Electrochemical and Chemical Dissolution Behavior of Zn-Fe Alloy Coatings and Its Application to the Analysis of Zn-Fe Alloy Layer

Abstract

Electrochemical and chemical dissolution behavior of Zn-Fe alloy coatings and steel sheets was investigated, and following two methods for selective dissolution of Zn-Fe alloy coatings from steel sheets were developed.
(1) Alkali-dissolution method : In a strong alkaline solution (NaOH-triethanolamine (TEA)-H₂O), Zn-Fe coatings are dissolved taking the form of ZnO₂^2- and [Fe(OH)₄·TEA]^-, while steel sheets and Fe-Zn (Fe-P) coatings are insolble because passive films are formed on the surface.
(2) Cathodic polarization electrolysis method : Under cathodic polarization (-1.2 Vvs. SCE) in a NH₄NO₃ electrolyte (NH₄NO₃-1, 10-phenanthroline (ο-phen)-MeOH-H₂O (pH=10 adjusted by NH₄OH)), Zn-Fe and Fe-Zn (Fe-P) coatings are dissolved taking the form of [Zn(NH₃)₄]²⁺ and [Fe-(ο-phen)₃]²⁺. At this potential, dissolution of steel sheet is prevented by the ο-phen films adsorbed on the steel surface.
Alkali-dissolution method is applicable to quantitative analysis of Zn-rich Zn-Fe layer (Fe<40%) coatings and upper/down layer of Fe-Zn (Fe-P)/Zn-Fe double layer coatings.
Cathodic polarization electrolysis method is applicable to quantitative analysis of Fe-rich Zn-Fe layer (Fe<70%) coatings.