GB2178065A - Process for treatment of zinc-aluminium alloy coatings - Google Patents

Description

SPECIFICATION Process for treatment of zinc-aluminium alloy coatings It is known from GB 1,042,108 to treat metal surfaces with an aqueous alkaline solution having pH above 11 and containing polyvalent metal ions including ions selected from nickel, iron, cobalt and manganese and containing a sequestering agent in an amount sufficient to keep the metal ions in solution. It is described in that that the treated surface can be subsequently treated with an aqueous chromate solution.

A conventional treatment for zinc coated steel sheet involves treating the sheet with a chromate solution containing hexavalent chromium as its main component. This chromate treatment is intended to improve paint film performance. Unfortunately it involves the discharge of large amounts of aqueous chromate and this is environmentally undesirable. This process therefore is tending to be replaced by one in which the aqueous chromatesolution contains silica and the applied solution is dried on to the zincsurface.

For many purposes it is accepted that it is preferred for the steel to be coated with a zinc aluminium alloy instead of zinc. Such a coated sheet has better corrosion resistance than that of a conventional hot dip galvanised steel sheet. Howeverthere tends to be little advantage on paint film performance between galvanised steel and zinc-aluminium alloy coated steel when the surface has a dried-on chromatesolution applied to it Also, a particular problem with steel sheets having a zinc aluminium alloy coating is atendency forfailure ofthe alloy coating, orof paint coating over it, from the edges ofthe sheet, andthisapplies irrespective of the type of chromate coating that is applied.

It would be desirable to devise a surface treatmentfor steel sheets having a zinc-aluminium alloy coating and which permitted satisfactory application of a paint coating and, especially, that minimised edgefailure.

A process according to the invention forthe surfacetreatment of a steel sheetcarrying a coating of an alloy of zinc with 1 to 70% aluminium comprises treating the alloy coating with an aqueous alkaline solution having pH above 11 and containing polyvalent metal ions including ions selected from nickel, iron, cobalt and manganese and containing sequestering agent in an amount sufficientto keep the metal ions in solution, applying an aqueous chromate solution containing silica, and drying the applied solution on to the surface. By this process it is possibleto provide a good basefora subsequently applied paintfilm and, especially, to minimise failure from edges ofthesheet.

The pretreatment solution may be a solution as described in GB 1,042,108. It will contain metal ions selected from nickel, iron, cobalt and manganese but may also contain any ofthe other polyvalent metal ions proposed for incorporation in the alkaline solution described in GB 1,042,108.

The metal ions are generally introduced as inorganic salts such as nitrates, sulphates, carbonates or chlorides or any other compound that is soluble in the aqueous alkali. The concentration of the metal ions b generally in the range 0.01 to 10, preferably 0.05 to 5, g/I. The nickel, iron, cobalt and manganese are generally present in these amounts.

It is necessary to include a sequestering agent to keepthe metal ions in the dissolved state. The amount is generally 0.1 to 20 gil.

Suitable sequestering agents include condensed phosphates, ammonia orthe like, and organic sequestering agents such as dicarboxylic acid,for example, malonic acid, fumaric acid and the like, amino acids, for example, glycine orthe like, hydroxycarboxylic acid, for example, citric acid, gluconic acid and lactic acid, polyhydroxyaliphatic compounds, for example, sorbitol and 1, 2-ethanediol, phenolic carboxylic acids, for example, salicylic acid and phthalic acid, amine carboxylic acids, for example ethylenediaminetetraacetic acid, polyamino acids, for example, diethanolaminomethane phosphonate, lignosulfonate and the like.

Particularly preferred agents are hexahydroxyheptonic acid, sodium gluconate, sodium ethylendiaminetetraacetate.

The alkaline solution can be applied to the alloy surface under conditions generally as described in GB 1,042,108. Thus is may be applied at temperatures between room temperature and boiling by spray or immersion methods. Often the treatment temperature is 50 to 80"C and the duration oftreatment may be 4to 10 seconds on a continuous strip line, but can be shorter if the concentration ortemperature ofthe solution is increased.

