Investigation of stress-corrosion cracking of welded joint of H70 steel under cathodic polarization in near neutral environment

Abstract

Study of stress-corrosion cracking of welded joints made of X70 steel at cathodic polarization in near neutral solution NS4 was carried out. It was established that the tendency of base metal of X70 steel to stress-corrosion cracking, estimated by KS coefficient, increases from 1.07 to 1.13 whereas polarization potential changing from the corrosion potential to maximum protective potential -1.05 V. Such regularity correlates with increasing of hydrogen penetration through X70 steel, which at maximum protective potential -1.05 V is equal to 0.000518 mol/m3 . Susceptibility of welded joint is lower than the base metal, KS coefficient change not much near the value 1.0. With increasing of polarization potential from -0,75V to -1.05V fracture surface of X70 steel characterized by decreasing in the size of holes and the appearance of flat areas, through that the rupture occurred. A similar nature of the rupture was observed for the welded joint, but visually larger proportion of flat areas can be noted. It was established that the rupture of the welded joint occurs on the base metal, which allows to propose stress-corrosion cracking susceptibility factor KS to be legitimately used for estimation to stress-corrosion cracking of welded joint, provided the weld is performed in a high-quality manner. Stress-corrosion cracking results of welded joint of X70 steel correlate satisfactorily with the experience of stress-corrosion cracking on main gas pipelines, where stress-corrosion cracks form and develop along the base metal of gas pipelines.

Keywords

• pipe steel x70
• welded joint
• near-neutral environment
• cathodic polarization
• stresscorrosion cracking
• slow strain rate test
• voltammetry
• fractography