter of "PEGbiCOOH 600" at, respectively, 20, 50 and 80 A / dm2.

In each figure, the carbon content curve is identified by comparison of this figure with that of Figure 4 described in detail at Example 6.

The results obtained are summarized in Table XII.

Table XII shows that the current density and the concentration of polyethylene glycol polymer in the bath affect the amount of organic compound incorporated into the coating.

The influence of the concentration of polyethylene glycol polymer in the bath had already been illustrated by example 4 at a high current density (80 A / dm2).

At low current density (20 A / dm2, Figures 8 and 13), the coating incorporates little organic compound.

For higher current densities (J> 0.25 x J _lim. ), The evolution of the incorporation of organic compound as a function of the current density is different depending on whether it is incorporation in the core of the coating or at the coating-steel interface, as illustrated in Figures 9 to 12, 14 and 15; in FIG. 12, the carbon concentration at the interface reaches almost twice the carbon concentration in the core of the coating (cf. pic “Ci” in FIG. 4, very pronounced here).

• la teneur en carbone dans l'épaisseur de la couche, en dehors de la zone d'interface acier-zinc (zone du pic « Ci »), croit avec la densité de courant puis reste à peu près constante au delà d'une densité de courant située 80 et 110 A/dm2 (« effet de seuil ») ;
• la teneur en carbone à l'interface acier-zinc (hauteur du pic « Ci ») croít régulièrement avec la densité de courant, sans effet de seuil.

The following elements can be deduced from these results:

• the carbon content in the thickness of the layer, outside the zone of steel-zinc interface (zone of the peak "Ci"), increases with the current density then remains approximately constant beyond a density current between 80 and 110 A / dm2 ("threshold effect");
• the carbon content at the steel-zinc interface (height of the peak "Ci") increases regularly with the current density, without threshold effect.

By comparing Figures 8 to 12 and Figures 13 to 15, we see that the threshold of carbon concentration in the thickness of the layer is higher at the concentration 2.10-3 M / I than at the concentration 1.10-3 M / I of "PEGbiCOOH 600"; now, the results of Tables X and XI (Example 6) concerning this polymer show a greater improvement in the specific efficiency of protection against corrosion (T _ps ) as soon as the carbon content exceeds 0.65% in the coating; it is estimated that, in this case, the carbon content, measured only in the thickness of said layer and outside the steel-layer interface zone, is greater than or equal to 0.5% by weight; this “carbon content in the thickness of the layer outside the steel-layer interface zone” is evaluated by measuring the area under the curve “C” without taking into account the interface peak “Ci”.

Example 8 :

The purpose of this example is to illustrate the impact of the polymer additive introduced into the electroplating baths according to the invention and the incorporation of this polymer into the electrodeposited coating from this bath, on the tribological properties of the coating.

Tribological tests are carried out in a suitable conventional tribometer to measure the "plane-plane" type coefficient of friction of a sample against a "standard" surface by gradually increasing the pressure of clamping of this sample against this surface.

All the samples are coated as in Example 7 under a current density of 80 A / dm2.

Before the friction test, the samples are all oiled with the same using an oil referenced 4107S from the company FUCHS which is not not considered as an oil specially adapted for stamping.

The results obtained are shown in Figures 1 to 3 and summarized in Table XIII. tribological properties of the coating. Electrogalvanizing bath cf. Coefficient of friction Additive Conc. c M / l figure mini way max Reference - 1 0.05 0.14 0.23 PEGbiCOOH 600 2 10 ^-3 2 0.06 0.13 0.18 PEGbiCOOH 250 5 10 ^-2 3 0.09 0.14 0.20

We therefore observe, especially when analyzing the figures, that the addition of polyethylene glycol in an electrodeposition bath according to the invention makes it possible to make zinc-based coatings which have better properties tribological: in fact, friction on coatings according to the invention has much less chatter than friction on a coating classic made from the same bath but without additives.

This advantage is particularly valuable for shaping, in particular by stamping, electro-galvanized sheets.

