Compositions and processes for producing chromium conversion coatings on surfaces of zinc or cadmium

Abstract

In a process for producing a chromium protective coating on a zinc or cadmium surface the surface has applied thereto an aqueous composition including chromium, perchlorate and fluoride ions with the chromium ion being the only metal ion present before application to the surface. The apparatus by which the composition is applied to a strip moving through an application station includes a provision for rinsing the surface if the strip is stopped for longer than a predetermined time in the station.

Description

FIELD OF THE INVENTION

This invention is concerned with improvements in or relating to compositions and processes for producing chromium conversion coatings on surfaces of zinc or cadmium.

REVIEW OF THE PRIOR ART

A continuing problem exists in the field of protective coatings of zinc and cadmium applied to a substrate, such as a steel strip or article, to protect is against corrosion. The zinc or cadmium protective coating itself will corrode in time to give a white or whitish deposit commonly known as "storage stain" or "white rust." It is common therefore to apply a thin protective coating to the zinc or cadmium surface which usually consists of a chromium compound of compounds and is known as a "conversion" coating.

The conversion coating usually is formed by applying a suitable aqueous solution containing chromium to the zinc or cadmium surface, the solution reacting with the surface to form a thin gel-like film of complex chromates. The film is substantially non-porous to moisture, and also is non-crystalline, so that it provides a good paint bond without absorption of the paint onto the surface. Examples of processes for producing such a coating are given in U.S. Pat. Ser. No. 3,457,124, issued July 22nd, 1969 to Cowles Chemical Company, the solution including specified amounts selected from sodium diachromate, chromic acid, sulfamic acid, ammonium sulphate, boric acid, sodium silicofluoride and nitric acid.

SUMMARY OF THE INVENTION

It is the principal object of the present invention to provide a new composition for use in the production of a protective coating on zinc or cadmium, and a new process making use of such a composition.

In accordance with the present invention there is provided a composition for producing a chromate protective coating on a surface comprising zinc or cadmium, the composition is may be added in the form of the 60 percent perchloric acid to provide the required amount of ion of 0.5 to 0.85 grams per liter, and for this concentration the solution should contain , requiring the specified amount of from 0.3 to 0.5 ml per liter of the said 60 percent perchloric acid. The fluoride ion conveniently is added as the 48 percent hydrofluoric acid to provide from 0.1 to 0.27 grams per liter of the fluoride ion, preferably 0.15 to 0.27 grams per liter, and this amount of ion requires , requiring the addition of the specified amount of about 0.1 to 0.25 ml per liter of the acid, preferably 0.15 to 0.25 ml per liter.

The usable operating temperature range for the solution is 150° F to 200° F, while the preferred temperature range is 170° F to 190° F. With the high reactivity the time of application is of the order of 1 to 10 seconds only, and the provision for rinsing described above is made operative if the exposure time increases to above fifteen seconds. An excessive exposure time is indicated by brown discolouration and even etching of the zinc surface.

The exact mechanism for the formation of the chromate film is not known to the applicants at this time, and accordingly the invention disclosed and claimed herein is not to be limited by any mechanism which is postulated at this time. The composition solution in contact with the zinc or cadmium surface generates hydrogen which is believed to reduce some of the hexavalent chromium to trivalent chromium, some of which may then be oxidised by the perchlorate ion back to the hexavalent state to balance the relative concentrations in the solution. The balance of the trivalent chromium goes into film formation on the zinc or cadmium surface. The trivalent chromium is believed more effective than the hexavalent form in combining with the fluoride ion, with which it complexes readily during film formation.

As will be appreciated from the small exposure times required the reaction involved takes place extremely rapidly and it appears to be a characteristic of films produced in accordance with this invention that the ratio of trivalent to hexavalent chromium therein always is about 2:1. However, excellent performance has been achieved with film weights outside this ratio, so that some form of balanced mechanism appears to be operative, whereas no consistent ratio is obtained with prior processes known to the applicants.

An examination of typical films produced shows trivalent chromium in the amount of from about 5 to about 12 micrograms per sq. inch (m.s.i) while the hexavalent form is present in the amount of from about 2 to about 7 m.s.i, giving a total chromium content of from about 7 to about 19 m.s.i., most usually in the neighbourhood of about 16 m.s.i. This thickness may be compared to the usual thickness of about 9 m.s.i. obtained with known systems. Excellent results have also been achieved with film weight outside these values.

