Light scattering analysis, a NDT method for the study of corrosion

Material anomalies due to corrosion have often led to unforeseen failures of different kinds of mechanical parts. Specific NDT methods are used for each case of corrosion, based on the type and site of corrosion and the access to corroded areas. These methods differ in some aspects: automation, speed, precision and cost. But generally speaking the quest for a rapid, sensitive, and automatic NDT technique still remains unsatisfied. The technique proposed is the result of our studies of rough surfaces by means of light scattering(1–7). It deals with the study of the first and second order statistical properties of laser light scattered by a rough surface. In this paper we present recent results obtained with a model with different stages of electroerosion (as a pitting corrosion) in different metallic samples. First, we study the mean intensity light scattered in the far field region by means of two perpendicular linear scans with a photodetector. Samples of different materials affected from 0% to 100% of the corroded surface were studied. We show the scattered light has two curves superimposed, one of them due to the surface free of corrosion and the other one due to corroded surface. Both curves change in different ways with the corrosion state. It is possible to evaluate a T/σ value (ratio between the correlation length T and the surface roughness σ) from these curves. Second, two speckle patterns are captured and registered with a CCD camera, before and after the illuminating angle changes. Then, a computer correlation algorithm compares the two speckle patterns. The intensity correlation of the speckle patterns gives a measure of the roughness. We show the evolution of the surface roughness σ with different states of electroerosion. We have a good correlation between the quantity of corroded surface and the measured roughness.