Stress Corrosion Cracking Susceptibility Of Steel Research Articles

Role of Mexican Clay Soils on Corrosiveness and Stress Corrosion Cracking of Low-Carbon Pipeline Steels: A Case Study

This work analyzed the physicochemical effect of different types of Mexican clay soils on corrosion and stress corrosion cracking (SCC) behavior in contact with X60 and X65 steels. Four soils were obtained from the right of way land in southern Mexico at 1.5 m depth close to pipelines. Two soils were from the state of Oaxaca (SO1 and SO2), and two others from the state of Veracruz (SV1 and SV2). Physicochemical and textural analysis of soils was performed and correlated to SCC susceptibility and corrosion mechanism. It was observed that soil texture might be related to corrosivity. A texture index (ratio between sand and silt + clay), which was seen to have a relationship with the corrosive tendency of soils, was estimated. It showed that soil with a higher index (SV1) has a higher corrosion rate. Electrochemical impedance spectroscopy and polarization curves were performed and correlated to the corrosion rate and the SCC susceptibility of steels. Steels exposed to SV1 soil exhibited a higher corrosion rate related to a higher content of chlorides and acid pH than those seen in other soils, which resulted in the pitting of such steels. Two types of corrosion were observed. Soils from SV1 and SV2 generated pitting, and soils from SO1 and SO2 produced general corrosion. Inclusions caused pitting in the gauge section of X60 and X65 steels exposed to SV1 soil by anodic dissolution. Galvanic coupling between inclusions and the base metal and dissolution of the inclusions might have enhanced the nucleation of pits at these sites. SCC susceptibility was evaluated using slow strain rate tests (SSRT). After SSRT, the fracture surfaces were analyzed through scanning electron microscopy. The SCC index obtained from SSRT indicates that X60 and X65 steels exhibited good resistance to SCC. A highly corrosive soil, such as SV1, causes the formation of pits instead of cracks, which is attributed to the dissolution process; however, lower SCC indexes were obtained for this system. The higher corrosion resistance of X60 steel is related to a more homogenous microstructure and a higher content of elements, such as Ni and Cr, than those of X65 steel that decrease the corrosion rate.

Long-Term Evolution of Corrosion Potential of Carbon Steel in Alkaline Radioactive Waste Environments

The long-term shifts of corrosion potential are important in predicting the likelihood of localized corrosion and stress corrosion cracking (SCC) of carbon steel used for storing radioactive wastes in underground storage tanks. Although considerable work has been done in understanding the passivity and corrosion potential of steel in various electrolytes, an important aspect of the current work is in assessing the effects of multiyear exposures of steel in waste simulants and their effects on corrosion potential. It is shown that SCC susceptibility of steel in nitrate increases at the long-term corrosion potential in solutions without organics (either by applying that potential or letting the corrosion potential increase over time). The long-term increase in corrosion potential results principally from a decrease in the passive current density with time of exposure. The present work shows that such a reduction in passive current density is accompanied by changes in the semi-conductive properties of the passive film, which itself may be a result of changes in stoichiometry of the film over time. Nitrite reduction is the most likely cathodic reaction with a small contribution from oxygen reduction. However, the presence of organic species in the environment can result in additional anodic reactions that may decrease the corrosion potential.

Influence of AC waveforms on stress corrosion cracking behaviour of pipeline steel in high pH solution

The effect of various waveforms AC on stress corrosion cracking (SCC) behaviour and mechanism of X80 pipeline steel was investigated in high pH carbonate/bicarbonate solution by polarisation curves and slow strain rate tensile tests. Under various waveforms AC, the passivity of the steel degrades severely, and pitting corrosion easily forms on the electrode surface. With or without AC application, the SCC behaviour and mechanism of the steels in the solution are distinctly different. Without the presence of AC, the cracks propagation are intergranular, and the mechanism is anodic dissolution. AC significantly increases the SCC susceptibility of pipeline steel. The order of the SCC susceptibility of the steel at various waveforms AC is: sine wave < square wave < triangular wave. Under a long term AC interference, the fracture mode is transgranular, and the mechanism of the steel in high pH solution is controlled by anodic dissolution and hydrogen embrittlement.