Surface treated aluminium@ alloy sheet for motor car construction - used in making decorative coloured items, zinc phosphate layer having fine grained structure of uniform thickness

摘要

Two stage process is used for surface treatment of an Al/Mg alloy sheet (3.5-5% by wt. Mg) to improve properties on phosphating, and also ways of using this sheet in making motor car components. In the first stage the sheet is surface cleaned by washing in a weak alkali (e.g. Na phosphate) soln. so that the oxide layer is removed and the surface Mg content falls in the range 0.035-0.5%, then in the phosphating soln. the plate is subjected to the effects of a soln. which has a free fluoride content of 75-800 ppm. USE/ADVANTAGE - Gives fine crystalline Zn phosphate layer of uniform thickness and high corrosion resistance. The coated plate can take colouring and be made into decorative car parts, e.g. mud guards. In an example, an alloy sheet was dipped (for 2 mins.) in a weak alkali (pH 10.5) at 40 deg.C; then 2 mins. in a Ti colloid soln. (1.5 g/l) at room temp.; and finally phosphate dipped (2 mins.) in a Zn phosphate soln. at 42 deg.C. This gave a fine grained Zn phosphate plating on the alloy sheet. Corrosion testing over a long period in salt water spray and other tests in different environmental humidities and temps. in competition with commercially available alternatives showed expected improvement.