GB2179960A

GB2179960A - Passivation of zinc and zinc-aluminium alloys

Info

Publication number: GB2179960A
Application number: GB08621560A
Authority: GB
Inventors: Gary A Reghi
Original assignee: Parker Chemical Co
Current assignee: Henkel Corp
Priority date: 1985-09-06
Filing date: 1986-09-08
Publication date: 1987-03-18
Anticipated expiration: 2006-09-08
Also published as: DE3628660A1; AU590885B2; JPS6256579A; GB8621560D0; EP0213590B1; DE3680263D1; CA1274754A; ES2001659A6; EP0213590A1; KR870003229A; AU5875286A; GB2179960B; NZ216539A; ATE65266T1

Abstract

An aqueous acidic treating solution for forming a chromium-containing passivate coating on the surface of zinc or zinc-aluminium alloys contains chromium ions of which the predominant proportion are in the hexavalent state, preferably in an amount 2-20 g/l., nitrate ions, preferably in an amount 0.5-6 g/l., and phosphate ions, preferably in an amount 1-15 g/l. The solution may also contain fluoride ions, molybdate ions and tungstate ions. The phosphide may be an alkali metal or ammonium phosphate, a polyphosphate or phosphoric acid.

Description

1 GB2179960A 1

SPECIFICATION

Passivation process and composition for zinc and zinc-aluminium alloys 17 t h r The present invention relates to a process and aqueous acidic treating solution for the treatment 5 of surfaces of zinc or zinc-aluminium alloys so as to form a passivate coating and to enhance the corrosion resistance of the metal surface.

In this specification the term -zinc surface- is intended to encompass surfaces of relatively pure zinc as well as of zinc containing minor quantities of alloying elements.

It is known to treat zinc surfaces such as galvanized hot dipped and electrolytic zinc-coated 10 ferrous metals such as steel with aqueous acidic passivate solutions containing chromium so as to deposit a chromium-containing passivate coating on the surface and to substantially improve the resistance of the galvanized material to corrosion, discolouration and white rust formation. An aqueous treating solution which is commercially employed for this purpose employs chromate ions, boric acid and hydrofluoric acid as the principal ingredients.

While known treating solutions have been found satisfactory for treating substantially pure zinc metal surfaces, they are unsatisfactory for the treatment and passivation of zinc-aluminium alloys which are now in widespread commercial use as a substitute for conventional zincgalvanized steel sheet. Zinc aluminium alloys contain a substantial amount of aluminium, eg above 30%.

One such zinc-aluminium alloy is marketed under the brand name -Galvalumeand nominally contains above 55% by weight aluminium, 43.5% by weight zinc and about 1.5% by weight silicon. It has been observed that conventional passivation treatments of the types heretofore employed on substantially pure zinc-galvanized stock do not effectively prevent rapid darkening and/or discolouration of such zinc-aluminium alloy surfaces in an unpainted condition when exposed to normal atmospheric conditions. Such discolouration and/or darkening of the surface 25 is objectionable.

An aqueous acidic solution according to the invention contains chromium ions of which the predominant proportion are in the hexavalent state, nitrate ions and phosphate ions which are present in an amount sufficient to form a chromium-containing passivate coating on the zinc or zinc-aluminium alloy surface. The aqueous acidic treating solution can optionally further contain 30 fluoride ions, molybdate ions and tungstate ions present in an amount effective to accelerate the coating action of the solution.

The treatment solution can be prepared employing a liquid concentrate which can conveniently be diluted with water to form an operating solution of the desired concentration.

A process according to the invention comprises applying the treating solution to a clean zinc 35 or zinc-aluminium surface such as steel sheet coated with zinc or a zinc- aluminium alloy and drying the resultant liquid film. The liquid film should be applied to a desired substantially uniform thickness. The time of application, temperature of the solution, concentration of the active constituents and the thickness of the liquid film are controlled so as to provide a chromium-containing coating of the desired weight.

