Corrosion Fatigue of AZ91E-T6 Cast Magnesium Alloy in a 3.5 Percent NaCl Aqueous Environment

Abstract

The objective of this research was to obtain and compare constant and variable amplitude fatigue behavior of AZ91E-T6 cast magnesium alloy in both an air and 3.5 percent NaCl aqueous corrosive environment. An additional objective was to determine if commonly used models that describe fatigue behavior and fatigue life are applicable to this material and test environment. Fatigue tests included constant amplitude strain-controlled low cycle fatigue with strain ratio, R, equal to 0, −1 and −2, Region II constant amplitude fatigue crack growth with load ratio, R, equal to 0.05 and 0.5 and variable amplitude fatigue tests using keyhole notched specimens. In all fatigue tests, the corrosion environment was significantly detrimental relative to the air environment. Mean strains influenced fatigue life only if accompanied by significant mean stress. The Morrow and Smith, Watson, and Topper mean stress models provided both accurate and inaccurate fatigue life calculations. Likewise, variable amplitude fatigue life calculations using the local strain approach and based upon the formation ofal mm crack at the keyhole notch were both accurate and fairly inaccurate depending on the specific model used.

Fatigue of Magnesium Alloys

“…[26] This effect was also found in some magnesium alloys, which are mainly based on the MgAl [4,14,17,19,23,27,28] or Mg-Li systems. [29,30] In further publications the attention is additionally drawn to different heat treatments and their microstructures.…”

“…[48] The corrosion fatigue has been tested in different aqueous solutions, but most investigations have been performed with sodium chloride solutions. [6,8,10,27,28,46,48,49] Generally, the influence of corrosion fatigue shows a significant reduction in fatigue life of magnesium components with different impacts depending on the selected solution. [31,50,51] Chloride, nitrate and phosphate solutions are somehow the most severe solutions, because the ions of these salts tend to dissolve the developed corrosion layer of magnesium hydroxide (Mg(OH) 2 ).…”

“…[39] The models of Smith-Watson-Topper (SWT) and Morrow were also tested for corrosion fatigue, but with non-satisfying results, because both accurate and inaccurate fatigue life calculation were received. [27] To improve the reliability of fatigue life models, new approaches are based on micro structural life-prediction concepts. One concept has been successfully developed for magnesium alloys by Eisenmeier et al using a derivation from Basquin and Manson-Coffin equations.…”

A Review on Corrosion behaviour of Mn added Magnesium and its alloys

“…The excess addition of Zr can lead to precipitation of Zr in the matrix, which is detrimental to the corrosion [23]. In addition, rare earth improves also the corrosion resistance of magnesium alloys, but is affected by medium and pH value [21]. …”

“…This is important, when present in Mg, any Fe is nominally insoluble and forms a pure-Fe (BCC) phase in the Mg matrix. This pure Fe has a large potential difference compared to the Mg and is able to support cathodic reactions efficiently and hence accelerates the corrosion rate dramatically [21] .…”