Influence of surface pretreatment on phosphate conversion coating on AZ91 Mg alloy

Abstract

Surface pretreatment is generally applied before application of protective coatings on Mg alloys, which influences surface microstructure and electrochemical activity of the substrate and has an effect on the coating properties. The effect of various pretreatment processes (sand-blasting, grinding and polishing) on the microstructure and corrosion protection performance of phosphate conversion coating (PCC) on AZ91D Mg alloy was investigated in the present study. Sand-blasting cleaning significantly increases the surface roughness and electrochemical activity of the substrate, leading to formation of a porous PCC with inferior corrosion protection performance. In the case of ground/polished Mg alloy, the uniformity and corrosion resistance of the resultant conversion coating are mainly related to the surface roughness. Relatively low surface roughness of the substrate facilitates formation of a corrosion protective PCC.

Chromate-Free Corrosion Protection Strategies for Magnesium Alloys—A Review: PART I—Pre-Treatment and Conversion Coating

“…It is a popular method in the industry due to its fast and convenient process and universal applicability to various metallic substrates. However, adverse effects of sandblasting on corrosion resistance have frequently been reported for Mg and its alloys [ 13 , 14 , 15 , 16 ]. The reason for these adverse effects is mainly attributed to the contamination introduced to the magnesium surface during sandblasting.…”

Corrosion resistance of PEO and primer coatings on magnesium alloy

“…Another method consists of conversion coating treatment with a permanganate phosphate solution, anodic oxidation or galvanic deposit coatings [4,5]. The most important of them utilize surface modification techniques such as anodizing electroless plating solutions containing hexavalent chromium compounds, ion implantation and organic coatings [6][7][8][9][10][11].…”

Progress in bioactive surface coatings on biodegradable Mg alloys: A critical review towards clinical translation

“…Therefore, these coatings are not preferable as a top layer. However, as a primer layer, they are well established for better corrosion resistance and the middle layer for strengthening the adhesion between the substrate and the outer layer [ 200 ]. In contrast, some studies utilized it as a post-treatment technique to convert amorphous dicalcium phosphate dehydrate ( DCPD) coatings to highly crystalline HA coatings, which efficiently shielded the Mg substrate in physiological media [ 3 , 141 , 201 , 202 ].…”