GB2188946A

GB2188946A - Chromoting metal surfaces

Info

Publication number: GB2188946A
Application number: GB08707748A
Authority: GB
Inventors: George Higgins
Original assignee: Pyrene Chemical Services Ltd
Current assignee: Pyrene Chemical Services Ltd
Priority date: 1986-04-08
Filing date: 1987-04-01
Publication date: 1987-10-14
Anticipated expiration: 2007-04-01
Also published as: GB2188946B; US4801337A; GB8707748D0; ES2018188B3; DE3711095A1; EP0240943B1; EP0240943A2; DE3765445D1; GB8608508D0; CN87103418A; EP0240943A3; KR870010219A; CA1284762C

Description

GB2188946A 1

SPECIFICATION

Processes and compositions for coating metal surfaces There is described in British Patent Specification No. 1,042,108 a process for forming a corro- 5 sion resistant coating on a zinc or zinc alloy surface comprising treating the surface with a chromate-free aqueous alkaline solution having a pH above 11 and which contains a metal of a group of the periodic system other than group 1 and that cannot exist in the alkaline solution in free cationic state but that is complexed with an organic complexing agent and is in solution. It is stated in that specification that hexavalent chromium interferes with the coating and so must 10 be absent, but that it is desirable to give the coating a subsequent rinse in dilute aqueous chromic acid.

It is stated that at least two complexed metal ions may be present. The complexed metals that are exemplified are combinations of iron with cobalt, magnesium, cadmium, tin, titanium, antimony, bismuth, molybdenum, tungsten and manganese and blends of cobalt with arsenic and 15 cerium. The only other complexed metals that are exemplified are ferrous and vanadium although aluminium, chromium and nickel are also mentioned as metals that can be used in the process.

A process that has been used successfully involves the treatment of continuous strip galvanised steel at, for instance, above 50'C using an alkaline solution of sodium heptonate, cobalt and ferric iron, followed by a chromate rinse. 20 It would be desirable to improve the corrosion resistance or adhesion or both of the coating and it would be desirable to be able to obtain a good coating either using less concentrated solutions or lower coating temperatures or both.

We have now surprisingly found that these objectives can be achieved if trivalent chromium is included with at least two other polyvalent metals. 25 An aqueous alkaline solution according to the invention for forming a corrosion-resistant coating on a surface of zinc or zinc alloy has a pH above 11 and contains at least three polyvalent metals that cannot exist in the free cationic state in the alkaline solution and that are complexed with a complexing agent and are in solution, and one of the polyvalent metals is trivalent chromium. 30 The other polyvalent metals may be any of those named in British Patent Specification No.

1,042,108 but are preferably selected from iron, cobalt and nickel. There is particular benefit in the use of chromium when cobalt is also present, preferably in combination with iron. Best results are achieved when the solution contains, in addition to trivalent chromium, divalent cobalt and trivalent iron although useful results can also be obtained if the cobalt is replaced wholly or 35 in part by nickel.

Any suitable complexing agent or blend of complexing agents may be used, generally selected from the materials described in GB 1,042,108. A typical complexing agent is gluconic acid but best results are generally achieved using heptonic acid. It is usually introduced in the form of a water soluble salt, especially as sodium heptonate. 40 It may be found that if the metals are introduced only as salts with inorganic acids, generally nitrate, the amount of heptonate or other complexing agent needs to be rather large for best results. Good results can be achieved with lesser amounts if at least one of the polyvalent metals is introduced in the form of a water soluble salt with an organic acid or if free organic acid (as a sodium or other salt thereof) is added to the composition. This organic acid may be a 45 complexing agent but the preferred systems, from the point of view of cost and effectiveness, are those in which the organic acid is formic acid or acetic acid. Thus one preferred process involes introducing chromium as chromium acetate or formate, with the other polyvalent metals generally being introduced as nitrate or other suitable inorganic acid salt. Another involves the use of sodium formate or acetate in combination with inorganic acid salts of all the metals. 50 Although it is preferred to introduce all the polyvalent metals in the form of cations, any that can exist in the form of anions may be introduced in this state if desired.

The solution must have pH above 11, preferably above 12, with best results generally obtained at pH 12.2 to 13.3. Although a variety of alkaline compounds can be used (such as are mentioned in GB 1,042,108) it is preferred to use alkali metal hydroxide, especially sodium 55 hydroxide. The amount will be selected to give the desired pH and typically is in the range 5 to 9/1.

The amount of complexing agent will depend on the amount of polyvalent metal in the solution during use, since the amount should be sufficient to complex all heavy metals that are in the solution. Generally it is from.05 to 10, preferably 1 to 5, 9/1 when the complexing agent 60 is sodium heptonate and equivalent amounts of other complexing agents may be used.

The total amount of polyvalent metal ion is generally in the range 0.3 to 3 g/1, most preferably 0.4 to 1 g/1.

The amount of trivalent chromium is generally in the range 0.04 to 0.4, most preferably 0.1 to 0.3 9/1. When trivalent iron is present its amount is generally from 0. 1 to 0.3 and any third 65 2 GB2188946A 2 metal, for instance divalent cobalt, is generally present in an amount of from 0.15 to 0.4 g/1. When acetate or other organic acid salt is to be introduced it is preferably introduced as the salt of polyvalent metal that provides 15 to 60%, preferably 20 to 40%, by weight of the total polyvalent metal ion. Alternatively the equivalent amount of free organic carboxylic acid may be introduced. 5 The surface that is to be treated can be any of the surfaces described in British Patent Specification No. 1,042,108. Generally the zinc is present as a coating over iron (including steel).

