GB2204067A

GB2204067A - A process for phosphating steel or galvanized steel prior to electro-immersion painting

Info

Publication number: GB2204067A
Application number: GB08808478A
Authority: GB
Inventors: Gerhard Mueller; Werner Rausch
Original assignee: ARDROX PYRENE Ltd
Current assignee: ARDROX PYRENE Ltd
Priority date: 1987-04-11
Filing date: 1988-04-11
Publication date: 1988-11-02
Anticipated expiration: 2008-04-11
Also published as: ES2030493T3; JPS64277A; CA1332910C; JP2674075B2; EP0287133B1; DE3869455D1; GB2204067B; BR8801679A; EP0287133A1; AU1442788A; DE3712339A1; GB8808478D0

Description

2204067 60/2715/01 A PROCESS FOR PHOSPHATING PRIOR TO ELECTRO-IMMERSION

PAINTING The invention relates to a process for phosphating steel or partially galvanised steel workpieces as a pretreatment for electro-immersion painting, during which the cleaned and rinsed workpieces are initially activated with an aqueous, weakly alkaline solution containing titanium phosphate and are subsequently treated with an aqueous acidic phosphating solution containing zinc phosphate.

it is known to treat steel with aqueous acidic phosphating solutions based on zinc/iron(II)/nitrate/ phosphate by immersion prior to painting. However, there are considerable drawbacks when such a process is used in conjunction with electro-immersion painting. For example, the phosphate layer produced in such known processes is of varying thickness and.the subsequent electro-immersion painting gives paint layers of uneven thickness, with streaks, runs and pits. Moreover it is frequently found that the corrosion protection of these uneven lavers is insufficient to fulfil the necessary requirements.

An object of the invention is to provide a process for phosphating steel or partially galvanised steel which produces phosphate layers that are more even, thus rendering the process particularly suitable as a pretreatment for electro-immersion painting.

A process for treating clean steel or partially galvanised steel surface, suitable as a pretreatment for electro-immersion painting, initially activating the surface with an aqueous, weakly alkaline solution containing titanium phosphate and then forming a phosphate coating bg -reating the sulfacf-i at a of 40 to 600C with an aqueous acidic phosphating solution containing from 1.8 to Sg/1 of Zn, from 0.1 to 7g/1 of Fe(II), from 8 to 25g/1 of P,10 5, and from 5 to 30g/1 of NO 3 and in which the free acid to total acid ratio is adjusted to 0.04 to 0.07.

The process according to the invention is used for treating steel, for example cold-rolled strip and sheets of soft, unalloyed steel and higher strength cold-rolled fine sheets of phosphor-treated steels, micro-alloyed steels and dual-phase steels. The galvanised layers on galvanised steel include, for example, those produced by melt-immersion methods based on Zn, Zn + Fe, Zn + Al, Zn + Al + Si and those obtained by electrolytic methods based on Zn, Zn + Ni, Zn + Fe.

The process according to the invention can be applied to workpieces of varied types and shapes, for example flat products, deep drawn articles, welded, folded, flanged and adhered constructions. In order to ensure that the treatment of the internal and external surfaces of hollow bodies is consistent throughout, it is desirable to provide good ventilation and ease of discharge of the fluid. For example car bodies are typical workpieces having complex shape and consisting of greatly differing materials.

The surface must be clean and so will normally be firstly cleaned in a conventional manner, for example using alkaline degreasing agents, and are subsequently rinsed in water. This is followed bv treatment with an aqueous, slightly alkaline solution in which activating titanium phosphate is very finely dispersed.

Application of the phosphating solution is carried out at a temperature of from 40 to WC. Below this temperature, the rate of this phosphating process is unacceptably slow, and above WC, the energy losses 3 increase greatly and there is an increased risk of undesirable drying and incrustation. The solution is preferably applied by immersion and the workpieces are optionally sprayed before or after immersion.

