Hydrogen flux and corrosion rate measurements on hydrogen induced cracking susceptible and resistant A516 steels in various sour environments

 Two A516 plate steels of 16 mm thickness were simultaneously exposed during 14 day trials to various sour saturated solutions at 20 ^oC. One did not show any evidence of hydrogen induced cracking (HIC), the other consistently cracked.   Flux and corrosion rates of the two plates decreased after a few days in non-buffered acid solutions, whereas in buffered acid conditions they reached and maintained a steady state. After these trials, the crack section ratio (CSR) of four sectioned, HIC susceptible plates were approximately proportional to total through wall flux passed, suggesting crack initiation in the first few days of exposure, and progressive cracking proportional to flux passed, thereafter.  In ammonium sulfide solutions, flux was only induced by addition of cyanide; the flux then subsided, probably due to reaction with sulfide.

In all trials flux from the two plates co-trended extremely closely.  A significant diurnal temperature effect was observed consistent with steel temperature variation of a few degrees, underlying the temperature sensitivity of hydrogen permeation.    Early modeled flux data from the trials indicated the hydrogen diffusivities, entry concentrations, and permeabilities of the HIC resistant and susceptible plates were similar.  Diffusion coefficients varied between 8 and 12 x 10^-6 cm².s^-1, consistent with other data for A516 steels.    There was good correlation between corrosion rate and steady state flux obtained from modeled data pertaining to the first 24 hr of measurement, except from pH 2.7 solutions, where an ‘additional’, non-hydrogen occluding corrosion of  0.5 mm/yr corrosion rate was attributed to similar corrosion measured prior to sour saturation. Apart from this, no pH effect on the correlation was evident between pH 2.7 and 9.  The same correlation curve was approximately observed between coincidently measured flux and corrosion rates, when allowance was made for pre-existing sour corrosion at low pH, and the brevity of a flux episode at high pH.