GB2208876A

GB2208876A - Process for forming phosphate coatings on metals

Info

Publication number: GB2208876A
Application number: GB8819791A
Authority: GB
Inventors: Werner Rausch
Original assignee: ARDROX PYRENE Ltd
Current assignee: ARDROX PYRENE Ltd
Priority date: 1987-08-19
Filing date: 1988-08-19
Publication date: 1989-04-19
Anticipated expiration: 2008-08-19
Also published as: ZA886167B; JP2680618B2; CA1333147C; JPS6468481A; EP0304108B1; ES2036666T3; EP0304108A1; GB8819791D0; DE3871031D1; GB2208876B

Description

4, 1 9 1 Ardrox Pyrene Limited Processes for Forming Phosphate Coatings on

Metals U 60/2749/01 This invention relates to processes for forming 5 phosphate coatings on surfaces which are of steel and/or galvanised steel and which may also include aluminium surfaces. The coatings are particularly intended as a base for subsequent painting.

It is standard practice to use zinc phosphate solutions for this purpose, and different effects can be obtained by different ratios of zinc to phosphate and by the incorporation of other metals in the solutions. Thus it is known that phosphate coatings can be formed using zinc-magnesium phosphate solutions but when these are the only divalent cations various disadvantages occur.

It is also known that low zinc phosphating processes can be used, especially prior to painting, to produce coatings having a high phosphophyllite content and that this can lead to beneficial effects in paint adhesion and in infiltration resistance of the film of paint when subjected to corrosive surroundings. Nickel can be included in the low zinc phosphating solutions and can result in the production of coatings that appear perfect from the visual point of view. However the adhesion of the paint films, especially on to zinc surfaces, tends to be unsatisfactory in the presence of moisture. Also there may be defects in the phosphate coating that is formed on zinc surfaces. These defects appear to be due to local crystalline efflorescence, which appears as specks, and impairs the formation of a uniform film of paint. This is a particular problem when application is by an immersion process.

4 1 1, A 2 other problems arise when the surfaces include aluminium surfaces, especially when the phosphate coating is based on zinc and magnesium.

There is therefore a need to devise a process that is capable of giving satisfactory coatings, especially as a base for painting, on metal surfaces that can be of steel andlor galvanised steel, and optionally can include aluminium surfaces.

In the invention, a phosphate coating is formed on a surface of iron and/or zinc, and optionally also of aluminium, by contact of the surface with a phosphating solution that is an aqueous solution containing 0.4 to 2g/ 1 Zn, 0.4 to 5g/1 Mg, 0.05 to 2g/1 Ni, 8 to 20g/1 P2 0 5 and at least one oxidising agent, and which has a free acid:total acid ratio of 0.02:1 to 0.15:1 and in which, when the surface includes aluminium, contains from 80 to 400mg/1 of fluoride as determined by a fluoride-sensitive electrode immersed in the solution.

The surface, or the part of the surface, that is of iron is generally of steel and this may be a non-alloyed or a low-alloyed steel. The surface, or the part of the surface, that is of zinc is generally a galvanised surface, and in particular is generally galvanised steel. Thus the surface may be formed from steel that has been coated with zinc or zinc alloy coatings, for instance having been coated with pure zinc, zinc nickel, zinc iron or zinc aluminium coatings by a suitable process, such as melt immersion. When the surface, or part of the surface, is aluminium, the aluminium may be pure metal or may be an alloy of aluminium with, for instance, magnesium, zinc, copper, silicone or manganese.

The invention is of particular interest when the surface or surfaces that are being coated include components of steel and components of galvanised steel.

It is also of interest when the surface or surfaces 14 3 1 1 include components of aluminium with steel, aluminium with galvanised steel or aluminium with steel and galvanised steel.

Fluoride can be present in the solution when the 5 surfaces being treated do not include aluminium surfaces. However when the surfaces include aluminium surfaces the solution must include fluoride in an amount of 80 to 400mg/1 as determined by a fluoride-sensitive electrode.

The presence of fluoride has the advantage of accelerating coating formation on all surfaces, and is particularly important when aluminium is present. The fluoride can be introduced as simple and/or complex fluoride, for instance as fluoroborate or fluorosilicate.

The concentration of fluoride detected by the fluoride-sensitive electrode corresponds approximately to the content of dissociated fluoride (F present in the phosphating solution. To adjust a F(el) value of from 80 to 400m9/1 in the conventional phosphating bath pH values of the baths, about 0.4 to 0.9g/1 of NH 4 HF 2 or equivalent quantities of other simple fluoride containing compounds must be added. The bath is preferably made up by adding sufficient simple fluoride containing compound to the phosphating bath to attain the desired value of F(el).

The cations zinc, magnesium and nickel are introduced into the phosphating solution, for example as metal oxide, carbonate, phosphate, nitrate, chloride and/or complex fluoride. The phosphate content is supplied to the solution via phosphates and/or phosphoric acid. Further cations, for example Na, K, NH 4 or anions, for example nitrate, chloride, sulphate are also used in order to adjust the desired ratio of free acid to total acid of (0.02 to 0.15):l.

