AU633728B2

AU633728B2 - Formation of conversion coatings on surfaces of zinc or zinc alloys

Info

Publication number: AU633728B2
Application number: AU76178/91A
Authority: AU
Inventors: Thomas Ing. Kolberg; Werner Dr.-Chem. Rausch; Peter Dr.-Chem. Schubach; Thomas Ing. Wendel
Original assignee: Metallgesellschaft AG
Current assignee: Chemetall GmbH
Priority date: 1990-05-29
Filing date: 1991-04-29
Publication date: 1993-02-04
Anticipated expiration: 2011-04-29
Also published as: EP0459549B1; JPH04231479A; AU7617891A; CA2041892C; DE4017186A1; JP3083872B2; DE59102544D1; BR9102173A; ZA914085B; ES2057734T3; EP0459549A1; CA2041892A1

Description

3 3728 Form COMMONWEALTH OF AUSTRALIA PATENTS ACT 1952-69 COMPLETE SPECIFICATION

(ORIGINAL)

Class Int. Class Application Number: Lodged: Complete Specification Lodged: Accepted: Published: Priority Related Art Name of Applicant 4 o Address of Applicant METALLGESELLSCHAFT AKTIENGESELLSCHAFT Reuterweg 14, D-6000 Frankfurt/Main, Federal Republic of Germany THOMAS KOLBERG, WERNER RAUSCH, PETER SCHUBACH and THOMAS WENDEL Actual Inventor Address for Service: WATERMARK PATENT TRADEMARK ATTORNEYS.

LOCKED BAG NO. 5, HAWTHORN, VICTORIA 3122, AUSTRALIA Complete Specification for the invention entitled: FORMATION OF CONVERSION COATINGS ON SURFACES OF ZINC OR ZINC ALLOYS The following statement is a full description of this invention, including the best method of performing it known to :-US I I A 7470 P 40 17 186.8 Metallgesellschaft AG Frankfurt, dated May 28, 1990 Reuterweg 14 6000 Frankfurt-on-Main 1 Case No. 90 00 51 Title: Formation of Conversion Coatings on Surfaces of Zinc or Zinc Alloys i t

DESCRIPTION

This invention relates to a process of forming conversion coatings on surfaces of zinc and zinc alloys, in which the surfaoes are contacted in a first stage with a solution which contains at least two different polyvalent metal ions and also contains such 4s4* an amount of complexing agent that the polyvalent metal 4 o a 0 ions are kept in solution, and which has a pH value of at least 11, and the surfaces are treated in a succeeding stage with a postrinsing solution, and also relates to the use of the process for a pretreatment before a succeeding painting, film laminating or adhesive coating operation.

Such a process of forming a conversion layer is known from LE-C-1 521 854 particularly as a pretreatment before a painting or film laminating operation.

n-2- The layer is formed on the surface of zinc or zinc alloy by a treatment with aqueous alkaline solutions, which ccntain so-called non-alkali metal ions consisting of ions of one or more of the metals silver, magnesium, cadmium, aluminum, tin, titanium, antimony, molybdenum, chromium, cerium, tungsten, manganese, cobalt, iron, and nickel. It has been emphasized that particul:arly suitable solutions contain nonalkali metal ions consisting of ions of iron or of cobalt together with another of the stated metals. The solutions also contain organic complexing agents in an amount which is iufficient to keep the non-alkali metal ions in solution. The conversion coalinps formed by means of said ions will increase the resistance to corrosion ana ,ill improve the adhesion of subsequently applied organic coatings.

The resistance to corrosion and the bond strength will be further improved if the surfaces on II 0 Swhich the conversion layer has been formed are postrinsed with ancid solution which contains hexavalent chromium and optionally contains also trivalent chromium.

Vhereas that known process produces good results as regards the resistance to corrosion and the bond strength, the use of trivalent chromium ions and particularly of hexavalent chromium ions in the passivating post rinse solution is most undesirable because hexavalent chromium is toxic and requires a special disposal.

It is an object of the invention to provide for the formation of conversion coatings on surfaces of zinc or zinc alloys a process by which the disadvantages of The known process are avoided and, in particular, the environment is not polluted or is only very slightly polluted and which produces at least equally good results as regards the resistance to corrosion and the adhesion of paint.

That object is accomplished in that the process of the kind described first hereinbefore is carried out in such a manner in accordance with the invention that the solution used for post rinsing contains aluminum, zirconium, and fluoride and has been adjusted to a pH vqlue not in excess of 5 he process in accorcance with the in- S**AI venticn can be tsed to treat all surfaces which contain zinc or zinc alloys, such as materials of massive zinc or massive zinc alloys, but also to treat surfaces which have been plated with zinc or zinc alloys electrolytically, by deposition from the gas phase or by hot dip coating. Elements which can be alloyed with zinc include particularly aluminum, silicon, lead, iron, nickel, cobalt and manganese. Sheet materials may be plated with zinc or with a zinc alloy on one side or on both sides.

