Sulphide stress corrosion in SAE 1070 steel wire used in the tensile armour of flexible pipelines: Effect of induced superficial defects

Abstract

This research aims to investigate and quantify potential damage resulting from induced superficial defects on the surface of wires use on tensile armor of flexible pipes. Different values of kt factors and H2S concentration in aggressive gas mixtures with CO2, were analysed to determine the operational conditions that do not result in rupture of the tensile reinforcement during service, under the conditions studied. The tests were based on NACE TM0316 standard, using a 4-point bend test. Two superficial notches were created, which generated known stress concentration factor Kt values of 1.5 and 2.0, and three gaseous concentrations (220, 700, and 1100 ppm of H2S in CO2) were used. After bending tests, the samples were evaluated by metallographic and fractographic analysis, weight loss and liquid penetrant inspection. The results were then analysed using analysis of variance (ANOVA). Only the samples exposed to 220 ppm of H2S in CO2 did not rupture at both Kt values. However, for the Kt value of 2.0, cracks were observed in the longitudinal direction of the wires, originating from the machined notch. Additionally, the samples exposed to 700 ppm and 1100 ppm of H2S had a significantly higher damage score (approximately 15x) than the samples exposed to 220 ppm, regardless of the Kt value.

Keywords

• Corrosion
• Induced superficial defects
• Flexible pipes
• H2S