It appearsthatthe pretreatment selectively removesaluminiumfrom the alloy coating and so the duration and temperature must be sufficiently greatto permitthe desired degree of aluminium removal. It seemsthat the removal of aluminium occurs preferentially from the surface, for instance when it is segregated abovethe alloy coating.

The surface is generally rinsed with water after the pretreatment.

The surface is then treated with the chromate solution containing silica. Generallythe hexavalentchromium ion is derived from chromic acid anhydride and the silica from colloidal silica. The aqueous chromate solution preferably contains a chromium (III) ion and/or phosphate ion. The chromium (III) ion may be formed by adding a reducing agent such as starch, oxalic acid, tannin, alcohols orthe liketo an aqueouschromate solution containing hexavalentchromium. The weight ratio of the silica/chromium (VI) ion in the aqueous chromatesolution is preerably in the range of0.6-1 5. Theweightratio ofthe chromium (Ill) ion/chromium (VI) ion in the aqueous chromate solution is preferably in the range of 0.2-1.The weight ratio of the phosphate chromium (VI) ion in the aqueous chromate solution is preferably 0.6-3.

The composition and coating conditions are preferably such that the resultant dried chromate-silica coating has a dry weight of 10 to 400 mg/m2. If the coating weight is too low then it does not exhibitthe benefits ofthe invention but if it is too high the paint adhesion is reduced.

Application can be at ambienttemperature by any convenient application technique such as dipping or, preferably, roll coating, and the surface is then dried.

Suitable compositions and their application are described in GB 1,234,181.

The alloy on the steel sheet generally contains at least3% aluminium, typically 3to 20% aluminium, with the balance being zinc.

The following are some examples. Example 1 demonstrates the effect on the AI/Zn ratio of the pretreatment and the remaining examples show the effect of combined pretreatment and chromate coatings.

Example 1 Acommercially vailable Zn-Al alloy coated steel sheet (Zn =90-96.5%, AI=3.5-10%; trade name "SUPERZlNC") was sprayed at 57-630C for 5 seconds with various alkaline solutions having a pH value of 13.5-13.7 and containing 22 g/l sodium hydroxide and sodium hexahydroxyheptone as sequestering agent.

The solutions also contained various metal ions. Iron was used in theform of Fe (NO3)3, nickel in the form of Ni(NO3)2 and cobalt in the form ofCo(NO3)2, respectively.

The Al-Ka count value of the treated sheet surface was measured with a fluorescence X-ray analyser (FXA) using a Cr-tube and spectroscopiccrystal =EDDT underthe condition of voltage: 50 KV, electric current: 30 mA, integration time: 20 seconds and the Zn-K(x count value was measured with above analyser. The condition for measurement is as follows: tube=W, spectroscopic crystal LiF, voltage=20 KV, electric current=2 mA, integration time=10 seconds. The ratio of Al/Zn was calculated from the countings of Al and Zn.

The amounts of metal ion and sequestering agent (in grammes per litre) andtheAl and Zn countingsare shown in Table 1.

TABLE 1 No. Metal iron Sequestering Al-Ko Zn-Ko Ratio Agent counting counting AllZn 1 0.4 Fe 1.7 5304 88368 0.060 2 0.24 Ni 1.7 4961 92014 0.054 0.17 Fe 3 0.24 Co 1.7 3690 89128 0.041 0.17 Fe 4 0 1.7 6660 83260 0.080 5 0 0 9826 82899 0.120 6 Untreated 10646 78171 0.136 This table clearly demonstrates thatthe pretreatment solutions used in the invention (solutions 1,2 and 3) give lowerAI/Zn ratiosthantheothertreatments.