If the surface of zinc or zinc alloy is only slightly greased or soiled, there may be no need for a preceding cleaning and degreasing of the surface.

In said cases, surfactants may be added in the first stage so that the cleaning and degreasing are effected with the treating scluticn used also to form the conversion coating. That embodiment affords mainly the advantage that the entire pretrea-;ment the- ;rfece can be carried out in fewer stages because the separate cleaning and the associated rinsing with water are omitted.

The alkaline solution used in the first stage may be applied, by spraying, dipping or flooding.

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It has been found that alkaline solutions which contain iron(III) ions and additionally contain cobalt ions and/or nickel ions and/or chromium (III) ions and/or aluminum ions in a total content between 0.3 and 3 g/l, preferably between C.4 and 1.2 g/1, are particulTly suitable. The polyvalent metal ions may be used as salts of inorganic acids, such as nitric acid, or as salts of organic acids, such as formic acid and particularly also of acetic acid. It is also possible to use salts of such organic acids which also serve as complexing agents. Amphoteric metals, such as aluminum, may be dissolved in the form of the hydroxy complex even without an additional anion or complexing agent.

-c a~ulira~-q; Due to the pickling attack during the treatment in the first stage, additional polyvalent cations, which are present in the surface to be treated and are not contained in the freshly prepared bath solution, may enter the treating solution from the surface of zinc or zinc alloy. On surfaces plated with zinc by hot dipping such cations consist of e.g. zinc, aluminium and lead. The total concentration of such cations may increase up to a few grams per liter and in general this will not o 0 ,ooo° disturb the formation of the conversion coating.

Suitable complexing agents. include particularly organic chelating agents of various kinds, for instance, dicarboxylic acids (malonic acid, fumaric acid 0 4S*"Oo etc.); amino acids glycine); hydroxycarboxylic 00 acids citric acid, gluconic acid, lactic acid); 1,3-diketones (e.go, acetylacetone); aliphatic polyalcohols sobitol, 1,2-ethanediol); aromatic carboxylic acids salicylic acid, phthalic acid); amino- 0, carboxylic acid ethylenediamine-tetraacetic acid).

oo Other complexing agents, such as methanephosphonic acid diethanolamide, may also be used. The amount of the complexing agent contained in the solution must be sufficient for a complex binding of all polyvalent metal ions which are present. For this reason the content of complexing agent must be increased too when the content of polyvalent metal ions in -he solution increases. Because increasing amounts of certain complexing agents, which are inherently acidic, may decrease the alkalinity of the -6solution, it is preferred to use complexing agents in the form of neutral salts, particularly of the alkali metal salts. It has been found that no advantage will be afforded by the use of complexing agents in a surplus.

Particularly desirable results will be produced by the use of complexing agents consisting of °0 salts of gluconic acid, particularly of hexahydroxy- 0 heptanoic acid. The solution should contain complexing 0 agents in an amount between 0.05 and lo g/1, in most cases between 1.5 and 5.5 g/l (based on the sodium salt 0o of hexahydroxyheptanoic acid).

The aqueous solution must have a pH value of at least 11. Best results will be produced with pH values in the range from 12.2 to 1533. The pH value may be adjusted, by triethanolamine, alkali hydroxides, alkali carbonates, alkali phosphates, alk ,li polyphosphates, alkali pyrophosphates, alkali borates, 0 alkali silicates or mixtures thereof. But the use of al- 00 0 °o kali hydroxides, particularly sodium hydroxide, is most desirable.

The temperature of the solution used in the first stage may generally lie between 20 and 90 °G.

The preferred temperature range is about 45 to 65 0C.

The treating time is generally about 2 to 60 seconds, as a rule, and preferably 5 to seconds. It will depend, inter alia, on the application r 7 1 -7technology that is employed. For instance, the treating time will be shorter in a spraying process than in a dipping process under conditions which are equal in other respects.

In general, solutions having a lower metal ion concentration must be used at higher temperatures and for longer treating times than solutions having a higher meal ion concentration.

Shen a conversion coating has been formed, any surplus treating solution should be removed as completely as possible from the surface of zinc or zinc alloy. That may be effected, by dripping, squeegeeing, blasting or rinsing with water or with an aqueous solution, which may have been acidified, by an int organic of organic acid (hydrofluoric acid, boric acid, *nitric acid, formic acid, acetic acid, etc.).

