GB2152955A - Phosphate treatment of zinc-aluminium plated materials - Google Patents

Abstract

The plated material is treated beforehand with an alkaline aqueous solution with a pH of 11 or more which contains ions of one or more of the metals selected from the group consisting of nickel, iron, cobalt and manganese and a complexing agent to an amount sufficient to dissolve said metal ions. The treatment removes segregated and concentrated aluminium at the plated material surface, which would otherwise tend to prevent phosphate formation.

Description

SPECIFICATION Method of phosphate treatment The present invention relates to a phosphating process in which zinc-aluminium plated material is treated with an alkaline aqueous solution and then is phosphated.

It is known conventionally to effect phosphating in order to improve the corrosion resistance and painting undercoat characteristics of zinc plated steel plates, and this process is widely practised.

Steel plates plated with a variety of alloys with the object of improving the corrosion resistance of the actual steel plates themselves have been developed in recent years, one type that has attacted attention and has also come onto the market being zinc-aluminium alloy plated steel plates. However, in provision of a phosphate film on zinc-aluminium alloy plated material, there is a tendency for phosphate film formation to be prevented due to segregation and concentration of Al at the plated material's surface. In continuous processing, phosphate film formation is further prevented because Al is eluted into and accumulates in the phosphating bath and when the accumulated Al reaches a certain amount it causes poor formation and deterioration of properties.If, therefore, the amount of Al dissolved in the phosphating bath is not kept below a certain set level, the result is failure to produce a good phosphate film and undesirable properties following painting. The mechanism by which dissolved Al prevents phosphate film formation is not clear, but there is stable formability if the amount of dissolved Al is kept at a level below that at which poor formation occurs. A measure that has been proposed in this respect is to throw away and renew a portion of the phosphating bath but considerations of cost and environmental pollution mean that this cannot be described as a good method.

It is the object of the present invention to provide a process for forming a good phosphate film on the surface of zinc-aluminium plated material which eliminates these drawbacks.

The present invnetors investigated preliminary selective removal of Al at the surface of zinc-aluminium plated material in order to eliminate these drawbacks in the process of phosphating.

As a result of this investigation, it was found that the object of the invention is achieved by pretreating zinc-aluminium alloy plated material with an alkaline aqueous solution with a pH of 11 or more which contains metal ions, preferably ions of one or more of the metals cobalt, iron, nickel and manganese, and a sufficient amount of a complexing agent to dissolve these metal ions, in order to remove segregated and concentrated Al at the plated material surface, and then effecting phosphating.

Examples one may cite of the alkali contained in the alkaline aqueous solution in the invention are alkaline metal hydroxides, carbonates, phosphates and borates, etc., sodium hydroxide being particularly suitable, and normally a concentation of 1-60 gn is used. If the pH of this alkaline aqueous solution is not 11 or more, etching removal of the Al at the zinc-aluminium alloy plated material's surface is difficult.

The metal ions contained in the alkaline aqueous solution in the invention are provided in the form of nitrates, sulfates, carbonates, chlorides or other inorganic salts or as other water-soluble or alkali-soluble compounds and their concentration is 0.01-10 g/l, preferably 0.05-5 g/l. Sufficient complexing to maintain these metal ions in a dissolved states in the alkaline aqueous solution must be effected, and the complexing agent for this is normally at a concentation of 0.1-20 g/l and is, for example, a condensed phosphate, amonia or similar inorganic complexing agent or an organic complexing agent such as malonic phthalic acid or similar dicarboxylic acid, glycine or similar amino acid, citric acid, gluconic acid, lactic acid or similar hydroxycarboxylic acid, sorbitol, ethane-1,2-diol or similar polyhydroxy aliphatic compound, salicylic acid, phthalic acid or similar phenol carboxylic acid, ethylene diamine tetra-acetic acid or similar amine carboxylic acid, diethanolamine methane phosphonate or similar polyamino acid or lignoosulfonic acid, etc. Agents that are particularly suitable are hexahydroxy heptonic acid, sodium gluconate and a sodium salt of diamine tetra-acetic acid.

The treatment stages in the invention comprise stages in which zinc-aluminium alloy plated material is treated, by spray or dip process, with an alkaline aqueous solution, which is at normal temperature to boiling temperature in the invention, for a sufficient time to effect selective etching removal of Al from the surface of the plates material, in which water washing or water washing followed by surface adjustment (e.g., by treatment with a titanium colloid aqueous solution) is effected, and in which phosphating is effected.

In the case of a continuous hoop line, it is normally satisfactory for practical purposes if pretreatment with an alkaline aqueous solution according to the invention is effected for a treatment period of 5-15 seconds with the treatment solution temperature at 50-80,C but the time can be made still shorter by using a high temperature, high concentration alkaline aqueous solution.

