Activation and inhibition of Zn–Al and Zn–Al–Mg coatings on steel by nitrate in phosphoric acid solution

The effect of nitrate on the reactivity of commercial Al and Mg containing Zn based coatings on galvanized steel in phosphoric acid has been investigated. For alloy coatings, Zn and Mg dissolution rates increase dramatically when nitrate is present while Al dissolution is strongly inhibited. Pure Zn and Mg reveal a similar accelerating effect but no inhibitive effect is observed on pure Al. Titrations are used to clarify the precipitation chemistry of the different species involved. The results are interpreted in terms of the increased interfacial pH and the relative solubilities of the component cations.

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► Effect of nitrate in H3PO4 on Zn, Al, and Mg dissolution of galvanized steel alloys.
► Alloys: Nitrate accelerates Zn and Mg dissolution but inhibits Al dissolution.
► Pure Metals: nitrate accelerates Zn and Mg dissolution but has no effect on Al.
► Results interpreted in terms of interfacial pH changes induced by nitrate.