A technique for studying the 3-dimensional shape of corrosion pits

A transmission electron microscopy study of constituent-particle-induced corrosion in 7075-T6 and 2024-T3 aluminum alloys

“…Conceptually, corrosion around a single cathodic constituent particle proceeded under cathodic control at a constant current density over the particle surface, as illustrated schematically in Figure 9(a). [1,2,5,6] Matrix dissolution within such a pit is expected to be enhanced by the same particle-matrix galvanic couple, although the current densities would reflect the local electrochemical conditions within the pit. A similar schematic model for dissolution of an anodic particle is shown in Figure 9(b).…”

“…[1][2][3][4] The combination of pitting corrosion and subsequent fatigue crack growth can significantly reduce the lives of structural components made of these alloys in service. [5,6] In this study, a more direct observation of the nature and extent of particle-matrix interactions in an electrolyte was made with the aid of transmission electron microscopy (TEM). In previous studies, pitting in 7075-T6 and 2024-T3 aluminum alloys has been identified by optical microscopy (OM) and scanning electron microscopy (SEM) with galvanic coupling between the matrix and the constituent particles in these alloys.…”

“…Examples of particle-induced corrosion in aluminum alloys include those of iron-rich particles (such as Al 3 Fe), [13,14] Al 7 Cu 2 Fe, [15] and MgZn 2 . [1,2,5,6] For the TEM studies, 3-mmdiameter disks were cut by electro discharge machining from the alloys, with the disk plane parallel to the sheet surface. Particles that contain Al, Cu, and Mg tend to be anodic relative to the alloy matrix, while those that contain Al, Cu, Fe, and Mn tend to be cathodic relative to the matrix.…”

Probabilities of occurrence and detection of damage in airframe materials

“…The micrograph is a typical localized severe pit in which constituent particles are visible. Clusters of particles determine the rate and extent of pit growth, and naturally, the larger clusters lead to more severe damage 6 , 7 . Severe pits that occur in high‐stress regions of components or structures are sites from which corrosion fatigue cracks nucleate and grow.…”

Probability modelling and statistical analysis of damage in the lower wing skins of two retired B‐707 aircraft

“…The early stage of the model is dominated by pitting corrosion and the later by corrosion fatigue crack growth. Clusters of particles determine the rate and extent of pit growth, and naturally, the larger clusters lead to more severe damage 7 , 8 . Severe pits that occur in high stress regions of components or structures are sites from which corrosion fatigue cracks nucleate and grow.…”