Galvanically Induced Atmospheric Corrosion on Magnesium Alloys: A Designed Experiment Evaluated by Extreme Value Statistics and Conventional Techniques

Galvanic corrosion of high purity die cast magnesium alloys AM50 and AZ91 was examined in accelerated atmospheric corrosion testing according to Volvo STD 1027,1375 for 6 weeks involving cycling of the relative humidity between 90% and 45% in combination with intermittent immersion in one of two NaCl-solutions (0.3% or 1.0%). The exposures were performed at two different CO₂ levels; 0.01% and 0.3%.

The initial general corrosion rate of the AM50 alloy is 50-100% higher than that of AZ91 depending on surface preparation. The corrosion weight loss of both materials depends linearly on salt load in the investigated range. CO₂ has a moderate accelerating effect, being higher with decreased salt load.

Extreme value analysis was used to evaluate the deepest pit distribution around the perimeter of mounted bolts in panels of AZ91 and AM50. Quite contrary to the general corrosion results, AZ91 showed 30% deeper pits than AM50. The effect of Sn/Zn coating on the attached bolts reduced pit depth with 20% compared to uncoated steel bolts. The analysis confirmed that extreme value statistics applied to pit depth measurements is a viable method to evaluate galvanic corrosion. Assessing galvanic susceptibility on the basis of total weight loss measurements is evidently misleading.