Journal of the Korean institute of surface engineering (한국표면공학회지)

The Korean Institute of Surface Engineering (한국표면공학회)
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Characteristics of Environmentally-Friendly Conversion Coating of AZ31 Magnesium Alloy by a Alkaline Phosphate-Permanganate Solution

알카리성 인산-과망간산 용액을 이용한 AZ31 마그네슘 합금의 친환경 화성 처리 및 화성 피막의 특성 평가

  • Kim, Myung-Hwan (Department of Chemical Engineering, Pukyong National University) ;
  • Lee, Man-Sig (Korea Institute of Industrial Technology [KITECH], Green Technology Center) ;
  • Kwag, Sam-Tag (Department of Industrial Chemistry, Pukyong National University) ;
  • Moon, Myung-Jun (Department of Industrial Chemistry, Pukyong National University)
  • Received : 2011.03.12
  • Accepted : 2011.06.29
  • Published : 2011.06.30
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Abstract

A uniform chromium-free conversion coating treated with an alkaline phosphate- permanganate solution was formed on the AZ 31 magnesium alloy. The effect of acid pickling on the morphology and on the corrosion resistance of the alkaline phosphate-permanganate conversion coating was investigated. The chemical composition and phase structure of conversion coating layer were determined via optical microscopy, SEM, EDS, XPS and XRD. Results show that the conversion coatings are relatively uniform and continuous, with thickness 1.8 to $2.4\;{\mu}m$. The alkaline phosphate-permanganate conversion coating was mainly composed of elements Mg, O, P, Al and Mn. The conversion-coated layers were stable compounds of magnesium oxide and spinel ($MgAl_2O_4$). These compounds were excellent inhibitors to corrosion. The electrochemical corrosion behaviors of coatings in 3.5 wt.% NaCl solutions were evaluated by electrochemical impedance spectroscopy, potentiodynamic polarization technique. EIS results showed a polarization resistance of $0.1\;k{\Omega}$ for the untreated Mg and $16\;k{\Omega}$ for the alkaline phosphate-permanganate conversion treatment sample, giving an improvement of about 160 times. The results of the electrochemical measurements demonstrated that the corrosion resistance of the AZ 31 magnesium alloy was improved by the alkaline phosphate-permanganate conversion treatment.

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