Hydrogen Permeability Of Tubing Steel And Its Effect On Stress Corrosion Cracking

Steel-made equipments occur electrochemical corrosion in corrosive medium containing CO2, which cathodic hydrogen evolution reaction occurs, producting hydrogen atoms on the steel surface. Because the diameter of a hydrogen atom is much smaller than metal atom’s, part of the hydrogen atoms diffuse into the lattice of steel substrate based on the adsorption-absorption process. The dissolved hydrogen can cause embrittlement of the material, material cracking and loss of the carrying capacity. A synergy of material damage can occur when hydrogen, stress and corrosive environments exist together. For instance, hydrogen diffusion in the material can cause embrittlement under stress or hydrogen induced cracking at nonmetallic inclusions. In this article, both the hydrogen permeation and stress corrosion cracking of80SS and110SS tubing steels in simulating oilfield water saturated with CO2were studied. Combined with electrochemical methods, SSRT and a variety of modern analytical techniques, influence of various factors on the hydrogen permeation and the stress corrosion cracking(SCC) were explored; susceptibility to stress corrosion cracking was predict; relationship between hydrogen permeation and SCC was established.Devanathan-Stachurski double cell electrochemical hydrogen permeation process was chosen to measure the hydrogen permeation curves following strictly the "five steps" in good order. Besides, the influence of poisoning substance, hydrogen trap, heat treatment, pH, temperature, salinity, Cl-on hydrogen permeation behavior was studied. The results show that: i) Both of110SS and80SS are made of tempered sorbite, while the former has smaller grain size, more boundary, longer hydrogen permeation time, smaller hydrogen diffusion coefficient, more hydrogen diffusion resistance and better resistance to hydrogen induced cracking sensitivity than the latter one. ii)"ferrite+pearlite","ferrite+patenting" and "martensite" are obtained after normalizing, annealing, quenching, respectively, among which trapping density of hydrogen and susceptibility to hydrogen embrittlement increasing in turns, iii) steady current increases and hydrogen diffusion coefficient decreases with the strength/pH decreasing or T increasing; All hydrogen permeation parameters of P110are in the same order of magnitude as110SS’; Effects of salinity and Cl-on hydrogen permeation have no an uniform rule. Among them, there is a linear relationship between concentration of diffusion hydrogen and applied polarization current, and Arrhenius relationship between hydrogen diffusion coefficient and T (298-363K).Through the method of potentiodynamic polarization curve speed scan test, it can be predicted that80SS and110SS have respectively stress corrosion cracking susceptibility when potential greater than-589mV and-541mV (vs. SCE) using Parkins boundary conditions and stress corrosion cracking parameter Pi. The SCC is controled by the anodic dissolution mechanism under the chosen potential before.SSRT was used to caculate the effects of hydrogen on SCC. The investigation indicated that both80SS and110SS appear hydrogen-induced damage phenomenon, and embrittlement phenomenon of80SS with concentration of diffusion hydrogen increasing is more obvious: When concentration of diffusion hydrogen is3.99mol H/m, concentration of diffusion hydrogen is1.922×10-6cm2/s, yield strength has a reduction in25.77%, tensile strength decrease by7.92%, strain hardening index is0.21, integration of stress and strain reduce a magnitude of62.0%, loss rate of elongation is up to50.40%, and a brittle fracture occurs; However, the decrease degree of110SS on strength and plasticity with the increasing diffusible hydrogen concentration is much lower than the80SS’, which indicates a stronger hydrogen permeation resist and a smaller influence by hydrogen permeation.SCC mechanism of80SS and110SS in produced water with saturated CO2is hydrogen assisting anodic dissolution type stress corrosion cracking. Cracking process is as follows: Corrosion happens to form an uniform dense FeCO3and CaMg(CO3)2product film→Partial destruction ocurrs on film and the microcrack forms→Corrosion, stress and hydrogen atoms coordinate with each other to make the crack grow until it breaks.