The post rinse solution may be applied, by dippin.:, flooding, spraying or roller coating.

In a preferred embodiment of the invention the solution employed for the aftertreatment contains aluminum, zirconium andfluoride in a total concentration of Al Zr F between 0.1 and 8 g/l preferably between 0.2 and 5 g/l. The Al i Zr F mole ratios should desirably be adjusted to (0.15 to 8) 1 (5 to 52), particularly to (0.15 to 2.0) 1 (5 to 16). In a particularly preferred embodiment of the process the I i -8- Al Zr F ratio in the post rinsing solution is (0.15 to 0.67) 1 (5 to According to a further preferred featze of the invention the pH value should be adjusted to 2 to The post rinsi.ng solutions used in the process in accordance with the invention contain, inter alia, acid aluminum fluoride zirconates and in case of a surplus of aluminum additionally contain other salts of aluminum, such as fluorides, tetrafluoroborates, nitrates, They may be produced, in that zirconium metal or zirconium carbonate is initially dissolved in aqueous hydrofluoric acid to form complex fluorozirconium acid. Aluminum metal or aluminum hydroxide or an aluminum salt, such as the ni.rate, fluoride, tetrafluoborate, formate, acetate, is uhen added, zreferably in dissolved 44" form, and is optionally dissolved. A possible slight cloudiness of the solution will not adversely affect its effectiveness. Whereas the preparation is referably ef- 1 fected on the described route, the solutions may alternatively be prepared in a different manner.

The pH value of the solution is preferably adjusted with cations of volatile bases. These include particularly ammonium, ethanolammonium and diand triethanolammonium. The adjustment particularly to relatively nigh pH valuew in the statea pH range and to relatively high concentrations in the range stated for r -9the total concentration of Al Zr F may result in a cloudiness of the solution but this will not adversely affect the effectiveness of the process.

Accordinr: to a further desirable feature of the invention the surfaces provided with a conversion coating are rinsed with an aqueous solution which additionai.y contains at _ea:3t one of rhe anions benzoate, caprylate, ethyl hexoate, salicyate, preferably in a total concentration of C.05 to o05 5/1.

This will further increase the bare corrosion protection. The anions may be introduced by means of the corresponding acids or their salts.

According to a further desirable feature of the invention the post rinsing solution is applied for between about 1 :ind L20 :--conds, particularly for between 1 and 50 seconds. The solution may be applied at a cemperature between 20°C and about 80° Temr'urt-e between 20 and 50 O are preferred.

Deionized vater or low-salt water is preferably used to prepare the post rinsing bath. Water having a high salt content is less suitable for the preparation of the bath.

After -he passivating aftertreatment the surface may be dried, on the air or in an oven and optionally may be rinsed before with deionized water.

According to a preferred embodiment of the invention the surface which has been subjected to the passivating aftertreatment is subjected to an accelerated drying, with~ hot air or by infrared radiation.

Lhe process in accoraance with the invention serves primarily to pretreat surfaces of zinc or zinc alloys before they are painted 9 laminated with a film or coatCed with an adhesive. it Will iMprove the adhesion of the organic films on th- e metallic 3ucs-craze and.

will also improve uneir resistance -to format-'on of blisters under orrosive action and will i,itit subsurface corrosion proceeding from -efects in the film.

!'he p-rocess in. acc-orda-nce '~zhhe L'Xvention will be oxoilained more ;I.euail ana by way of example with reference tio the fol2.owin,:; example.

Example '2o form a convers-zion no-atin:- on cleaned and depgreased sheets of h.-ot galvanized steel they sere dipped for 30 seconds into solut.'.on -which temperature of C and had the following composition: 7,1 e0.2 -/l .3oti-un salt of hexahydroxyhept.anoic acid 2.2 ,/1 NaOH 27.4~ S/1 Tlhereafter the sheets Were rinsed With -11- I-ll water and were then subjected to a pasjivating aftertreatment. To that end the sheets were dipped into the post rinse solution for 5 seconds and were subsequently squeegeed for a removal of surplus solution.

After a drying at 7500 in an oven operated with circulating air for 0.5 minute ,he pretreated sheets were painted with an epoxy primer ana an a-crylate top coat. The total coating had a thickness of about 25 un.

The created sheets were suosequently sabjected to the following tests: The adhesion of paint was determined by the T-Bend Test, in which the sheets were bent through 180C and the various radii of curvature (Tn) were stated as amounting to n times the sheet metal thickness 1,2 The test result is stated as the percentage of the flaked-off paint surface area in the total curved surface area.