The phosphating process according to the invention is one of the group of processes on the "iron side" and is distinguished by comparatively slight sludge formation. For example for application by immersion the initial iron(II) concentration in the immersion baths can lie below 0.1g/1 because the concentration increases during the process and rapidly reaches the range according to the invention owing to corrosion of the steel surface.

The concentration ranges for Zn, Fe(II), P 2 0 5 and NO 3 are essential in order to produce even phosphate layers which are suitable for subsequent electro-immersion painting.

The concentration of zinc is from 1.8g/1 to Sg/1. If the zinc concentration falls below this range, the phosphate layers produced are incomplete and do not form an overall covering. if the zinc concentration is above Sg/1 the phosphate layers produced are too thick to allow uniform paining.

The concentrati on of iron (II) is from 0.1 to 7g/1.

At concentrations greater than 7g/1 the cuality of the phosphate layer is considerably diminished having regard to the subsequent electro-immersion.

Phosphate in the solution is at a concentration of from 8g/1 to 25g/1. Below this range the phosphate content is too low to produce an even phosphate layer, and there is no further commercial adl,,antage to a concentration higher than 25g/1.

The nitrate concentration is from 5g/1 and 30g/1.

Below Sg/1 the phosphating solution lacks the necessary acceleration for formation of the phosphate layer and a 4 concentration above 30g/1 provides no further exploitable increase in the rate of phosphate layer formation.

An important feature of the invention is that the ratio of 1.

"ree acid to total acid in the phosphating solution is from 0.04 to 0.07. If this ratio is below 0.04 there is an increase in sludge formation and valuable constituents of the phosphating solution are lost. However above 0.07, the phosphating rate decreases significantly. This ratio is adjuste1 by determining the concentration of the components'in the immersion bath and optionally using further cations, for example of Na, K, NH 4 or further anions, for example of Cl SO 4 According to a preferred embodiment of the invention particularly desirable results with respect to subsequent electro-immers-Jon painting are achieved il- the workpieces are brought into contact with a phosphating solution containing a maximum of 3g/1 of Zn and preferably from 0.5 to 5g/1 of Fe(II).

Further divalent cations, for example from the group comprising Ca, Co, Cu, Mg, "Mn, Ni can he added to the phosphating solutions to modify layer formation. A preferred phosphating solution according to the invention addittionally contains manganese in a quantity of up to 3g/1 or magnesium in a quantity of up to 3g/1. Addition of cobalt, up to 0.3g/1 and of nickel up to 0.15g/1 are also preferred. However, striped phosphate layers may be formed on steel if the concentration of cobalt is above 0.3g/1 or if the concentration of Ni is above 0.15g/1.

A further advantageous embodiment of the invention involves contacting the surface with a phosphating solution containing up to 3g/1, preferably at least 0.3g/1 of hydroxylamine. In this case, the concentration of Ni can be increased to 6.5g/l if at least 0.3g/1 of hydroxylamine are present at the same time. Hydroxylamine has an accelerating effect on the phosphating process.

If an increase in the phosphating rate is required and in order to optimize layer formation of aluminium-containing zinc surfaces the aggressiveness of the phosphating solution can be increased, according to a further advantageous embodiment of the invention wherein the workpieces are brought into contact with a phosphating solution containing up to 3g/l SiF 6 and/or up to 3g/l of BF 4 and/or up to 1.5g/l of F.

Addition of tartaric acid and/or citric acid, preferably of up to 3g/l, serves to reduce the weight per unit area of the phosphate layers and gives further acceleration to layer formation.

it is also advantageous to use phosphating solutions containing up to 0.5g/l, preferably from 0.05 to 0.35g/l of m-nitrobenzene sulphonate. m-nitrobenzene sulphonate causes a marked acceleration in the rate of phosphating and at the same time results in a considerable reduction in the thickness of the phosphate layer.

