Effects of Solution pH on the Growth Characteristics of Phosphate/Permanganate Conversion Coatings on AZ31 Magnesium Alloys
Date Issued
2008
Date
2008
Author(s)
Chu, Yu-Ren
Abstract
Magnesium alloys have become a group of the most widely used materials for their outstanding mechanical properties. However, the poor corrosion resistance is a critical weakness of most magnesium alloys. Among the various surface modification treatments, chromate conversion coating has received most attention in the past decades; however, the use of hexavalent chromate is prohibited due to environmental concerns. The phosphate/permanganate system, which is more environmental friendly and has been shown to have comparable corrosion resistance as compared to the chromate counterpart, is now the developing system with most expectation. In former researches, the pH value has been mentioned to be the key factor of the phosphate/permanganate conversion coating. Nonetheless, the pH effects on both growth behavior and coating properties are less well studied. Therefore, this study, with the aids of microstructural analyses and electrochemical measurements, aims at detailing the pH effects on the phosphate/permanganate conversion coating on AZ31 magnesium alloys. The results show that the coating formed under pH 4.0 is composed of two layers: the inner porous layer contacting the substrate, and the outer cellular layer, which shows litter protection during the conversion coating treatment. On the contrary, the coating formed under pH 5.7 is covered by crystalline struvite (MgNH4PO4‧6H2O) precipitates, which seems to inhibit the formation of the cellular layer. Compared to the coating formed under pH 4.0, the thinner coating with lighter color formed under pH 5.7 shows enhanced adhesion property and better performance in corrosion test.
Subjects
magnesium alloys
phosphate/permanganate conversion coating
pH value
crystalline precipitates
non-chromate conversion coating
SDGs
Type
thesis
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