The measurement of shape change during early stages of corrosion pit growth

S.E.M. stereo photogrammetry methods were used to measure the shape of corrosion pits grown on pure iron in 1 N H₂SO₄ containing 0.003 N NaCl at +0.65 V (Ag/AgCl). The method involved analysis of matched S.E.M. stereo pairs with use of the 'floating dot' technique. The method was found to be accurate and precise to within 3%. Analysis of 76 pits in the size range 7-200 μm diameter indicated that corrosion pits in the model system studied were essentially hemispherical up to 50 μm diameter, and were more shallow above this size.