Anodic Polarization Characteristics of Phosphated Steels

Abstract

The problem of providing a meaningful evaluation of the corrosion resistance of phosphated steel for the dual purposes of quality control and of improving the phosphate coating has a long and arduous history. The electrochemical characteristics of zinc-phosphated mild steels have been investigated to determine the repeatability, significance, and possible applications of such data. This investigation resulted in a precise electrochemical procedure that can distinguish between steels with zinc phosphate coatings of different quality and crystal structure. When zinc-phosphated mild steels are polarized anodically in 0.6M ammonium nitrate solution, different steels with the same nominal composition and phosphate treatment exhibit active-passive corrosion behavior with current density values that vary considerably over a narrow potential range. These critical current densities, with certain exceptions, vary in relation to the rust creepage corrosion evaluations of coated specimens in salt spray and to phosphate crystal structure. The effects of phosphate ions and of pH changes on the unique anodic polarization characteristics of phosphated steels were studied by simulating conditions that exist at the steel-phosphate-solution interfaces. Anion adsorption is discussed as a basis for the complicated behavior reflected in the anodic polarization data.