Effect of the pH Value of Phosphating Solution on the Formation and Corrosion Resistance of Phosphate Conversion Coating on the Surface of AZ91D Magnesium Alloy

AN Yanli, LIU Miaomiao, DU Huayun

doi:10.16577/j.issn.1001-1560.2022.0302

2022 , Vol. 55 >Issue 11: 14 - 21

DOI: https://doi.org/10.16577/j.issn.1001-1560.2022.0302

Effect of the pH Value of Phosphating Solution on the Formation and Corrosion Resistance of Phosphate Conversion Coating on the Surface of AZ91D Magnesium Alloy

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(1. College of Preclinical Medical, Shanxi Medical University, Taiyuan 030001, China;2. College of Materials Science and Engineering, Taiyuan University of Technology, Taiyuan 030024, China;3. Xingxian Economic Development Zone Aluminum Magnesium New Material R & D Co., Ltd., Lvliang 035300, China)

Received date: 2022-05-17

Revised date: 2022-06-22

Accepted date: 2022-07-19

Online published: 2023-12-25

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Abstract

For further revealing the effect of pH value on the formation of conversion coating on magnesium alloy surface, the formation mechanism of phosphate chemical conversion coating on AZ91D magnesium alloy surface was studied, and the influence of the pH value of phosphating solution on the morphology and corrosion resistance of the conversion coating was mainly discussed. The potentiodynamic polarization test data and electrochemical impedance spectroscopy of the conversion coatings were compared, and the corrosion resistance of phosphate conversion coatings prepared in 3.5% NaCl solution under different pH conditions was analyzed by soaking test. Results showed that the key to the film formation of phosphate was the combined action of the hydrolysis equilibrium process of manganous dihydrogen phosphate [Mn(H2PO4)2] and the hydrolysis condensation process of ammonium metavanadate (NH4VO3). The conversion coating was greatly affected by the pH value of the phosphating solution. With the increase of pH value, the corrosion resistance of the conversion coating was gradually improved, but the thickness showed a trend of increasing first and then decreasing after reaching the extreme value. When the pH value of phosphating solution was 4.0, the coating had the best quality and the optimal corrosion resistance.

Key words： AZ91D; phosphate conversion coating; pH value; thickness of coating; corrosion resistance

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AN Yanli, LIU Miaomiao, DU Huayun . Effect of the pH Value of Phosphating Solution on the Formation and Corrosion Resistance of Phosphate Conversion Coating on the Surface of AZ91D Magnesium Alloy[J]. Materials Protection, 2022 , 55(11) : 14 -21 . DOI: 10.16577/j.issn.1001-1560.2022.0302

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