The chromium ions can be introduced in the form of any solution soluble chromium compound such as chromic acid, alkali metal and ammonium chromates and dichromates as well as mixtures thereof. Preferably, the chromium ions are introduced as chromic acid or chromium trioxide which further imparts acidity to the solution. The chromium ions in the passivate solution are predominantly in the hexavalent state. Although the chromium is added as hexavalent chromium, some trivalent chromium may be present in the treating solution due to a reduction of the hexavalent chromium to the trivalent state during the coating reaction. The chromium ion concentration in the treating solution can broadly range from about 2 up to about 20 grams per litre (g/1) with a concentration of about 11 g/1 being typical.

The nitrate ions can also be introduced into the treating solution in the form of any solution 50 soluble and compatible nitrate compound such as nitric acid, alkali metal and ammonium nitrates as well as mixtures thereof. Nitric acid itself constitutes a preferred compound. The nitrate ions can be present in an amount of about 0.5 up to about 6 g/1 in the operating solution with concentrations of about 3 g/1 being typical.

The phosphate ions can also be introduced in the form of any solution soluble and compatible 55 phosphate compound including alkali metal and ammonium phosphates and polyphosphate as well as phosphoric acid itself which constitutes a preferred compound. Generally, the concentra tion of the phosphate ions in the treating solution can range from about 1 up to about 15 g/1 with concentrations of about 7 g/1 being typical.

The treating solution is on the acid side with an operating pH of less than about 2 being 60 typical. The appropriate acidity can be achieved by employing acidic components of the essential ingredients or in the case of the addition of salts, appropriate pH adjustment can be made utilizing a suitable acid.

It has been found by experimentation that an aqueous acidic treating solution of the foregoing composition is satisfactory for treating zinc or zinc-aluminium alloys. Accordingly, the treating 65 2 GB2179960A 2 solution can be employed for passivating both conventional zincgalvanized steel strip as well as steel strip coated with a zinc-aluminium alloy, for example, thereby requiring only one treatment solution for both types of coated strip.

In the treatment of zinc-aluminium alloy surfaces, it has been found advantageous to further include in the operating solution activating ions which accelerate the coating reaction and also contribute to improved passivate coatings on the treated surface. Such optional additives include fluoride ions typically present in an amount of about 0.5 g/1 which can be conveniently added as hydrofluoric acid; molybdate ions typically present in an amount of about 0.5 to about 0.7 9/1 which can be introduced as the alkali metal and ammonium salt or molybdic acid of which sodium molybdate itself constitutes the preferred material for economic considerations; tungstate 10 ions which can be conveniently introduced as an alkali metal tungstate salt of which sodium tungstate constitutes a preferred material and can be introduced with a concentration typically about 0.2 g/L The operating treating solution is conventionally prepared by first forming a concentrated solution of the essential ingredients which can subsequently be diluted with water to form an 15 operating solution of the desired concentration. A suitable concentrate comprises 30.4% by weight of chromium trioxide (about 219 g/1 chromium ions) about 4.35% by weight of 100% nitric acid (about 59 9/1 nitrate ions), about 10.65% by weight of 100% nitric acid (about 59 g/1 nitrate ions), about 10.65% by weight of 100% phosphoric acid (about 142 g/] phosphate ions) and the balance water. A concentrate of the foregoing composition has a specific gravity of 20 about 1.383.

The concentrate can be diluted with sufficient water that the concentrate comprises from about 1% up to about 10% by volume of the treating solution.

The treating solution is applied to a clean zinc or zinc-aluminium alloy surface such as by spray, immersion, flooding, brushing, roller coating, or the like after which the excess treating 25 solution is removed such as by squeegee. The squeegee control of the applied solution produces a relatively thin uniform liquid film which can be controlled in thickness so as to provide the desired coating weight of the resultant chromium-containing coating. The coating weight of the chromium-containing coating is also influenced by the length of time the solution is in contact with the surface, the temperature of the treating solution and the concentration of the active 30 constituents therein. Typically the coating weight is of the order of 1 Mg/ft2.

Generally, the treating solution is controlled within a temperature ranging from about WC up to about WC with temperatures from about WC up to about 71'C being preferred. Typically, the treating solution is controlled at about 6WC.

The time of contact can range from as little as about 0.5 seconds up to about 1 minute or 35 longer depending on the desired thickness of the chromium-containing passivate coating desired.