During the phosphating process autocatalytic nitrite formation may occur. In order to prevent the phosphating from passing from the "iron side" to the "nitrite baths.

side", nitrite-destroying compounds, for example urea or amidosulpho acid, are preferably added to the baths.

In order to avoid an increase in the iron (II) concentration beyond the desired concentration, it may be necessary to oxidize a proportion of the iron(II) which has passed into solution due.to corrosion of the steel or iron surface of iron(III). Iron(III) is then precipitated out of solution in the form of substantially insoluble iron (III) phosphate sludge.

This oxidation may be carried out by contacting the phosphating solution with oxygen containing gas and/or addition of chlorate and/or peroxide compounds.

In order to reduce any excess free acid in the phosphating solution basic compounds, for example, alkali 6 hydroxides and carbonates, can be added. In particular, the use of zinc oxide, zinc carbonate and/or manganese carbonate is preferred as this introduces more layer forming cations into the phosphating solution and is particularly advantageous.

The phosphating process according to the invention can also be modified in such a way that immersion into the phosphating solution is preceded or followed by spraying with the phosphating solution. The immersion times usually lie in the range of from 2 to 5 min, whereas pre-spraying and/or after-spraying lasts for a few seconds to about 0.5min.

According to an advantageous embodiment, the phosphating process according to the invention is carried out such that phosphate layers having a weight per unit from 1 to 5g/m 2 are produced. optimum corrosion area of I protection is produced with good bending adhesion of the paint coatings.

The lavers formed by the process according to the invention provide a suitable base for adhesion of anodic and. cathodic electro-immersion paints. Particularlv desirable results are achieved in the process is used as a pre-treatment for cathodic electroimmersion painting and wherein a paint film thickness in the range of from about 15 to 40 ILM is to be achieved. The electro-immersion paint layers can serve as undercoat for further paint layers or may be single laver paints.

The invention is further described by way of the following Examples.

EXAMPLE

Sheets of car body steel and galvanised steel were degreased with an alkaline cleaner, rinsed in water, activated with an aqueous suspension composed of about 50g/l of titanium phosphate in an aqueous solution of Ig/l of disodium phosphate and 0.25g/1 of tetrasodium .7 pyrophosphate by immersion for 1 minute at 40'C and were then phosphated by immersion for I minute at 40"C and were then phosphated by immersion in the phosphating solutions 1 to 6 listed in the Table at 550C.

A minimum phosphating time of from 2 to 3 minutes was required on steel and of less than 1 minute on galvanised steel. The minimum phosphating-time is defined as the immersion time requ.ired for forming a visually uniform covering phosphate layer.

The weight of the layer was between 3.6 and 4.3g/m 2 in the case of steel and between 2.2 and 3.0 g/m 2 in the case of zinc.

The free acid (total acid) is defined as the quantity of ml n/10 NaOH required for neutralizing 10ml of bath sample reiati%re to dimethyl yellow (phenolphthalein). The ratio of free acid to total acid was (0.054 to 0.063):l.

After the phosphating treatment, the materials were rinsed with water, passivated with chromium-containing after-rinsing solution and rinsed with completely desalinated water prior to cathodic electro-immersion painting.

Uniform layers of paint were produced which adhered very well to the metallic substrate and provided excellent corrosion protection with or without further build up of paint. This quality was at least equal to the quality obtained by the known low zinc phosphating process carried out with chlorate and/or nitri.te acceleration in substantially iron(II)-free baths.

