Magnesium alloys have high strength/weight ratios, but poor corrosion resistance affects the viability of increased magnesium usage in aerospace and other applications. This review approaches the p...

Corrosion of magnesium

This review aims to provide a foundation for the safe and effective use of magnesium (Mg) alloys, including practical guidelines for the service use of Mg alloys in the atmosphere and/or in contact with aqueous solutions. This is to provide support for the rapidly increasing use of Mg in industrial applications, particularly in the automobile industry. These guidelines should be firmly based on a critical analysis of our knowledge of SCC based on (1) service experience, (2) laboratory testing and (3) understanding of the mechanism of SCC, as well as based on an understanding of the Mg corrosion mechanism.

A Critical Review of the Stress Corrosion Cracking (SCC) of Magnesium Alloys

Much of our understanding of the Mg corrosion mechanism is based on research using aggressive chloride based solutions like 3% NaCl, which are appropriate for understand the corrosion for applications such as auto construction. The chloride ions tend to cause break down of the partly protective surface film on the Mg alloy surface. The corrosion rate increases with exposure time until steady state is reached, which may take several weeks. An overview is provided of the aspects which determine the corrosion of Mg alloys: (i) measurement details; (ii) impurity elements Fe, Ni, Cu and Co; (iii) second phases; (iv) surface films and surface condition and (v) stress corrosion cracking (SCC). This understanding is used to help understand Mg corrosion for Mg as a biodegradable implant for medical applications. Solutions that elucidate these applications tend to form surface films and the corrosion rate tends to decrease with immersion time.

Corrosion mechanism applicable to biodegradable magnesium implants

Stress corrosion cracking (SCC) of the Mg–Al alloys AZ91, AZ31 and AM30 in distilled water has been characterised using constant extension rate tests (CERTs) and linearly increasing stress tests (LISTs). AZ91 consists of an α-matrix with extensive β-particles, whereas AZ31 and AM30 consist only of an α-matrix with an Al-concentration similar to that in AZ91. The presence of β-particles in AZ91 was associated with: (i) a lower threshold stress, σ SCC , for AZ91 (55–75 MPa) relative to AZ31 (105–170 MPa) and AM30 (130–140 MPa); and (ii) a different SCC initiation mechanism for AZ91 relative to AZ31 and AM30. The SCC velocity, V c , for AM30 (3.6 × 10 −10 to 9.3 × 10 −10  m/s) was lower than that for AZ91 (1.6 × 10 −9 to 1.2 × 10 −8  m/s) and AZ31 (1.2 × 10 −9 to 6.7 × 10 −9  m/s). This was attributed to the influence of Zn and second phase particles, which are more concentrated in AZ31, on the diffusivity of H in the α-matrix.

Characterisation of stress corrosion cracking (SCC) of Mg-Al alloys

Transgranular stress corrosion cracking (TGSCC) of the Mg alloy AZ91 in distilled water and 5 g/L NaCl solution has been evaluated using the linearly increasing stress test (LIST) and the constant extension rate test (CERT). The differences between these techniques, with respect to fractography and the measurement of SCC parameters, are discussed. The LIST and CERT techniques are both useful in identifying the occurrence of SCC and, when coupled with a technique for characterizing crack extension, measuring the threshold stress and crack velocity. During a LIST fast fracture ensues a relatively short time after the threshold stress is attained, whereas during CERT crack growth over a much longer time period is facilitated by a reduction in stress. Consequently, the LIST is typically 30–50% shorter in duration, whereas the CERT produces a larger SCC fracture surface.

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Comparison of the linearly increasing stress test and the constant extension rate test in the evaluation of transgranular stress corrosion cracking of magnesium

A fractographic study has been carried out on transgranular stress-corrosion cracks produced in both tensile and bend specimens of a Mg-7.5 Al alloy tested in an aqueous NaCl-K2CrO4 solution. It is concluded that the complex fractography reported in earlier papers is fully consistent with the view that failure occurs basically by cleavage on planes. Thus the coarse markings reported previously are shown to correspond to a stepped or pfeated topography consisting of alternate faces produced by cleavage and ductile rupture parallel to (0001), the latter exhibiting flutings of the type observed in other HCP alloys. The reasons for the occurrence of the ductile component are discussed.

D. G. Chakrapani

Papers

On the Fractography of Transgranular Stress Corrosion Failures in a Mg-Al Alloy