2D modelling of inclined intergranular stress corrosion crack paths

Abstract

Inclined high pH stress corrosion cracking (SCC) is a type of intergranular environmental cracking in gas pipelines, which differs from typical SCC by propagating at an angle from the wall direction. Investigations of Australian and Canadian inclined SCC colonies have not provided a clear indicator of a cause for the abnormal crack growth direction. This paper addresses the possibility of crack tip strain enhanced electrochemistry causing the inclination. Potentiodynamic tests were conducted to quantify the influence of strain on the electrochemistry, and strain was found to increase current density up to 300% in the SCC region. A model was developed that incorporates crack tip strain driven SCC growth, which showed good agreement with field grown cracks, and the aspect ratio of the grains was shown to have an effect on the inclination angle. The results indicate that crack tip strain enhanced electrochemistry is a plausible cause for inclined SCC.

High-pH inclined stress corrosion cracking in Australian and Canadian gas pipeline X65 steels

“…Through a numerical model under development by the authors, 23 the grain aspect ratio (AR, defined as height of the grain divided by the width) was identified as another possible parameter influencing the crack paths. The model operates under the assumption that the strain in the crack tip plastic zone enhances the current density for dissolution and reduces film stability.…”

“…The model operates under the assumption that the strain in the crack tip plastic zone enhances the current density for dissolution and reduces film stability. [23][24][25] The critical plasticity at which the enhanced dissolution offsets, the higher stress intensity factor in the radial direction was determined from metallographic studies. 7 It also incorporates the geometry of the grain boundaries and how they influence the growth direction for the intergranular growth.…”

“…7 occur via normal film fracture rate dependant SCC, with fracture occurring at the highest stresses. 26 After a certain depth, the crack tip strain increases such that the interplay between crack tip electrochemistry and plasticity [23][24][25] allows the crack preferential growth along an inclined angle. The growth mechanism switches back and forth between stress dominated and strain dominated growth until a depth where the growth angle stabilises and strain is the dominant factor.…”

Stress corrosion cracking behavior of micro-arc oxidized AZ31 alloy

“…3–6 During the studies on the corrosion fatigue behavior of magnesium alloys, it found that the stress loading rate had great influence on the dynamic mechanical corrosion properties of magnesium alloys, such as SCC. To date, investigations on SCC and protection properties of various steels and aluminium alloys have been performed, 7–9 whereas such investigations are just beginning for magnesium alloys. Therefore, exploring the SCC properties and stress corrosion protection method for magnesium alloys is particularly urgent.…”