Heterogeneous corrosion of mild steel under SRB-biofilm characterised by electrochemical mapping technique

Abstract

Heterogeneous corrosion of mild steel under sulphate reducing bacteria (SRB)-biofilm was characterised by wire beam electrode (WBE) technique and electrochemical impedance spectrum. The potential/current distributions of the WBE under SRB-biofilm showed that the potential maps could not indicate the localised corrosion of steels beneath biofilm due to the fact that all wire electrodes were short-circuited by the highly conductive sulphide precipitates embedded in SRB-biofilm. Instead, the galvanic current maps may give a good indication. The characteristic of super-capacitance (0.21 F/cm²) of SRB-biofilm was attributed to the huge specific surface area of conductive pore walls inside biofilm.

Introduction

Microbiologically influenced corrosion (MIC) related to sulphate reducing bacteria (SRB) is one of the main factors for the degradation of pipeline in oilfields [1], [2]. SRB tend to form biofilm, the complex colonies attaching to surfaces by self-produced extracellular polymeric substance (EPS) [3], which can affect the dissolution of metal through the SRB metabolites [4], [5]. The SRB adhering to the metal surface may modify the local chemistry beneath biofilm, leading to intrinsic heterogeneity and critical localised corrosion [4]. Videla [6], [7] proposed that the uneven colonisation of SRB on the steel surface reduced the protective performance of the outer layer of the corrosion products. Hamilton [8] presented a differential aeration cell between the uncolonised region and those regions under biofilm.

Iron sulphides (mackinawite, pyrrhotite) are the main corrosion products of mild steel in SRB environment [9]. The freshly produced iron sulphides on the metal surface may act as a protective film by limiting the ferrous ion diffusion through the sulphide film. However, through the film spalling or rupture, the sulphides can promote the localised dissolution of steel by cathodic depolarisation [10]. On the other hand, a few publications about MIC inhibition (MICI) have been reported [11]. Perez et al. [12] revealed that the strong adhesion of biofilm could inhibit the damage of the steel. Stadler [13] examined the corrosivity of different compositions of EPS extracted from biofilm, and found that EPS from different SRB species exhibited promotion or inhibition of the corrosion of mild steel. Depending on the structure and heterogeneity of biofilm, the inhibitory action of biofilm can be reversed to a corrosive action within bacteria colonies in the biofilm [14].

MIC is a typical localised corrosion where heterogeneous electrochemical processes beneath biofilm are very common [15]. However, conventional electrochemical methods (polarisation curves, electrochemical impedance spectroscopy, etc.) employing a single electrode could not differentiate the anodic and cathodic zones on the biofilm-covered metal surface. Therefore, it is difficult to employ them in heterogeneous electrochemical researches. Scanning vibrating electrode technique (SVET) is a non-destructive method for locating the position of anodic electrochemical activity on metal surface [16], [17]. Franklin et al. indicated that pit propagation was facilitated by Pseudomonas sp. via SVET [18]. Atomic force microscopy (AFM) can also characterise the heterogeneity of biofilm [19]. For example, Yuan found that the heterogeneity and thickness of biofilm increased with exposure time by AFM observation [20].

Compared to SVET, a multi-electrode device (also called wire beam electrode, WBE) was successfully applied in the studies of heterogeneous corrosion [21], [22]. Tan [23], [24] had studied the non-uniform characteristic beneath coating and found that the working surface of WBE could not only simulate the electrochemical processes occurring on a large area single electrode, but also provide cathodic/anodic information of single wire electrode. Wang [21] also illustrated that, as a non-disturbing electrochemical technique, the WBE technique was valuable in characterising the heterogeneous electrochemistry at the artificial biofilm/stainless steel interface.

Although heterogeneous corrosion under SRB-biofilm has been studied broadly, on-line monitoring methods of the localised corrosion are rarely reported. This work aims to study the heterogeneous electrochemistry at SRB-biofilm/mild steel interface using WBE technique and electrochemical impedance spectrum (EIS). The distributions of potential, galvanic current and impedance of mild steel beneath SRB-biofilm were mapped to indicate the evolution process of localised corrosion.