In other treated sheets, scratches extending as far as to the metal subsrate were made by means of a metal needle and the sheets were formed with a cut edge by plate shears. Thereafter the sheets were subjected to the salt soray test in accordance with DIN 50021 SS for 1008 hours. 2he test result was stated as the extent in mm of the migration under the paint from the crack or the cut edge.

To prepare the post rinsing solution to be employed, 1,6 g/1 and 20 g/1 respectively of an aqueous concentrate, which contained -12- 0.855% by weight Al and 8.62% by weight Zr and 10.7%/ by weight F, was diluted with deionized waterto yield post rinsing solution A and post rinsing solution 3. Both scjlu-zions were adjusted with ammonia to a pH value of about 3.6.

For comparison, a post rinsing solution C was employed, which contained Cr(IT) and Cr(IIT) and haQL a pH value of about 3.3.

"he post rinsing sclutions ad -he following compositions: P ost rinsing solution A: Al 0,014 g/1 Zr 0.14 g/ 1 F0-17 1;/

NH

4 0.016 x11 Post rinsing solution Al 0.17 1/ r 1.72 S/lI F 2.14 -7/ .,.,TH40.40 Post rinsing go-Lution 3 0.8 -/1 F 0.2 g/1 Zn 0.3 g/1l 14 i i ig i i: The test results are stated in the following Tables.

Table 1: Paint adhesion in T-Bend Test Post rinsing Solution Flaked off surface area in case of radius of curvature :n i1 T2 T3 T4 A (invention) B (invention) C (comparison) 100 100 100 Table 2: Resistance to corrosion in salt spray test Il U

II

Post rinsing solution A (invention) B (invention) 0 (comparison) migration under paint (mm) after 1008 h at the scratch at the edge 1 to 1 8 to 9 0 to 1 7 1 to 3 9 to A comparison of the tabulated data reveals that the data obtained with the process in accordance with the invention are just as good or better than those of the also tested comparison process in wnich an post rinsing solution wao used that contained Cr(VI) and Cr(III).

Claims

1. A process of fLlorming conversion coatings on surfaces or' Linc and zinc ailloys, in which the surfaces are contacted in a first stage with a solution which contains at least two different pol-yvalent metal ions and also contains such a~n amouirt of complexing agent that 7he zolyvalent metal ions are kepDt in solution, 7and which 1,1,s a pH value of atc le-qst 1.1, and the surfaces are treaedt in. a succeeding stage with a post rinsiny solution, characterized in fl0flsd3 0 (oy.rl tha~tcclto cdfratrrtn ccntains alu- minum, zirconium, 3nd fluoride and has been ao.justed to a pH value not in excess of

2. process according to claim 1, characteriz],L in that post rinsing solu,..>cns are used which contain aluminum, zirconium and fluoride in a to- -tal concentrati4on of 2.l Zr F betw~een 0.1 and 8 Ra/l, preferably between 0.2 and Sprocess according to ciaimi 1 or 2, characterized in that post rinsing solutions are used in which the Al Zr F mole ratios have bee-n _dtjusted to (0.15 to 8) :1 (5 to 52), particularly to (0.15 o 2.0) :1 (5 to 16). A process accorctinE4 to clain: 1, 2 or characterized in that post rinsing solutions are used in which the A.1 Zr mole ratios have been qdjusted to (0.15 to 0.67) :1 (5 to 7)> A process according to any of claims 1 to 4, characterized in that post rinsing solutions are used which have been adjusted to a pH value of 2 to

6. 1 process according to any of claims 1 to 5, characterized in that post rinsing solutions are used in which the DH value has been adjusted with cations of volatile bases, preferably with ammonium, ethanolamonium, di- or triethanolammonium. 0

7. process according to any of claims 1 to 6, characterized in zhat post rinsing solutions are used which additionally contain at least one of the anions benzoate, caprylate, ethyl hexoate, salicylate, in a total concentration of 0.05 to 0.5 g/1.

8. A process according to any of claims 1 000 to 7, charactezized in that the treatment with a post rinsing 9olution takes 1 to 120 seconds, preferably 1. to secondso

9. A process according to any of claims 1 to 8, characterized in that a post rinsing solution is used which 43 at a temperature of 20 to 80 0C, preferably of 20 to 50 0 C. A process according to any of claims 1 to 9, characterized in that the surface is dried when it has been aftertreated. -16-

11. The use of the process accoraing to an7 of claims 1 to 10 as a pretreatment before a painting, film-laminating or adhesive coating operation. DATED this 29th day of April 1991. METALLGESELLSCHAFr AKTIENGESELLSCHAFT WATERMARK PATENT TRADEMARK ATTORNEYS "THE ATRIUM" 290 BURWOOD ROAD HAWTHORN. VIC.3122. 4 I. 44 0 S ~0