Example 9 :

The purpose of this example is to illustrate the exceptional characteristics adhesion to paints offered by zinc-based coatings or zinc alloys produced according to the invention, in particular in the case of paints applied by electrophoresis.

• des échantillons dits de référence (USSP seulement comme additif dans le bain d'électrozingage),
• des échantillons selon l'invention dans un bain additivé de 1 10^-3 mole/litre de « PEGbiCOOH 600 ».

Under conditions analogous to those of Example 6, two types of samples of galvanized sheets are prepared:

• so-called reference samples (USSP only as an additive in the electrogalvanizing bath),
• samples according to the invention in an additive bath of 1 10 ^-3 mole / liter of "PEGbiCOOH 600".

• mode 1 : phosphatation (épaisseur 3 µm) puis enduction ;
• mode 2 : enduction directe, sans phosphatation préalable.

These samples are coated with paint by cataphoresis in two different ways:

• mode 1: phosphating (thickness 3 µm) then coating;
• mode 2: direct coating, without prior phosphating.

The coating conditions are identical in the two modes: in particular uses the same cataphoresis bath referenced PPG 742 (Company PPG).

• on plonge les échantillons peints dans l'eau bipermutée à 50°C pendant 10 jours ;
• puis, après séchage, à l'aide d'un outil coupant de type « cutter » (en langue anglaise), on effectue sur la couche de peinture des rayures suffisamment profondes pour atteindre le métal sous la peinture ; on réalise en fait un quadrillage de rayures sur une surface de 1 cm2 d'échantillon, les lignes de rayures étant équidistantes de 1 mm environ les unes des autres.
• on déforme ensuite l'échantillon au niveau de la surface quadrillée de la manière suivante : comme pour un test dit « Erichsen », on appuie un poinçon hémisphérique (diamètre : 20 mm) à tête polie sur la face opposée à la surface quadrillée et on l'enfonce sur 8 mm de profondeur, l'échantillon étant bloqué dans une matrice annulaire (serre-flan).
• on applique un ruban plastique auto-adhésif par pression (de type « Scotch ») sur le quadrillage à l'endroit de la déformation ;
• on arrache ensuite le ruban et on mesure, à l'endroit du quadrillage, la proportion de la surface qui n'est plus couverte par la couche de peinture.

On the painted samples, the paint adhesion tests are then carried out as follows:

• the painted samples are immersed in bipermuted water at 50 ° C for 10 days;
• then, after drying, using a cutting tool of the “cutter” type (in English), scratches are made on the paint layer deep enough to reach the metal under the paint; a grid of stripes is in fact produced on a surface of 1 cm 2 of sample, the stripe lines being equidistant from about 1 mm from each other.
• the sample is then deformed at the level of the grid surface in the following manner: as for a so-called "Erichsen" test, a hemispherical punch (diameter: 20 mm) with a polished head is pressed on the face opposite to the grid surface and push it in 8 mm deep, the sample being locked in an annular matrix (blank holder).
• a self-adhesive plastic tape is applied by pressure (“Scotch” type) to the grid at the location of the deformation;
• the tape is then torn off and the proportion of the surface which is no longer covered by the layer of paint is measured at the location of the grid.

The results obtained are reported in Table XIV. paint adhesion tests. Electroplating bath How to apply the paint: % of surface discovered after adhesion test. Reference (USSP) mode 1 90 to 100% id. mode 2 100% PEGbiCOOH at 1.10-3 M / l mode 1 0% id. mode 2 40%

It can therefore be seen that the layers of paint, especially when they are applied by cataphoresis, adhere very strongly to the coatings based on zinc or zinc alloys when made from baths electroplating according to the invention.

Example 10 :

The purpose of this example is to show that the introduction of certain additives in electroplating baths, in addition to polyethylene glycol according to the invention has the effect of annihilating or appreciably limiting the effects of the invention, in particular at the level of the incorporation of organic compound in coating and improving the specific protection efficiency of this coating.

