An improved radiographic-photographic technique has been developed for measuring the depths of corrosion pits in aluminum and other metals. Consideration and control of factors affecting the radiographic and photographic sensitivity have led to the achievement of substantially higher sensitivity than had been demonstrated in the past.
A series of high contrast positive prints of a corroded specimen and a calibrating stepped wedge, prepared by extinction printing from reversal negatives made from high sensitivity radiographs, are representative of specific depth layers of the corroded specimen. These are analogous to photographs that would be obtained from the metal surface if layers of incremental thickness were successively removed. The prints thus obtained are used for rapidly performing an accurate quantitative analysis, facilitating statistical studies of pit depth vs frequency.
Although sensitivity decreases rapidly with increasing specimen thickness, owing to the logarithmic nature of X-ray absorption laws, incremental sensitivities of 0.001 in. in a 0.060 in. aluminum specimen have now been attained. Suggested modifications are outlined which can increase the sensitivity and extend maximum specimen thickness examined, as a particular application might require.
