Micro-Biologically Induced Steel Corrosion and Corrosion Control in Simulated Marine Environment: Steel Electrochemical Response and Micro-Organisms Viability within Cathodic Protection Application

Abstract

Micro-biologically induced corrosion (MIC corrosion) is a complex and challenging phenomena, associated with the metabolism of microorganisms. Corrosion resistance, surface properties and overall performance of steel structures in sea water, fresh waters or soil are affected by the inevitable existence of various types of microorganisms. Methods, techniques and practical applications are available, however, they are limited with respect to one or another fundamental mechanism, expected results, environmental conditions etc. The complexity of the related challenges results from the fact that there is no single mechanism involved on one hand. On the other hand, despite available knowledge, the fundamental processes related to MIC corrosion still need to be clarified for every environmental condition and/or practical application. The topic of this work is linked to a broad research project on studying in detail the predominant mechanisms within MIC-related corrosion and corrosion control for low-carbon steel in marine environment. One of the main objectives of this research is to justify the potentially superior performance of an alternative impressed current cathodic protection (ICCP), based on pulse technology, in view of controlled microorganisms viability, hence efficient MIC control. With this respect, except steel electrochemical response in relevant conditions, the behaviour of microorganisms with regard response to cathodic polarization, in the course of bio-film formation and/or restricted bio-film growth, are of significant importance and need to be studied and clarified. To this end, the paper will discuss results from preliminary and on-going investigation, as follows: The global electrochemical response of steel in biotic and abiotic environment (simulated sea water), compared to response after pulse ICCP. In addition to recorded evolution of open circuit and on-potentials (within ICCP) with time, the results are in terms of polarization resistance and corrosion current density, respectively, derived from electrochemical impedance spectroscopy (EIS), linear polarization resistance (LPR), potentio-dynamic polarization (PDP) and cyclic voltammetry (CVA). The local electrochemical response, derived from scanning vibrating electrode technique (SVET) and scanning electrochemical microscope (SECM), adds-up to the global electrochemical behaviour of the tested steel. The results provide information for the distribution of ionic currents during bio-film formation or re-distribution (following ICCP), as well as alterations in oxygen availability due to microorganisms metabolism. Additionally, SVET and SECM provide in-situ information on altered ionic current flow after ICCP and/or subsequent changes in microorganisms viability. Finally, fluorescent microscopy supports the electrochemical tests and justifies the effect of cathodic polarization on microbial metabolism and restriction thereof, hence “visualises” the effect of the alternative ICCP as a MIC-control approach. The results from this work serve as a base for further investigation on the application of a hybrid, bio-based coating for control of excessive hydrogen evolution within ICCP. This is especially relevant for the practical situation, when bacterial metabolism reduces ICCP efficiency i.e. when bio-film formation in conditions of conventional ICCP increases the demand for enhancing cathodic polarization. It is expected that the bio-based coating, together with the alternative ICCP, will result in optimum protection with minimum overprotection for steel in marine environment. The concept of this novel solution will be also discussed.