A three layer corrosion protection system for Mg-9Al-Zn alloy offered by complex fluoride ions and precipitates-based electrolytic oxidation

Abstract

Fluoride species were historically used in conversion coatings to facilitate the initial chemical reactions on metal surface, however with excellent outcome only for complex fluoride species (Na2SiF6, K2ZrF6). In this study, Two-step electrolyte process was performed on AZ91 alloy in K2ZrF6 based alkaline solution and compared with samples prepared in graphite, borate and silicate based solutions. Coating formed in the colloidal K2ZrF6 based solution was found with pancakes of larger sizes, lower porosity and cracks on the surface. Micro-hardness of coatings in the respected colloidal solution was found similar to 1397.85 HV with a minima and maxima similar to 1023.5 HV and similar to 1588.4 HV respectively. Highest corrosion potential and lowest current densities similar to - 0.28 [V vs SCE] and 1.436 x 10(-7) A.cm(-2) were observed for the coatings formed in the K2ZrF6 based colloidal solution. A three level corrosion protection system including a) the top precipitates/amorphous layer that blocked the pores b) the dense PEO layers and c) the compact barrier layer formed because of complex fluoride ions surface activations offered by the K2ZrF6-based solution was suggested to be responsible for the high corrosion resistance of the respected coatings.