The degradation of phosphate conversion coatings by electrochemically generated hydroxide

The degradation of phosphate coatings by electrochemically generated hydroxide has been investigated for two synthetic conversion coatings on electrogalvanized steel simulating the pH increase occurring during corrosion or cataphoretic painting. The reactivity of the monocation and trication phosphate coating are presented and discussed. By comparing the elemental dissolution rates with the rate of cathodic hydroxide production it is possible to measure the stoichiometry of the degradation reaction: 15 hydroxides per phosphate for monocation and 95 hydroxides per phosphate for trication coatings. These experiments were compared with measurements involving direct exposure to 0.1 M NaOH solution and to multiscan cyclic voltammetry.

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► AESEC measurement of conversion coating stability to electrochemically generated hydroxide.
► Cathodic degradation of phosphate conversion coating.
► Higher stability of trication compared to monocation phosphate conversion coating.
► Quantitation of the hydroxide/phosphate ratio (15 for monocation, 95 for trication layer).
► Insight into degradation and passivation mechanisms through multiscan cyclic voltammetry.