Although it is often preferred that the solution is free of other polyvalent metals, various additives can be 1 4 included in the solution, usually in minor amounts. Such additives include copper, manganese, calcium, iron, cobalt, polyphosphates, polyhydroxycarboxylic acids, surface-active agents as well as agents for influencing 5 sedimentation.

When the surfaces do not include aluminium, the concentration of zinc in the phosphating solution is generally from 0.4 to lg/1 when the application is by spraying and from 0.9 to 1.5g/1 when the application is by spray immersion or immersion. When aluminium is present, the respective concentrations are generally 0.7 to 1.0g/1 and 1.4 to 2.0g/1.

The phosphating bath contains at least one oxidising agent. Preferred materials comprise nitrate, nitrite, chlorate, bromate, peroxide and nitrobenzene sulphonate, and the following quantities can be used, for example, from 2 to 20g/1 of nitrate; from 0.05 to 0.5g/1 of nitrite; from 0.5 to Sg/1 of chlorate; from 0.2 to 3g/1 of bromate; from 0.02 to 0.1g/1 of peroxide; from 0.1 to lg/1 of nitrobenzene sulphonate. The oxidising agents can also be combined with one another, for example, nitrate-nitrite, (nitrate-) chlorate-nitrite, nitrate-bromate, (nitrate-) chlorate-nitrobenzene sulphonate, nitrate-nitrobenzene sulphonate.

To prevent the formation of small spots of crystalline efflorescence (specks) on the phosphate coating during the phosphating of galvanised steel, particularly when adopting the spraying immersion and immersion process it is preferred to control the combinations of particular additives, notably chloride, bromide, nitrate and nickel. Thus preferably only two of the components selected from (chloride and/or bromide), (nitrate) and (nickel) should be present in respective concentrations of above 0.1g/1 while the third should be held at a concentration of not more than 1 1 0. lg/1. Chloride and bromide are counted as a single component and their concentrations are added together. The chloride can, for instance, originate from initial dilution water or from reduction of chlorate accelerator while bromide can originate from bromate accelerator.

- The values of free acid and total acid are the consumption of ml of OAN sodium hydroxide required for neutralisation of a 10m1 sample of the solution to, respectively, the first or second neutralisation points of phosphoric acid.

The surfaces that are to be treated should, as is conventional, be tree of contamination by, for instance, oils, lubricants and oxides and so should preferably have been cleaned in a manner conventional for surfaces that are to be phosphated. The surfaces may be activated, prior to phosphating, with a known activating agent such as an aqueous suspension containing titanium phosphate.

The activating agent can be used in the cleaning solution or as a separate stage between cleaning and phosphating.

The phosphating solution is preferably applied at a temperature of from 40 to 6WC. Application is generally by spray. spray immersion or immersion processes. The contact between the solution and the surfaces typically is 20 seconds to 10 minutes, this range being particularly suitable when the surfaces are of galvanised steel but the higher amounts, for instance 1 to 10 minutes generally being required when the surfaces are of steel and aluminium.

The phosphate coatings are preferably used as the base for a subsequent application of paint, often by an electro-immersion process, generally cathodic electro-immersion.

The resultant paint coatings have high resistance to corrosion caused by normal weathering, weathering accelerated by salt, moisture, salt mist or washing t, Z 6 liquor. The paint coatings also have good adhesion to various environments -for instance ordinary weathering, weathering accelerated by salt, moisture, salt mist and wash liquor even when subjected to scratching or impact before or after the exposure to corrosion conditions.

The process is of particular value for the pre-treatment of metal casings for domestic and industrial appliances, metal furniture, vehicle bodies and vehicle accessories, prior to painting.

The invention is illustrated in the following examples. Examples Steel sheets, electrolytically galvanised steel, hot galvanised steel and AlMgSi were treated at SWC for 2 minutes by spraying and for 3 minutes by immersion with the phosphating solutions listed in the Table after alkaline cleaning, water rinsing and activation in an aqueous titanium phosphate suspension. This was followed by rinsing with water, chromium (VI)-containing aqueous passivating solution, completely desalted water as well as a drying stage.

The Table shows that perfect coatings could be obtained on steel in the entire operating range. The same applies to galvanised steel, with the exception of the case in which nitrate, chloride and nickel were present in respective concentrations above 0.1g/1 (Examples 6 and 11), providing that the ranges essential to the invention for the Zn, Mg and Ni concentration were observed. A uniform crystalline layer was invariably formed on AlMgSi if the fluoride concentration measured electrometrically with a fluoride-sensitive electrode (F(el) lay in the rasnge of between 80 and 400mg/1 (Examples 4 to 6 and 13 to 20).

After visual evaluation, the coatings were provided with a cathodic electro-immersion paint and an automobile !kl 7 p paint composition and were subjected to the conventional application tests. Excellent test results were achieved in all cases where there were crystalline coatings on the metal substrate.