After the excess liquid film has been removed such as by squeegee, the liquid film remaining is allowed to dry.

In normal commercial operation, a steel strip is typically first cleaned employing a solvent cleaner to remove any protective oil films thereon whereafter the strip is subjected to a suitable 40 cleaning treatment such as by employing an alkaline cleaner. Thereafter, the strip is water rinsed and upon drying is submerged in a molten bath of a zinc-aluminium alloy to effect ahot dip galvanizing thereof. Upon solidification of the zinc-aluminium alloy coating, the strip can be directly contacted with the treating solution to effect passivation of the surfaces thereof.

In order to further illustrate the present invention, the following examples are provided. It will 45 be understood that the examples are provided for illustrative purposes and are not intended to be limiting of the scope of the present invention as herein described and as set forth in the subjoined claims.

Example 1

A hot dipped zinc-aluminium alloy ("Galvalume") coated steel test panel was treated in a treating solution in accordance with the present invention containing 0. 71% by weight of chrom ium trioxide (about 3.7 9/1 chromium ions), about 0.15% by weight of 68% nitric acid (about 1 9/1 nitrate ions) and about 0.33% by weight of 75% phosphoric acid (about 2.4 g/1 phosphate ions) and the balance water. Prior to treatment, the coated test panels were first wiped with mineral spirits to remove any protective oil film and contaminating substances and were there after subjected to a 10 second alkaline cleaning by spray application at a temperature of about WC employing a proprietary alkaline cleaner sold under the brand name Parco Cleaner 338 (Parco is a trade mark of Parker Chemical Company). Thereafter the cleaned test panels were hot water rinsed for a period of five seconds at a temperature of about 49'C. Following the rinse treatment, the panels were immersed in the treating solution at a temperature of about WC for a period of time to produce a chromium-containing passivate coating of approximately 1 milligram per square foot (Mg/ft2) of chromium when dried. The excess treating solution was squeegeed off and the residual liquid film was permitted to dry at room temperature.

The subsequent treated test panels were cut into test samples of a size of about 3.5 by 7.5 65 1 1 3 GB2179960A 3 inches and were oiled with a vanishing oil sold under the brand name Castrof Rustillo DW-924 HF.

The test samples were thereafter subjected to a corrosion test established by suppliers of zinc or zinc-aluminium alloy coated steel to predict resistance to atmospheric weathering. The samples were first rinsed with mineral spirits to remove the protective oil film and were thereafter placed in glass trays of a size of 4.5X8.5X3 inches deep to which 0.75 litres of distilled water was added. The panels were placed submerged in the water and placed in a humidity cabinet main"tained at a temperature of 38'C.

During the continuance of the test, the surface of the treated test sample was periodically checked to evalute the presence of any darkening and/or discolouration. The test panels were 10 rated on a scale of 1 to 8 in which 8 represents no discernable darkening whereas 1 represents complete darkening.

k Example 2

A second series of hot dipped zinc-aluminium alloy ("Galvalume") processed in accordance with the same procedure as described in Example 1 and subjected to accelerated corrosion testing. In this example, a treating solution was prepared containing 0. 65% by weight chromium trioxide (about 3.4 9/1 chromium ions), 0.14% by weight of 68% nitric acid (about 0.93 9/1 nitrate ions), about 0.33% by weight of 75% phosphoric acid (about 2.4 g/1 phosphate ions), about 0.07% by weight of 70% hydrofluoric acid (about 0.5 g/] fluoride ions), about 0.09% by 20 weight sodium molybdate dihydrate (about 0.6 g/i molybdate ions), about 0. 03% by weight sodium tungstate dihydrate (about 0.22 g/1 tungstate ions) and the balance water.

Example 3

For comparative purposes, a third series of hot dipped zinc-aluminium alloy (---Galvalume---) test panels were processed and tested in the same manner as previously described in Example 1.

