Evaluating hydrogen stress cracking of line pipe steels under cathodic protection using crack tip opening displacement tests

Crack tip opening displacement (CTOD, delta) 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 (delta (c)) curves, irrespective of types, pH and water content of the soils; delta (c) showed a minimum (delta (HSC)) when i >i(th) (i(th)=1 mA/cm(2)) in all the testing conditions. delta (HSC) increased with the increasing fracture toughness of the steel. Fluctuation of cathodic current density influenced delta (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 delta greater than or equal to delta (HSC).