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Abstract
This article presents an assessment of the susceptibility of the WE54 magnesium-based alloy to stress corrosion cracking in an 0.1 M Na2SO4 environment. In this work, the basis criterion for assessing the alloy’s behavior under complex mechanical and corrosive loads is the deterioration in its mechanical properties (elongation—ε, %, reduction in area—Z, %, tensile strength—Rm, MPa) along with a fractographic analysis of fracture surfaces. The WE54 magnesium-based alloy was subjected to the slow strain rate test (SSRT) under mechanical loads in corrosive environment (0.1 M Na2SO4 solution). The test was carried out in four variants: (a) SSRT in air, (b) cathodic hydrogen charging for 24 h at a current density of 50 mA/cm2 followed by SSRT in air, (c) SSRT in a corrosive environment under open-circuit potential conditions, and (d) SSRT in a corrosive environment under cathodic polarization at a current density of 50 mA/cm2. In each variant, the content of hydrogen in the alloy was determined. It was demonstrated that under SCC conditions, in the presence of hydrogen, the plastic properties of WE54 decreased significantly, whereas the alloy’s strength properties changed to a smaller degree. The change in the mechanical properties under SCC conditions in a corrosive environment was accompanied by a change in the fracture surface morphology and by the presence of numerous cracks, both on fracture surfaces and in the alloy’s microstructure.
Highlights
Magnesium and its alloys, with their low density and a good set of technological properties and mechanical properties, are a very attractive construction material in those industries where it is important to reduce the mass of a construction and at the same time retain its mechanical properties (Ref 1, 2)
The highest tensile strength (Rm % 290 MPa) was recorded for the specimens strained in air—both those that had not been exposed to the corrosive environment (Fig. 2a) and those that had been exposed to the corrosive environment (Fig. 2b)
The highest hydrogen concentration was recorded for the specimens tested under cathodic polarization (CH = 31.7 ± 0.6 ppm) and the specimens tested in air following exposure to the corrosive environment under cathodic polarization (CH = 31.3 ± 0.6 ppm)
Summary
With their low density and a good set of technological properties (workability, castability, machinability, and full recyclability) and mechanical properties (considerable specific stiffness and specific strength), are a very attractive construction material in those industries where it is important to reduce the mass of a construction and at the same time retain its mechanical properties (Ref 1, 2). Structural components made of magnesium-based alloys are often subject to mechanical loads, and to the action of a corrosive environment. In such service conditions, a dangerous phenomenon is stress corrosion cracking (SCC). Literature data (Ref 8, 9) concerning stress corrosion cracking suggest that the phenomenon may be caused by stresses that are below 50% of the yield strength of particular magnesium-based alloy being in service, in environments causing only negligible corrosion in those alloys (Ref 9-11). Hydrogen-assisted stress corrosion cracking is dangerous for the group of high-strength magnesium-based alloys containing rare earth elements. It is attempted to analyze stress corrosion cracking in a modern magnesium-based alloy containing rare earth elements—WE54—intended for use at temperatures of up to 300 °C. The mechanical tests were complemented by an analysis of the alloyÕs microstructure and the changes in the morphology of the fracture surfaces being the result of stress corrosion cracking
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