• bain n°1 : bain de référence identique à celui de l'exemple 6,
• bain n°2 conforme à l'invention, préparé à partir du bain n°1 auquel on a ajouté 2.10^-3 Mole/litre de « PEGbiCOOH 600 »,
• bain n°3, conforme au document US 5 575 899 déjà cité, préparé à partir du bain n°2 auquel on a ajouté 1 g/l de thiosulfate de sodium à titre de « composé possédant un doublet électronique libre ».

The procedure is carried out under the same conditions as in Example 6 so as to produce deposits with a thickness of 10 μm from the following different baths:

• bath No. 1: reference bath identical to that of Example 6,
• bath no 2 in accordance with the invention, prepared from bath no 1 to which 2.10 ^-3 mole / liter of "PEGbiCOOH 600" has been added,
• bath no. 3, in accordance with document US Pat. No. 5,575,899 already cited, prepared from bath no. 2 to which 1 g / l of sodium thiosulfate has been added as "compound having a free electronic doublet".

The results obtained in terms of specific protection efficiency Tps, incorporation of organic compound in the coating (C rate, in arbitrary units or "ua" - measurement of the area under peak "C _i " of the curve " SDL "of carbon content of the type of that of FIG. 4), and of morphology of the coating are given in Table XV.

The sulfur incorporation rate (“S rate”) in the coating was also evaluated by glow discharge spectroscopy, using the same measurement method as for carbon (“C rate”), in arbitrary units (“ ua ”). Impact of other additives on the coating and its properties. Bath Protection T. _ps (H / µm) Rate of C (in ua) Rate of S (in ua) Coating morphology # 1 10.5 ≤0,5 0.2 homogeneous and refined # 2 13.5 2.0 0.2 homogeneous and very refined (glitter) # 3 11.4 ≤0,5 3.0 heterogeneous: refined areas and unrefined areas.

It is deduced from these results that the introduction into the bath according to the invention of a "compound having a free electronic doublet" such as sodium thiosulfate completely inhibited the incorporation of compound organic in the coating (the carbon content is identical to that of the reference) and lost the improvement in specific protection efficiency of the coating.

We are therefore dissuaded from using the electroplating baths described in document US 5,575,899 for implementing the invention.

At the same time, it is noted that bath No. 3 incorporates a sulfur compound in place of the organic compound according to the invention.

Example 11 :

The purpose of this example is to illustrate the use of electroplating baths according to the invention for depositing a zinc alloy, in this case a zinc alloy and nickel.

• chlorure de zinc (ZnCl₂) : 2,8 Moles/litre - chlorure de nickel : 0,35 M/I.
• chlorure de potassium (Kcl) : 4,36 M/I.

The reference bath (n ° 1) used contains the following elements:

• zinc chloride (ZnCl ₂ ): 2.8 Moles / liter - nickel chloride: 0.35 M / I.
• potassium chloride (Kcl): 4.36 M / I.

• n°2 : 0,5 g/l de « PEGbiCOOH 600 »,
• n°3 : 1,0 g/l de « PEGbiCOOH 600 ».

Two baths are prepared according to the invention by adding to bath No. 1:

• n ° 2: 0.5 g / l of "PEGbiCOOH 600",
• n ° 3: 1.0 g / l of "PEGbiCOOH 600".

• densité de courant : 2 modalités : 50 et 70 A/dm2,
• vitesse d'écoulement de l'électrolyte par rapport à la tôle : 2 modalités : 100 m/min. et 150 m/min.

From these baths and using the electrodeposition device of Example 5, Zn-Ni coatings are prepared according to the following methods:

• current density: 2 modes: 50 and 70 A / dm2,
• flow rate of the electrolyte relative to the sheet: 2 methods: 100 m / min. and 150 m / min.

All the coatings obtained have the same nickel content (13% at 14% by weight), which shows that the polymer added to the bath in accordance with the invention (in this case, "PEGbiCOOH 600") has no influence on the content of alloying element (in this case, nickel) in the coating.

