Some magnesium alloys experience environment assisted cracking (EAC) in ambient atmospheric air, and hydrogen embrittlement is the dominant mechanism [
]. The highest susceptibility has been found in aluminum-containing alloys (6% max) [
]. EAC has also been observed in pure magnesium [
]. The EAC sensitivity of aluminum-free rare earth-containing alloys, such as WE43 has not been previously investigated in detail. Previous work has shown that plastic strain nucleates EAC in WE43-T6 by cracking of the intergranular intermetallic [
]. This paper reports a quantitative study of the crack nucleation mechanism in WE43-T6. This is part of a project that aims to predict the probability of nucleation and propagation of cracks at notches and micro-shrinkage porosity in cast engineering materials.