Microstructure and corrosion resistance of permanganate conversion coating on LZ91 magnesium alloy

• Main Authors: Sung-Mao Hung, 洪崧貿
• Other Authors: 林招松
• Format: Others
• Language: zh-TW
• Published: 2015
• Online Access: http://ndltd.ncl.edu.tw/handle/73958045143051425057

碩士 === 國立臺灣大學 === 材料科學與工程學研究所 === 103 === LZ series magnesium alloys have lower density and better formability than AZ magnesium alloys that have found many applications. The LZ magnesium alloys are thus potential for structural applications. However, the LZ alloy has poor corrosion resistance resulting from galvanic corrosion prevailing in the  and  phase. This study, therefore, aimed at developing a permanganate conversion solution for LZ91 alloys. Moreover, the effect of conversion bath temperature was detailed so as to prepare the permanganate conversion coating effectively enhancing the corrosion resistance of LZ91 alloys. Permanganate conversion coatings were formed on LZ91 alloys in the permanganate solution with pH value of 1.5 at different temperatures, 15℃、25℃、40℃、60℃, respectively. The surface and cross-section morphology of the permanganate coating were investigated by using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). XPS and Auger spectroscopy were employed to measure the composition of the permanganate coating. The corrosion resistance of the permanganate-coated LZ91 alloys was measured by potentiodynamic polarization, EIS, and salt spray tests. Results show that the permanganate coating was composed of MgO, MnO2, and Mg(OH)2 , indicating that permanganate ions was reduced to manganese oxides when magnesium and lithium were oxidized and dissolved. The thickness of the permanganate coating decreased with increasing bath temperature. the permanganate coating formed at 15℃ was relatively thick and suffered severe cracking after dehydration. Increasing permanganate bath temperature retarded the growth of the permanganate coating and coating thus formed had better corrosion resistance. Although the permanganate-coated LZ91 sustained after 12h of the salt spray test regardless of the permanganate bath temperature, the EIS and polarization measurements showed that the coatings formed at 40℃ afforded better protection over the LZ91 alloy. Finally, hydrogen evolution experiment showed that elevating permanganate bath temperature promoted the reaction and optimized the passivation of the coating. Nevertheless, the effect of dual phase induced galvanic corrosion on the conversion mechanism was not notable in such a strongly acidic solution. A full coverage of permanganate coating on the LZ91 alloy can be obtained in a short term of conversion treatment.