The effect obtained by the use of the bath according to the invention is therefore entirely different from that provided by the polyoxyalkylenated compounds of document EP 0 285 931 (already cited) or by the polymer additives described in the document EP 0 342 585 in zinc-chromium alloy plating baths, which are intended to increase the chromium content in the coating.

Furthermore, for all the filing methods according to this example, the coatings obtained have very good adhesion to the substrate (for the adhesion test, fold the coated sheet 180 °, apply a tape Scotch® adhesive on the folded edge then tear off the tape, and check that the coating driven by tearing).

Claims (6)

1. Chloride-based aqueous electrodeposition bath for the preparation of a zinc-based coating:

   the pH of which is greater than 4,

   the zinc ion molar concentration of which is greater than 1 mol/litre,

   not containing a "compound possessing a free electron pair" chosen from the group comprising sodium thiosulphate, nicontinic acid, urea, thiourea, nicotinamide and thioglycolic acid, considering that the bath does not contain a "compound possessing a free electron pair", whenever the measured concentration is less than 0.001 g/l,

   containing, in solution, at least one polyethylene glycol polymer satisfying the general formula R¹-O-(CH2-CH2-O)_n-R²,

   characterized in that:

   - 1) firstly, n ≤ 13,

   - 2) secondly, the concentration of the said polymer in the bath is between 10^-4 and 10^-1 mol/litre and

   - 3) thirdly:

      either R¹ and/or R² are hydrogen atoms and the said polymer has an average molecular mass M of less than 500 g/mol
      or R¹ and R² are different or identical chain-end substituent groups chosen from the group comprising:

   unsubstituted alkyl (-C_mH_2m+1), alkene (-C_mH_2m-1) or alkyne (-C_mH_2m-3) groups;

   alkyl (C_mH_2m-R³) , alkene (C_mH_2m-2-R³) or alkyne (C_mH_2m-4-R³) groups substituted in the terminal position;

      where -R³ = -O-R⁴ (ethers or "oxy"), -COO-R⁴ (esters or "carboxy") or -COOM (salt of a carboxylic acid) , -SO₃-R⁴ ("sulphonyl") or -SO₃-M (salt of sulphonic acid), -CO-R⁴ (ketone), -N=R⁴ or

   

   (amine), -S-R⁴ ("thio") or -C≡N (nitrile),
      where R⁴ and R⁵ are chosen from the group comprising H, an alkyl (-C_mH_2m+1), alkene (-C_mH_2m-1) or alkyne (-C_mH_2m-3) group,
   the value of m being low enough for the said polymer to be soluble at the said concentration in the bath.
2. Electrogalvanizing bath according to Claim 1, characterized in that the average molecular mass M of the said polymer is greater than 150 g/mol.
3. Method for the electrodeposition of a zinc-based coating on a strip of steel sheet, in which:

   the said strip is made to run through an electrodeposition bath according to either of Claims 1 and 2;

   the relative speed of the bath in the vicinity of the strip is greater than 30 m/min; and

   an electrical electrodeposition current is passed between the said strips serving as cathode and at least one anode placed in the said bath facing the said strip;

   characterized in that the mean current density, measured on that portion of the said strip facing at least one anode, is greater than 0.25 x J_lim, where J_lim is the limiting current density corresponding to the current density plateau in the "current-potential" curve characteristic of the said electrogalvanizing bath, for the relative speed of the bath with respect to the strip.
4. Steel sheet coated with a zinc-based corrosion protection coating produced by the process according to Claim 3, characterized in that the said coating contains more than 0.1% by weight (expressed as carbon) of an organic compound of the polyethylene glycol type, as defined in Claim 1.
5. Steel sheet according to Claim 4, characterized in that the said coating contains more than 0.65% by weight (expressed as carbon) of an organic compound.
6. Steel sheet according to Claim 4, characterized in that, in the thickness of the said coating and outside the steel/coating interface zone, the carbon content is greater than or equal to 0.5% by weight.

Images