SCANNING AUGER MICROPROBE STUDY OF HOT-DIPPED REGULAR-SPANGLE GALVANIZED STEEL: PART II. SURFACE COMPOSITION OF CHROMATED SHEET.

Zinc coastings produced on a hot-dipped-coating pilot line from a zinc bath containing small amounts of aluminum, antimony, and lead were treated with a commercial chromating solution and then examined with a scanning Auger microprobe. The results show that the chromating solution attacked the thin aluminum oxide precipitates in the zinc surface. The invisible conversion-coating film that was formed on the zinc matrix consisted mainly of oxygen, zinc, and chromium in order of decreasing atomic concentration. Surprisingly, the thickness and composition of the film was the same for treatment in 8 and 15 grams-per-liter chromate solutions. The film that was formed on precipitates of lead in the surface of the zinc coating was much thicker and richer in chromium than the film formed on the surrounding zinc. Examination of chromated surfaces exposed to water-saturated air at 100 degree F and to normal atmospheres showed that the surfaces corroded rapidly on a microscale.