Corrosion of magnesium alloys

Abstract

Magnesium alloys have very attractive mechanical properties, but their corrosion is a factor limiting their application. So understanding their corrosion behaviour is an essential step to improve their corrosion performance. In this doctoral dissertation, an effort was made on the understanding of the corrosion mechanisms and behaviour of pure magnesium, α and β phases, and commercial die cast AZ91D.In order to obtain sufficient information about the corrosion of magnesium and AZ alloys, the following examination techniques were employed: (1) Electrochemical polarization curve; (2) Electrochemical impedance spectroscopy (EIS); (3) Gas collection of hydrogen evolution from corroding samples; (4) Inductively coupled plasma atomic emission spectrophotometry (ICPAES); (5) Scanning tunnel microscopy (STM); (6) Optical microscopy; (7) Scanning electronic microscopy (SEM); (8) In-situ long focus optical microscopy.For pure magnesium, it was found that a partially protective surface film was a principal factor controlling corrosion. Film coverage decreased with increasing applied electrode potential. Application of a suitable external cathodic current density was shown to inhibit magnesium dissolution whilst at the same time the hydrogen evolution rate was relatively small. This showed that cathodic protection could be used to significantly reduce magnesium corrosion. A new definition was proposed for the negative difference effect (NDE).The experimental data of magnesium were also consistent with the involvement of the intermediate species Mg+ in magnesium dissolution at film imperfections or on a film-free surface. At such sites, magnesium first oxidised electrochemically to the intermediate species Mg+, and then the intermediate species chemically reacted with water to produce hydrogen and Mg2+ The presence of CI' ions increased the film free area, and accelerated the electrochemical reaction rate from magnesium metal to MgFor the alloy AZ501, AZ21 and AZ91, the corrosion rates increased in the following order: AZ501 < AZ21 < AZ91. The β phase, represented by AZ501, was very stable in the test solution. It served two roles, as a barrier and as a galvanic cathode. If the β phase is present in the a matrix as a second phase with a small volume fraction, the β phase mainly serves as a galvanic cathode, and accelerates the corrosion of the α matrix. If the fraction is high, then the β phase may mainly act as an anodic barrier to inhibit the overall corrosion of the alloy. The corrosion performance of α phase determines the corrosion behaviour of an alloy. The composition and compositional distribution in the α phase is crucial to the overall corrosion performance of dual phase alloys. Increasing the aluminum concentration in the α phase increases the anodic dissolution rate and also increases cathodic hydrogen evolution rate. Increasing the zinc concentration in the α phase might have the reverse effect.The skin of AZ91D die casting showed better corrosion resistance than its interior bulk. This might be due to relative higher ratio and more continuous β precipitate networks around finer a grains and lower porosity density in the skin layer than in interior of the material. It seemed that the casting method could affect the corrosion performance through changing the material microstructure.Based on the above results, some suggestions for the future research are also made.