Effect of organic corrosion inhibitors on the crack growth rate in pipe steel X70 in a weakly acidic solution

Abstract

The effect of a number of organic corrosion inhibitors (CIs) on the growth rate of a corrosion crack in pipe steel X70 in a weakly acidic (pH 5.5) citrate buffer solution in the absence and in the presence of 1 mM Na 2 S has been studied. A static straining load was applied to the specimens, with the stress intensity factor in the crack tip corresponding to the plateau of the kinetic curve of steel X70 destruction. The crack growth rate was determined from the change in the electric resistance of a specimen. The hydrogen penetration rate into metal was determined using the hydrogen electrochemical desorption method. It has been shown that CIs can considerably slow down the corrosion crack growth. In the solution containing no sulfide ions, the best protective properties are shown by IFKhAN-P-3 inhibitor (based on a salt of polyoxometalates with amines) and by a mixture of this inhibitor with phosphate (IFKhAN-P-8). In the presence of sulfide ions, the best inhibitors of stress corrosion cracking (SCC) of pipe steel include the IFKhAN-P-3 and IFKhAN-P-8 formulations, as well as modifications of the IFKhAN-29 inhibitor and a quaternary ammonium compound, Catamin AB. Good correlation is observed between the corrosion crack growth rate and the anodic dissolution rate of mild steel in electrolytes containing corrosion inhibitors. Only a poor correlation is observed between the crack growth rate and the rate of hydrogen penetration into the metal; the existence of this relationship is apparently due to the fact that all the CIs studied slow down not only the anodic dissolution of steel but also the penetration of hydrogen into the metal. The conclusion has been made that local anodic dissolution of the metal is the predominant mechanism of the growth of corrosion cracks in pipe steel in weakly acidic electrolytes. The ability of organic compounds to inhibit SCC can be estimated by the anodic current density at constant potential selected in the region of active dissolution of mild steel.