Effect of Mg17Al12 in Magnesium-Aluminum Alloy on the Microstructure and Properties of Phosphate / Permanganate Conversion Coating
Date Issued
2007
Date
2007
Author(s)
Tsai, Meng-Hsi
DOI
zh-TW
Abstract
The standard reduction potential of magnesium and its alloys are lower than the other structural materials. Therefore, magnesium alloys are highly susceptible to corrosion, giving rise to the limitation on their practical application. Many surface modification methods have, thus, been developed to protect magnesium alloys from corrosion. Among the conversion coating methods, chromate conversion caoting exhibits the best corrosion protective properties. Howerver, Cr(VI) is toxic to human body. Phosphate-permanganate conversion treatment is one of the methods that are potentially alternative to the chromate treatments. The microstructures and the properties of magnesium alloy can be controlled by its aluminium contents and the corresponding fabrication processes.
This research focuses on how the Mg17Al12 (β) phase in AZ31,AZ80 and AZ91 magnesium- aluminum alloys influences the surface morphology, microstructures and properties of the phosphate-permanganate coating formed in a short time (30, 60 and 120sec) of immersion. This research utilizes optical microscopy, SEM, TEM and low glancing-angle XRD to analyze the surface morphology and crystal structure of the coating, measures the electrochemical polarization curve and electrochemical impendence spectroscopy to analyze the corrosion properties of the coating, and uses ASTM D3359-97 method to study the adhesion of conversion coating. The results shows that the fabrication process and composition of the alloys markedly influence the shape, volume fraction and distribution of Mg17Al12 (β) phase which in turn, affects the surface morphology, microstructures, adhesion properties and corrosion properties of the coating.
Subjects
Mg17Al12
鎂鋁合金
微結構
磷酸鹽/錳酸鹽化成處理
無鉻化成處理
Magnesium-Aluminum alloy
Microstructure Phosphate/Permanganate Conversion Coating
Chromium-free Conversion Coating
Type
thesis
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