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Analysis of corrosion products in stress corrosion cracks Nikiforuk, Thomas Philip

Abstract

Circumferentially notched rods of three types of austenitic stainless steel were stress corroded under freely corroding conditions at their yield stress in boiling 154°C MgCl2, and boiling MgCl2 with additions of HCl, CoCl2, and FeCl3. Alloy types 304, 316, and 310 were chosen because of their known different stress corrosion susceptibility. The corrosion products formed on the stress corrosion fracture surface were analyzed by electron diffraction and energy dispersive x-ray analysis. Electron diffraction of corrosion products, both in situ and stripped from the fracture surface, showed the corrosion product was a spinel oxide in all cases. Qualitative x-ray analysis of corrosion products, in situ and stripped from the fracture surface, indicated the corrosion product formed on all alloys was enriched in chromium and contained lesser amounts of the elements iron, nickel, silicon, molybdenum, magnesium, phosphorous and chlorine. Observations led to the conclusion that the corrosion product formed in the cracks of the various alloys was similar, being predominantly a chromium enriched oxide spinel with possible traces of metal chlorides or a corundum type oxide. The presence of the spinel oxide was consistent with anticipated E-pH equilibrium within the crack. However, it was felt the variation in s.c.c. behaviour between the different alloys could not be adequately accounted for in terms of the composition of the oxide.

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