It may be an alloy, for instance an alloy of zinc with aluminium, generally as a coating on iron.

The surface is preferably precleaned and rinsed with water before treatment.

The treatment can be by any convenient technique such as dip or, preferably, spray. Although 10 the treatment temperatures can be up to, for instance, about 90'C it is preferably below about 60'C and most preferably below about 50'C. Generally it is above about 20'C. Although in many processes treatment temperatures of about 35 to 50'C, typically around 45'C, are pre ferred a particular advantage of the invention is that it is possible to obtain very good results at low temperatures of about 20 to about 35 or 40'C, typically around 25'C. 15 The duration of the treatment is generally from 2 to 60 seconds, preferably 5 to 30 seconds.

In general the more dilute solutions require higher temperatures or longer treatment times.

However a further advantage of the invention is that good results can be obtained using solutions that are more dilute than is required when the solution is free of trivalent chromium.

The solution is generally free of chromate and it is possible to obtain satisfactory results 20 without giving the coated surface a subsequent chromate rinse, although in some instances this is desirable, generally after rinsing the coated surface with water. The surface is eventually dried, optionally after a water rinse. The dried coating can then receive paint or other coating.

As explained in British Patent Specification No. 1,042,108 if the article that is being treated also has exposed iron or steel portions it may be desirable to give the article an acidic zinc 25 phosphate or alkali metal phosphate coating treatment after the alkaline treatment of the inven tion.

The following are some non-limiting examples of the invention.

Example 1 30

A treatment solution suitable for spray application at 45'C for 2 to 15 seconds is made by dissolving in water the following components.

C02 f 0.2 g] 1 -added as nitrate Cr3+ 0.2 gi 1 -added as acetate 35 Fe3 + 0.15 gi -added as nitrate Na Heptonate 2.2 gi NaOH 19.0 gi Example 2 40

A treatment solution suitable for spraying at 25'C for 2 to 15 seconds is made by dissolving in water the following components.

C02+ 0.3 gi 1 -added as nitrate CO + 0.15 91-1 -added as acetate 45 Fe3+ 0.2 gi-1 -added as nitrate Na Heptonate 2.9 gi-1 NaCH 25.0 g]- 1 Example 3 50

Hot dipped, galvanised steel panels having a zinc coating weight of 275 g/M2 and normal spangle were precleaned, sprayed for 20 second at 45'C with a solution according to example 1, rinsed with water, rinsed in a hexavalent chromium containing rinse, and were then dried and painted with an epoxy-primer and PVF2 finish coat.

These panels were cross-scored and subjected to salt-spray testing to ASTM B1 17 for 1000 55 hours. Blistering size and density was recorded as in BS3900 part H1. At the end of the test, no blistering of the paint was apparent, there was no paint removed from the score, and little white rust was produced.

Panels processed and tested in a similar fashion, except that the processing was in a solution where the chromium content was nil, evinced blisters of density 3 and size 3. White rustbleed 60 from the score had began to spread over the face of the panels.

Claims

1. An aqueous alkaline solution suitable for forming a corrosion resistant coating on a surface of a metal selected from zinc and zinc alloys and which has a pH above 11 and conrtains at 65 3 GB2188946A 3 least three polyvalent metals that cannot exist in the free cationic state in the alkaline solution and that are complexed with a complexing agent and are in solution, and in which one of the said polyvalent metals is trivalent chromium.

2. A solution according to claim 1 in which the polyvalent metals are trivalent chromium with at least two metals selected from iron, cobalt and nickel. 5

3. A solution according to claim 1 in which the polyvalent metals comprise trivalent chrom ium, cobalt and at least one other polyvalent metal.

4. A solution according to claim 1 in which the polyvalent metals comprise trivalent chrom ium, cobalt and iron.

5. A solution according to any preceding claim in which the total amount of the said 10 polyvalent metals is 0.3 to 3 9/1 and the amount of trivalent chromium is 0.04 to 0.4 g/L

6. A solution according to any preceding claim in which the complexing agent comprises heptonic acid.

7. A solution according to any preceding claim in which the complexing agent is selected from heptonic acid and gluconic acid and the amount of complexing agent is from 0.05 to 10 15 g/'.

8. A solution according to any preceding claim in which the solution includes an organic acid salt selected from formate and acetate in an amount that is equivalent to 15 to 60% of the total said polyvalent metal.

9. A process for forming a corrosion resistant coating on a surface of a metal selected from 20 zinc and zinc alloys comprising applying to the surface an aqueous alkaline solution according to any preceding claim.

10. A process according to claim 9 in which the treatment is conducted at 20 to 900C.

11. A process according to claim 9 in which the treatment is conducted at 20 to 500C.

12. A process according to any of claims 9 to 11 in- which the treatment is conducted by 25 spraying the solution on to the said surface.

13. A process according to any of claims 9 to 12 in which the said surface is a coating on iron of a metal selected from zinc and zinc alloys.

Printed for Her Majesty's Stationery Office by Burgess & Son (Abingdon) Ltd, Dd 8991685, 1987.

Published at The Patent Office, 25 Southampton Buildings, London. WC2A 1 AY, from which copies may be obtained.