Evaluating Hydrogen Stress Cracking of Line Pipe Steels under Cathodic Protection Using Crack Tip Opening Displacement Tests

Abstract

Crack tip opening displacement (CTOD, δ) tests were carried out for line pipe steels in buffer solutions, sand, and clay to evaluate initiation of hydrogen stress cracking (HSC) at surface defects in buried pipelines under cathodic protection. Four series of line pipe steels and two series of seam welds showed a similar tendency in cathodic current density (i) versus the critical CTOD (δ_c) curves, irrespective of types, pH and water content of the soils; δ_c showed a minimum (δ_HSC) when i>i_th (i_th=1mA/cm²) in all the testing conditions. δ_HSC increased with the increasing fracture toughness of the steel. Fluctuation of cathodic current density influenced δ_c when the maximum value of cathodic current density (i_max) was larger than i_th. HSC could be initiated at surface defects in pipelines only when i_max>i_th and δ≥δ_HSC.