The treating solution simulates a typical passivate solution heretofore employed for passivating galvanized steel and which has been found unsatisfactory for the passivation of zinc-aluminium alloys. The passivating solution contained 0.38% chromium trioxide (about 2 g/] chromium ions), about 0. 11 % by weight boric acid and about 0. 11 % by weight of 70% hydrof luoric acid. 30 Example 4

For comparative purposes, a hot dipped zinc-alumium alloy ("Galvalume") test panel was subjected to a mineral spirit wiping and alkaline cleaning followed by water rinsing in accordance with the same procedure as described in Example 1, but was not subjected to any passivation 35 treatment. The cleaned-only test panel was subjected to accelerated corrosion testing in the same manner as the test samples of Examples 1-3.

The comparative test results and the chromium-containing coating weights obtained on the several test samples are set forth in Table 1.

Table 1

BARE CORROSION Chromium 45 Coating Weight Test Results EXAMPLE (mg/ft2) 1100 hrs 2164 hrs 1 1.37 7.5 7.3 50 2 0.99 8 7.8 3 (comparative) 1.03 3.9 2.5 Cleaned-Only 1 (after - 55 168 hrs) The cleaned-only test sample had completely darkened and failed the test after only 168 hours of exposure. After 1,100 hours of exposure, the test sample treated with the passivate solution 60 of Example 1 exhibited only minimal darkening providing a rating of 7.5 while the test sample treated with the passivate solution of Example 2 retained a rating of 8 indicating no visible discolouration. In contrast, the test sample treated with a conventional passivate solution accord ing to prior art practice as described in Example 3 sustained severe discolouration after 1,100 hours having a rating of only 3.9. The benefits of the treating solution as exemplified in Example 65 4 GB2179960A 4 1 and 2 is further magnified by the results obtained at the conclusion 2, 164 hours of test evidencing clear superiority over that of Example 3.

Claims

1. An aqueous acidic solution for forming a passivate coating on the surfaces of zinc or zinc- 5 aluminium alloys containing chromium ions of which the predominant proportion are in the hexavalent states, nitrate ions and phosphate ions, each present in an amount sufficient to form a chromium-containind passivate coating on the zinc or zinc-aluminium alloy surface.

2. A solution according to claim 1 containing 2 to 20 g/1 of the chromium ions, 0.5 to 6 9/1 nitrate ions and 1 to 15 9/1 phosphate ions.

3. A solution according to claim 1 or claim 2 having a pH of less than about 2.

4. A solution according to any preceding claim including fluoride ions in an amount effective to accelerate the formation of a chromium-containing passivate coating.

5. A solution according to claim 4 in which the fluoride ions are present in an amount of about 0.5 9/1.

6. A solution according to any preceding claim including molybdate ions in an amount effective to accelerate the formation of a chromium-containing passivate coating.

7. A solution according to claim 6 in which the molybdate ions are present in an amount of about 0.5 to about 0.7 9/1.

8. A solution according to any preceding claim further including tungstate ions in an amount 20 effective to accelerate the formation of a chromium-containing passivate coating.

9. A solution according to claim 8 in which the tungstate ions are present in an amount of about 0.2 911.

10. An aqueous acidic concentrate suitable for dilution with water to form an operating solution according to any preceding claim.

11. A concentrate according to claim 10 containing about 219 g/] chromium ions, about 59 9/1 nitrate ions and about 142 9/1 phosphate ions.

12. A process for passivating a zinc or zinc-aluminium alloy to enhance the corrosion resis tance thereof which comprises applying an aqueous acidic solution according to any of claims 1 to 9 to a clean zinc or zinc-aluminium alloy surface and drying the resultant liquid film onto the 30 surface.

13. A process according to claim 12 in which the solution is applied at a temperature between 38T and 88T.

14. A process according to claim 12 in which the solution is applied at a temperature of between 55T and 71T.

15. A process according to any of claims 12 to 14 in which the chromiumcontaining coating has a weight of about 1 M9/ft2 of chromium when dried.

16. A process according to any of claims 12 to 15 in which the surface is a zinc-aluminium alloy.

Printed for Her Majesty's Stationery Office by Burgess & Son (Abingdon) Lid, Dd 8817356, 1987. Published at The Patent Office, 25 Southampton Buildings, London, WC2A 'I AY, from which copies may be obtained.

A