The stress-corrosion cracking of carbon steel in Co-C02-H20

Abstract

Stress-corrosion cracking of steel occurs in the CO-C02-H20 system when these three environmental components are present together. The dissolved carbon monoxide in the water forms an inhibitive layer on the surface of the steel. Rupture of the adsorbed layer by slip steps on the surface of the steel causes the exposure of uninhibited steel if plastic deformation occurs. Dissolved carbon dioxide accelerates the corrosion rate at this location. A crack forms when the corrosion is concentrated to form a crack tip, while corrosion of the crack flanks is inhibited. Due to the conservation of charge, crack growth relies on cathodic processes on the external surfaces. Cathodic reactions need not be confined to the crack tip and crack flanks, but may also occur on the external surfaces. To handle cracking in a practical context, critical stress intensities and corresponding growth rates are necessary for design purposes. The critical stress intensity for stress-corrosion cracking - K1scc - was determined by measuring the current flow from an external cathode, with comparisons with the crack growth rate. Crack propagation was measured with the potential drop method. The double cantilever beam specimens were coated with silicone rubber to limit corrosion to a small region around the crack. This allowed measuring of the current flow between the crack tip and two steel cathodes at the sides of the specimen. The current responses were correlated with the actual crack propagation. Experiments were conducted by testing C-Mn steel in distilled water with a 50°/o CO - 50°/o C02 gas mixture at 45°C and 800 kPa. It was found that K1scc can be predicted by using the current measurements. It appears that with such "electrochemical noise measurements" important factors that influence the cracking process can be investigated. The required inhibitor addition was evaluated and between 200 and 1000 ppm potassium bichromate was required to mitigate stress-corrosion cracks significantly. Constant deflection tests confirmed the low stress threshold of C-Mn steel exposed to industrial environments with carbon monoxide and carbon dioxide. Therefore, the ineffectiveness of stress relieving post-weld heat treatments to inhibit cracking is clear.