Effect of Alkali Metal Hydroxides on the Dissolution Behavior of a Zinc Phosphate Conversion Coating on Steel and Pertinence to Cathodic Delamination

Authors:
  • ASA. J. Sommer
    A. J. Sommer
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Abstract
Raman microprobe spectroscopy, scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and wet chemical methods of analysis were successfully used to determine the effects of high-pH environments on zinc phosphate conversion coatings. The more rapid dissolution of phosphate ions from the phosphate coating was pH dependent. At a pH of 12.5, phosphate ions were more rapidly dissolved from the phosphate coating while at 11.5 and 13.5 pH, zinc ions were more rapidly dissolved from the coating. Raman spectra confirmed the more rapid dissolution of the phosphate ions was a result of zinc oxide formation on the surface of the original phosphate conversion coating. The cation of the alkaline environment also proved to be an important factor in the dissolution of the phosphate conversion coating. Sodium hydroxide dissolved the phosphate coating at the greatest rate followed by hydroxides of potassium, cesium, and lithium. This cation dependence was related to the number of precipitation products formed in the different alkaline solutions.

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