The films produced by the application of the invention are found to be unexpectedly much more protective than those of known prior methods. One test known as the humidity cabinet test involves the use of a Cleveland condensation tester cabinet having its interior maintained at 100° F, wherein strips of galvanized steel are suspended above a water trough, the side exposed to the trough being at the said temperature of 100° F, while the other side is at the ambient temperature, usually about 70° F. The humidity of the cabinet interior is 100 percent and the water that condenses on the inner surface trickles back into the trough, giving the effect of a constant exposure to a rainy atmosphere. The strips are maintained under this condition until visual inspection shows that about 10 percent of the zinc coating has corroded to white rust. With the prior coatings known to the applicants an average exposure of 30 hours was sufficient to produce the specified corrosion, while this degree of attack is only obtained with the processes etc. of this invention after an exposure of about 400 - 1,000 hours.

In another test known as the salt fog test (A.S.T.M. No: B117-62) the strips are sprayed with a fine spray of 5 percent salt solution at 100° F. With known prior art films an exposure of about 10 hours was sufficient to produce the specified degree of corrosion, while strips treated in accordance with this invention only showed such attack after about 100 hours. It is known that a mere increase in thickness of the film does not necessarily increase the protection afforded thereby, and these improved results indicate that the films obtained are not only thicker, but also are more impervious to penetration by water vapour etc. It is known that the hexavalent chromium is considerably more mobile in the film forming complexes than the tivalent form, in addition to being more soluble, and does operate to provide a "sealing" mechanism whereby the mobile hexavalent ion migrates to potential "holes" in the film and reacts readily with the zinc to close such holes. However, it is the trivalent form complexed with the zinc and the fluorine that provides the essential protective film. The excess solution removed by the pressure rolls has the chromium predominantly in the hexavalent form because of its higher solubility and concentration.

Although the invention has been described as applied to the protection of zinc surfaces, it can also be applied to the corresponding protection of zinc alloy surfaces exhibiting the necessary reactivity toward the compositions, and to the protection of cadmium surfaces.

Claims

1. A composition for producing a chromate protective coating on a surface comprising zinc or cadmium the composition having a ph less than 1.15 and consisting of an aqueous solution of ph less than 1.15 containing demineralised water, a source of chromium ion, chromium being the only metallic ion present, the chromium ion being present in the amount of 5-15 grams per liter, a source of perchlorate ion, the perchlorate ion being present in the amount of 0.5 to 0.85 grams per liter corresponding to from 0.3 to 0.5 ml of 60 percent perchloric acid per liter, and a source of fluoride ion, the fluoride ion being present in the amount of 0.1 to 0.27 grams corresponding to from 0.1 to 0.25 ml of 48 percent hydrofluoric acid per liter.

2. A composition as claimed in claim 1, wherein the ph of the solution is in the range 1.15 to 0.85.

3. A composition as claimed in claim 2, wherein the ph of the solution is in the range 1.0 to 0.9.

4. A composition as claimed in claim 1, wherein the chromium ion is present in the amount of 9 to 13 grams per liter.

5. A composition as claimed in claim 1, wherein the fluoride ion is present in the amount of 0.15 to 0.27 grams corresponding to from 0.15 to 0.25 ml of 48 percent hydrofluoric acid per liter.

6. A composition as claimed in claim 1, wherein the ph of the solution is in the range 1.15 to 0.85, wherein the chromium ion is present in the amount of 5 to 15 grams per liter, wherein the perchlorate ion is present in the amount of 0.5 to 0.85 grams corresponding to from 0.3 to 0.5 ml of 60 percent perchloric acid per liter, and wherein the fluoride ion is present in the amount of 0.1 to 0.27 grams corresponding to from 0.1 to 0.25 ml of 48 percent hydrofluoric acid per liter.

7. A composition as claimed in claim 1, wherein the ph of the solution is in the range 1.0 to 0.9, wherein the chromium ion is present in the amount of 9 to 13 grams per liter, wherein the perchlorate ion is present in the amount of 0.5 to 0.85 grams corresponding to from 0.3 to 0.5 ml of 60 percent perchloric acid per liter and wherein the fluoride ion is present in the amount of 0.15 to 0.27 grams corresponding to from 0.15 to 0.25 ml of 48 percent hydrofluoric acid per liter.

