Studies of corrosion and stress corrosion cracking behavior of high-strength pipeline steels in carbonate-bicarbonate solutions

The electrochemical, mechanical and surface characterization techniques were used to study corrosion and stress corrosion cracking (SCC) behavior of X80 and X100 pipeline steels in carbonate-bicarbonate solutions. The parametric effects, including carbonate/bicarbonate concentrations, chloride ion concentration, CO2 content and cathodic polarization potentials were determined. It is expected to investigate comprehensively the fundamentals of corrosion and SCC of high-strength pipeline steels in carbonate/bicarbonate environments.;The steel specimens tested in air exhibited a ductile type of failure, and those tested in the diluted bicarbonate near-neutral pH solution showed regions of mixed ductile and brittle fracture with considerable decrease of reduction-in-area where a transgranular fracture path prevails. The susceptibility to SCC was demonstrated as a decrease in the mechanical properties. In the slow strain rate testing the ductility and time to failure is significantly reduced in more than 50% for every sample under different levels of cathodic polarization. Hydrogen permeation measurements were made and showed that the diffusion of atomic hydrogen was related to carbonate film formation and fracture appearance. The average hydrogen content in X80 and X100* pipeline steels charged in near-neutral pH solution depended on the charging time and cathodic potentials. Hydrogen content appeared less sensitive to cathodic potential at a short charging time. The hydrogen permeation increased with the increasing of cathodic polarization up to -1.1 V. Even small levels of cathodic protection generated enough hydrogen that substantially changed the mechanical properties of the material.