Influence of pH value on chromate-free conversion coating for magnesium alloys

• Published in: Transactions of Nonferrous Metals Society of China Vol. 16; no. A03; pp. 1819 - 1822
• Main Author: 翁文凭 陈琦 张慧 钟皓 闫蕴琪
• Format: Journal Article
• Language: English
• Published: Suzhou Institute for Nonferrous Metals Processing Technology, Suzhou 215026, China 01.12.2006
• Subjects: 微观结构; 铬酸盐; 镁合金; magnesium alloys; conversion coating; pH value; phosphate-permanganate; AZ31 alloy
• ISSN: 1003-6326

Many factors were found to have effects on the conversion coatings for AZ31 alloy, alloy the most important one in producing high quality conversion coatings is found to be the control of the pH value. The influence of pH value on the conversion coating including color, thickness, adhesion and surface morphology was studied. The performance of conversion coating was examined by cross cut test, SEM method and salt immersion. The results show that the variation ofpH value causes surface treatment process unstably. The conversion coating can obtain as pH value ranging from 3.0 to 5.0, while it presents dark, thick and bad adhesion under lower pH value. The conversion coatings have good combination of thickness and adhesion when pH value ranging from 4.0 to 4.5, and it exhibits a good corrosion resistance.