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Metal corrosion, with the exception of pitting, is generally a slow process. However, it can combine with mechanical loading to produce a particularly aggressive form, stress corrosion cracking SCC. A tensile stress or residual stress from component manufacture helps open the crack tip to allow the attacking elements like chloride ions or ammonia species in and allows concentrated chemical attack. The corrosion component of SCC allows areas of crack growth to continue even when the stress is lowered. Combining cyclic loading fatigue with corrosion is probably the most catastrophic form of attack affecting aluminium alloys in aircraft or in steel weld heat affected zones HAZ or or in brass plumbing fittings for example.

LPD Lab Services can investigate this form of corrosion and failure by metallurgical techniques like SEM/EDX, AES Auger Electron Spectroscopy, Optical microscopy, metallography and ion chromatography.

Works completed have included:

• Stress corrosion cracking of brass and bronze.
• Failing welds on lifting gear and cranes with characteristic tide marks in fractography.