A a Table

Example go. t 04 10 91 12 1) 140 1 94 17 at 19 20 Z0 (gill 0.3 Ol$ 1 0.3 0.5 015 f. 1. 1.4 1,4 &,% AA 9.4 1.5 119 1 AA A's &.a 018 Mg (am 1 1 1 1 2.$ Z.) a 2.3 21) 4P th a 1 A 3 3) $ NA (alit 0.1 0.7 011 O.P 0.1 0.7 0.) 0.7 0.3 0.7_ 0.3 0.7 0,0 o') 0,7 G3- 0,1 0.7 AJ a Ns (ale) 116 0.8 2.0 1.7 0.4 0.) 1,4 B.9 0.4 0,3 0,1 2j, AW6 Gy 1.1 0,3 0, 3 P 01 tall) 12 or 4 14 a th 14 As #5 13 A 13 as 93 is A a 3 As A 3 as at _U NO, (611) a 3 & it 4 a A 3 o.t t 0.21 0,31 0.1ht 7 T 0,21 7 r 0.21 0,21e 0.2t.at C&O 1 11 1 1.3 2 a NAN02 (all) 0.1 0.1 0,1 0.1 0.1 O.A 0.1 0.1 O.A 0.1 0. 0.b 0.1 1 0.) 0.3 0.3 094 0,04 0.4 0.4 Nitro benzene nulphonate-all 0.13 0.1) O.As 0,13 0,13 0,93 01,93 010 @,A)- 0j) 0j) F( ) (01)- SAP& 013 0.9 013 018 &,a 0.9 (911) -- 0.4 0.4 013 0.9 0.7 017 1.6 116 11) 7.3 211 all)43 lee bet all 116 a& 213 113 Fro Acid.

Total Acid 17.6 18.6 21.2 7212 2113 7213 2h.3 23.1 24,9 29.0 20,7 79,4 2A. 4 23.3 32,2 77.3 23.6 24.6 is. 4 55 Free Acid 0.07 10.04 0.0h 0,0) 0.0) 0.07 0.06 0.03 0.09 0,07 0,07 o.al 0. 04 0j) 0.0 1 0.06 0,04 0,09 0,03 Total Acid Application 2)-- spr Sw spr Spr Spr Spir ST To ST To TA To To To To TA TO To TA TO Steel 3) GooO Good Good Good Good Good Good Good Good Good Good Good Good Good Gond Gond Good Good Good Galvanised Goodi Good Good Good Good (Good Good Good Good Good (Coo Good Good Good Good Gno Gond Good Good Good Steel 3) 1 Good ^emits& 3) Good, ('.nnd 1 Good A Good G00.1 Good Good!good 1 Good Good - - -- - - Good Key 1) t additional concentration of Cl bv chloratn reduction diering throughput 2) Spr Spraying Ta - Im^rsion ST - Spray Intersion 3) Coating Formation:

Good: Good, uniforr.ly covering phosphate coating.

(Good). ap above, but appearance of topical crystalline efflorescence (specks) in possible).: Tin rr.51...elline coating on AIMcSi jA 00 m"UIK k 9

Claims

1. A process for forming a phosphate coating on a surface of iron and/or zinc, and optionally also aluminium, by contact of the surface with a phosphating 5 solution which is an aqueous solution containing 0.4 to 2g/1 Zn, 0.4 to 5g/1 Mg, 0.05 to 2g/1 Ni, 8 to 20g/1 P 2 0 5 and at least one oxidising agent, and which has a free acid:total acid ratio of 0.02:1 to 0.15:1 and which, when the surface includes aluminium, contains from 80 to 400mg/1 fluoride as determined by a fluoride-sensitive electrode immersed in the solution.

2. A process according to claim 1 in which the surface is free of aluminium and in which either the solution is applied to the surface by spraying and has a zinc is concentration of 0.4 to lg/1 or is applied by spray immersion or immersion and has a zinc concentration of 0.9 to 1.5g/1.

3. A process according to claim 1 in which the surface includes aluminium and in which either the solution is applied by spraying and has a zinc concentration of from 0.7 to 19/1 or is applied by spray immersion or immersion and has a zinc concentration of 1.4 to 2g/1.

4. A process according to any preceding claim in which the solution contains simple or complex fluoride.

5. A process according to any preceding claim in which at least one of the three components (chloride andlor bromide), (nitrate) and (nickel) has a concentration of not more than 0.1g/1.

6. A process according to any preceding claim in which 30 the contact of the solution with the surface is at a temperature of 40 to 600C.

7. A process according to any preceding claim in which the surface is subsequently painted.

8. A process according to claim 7 in which the painting is by electro-immersion.

g. A process according to claim 7 in which the painting is by cathodic electro-immersion It Pubb.shed 1988 at The Patent O:Mce. State House. 66 71 Hig!i HcOborn. London WClR 4TP Flarther copless njkv be obtained froYn The Patent Office. Sales Branch. St Mary Cray. Orpington. Rent BR5 3RD. Printed by Multiplex techniques ltd. St ma:7 Cray, Kent Con. 1/87.