This study is a continuation of the investigation initiated in the lab a couple of years ago to understand the value of using electrochemical techniques such as cyclic polarization (CP) and electrochemical impedance spectroscopy (EIS) to assess, rank, and predict the tendency of corrosion inhibitors (batch and continuous) to protect carbon steel surfaces from localized corrosion (pitting). The first part of this study proved out the good correlation between the results obtained with electrochemistry methods and the long-term immersion/autoclave testing. General corrosion rates from weight loss using rotating cage autoclave (RCA) tests and CP/EIS scans using rotating cylinder electrode (RCE) tests showed similar tendencies in most of the products tested (87% correlation was achieved). Based on the promising results obtained in Phase I of this investigation, Phase II focused on the 13% of the cases where there was no correlation and the changes needed to be made from the experimental standpoint to improve the correlation rate to 100%. For that purpose, in this study, one corrosion inhibitor formulation was tested at three different dose rates and the electrochemical methods were successfully used to identify at which dose rate no pitting was observed.
Electrochemical methods have long been utilized to provide real-time corrosion information but have rarely been used to study localized corrosion. Most recently, electrochemical methods such as electrochemical impedance spectroscopy (EIS) and cyclic polarization (CP) were investigated to predict pitting tendencies and the work was presented in 2022.1
Based on this work, it was proved that CP technique can be used to predict localized corrosion tendency of continuous corrosion inhibitors. The technique was evaluated for carbon steel that does not form a passive layer and preliminary studies demonstrated that this technique is a suitable way to determine the corrosion protection of carbon steel with corrosion inhibitor actives in the corrosive media. In addition to CP, EIS measurements were performed to determine the relative ranking of the corrosion inhibitor from the general corrosion (GC) standpoint.
In continuation of the previous work, CP and EIS techniques were conducted to further validate these methods in different set of test conditions. In these new test conditions, a good correlation of these techniques with RCA results was also obtained, emphasizing the utilization of the techniques for understanding the localized performance of corrosion inhibitors in a relatively short duration.
