An investigation into the failure of a series of cycled 40Ah valve regulated lead acid batteries, identified a number of different defect types present in the corrosion layer. In this paper, a detailed description of the materialographic preparation method used to produce cross-sections of the corrosion layer is given. Each of the defect types identified is described and illustrated in an appropriate micrograph. Variations between the defect types suggest that there is more than one mechanism for defect initiation and growth. Possible mechanisms for sources of the defects and their subsequent growth are proposed.