09121 Mechanistic Investigation of Deflected Stress Corrosion Cracking in Pipeline Steels

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Product Number: 51300-09121-SG

ISBN: 09121 2009 CP

Author: Robert Worthingham, Millen Sen, Lin Yang, Fraser King, Jeffrey Xie

Publication Date: 2009

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Metallurgical examination of a pipe from a hydrotest failure site revealed deflected stress corrosion cracking (SCC) cracks. The deflected SCC cracks changed direction at a depth of 200 – 600µm from the outer surface with an initial straight section in axial direction, then an inclined segment to the circumferential direction. The deflection angles were in the range of 30 – 50°. Statistically more than 50% of all the cracks with a straight length of 200 – 600µm were deflected. Crack branching was present during the propagation of cracks at the deflection point and/or at the crack tip. The branched cracks were roughly 90° to each other. All SCC cracks were in longitudinal direction on the surface and the entire cracks length is located within a layer of ca. 2.5mm from the surface.

Corrosion pits and intergranular corrosion formed on the surface and/or inside of the cracks, resulting in accumulation of corrosion products inside the crack and material loss due to isolated grains from the intergranular corrosion.

To understand the cause of the deflected SCC cracks, detailed investigations were conducted on the morphology and distribution of SCC cracks, initiation and propagation of SCC cracks, microstructural and microhardness characterization, mechanical properties. It is revealed that there were no unusual changes in microhardness, mechanical properties and microstructural characterization along the pipe wall thickness. A condition of residual shear stress is believed to have been established at the surface region of 200 – 600µm, thus promoting the deflection of the SCC cracks.

Keywords: Stress corrosion cracks, pipeline steel, corrosion, deflection