8 LAJU xample 2 3 4 5 6 n 2 2 2 4 4 4 0(11) (9/1) 1 2.5 5 1 2.5 5 1 (9/1) 0'1 0.1 0.1 0.1 0.1 Oo 2 0 5 (9/1) 16 16 16 _1 6 16 16 0 3 (9/1) 11 11 11 11 11 11 rtaric 1 1 1 1 1 1 (9/1) cid -T 1.4 IATIA 1.4 i 1.4 1.4 4 (9/1).4 The quantity required for adjusting the following titration data ree Acid (FS) 1.6 2.0 2.0 1.7 1.9 1 2 - 2 otal Acid (GS) 28.1 32.0 31.5 31.5 34.2 136.0 ree Acid (FS) 0.057 0.063 0.060 0.054 0.056 0.060 otal d(GS) inimurn Phos hating Time 3 3 3 2 2 2 or Steel (min) I I I Layer Weight 0 3.6 4.o 3.6 1 3.6 4.3 4.1 I I I For Steel (g/m') Layer Appearance Grey throughout, finely crystalline and For Steel uniform Minimum Phosphati Lower than I throughout Time for Zinc (miln - - JLayer Weight 0 2.2 2.6T2.2 2.5 3.0 2.9 For Zinc (g/m-) -ayer Appearance Grey throughout, finely crystalline and IFor Zinc uniform - 9

Claims

1 A process for treating clean steel or partially galvanised steel surface with an aqueous, weakly alkaline solution containing titanium phosphate and then forming a phosphate coating by treating the activated surface at a temperature of 40 to 601C with an aqueous acidic phosphating solution containing from 1.8 to Sg/1 of Zn, from 0.1 to 7g/1 of Fe(I1), from 8 to 25g/1 of P 2 0 51 and from 5 to 30g/1 of NO 3 and in which the free acid to total acid ratio is adjusted to 0.04 to 0.07.

2. A process according to claim 1, wherein the phosphating solution contains a maximum of 3g/1 of zinc.

3. A process according to claims 1 and 2 wherein the phosphating solution contains from 0.5 to 5g/1 of iron 01!).

4. A process accordIng to any preceding claim wherein the phosphating solution additionally contains manganese in a quantity of up to 3g/1.

5. A process according to any preceding claim wherein the phosphating solution additionally contains magnesium in a quantity of up to 3g/1.

6. A process according to any preceding claim wherein the phosphating solution additionally contains up to 0.3g/1 of cobalt and'/or up to 0.15g/1 of nickel.

7. A process according to any preceding claim wherein the phosphate solution contains from 0.3 to 3g/1 of hydroxylamine.

8. A process according to claim 7 wherein the solution contains nickel in an amount of not more than 0.5g/1.

9. A process according to any preceding claim wherein the phosphating solution contains up to 3g/1 of SiF 6 and/or up to 3g/1 of BP 4 and/or up to 1.5g/1 of F.

10. A process according to any preceding claim wherein the phosphating solution contains up to 3g/1 of tartaric acid and/or citric acid.

11. A process according to any preceding claim wherein the phosphating solution contains from 0.05 to 0.5g/1 of m-nitrobenzine sulphonate.

12. A process according to any preceding claim wherein the phosphating solution contains nitrite-destroying substances.

13. A process according to any preceding claim wherein the content of divalent iron in the phosphating solution is adjusted by precipitation of excess divalent iron in the form of iron-III phosphate using oxygen-containing gases and/or chlorate and/or peroxide compounds.

14. A process according to any preceding claim wherein the content of free acid in the phosphating solution has been adjusted by addition of zinc oxide, zinc carbonate and/or manganese carbonate.

15. A process according to any preceding claim wherein the surface is treated with the phosphating solution by immersion.

16. A process according to claim 15 in which the surfaces are treated with the phosphating. solution by spraying prior to and/or after the immersion treatment.

17. A process according to any preceding claim wherein the treatment with the phosphating solution results in the formation of a phosphate layer having a weight per unit area of from I to 5g/m

18. A process according to any preceding claim comprising the subsequent step of cathodic electro-immersion painting.

Published 1988 at The Patent Office, State House, 66.171 High Holborn, London WClR 4TP. Further copies may be obtained from The Patent Office, Sales Branch, St Mary Cray. Orpington, Kent BR5 3RD. Printed by Multiplex techniques ltd, St Mary Cray, Kent. Con. 1187.