Maocheng Yan

Electrochemical measurements using combination microelectrode in crevice simulating disbonded of pipeline coatings under cathodic protection

Abstract A novel combination microelectrode was constructed based on an iridium pH microelectrode and an Ag/AgCl microelectrode, and was applied to simultaneously monitor the steel potential, pH and chloride ion concentration in a crevice simulating disbonding of pipeline coatings with cathodic protection (CP) applied or removed. The sensing characteristics and sensitivity of the microelectrode are described as well as the calibration procedures. A simulated crevice was formed between a pipeline steel surface and a polymethylmethacrylate plate. The results showed that in CP conditions, the evident potential gradient resulting from the IR potential drop was concentrated at the crevice opening. A protective local environment induced by CP under disbonded coatings was characterised by an increase in pH and removal of Cl−. After CP is removed, the protective effects can persist for a period of time, depending on the diffusion of soluble species.

Microbiologically Induced Corrosion of X80 Pipeline Steel in a Near-Neutral pH Soil Solution

MIC of X80 pipeline steel in a near-neutral pH soil solution in the presence and absence of sulfate-reducing bacteria (SRB) was monitored by electrochemical techniques and microbiological tests. The results show that soil solution is more close to the complex soil environment around pipeline. The activity of SRB leads to the shift of the phase response to low frequency, the decrease of electrolyte resistance and the alteration of dielectric constant of the film. Both the activity and metabolite of SRB influence the corrosion behavior of the steel. The steel surface undergoes localized attack in the SRB-inoculated soil solution, whereas only slight uniform corrosion occurs in the sterile soil solution.

A study on bacteria-assisted cracking of X80 pipeline steel in soil environment

ABSTRACT In the paper, bacteria-assisted cracking by sulphate-reducing bacteria (SRB) of X80 pipeline steel in a meadow soil solution was studied under different applied stresses by electrochemical impedance spectroscopy, linear polarisation resistance and scanning electron microscope. The obtained results indicated that the physiological activity of SRB and the applied stresses not only play individual roles in corrosion of the steel, but also had intense synergistic effects in promoting the corrosion process. In the SRB-inoculated soil solution, the number, average length and width of micro-cracks increase to a certain extent. Under the combined actions of SRB and plastic stress, localised corrosion may not only extend to steel matrix but also to lateral direction. The generation processes of bacteria-assisted cracking were discussed in the paper.

Effect of Temperature on Corrosion Behavior of X80 Pipeline Steel in Acidic Soil

Corrosion of X80 pipeline steel in an acidic red soil collected from Yingtan area of Southeast China was studied in a temperature range 20~75 ℃ by electrochemical impedance spec- troscopy (EIS). Theory concerning corrosion dynamics and transition state was applied to analyze the process and kinetics of the corrosion reaction. The results show that EIS of X80 steel in the red soil contains a capacitive arc related with soil at the high frequency region and a capacitive arc from the interface process at low frequency region. With increasing temperature, the soil resistivity and charge transfer resistance decrease, and the corrosion rate increases. Kinetics analysis shows that corrosion of X80 steel in the acidic red soil is an endothermic reaction system companied with decrease of disorder degree.