The Influence of The Oxide Layer on The Stress Corrosion Cracking Initiation in The Case of Incoloy 800 in The Alkaline Environment

Abstract

Oxidized and non-oxidized C-rings cuts from Incoloy 800 tubes were tested with the purpose to study the influence of the oxide layer in the first step of the stress corrosion cracking (SCC) mechanism. A mechanical pre-crack of 100mm depth was executed on the external side of the C-rings in order to create some stress concentrations. The oxidized C-rings were obtained by autoclaving for 20 days in demineralised water adjusted with hydrazine (pH=9.7), at 260⁰C and 50atm. By X-rays diffraction there are emphasized iron dichromium oxide, nickel dichromium oxide, magnetite and hematite. Using the scanning electron microscopy we distinguished the presence of a double layer: the inner layer compact and adherent, containing fine grains associated with iron dichromium oxide, nickel dichromium oxide and the outer layer composed of other greater and less uniform particles associated with magnetite and hematite_. The SCC tests were performed in 10% sodium hydroxide solutions (pH=13) at 260⁰C and 50atm, for 57 days. After the SCC test, it was observed that the presence of the oxide layer led to a transgranulary SCC initiation, direct from a fissure in the oxide layer; the length of the SCC cracks is about 80–100mm. In the absence of the oxide layer, the SCC cracks start intergranulary, from a local corrosive attack zone and their depth is approximately 200mm. In both cases the SCC cracks have transgranular propagation.

Keywords: caustic environment, stress corrosion cracking, Incoloy 800, oxide layers