An aqueous solution containing 0.1-1% Zn, 0.6-4% P04, 0.1-4% NO2, 0.01-0.4% Ni and 0.01-0.3% F can be used as a phosphate aqueous solution in the invention. If a spray process is used, treatment is effected for a period of 2 seconds-2minutes with the solution temperature at 30-80"C and if a dip process is used, 2 second-2 minute treatment with the solution temperature at 40 - 90"C is effected. After formation of a phosphate film, water washing or water washing followed by chrome sealing treatment and drying are effected.

With zinc-aluminium alloy plated material treated by the method of the invention, Al that has segregated and concentrated at the surface of the material is selectively dissolved, whereby elution of Al into the phosphating bath in the subsequent stage is prevented and it is also made possible to improve the performance of the phosphate film that is formed.

Next, experiments on the selective etchability of Al at the surface of zinc-aluminium plated material by alkaline aqueous solutions according to the invention will be described.

Experiments Steel plates plated with zinc-aluminium alloy marketed under the trade name of "Super Zinc" (90-96.5 Zn, 3.5-10% Al) were spray treated for 5 seconds with pH 13.5-13.7 treatment solutions as noted in Table 1 at a bath temperature of 57-63"C, FE(N03)2 was used for the iron, Ni(N03)2 and Co(NO3)2 for the cobalt. The various metal ion characteristics are metal conversion values for the preparations used.The treatment was followed by water washing and drying and, using a fluorescent X ray analyser (FXA) the treated plated surface AI-Ka was measured in the conditions tube: Cr, analysing crystal: EDDT, voltage: 50kV, current: 30mA, integration time: 20 seconds and Zn-Ka was measured in the conditions tube: W, analysing crystal: LiF, coltage: 20 kV, current: 2 mA, integration time: 10 seconds. The AllZn ratio was calculated from the Al count and the Zn count. Findings are shown in Table 1.

It is seen from Table 1 that the amount of Al at the surface of zinc-aluminium alloy plated stell plates is less in plates treated with an alkaline aqueous solution according to the invention (No.1, No.2, No.3) than in untreated material.

Next, an example of the invention will be described.

Example Steel plate plated with the zinc-aluminium alloy marketed under the trade name of "Super Zinc" was sprayed for 6 seconds with an alkaline aqueous solution which contained 0.17 g/l Fe(NO3)2 calculated in terms of Fe, 0.24 gtl Co(N03)2 calculated in terms of Co, 1.7 g/l hexahydroxy heptone soda and 22 g/l caustic soda, had a pH of 13.6 and was at a solution temperature of 57-63"C, water washed, sprayed for 2 seconds with a colloidal titanium aqueous solution (30"C, 5 gll Parcolene Z manufactured by Nihon Parkerizing KK), and then phosphated by spray process (Bonderite 3310 manufactured by Nihon Parkerizing KK), water washed, chrome sealed (Parcolene 62 manufactured by Nihon Parkerizing KK) by 2 second spraying at 47-53"C and dried.

By way of comparison, treatment was effected in the same way as in the Example but with omission of the alkaline aqueous solution treatment.

The above treatment was effected in continous processing at 75 m2 (plated steel plate)/l (phosphating bath) and the amount of Al eluted into the phosphating bath and the weight of the treated plate's phosphate film were measured. Findings of these measurements are shown in Figure 1 and Figure 2.

Next, epoxy undercoat paint was applied by bar coater to give a film thickness of 5 > on treated plate at the start of treatment and after 60 m2!1 treatment and was baked at a treated plate temperature of 1900C and then polyester top coat paint was applied by bar coaterto give a film thickness of 13 > and baked at atreated plate temperature of 200"C. The findings of coating adhesion and corrosion resistance tests conducted on these coated plates are shown in Table 2.

The tests were conducted as follows.

(1) Bend test, impact test In the bend test, a coated test plate is bent in two, two plate pieces with the same thickness as the test plate are inserted in the bend portion and clamped in a vice and Scotch tape is stuck on the bend surface and peeled off sharply. After this test, the bend surface is given an assessment score based on the peel-of area of the applied film.

The impact test is effected with a Dupont impact tester, with a 1/2" impact point and 1 kg load and at a height of 50 cm, and Scotch tape is stuck on the indented surface and peeled off sharply. After this test, the indented surface is given an assessment score based on the peel-off area of the applied film.

The classification of scores is as follows.

Bend Test and Impact test SCORE JUDGEMENT STANDARD (after tape peel-off) 5 No abnormality but cracks accompany peel-off.

