Abstract
The structure of phosphate crystals that exerts a favorable effect on the scab corrosion resistance on the painted steel surface was investigated and characterized by 1) a low Zn/P intensity ratio measured by glow discharge spectrometry, 2) a low preferred orientation of the (100) plane in phosphophyllite, and 3) angular and granular phosphate crystal morphology.
The aforementioned structure of phosphate crystals depends on the crystal growth process. When the dissolution of iron from the steel sheet during the phosphating treatment is insufficient, the hopeite epitaxial growth plane (020) conforms to the phosphophyllite epitaxial growth plane (100), and the mixed crystals with a high Zn/P ratio grow along the steel surface. When the dissolution of iron from the steel sheet is sufficient, the iron ion concentration in the solution increases, the intrinsically free precipitation of phosphophyllite without the restraint of epitaxy becomes predominant, and phosphate crystals with a lower Zn/P ratio grow.
The dissolution of iron from the steel sheet is accelerated by the oxides of manganese and silicon in a surface layer of up to 0.1 μm deep that is dissolved during the phosphating treatment.