8. A process for producing a conversion coating on a metal surface comprising zinc or cadmium, including applying to the surface for a period of from 1 to 10 seconds at a temperature of from 150° F to 200° F a composition having a ph in the range 1.0 to 0.9 and consisting of an aqueous solution of ph in the range 1.0 to 0.9 containing demineralised water, a source of chromium ion, the chromium ion being present in the amount of 5-15 grams per liter, chromium being the only metallic ion present, a source of perchlorate ion, the perchlorate ion being present in the amount of 0.5 to 0.85 grams corresponding to from 0.3 to 0.5 ml of 60 percent perchloric acid per liter, and a source of fluoride ion, the fluoride ion being present in the amount of 0.1 to 0.27 grams corresponding to from 0.1 to 0.25 ml of 48 percent hydrofluoric acid per liter.

9. A process for producing a conversion coating on a metal surface comprising zinc or cadmium, including applying to the surface for a period of from 1 to 10 seconds at a temperature of from 105° F to 200° F a composition containing an aqueous solution of ph less than 1.15 containing having a ph less than 1.15 and consisting of demineralised water, a source of chromium ion, the chromium ion being present in the amount of 5 to 15 grams per liter, chromium being the only metallic ion present, a source of perchlorate ion, the perchlorate ion being present in the amount of 0.5 to 0.85 grams corresponding to from 0.3 to 0.5 ml of 60 percent perchloric acid per liter, and a source of fluoride ion, the fluoride ion being present in the amount of 0.1 to 0.27 grams corresponding to from 0.1 to 0.25 ml of 48 percent hydrofluoric acid per liter.

10. A process for producing a conversion coating on a metal surface comprising zinc or cadmium, including applying to the surface for a period of from 1 to 10 seconds at a temperature of from 105° F to 200° F a composition having a ph less than 1.15 and consisting of an aqueous solution of ph less than 1.15 containing demineralised water, a source of chromium ion, the chromium ion being present in the amount of 9 to 13 grams per liter, chromium being the only metallic ion present, a source of perchlorate ion, the perchlorate ion being present in the amount of 0.5 to 0.85 grams corresponding to from 0.3 to 0.5 ml of 60 percent perchloric acid per liter, and a source of fluoride ion, the fluoride ion being present in the amount of 0.1 to 0.27 grams corresponding to from 0.1 to 0.25 ml of 48 percent hydrofluoric acid per liter.

11. A process for producing a conversion coating on a metal surface comprising zinc or cadmium, including applying to the surface for a period of from 1 to 10 seconds at a temperature of from 105° F to 200° F a composition having a ph less than 1.15 consisting of an aqueous solution of ph less than 1.15 containing demineralised water, a source of chromium ion, the chromium ion being present in the amount of 5-15 grams per liter, chromium being the only metallic ion present, a source of perchlorate ion, the perchlorate ion being present in the amount of 0.5 to 0.85 grams corresponding to from 0.3 to 0.5 ml of 60 percent perchloric acid per liter, and a source of fluoride ion, the fluoride ion being present in the amount of 0.15 to 0.27 grams corresponding to from 0.15 to 0.25 ml of 48 percent hydrofluoric acid per liter.

12. A process for producing a conversion coating on a metal surface comprising zinc or cadmium, including applying to the surface for a period of from 1 to 10 seconds at a temperature of from 150° F to 200° F a composition having a ph in the range 1.15 to 0.85, consisting of an aqueous solution wherein the ph of the solution is in the range 1.15 to 0.85, demineralised water, wherein chromium ion is present in the amount of 5 to 15 grams per liter, chromium being the only metallic ion present, perchlorate ion is present in the amount of 0.5 to 0.85 grams corresponding to from 0.3 to 0.5 ml of 60 percent perchloric acid per liter, and fluoride ion is present in the amount of 0.1 to 0.27 grams corresponding to from 0.1 to 0.25 ml of 48 percent hydrofluoric acid per liter.

13. A process for producing a conversion coating on a metal surface comprising zinc or cadmium, including applying to the surface for a period of from 1 to 10 seconds at a temperature of from 170° F to 190° F a composition having a ph in the range 1.0 to 0.9 consisting of an aqueous solution wherein the ph of the solution is in the range 1.0 to 0.9, demineralised water, wherein chromium ion is present in the amount of 9 to 13 grams per liter, chromium being the only metallic ion present, perchlorate ion is present in the amount of 0.5 to 0.85 grams corresponding to from 0.3 to 0.5 ml of 60 percent perchloric acid per liter and fluoride ion is present in the amount of 0.15 to 0.27 grams corresponding to from 0.15 to 0.25 ml of 48 percent hydrofluoric acid per liter.