Microstructure and Properties of Cr-free Conversion coating on AZ91D Magnesium alloy
Date Issued
2011
Date
2011
Author(s)
Lee, Yueh-Lien
Abstract
In the first part of this paper, the effect of the different die-casting microstructures on the stannate conversion coating treatment has been investigated on AZ91D magnesium alloy, with emphasis on the die-chill skin and α/β (Mg17Al12) eutectic structure. Prior to the immersion in the stannate bath, AZ91D substrates were pickled in 11wt﹪HF solution at room temperature, respectively. After thorough cleaning in running water, the substrates were immersed in a stannate bath for 60s and 600s. SEM, TEM, EDS and XRD were employed to analyze the surface morphology and detailed microstructure of the coating on the AZ91D substrate. In addition, the electrochemical polarization measurement was performed to compare the corrosion resistance of the coating on the AZ91D with and without the die-chill skin. The results showed that the die-chill skin contain a lot of pores and those pores are covered with oxide. Besides, the stannate coatings on the substrate with die-chill skin after 10 minutes immersion show higher corrosion potential and smaller anodic current.
In the second part, the formation characteristics of conversion coating on the AZ91D magnesium alloy were discussed. Comparing to the α phase, the hydroxides films formed on the β phase at the beginning of conversion process provide insufficient protection. Since the galvanic effect between β phase and eutectic α + βcould occur most easily, the nucleation of stannate conversion coating depending on sufficient Mg2+ took place predominantly in the vicinity of β phase. In the contrast, formation of ceirum conversion coating on AZ91D was largely relied on the increase of the pH value. Since α phase is more active than β phase in the acid solution, the severe Mg2+ dissolution causes the production of H2 bubbles, leading the formation of blisters on the α phase at the beginning of cerium conversion process.
In the last part of this papar, a new approach for alternative of toxic hydrofluoric acid (HF) in the magnesium pretreatment process was investigated. The hydrochloric acid, hydrofluoric acid and vanadate solution were made on the AZ91D magnesium alloy before cerium/hydrogen peroxide conversion coating respectively. The microstructure of coatings was observed by transmission electron microscopy (TEM), and the electrochemical impedance spectroscopy (EIS)、Polarization curves and salt spray test (SST) were carried out to evaluate the corrosion resistance. The results show that, comparing with HF and HCl pretreated samples, the passive layer formed in vanadate-containing bath (pH=8) effectively restrained the reaction rate of Ce conversion coating and apparently reduced the number of blisters and partial peel-off of the coating. Both the EIS and SST results show that the better corrosion resistance coating can be obtained when the samples were pretreated in the vanadate-containing bath.
Subjects
AZ91magnesium alloy
die-chill skin
pretreatment
stannate conversion coating
cerium conversion coating
EIS
Type
thesis
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