Example 2 Acommercially available zinc-aluminium alloy coated steel sheet (trade name: "SUPERZINC") was sprayed at 57-63 C for 10 seconds with an aqueous alkaline solution containing 0.17 g/l Fe added as Fe(NO3)3, 0.24g/l Ni ion added as Ni(NO3)2, 1.7 g/l sodium hexahydroxyheptone and 22 g/l sodium hydroxide and having a pH value of 13.6. The sheet was rinsed with water and coated with an aqueous chromium solution containing 17.7 g/l chromium (VI), 9.9 g/l chromium (III) and 15.8 g/l silica and having a pH value of 3 using a roll coater and dried.

The coating film weightwas 190 mg/m2and a chromium contentof53 mg/m2.

Example 2a The process of example 2 was repeated except that the alkaline pretreatmentwas omitted. The chromium contentofthefinal coating was 50 mg/m2.

Example 2b The coated steel sheet was left untreated.

Example 3 The same zinc-aluminium alloy coated steel sheet (SUPERZINC) as in example 2 was sprayed at 55-60"C for6 seconds with an aqueous alkaline solution containing 0.17 g/l of Fe ion added as Fe(NO3)3, 0.24 g/l Co ion added as Co(NO3)2, 1.7 gil sodium hexahydroxyheptone and 22 g/l sodium hydroxide and having a pH value of 13.6, rinsed with water and coated with an aqueous chromium solution containing 16.5 gil of chromium (VI), 9.9 g/l of chromium (III), 54.2 g/l of silica and 25.5 g/l ofPO4and having a pH value of 1.5 using a roll coater, andwas dried. The coating film weightwas 280 mg/m2 and the chromium content ofthe film was 41 mg/m2.

Example 3a The process of example 3 was repeated except that the alkali pretreatment was omitted. The chromium content of the coating was 39 mg/m2.

Each of the steel sheets was then subjected to one of two types of paint coating. One type was 2 coat system in which it was coated with a preferred primer, namely an epoxy resin primer coat, using a barcoaterso asto attain a thickness of 5Fm and baked at 1 900C and then a polyester top coat was coated using a bar coater so as to attain thickness of 1 3 > m and baked at 200"C. The othertype was a one coat system, namely, the chromated sheets were coated with a polyester top coat paint using a bar coater so as to attain a thickness of 1 4'im and baked at 21 0 C. Paint adhesion test and corrosion resistance test were carried out with reference to these painted sheets. The results are shown in Table 2.

The painted panels were then subjected to bending, impact and coin scratching paint adhesion tests and to corrosion resistance tests at 5% salt spray and 3 year outdoor exposure from scratch surfaces, edges and bent surfaces, all in accordance with standard procedures. The properties of the panels of example 2b were greatly inferior on most tests. The panels of example 2 and 3 gave better resistance to corrosion resistance from the edges of the sheets than, respectively, the panels of examples 2a and 3a (which had not been subjected to the specified alkaline pretreatment).

Claims (8)

1. A process forthe surface treatment of a steel sheet carrying a coating of an alloy of zine with 1 to 70% aluminium,the process comprising treating the alloy coating with an aqueous alkaline solution having pH above 11 and containing polyvalent metal ions including ions selected from nickel, iron, cobalt and manganese and containing sequestering agent in an amount sufficient to keep the metal ions in solution, applying an aqueous chromate solution containing silica and drying the applied solution on to the surface.

2. A process according to claim 1 in which the dried coating comprising chromate and silica weighs 10to 400 mg/m2.

3. A process according to claim 1 orclaim 2 in which the aqueouschromate solution containstrivalent chromium and/or phosphate.

4. A process according to claim 3 in which the aqueous chromate solution contains trivalent chromium and the ratio Si:Cr6 is 0.6 to 15 and the ratio trivalent: hexavalent chromium is 0.2to 1.

5. A process according to claim 3 in which the aqueous solution contains phosphate and the ratio P04: Cr6 is 0.6 to 3.

6. A process according to any preceding claim in which the coated surface is subsequently painted.

7. A process according to any preceding claim in which the paint comprises an epoxy resin primer.

8. A process according to any preceding claim in which the alloy contains at least 3% aluminium.