4 Point peeling in several places; < 10% in terms of area 3 Peel-off surface area 11-25% 2 Peel-off surface area 26-50% 1 Peel-off surface area > 51% (2) Coin scratch test The coated surface is scratched with a 10 Yen coin.

After this test, an assessment score is assigned on the basis of the peel-off area of the coated film of the test surface. The classification of scores is as follows.

SCORE JUDGEMENTSTANDARD 5 No abnormality (state in which the primer is not visible 4 State in which the ground material is not visible 3 The ground material is slightly visible; < 25% in terms of area 2 The ground material is quite visible: 26-50% in terms of area 1 The ground material is completely visible; > 51% in terms of area (3) Corrosion resistance test A portion where the coated surface of a coated test plate has been cut through to the ground material with a knife, a cut-off portion and a bent portion are subjected to a 1,000 hour 5% salt water spray test in accordance with JlS-Z-2371 and an assessment is made in the basis of the following standards.

Salt Water Spray Test Cut portion and cut-off portion swelling SCORE JUDGEMENT STANDARD (maximum number of mm on one side 5 < Imm 4 2-4mm 3 5-7 mm 2 8-10mm 1 11 mum Salt Water Spray Test White rust area from bent portion SCORE JUDGEMENT STANDARD (white rust area) 5 No abnormality 4 White rust or swelling area within 10% 3 White rust or swelling area within 11-25% 2 White rust or swelling area within 26-50% 1 White rust or swelling area within > 51% As described above, practice of the present invention in phosphating of zinc-aluminium alloy coated material reduces the amount of Al eluted into the phosphating bath and makes it possible to improve the performance of the formed phosphate film.

Figure 1 and Figure 2 are graphs plotting the results of test on examples of the invention.

TABLE 1 Alkaline aqueous solution Plate surface AllZn composition g/l count values ratio determined by FXA Metal Complexing Alkali AIKa ZnKe ions agent count count Iron Hexahydroxy Caustic 5304 88368 0.060 :0.4 heptone soda soda :1,7 : :22 Nickel 0.24 4961 92014 0.054 Iron 0.17 Cobalt 0.24 3690 89128 0.041 Iron 0.17 - " " 6660 83260 0.080 - - " 9826 82899 0.120 Untreated 10646 78171 0.136 TABLE 2 Example Treated Film Applied film adhesion Corrosion surface weight tests resistance tests area slum2 bend impact coin cut cut-off bend scratch portion portion portion Example At start 1.0 5 5 4 5 4 2 of the invention 60 m2/l 1.0 5 5 4 5 4 2 Compar- At start 1.0 4 5 4 5 3 2 ison example 60m2/l 0.2 4 5 1 4 1 2

Claims (12)

1. Phosphating process wherein, in phosphating of zinc-aluminium alloy plated material, said plated material is treated beforehand with an alkaline aqueous solution with a pH of 11 or more which contains ions of one or more of the metals selected from the group consisting of nickel, iron, cobalt and manganese and a complexing agent to an amount sufficient to dissolve said metal ions.

2. Process for treating the surface of a zinc-aluminium alloy in which the surface is pretreated with an aqueous alkaline solution having a pH of 11 or more containing one or more metal ions selected from the group consisting of nickel, iron, cobalt and manganese and a complexing agent in an amount sufficient to dissolve the metal ions and the treated surface is then phosphated.

3. Process according to claim 1 or claim 2 in which the surface is treated by being sprayed with the aqueous alkaline solution.

4. Process according to any one of claims 1 to 3 in which the pretreatment is carried out at a temperature in the range 30 to 100"C, preferably 50 to 80"C, for a period of between 5 and 15 seconds.

5. Process according to any one of claims 1 to 4 in which the alkali is present in the solution in an amount between 1 and 60 grams per litre, preferably 10 to 40 grams per litre.

6. Process according to any one of claims 1 to 5 in which the alkali is sodium hydroxide.

7. Process according to any one of claims 1 to 6 in which the metal ions are each present in an amount in the range 0.01 to 10 grams per litre, preferably 0.05 to 5 grams per litre.

8. Process according to any one of claims 1 to 7 in which the total amount of metal ions in the aqueous solution is in the range 0.05 to 5 grams per litre.

9. Process according to any one of claims 1 to 8 in which the phosphating step is carried out in a solution containing 0.1 to 1% zinc and 0.6 to 4% phosphate ions.

10. Process according to claim 9 in which the phosphating solution contains nitrite ions, fluoride ions and/or nickel ions.

11. Process according to any one of claims 1 to 10 in which the phosphating step is carried out by spraying the surface.

12. Process according to any one of claims 1 to 11 in which the surface is treated after the treatment with an aqueous alkaline solution and before the phosphating treatment with an aqueous solution of titanium.