Pitting and intergranular corrosion

The orientation of the cracks reveals that cyclic bending stresses or cyclic axial stresses were active. The intensification of these stresses at pits and intergranular corrosion sites produced the cracks observed. 

Since austenitic stainless steels are susceptible to pitting and intergranular corrosion in the presence of chloride ions, other materials were examined for caustic service [43-47]. These include Fe-Cr alloys, which are resistant to SCC. However, these alloys are brittle and suffer from 475°C embrittlement and sigma embrittlement. A popular alloy that was examined for the caustic evaporator service was E-Brite-26-1, containing 26% Fe and 1 % Mo, which exhibited performance characteristics comparable to that of... 

Pitting and intergranular corrosion can be mitigated by cathodic protection and Inhibitor release... 

Pylon Tank Rliform corrosion caused die prafixa-don of one area of an aluminum alloy 6061-T6 pylon tank (Fig. 12). Pitting and intergranular corrosion were also detected on die pylon tank during die investigation of this proUon (Fig. 12c). The aircraft that operated with this tank had been flying in the hot and humid Mediterranean environment The proper qipli-cation and maintenance of epoxy or polyurethane paint finishes would have minimized the deterioration on this strucniie. 

Stress-coriDsioii cracking (SCC) in an aluminum alloy 7079-T6 main landing gear Blink, (a) Overall view of Blink, (b) Pitting and intergranular corrosion that initiated SCC. Source Ref 17... 

Localized deterioration Corrosion (especially pitting and intergranular attack), erosion, cavitation, mechanical wear, and so on (see Case History 9.8). 

Localised Corrosion (or localised attack) accelerated corrosion at certain sites only of a metal surface, usually induced by spatial separation of the anodic and cathodic sites. Examples include pitting corrosion, stress-corrosion cracking and intergranular corrosion. 

H. Kaesche. Pitting Corrosion of Aluminum and Intergranular Corrosion of A1 Alloys, in Localized Corrosion. 1971. Williamsburg, VA NACE, Houston, TX. 

Unalloyed zinc corrodes uniformly unless in contact with more cathodic materials, in which case a more localized galvanic corrosion can occur at the interface. In other cases, nonunifoim corrosion is generally not encountered. Zinc alloys, particularly those with aluminum, can suffer from pitting and intergranular attack. This tends to be influenced by the fineness of the alloy microstructure. 

Accelerated tests should produce the types of corrosion, pitting, or intergranular corrosion (IGC) that occur in service, and contain at least the major contaminants and environmental variables expected. Engineering requirements generally dictate the type of corrosion of most concern and the most significant method of evaluation. For example, for liquid containers the primary concern may simply be to avoid perforation and leaking. Depth of corrosion is the major concern. However, if the liquid is under pressure, the type of corrosion can be of importance as this csin affect burst strength. 

---