Electrochemical techniques including linear polarisation resistance, harmonic distortion analysis and electrochemical noise have been used to study the corrosion of carbon steel due to sulphate reducing bacteria (SRB) in typical seawater injection systems. The effects of SRB activity on the electrochemistry of the corroding interface and the effects of biocide treatments have been examined. Electrochemical monitoring provides a reliable means of detecting corrosion due to SRB activity, and provides new information regarding the effectiveness of biocide treatments.
This study was instigated to provide a controlled laboratory investigation into bacterial metabolic activity and the associated corrosion activity of carbon steel as determined by electrochemical corrosion monitoring. SRB activity results in the corrosion of carbon steel, primarily due to the formation of hydrogen sulphide. In sea-water injection systems bacterial activity is usually monitored by sampling techniques, which is a time consuming process. Biocide activity may be controlled using a variety of biocide treatment regimes. Inadequate biocide treatment can result in excessive SRB activity, which may result in catastrophic corrosion of plant and pipe-work. Electrochemical corrosion monitoring has the advantage of having a quick response time, and modern techniques may be used to assess both uniform and localised (pitting) corrosion activity.
The objectives of this study were to examine the relationship between bacterial metabolic activity (SRB), bacterial biofilm numbers (SRB and Glycogen Accumulating Bacteria - GAB) and electrochemical corrosion monitoring(1) using once through loops to simulate a typical North Sea Water Injection System. The laboratory based NCIMB Ltd continuous biocide testing apparatus was used for this work (Figure 1).
The source of enriched mixed culture used in the work, was the Statfjord A Platform. Continuous electrochemical corrosion monitoring using a simple three identical electrode arrangement and SmartCETÔ 1 corrosion monitors were used to follow the evolution of the corrosion processes in the presence and absence of SRB activity, and during periods of biocide treatment. The electrochemical techniques employed were, Linear Polarisation Resistance (LPR), Harmonic Distortion Analysis (HDA), and Electrochemical Noise (EN) updating each parameter every twelve minutes for the duration of the test (~50 days). In addition to the electrochemical corrosion monitoring, regular checks were made on hydrogen sulfide concentrations, and biofilm numbers.
The system consisted of a synthetic seawater supply system which fed into towers where dissolved oxygen was stripped by sparging with oxygen free nitrogen (OFN). The seawater was then passed into mixing chamber 1, where oxygen scavenger and CoCl2 catalyst were added. Control loop 1 was fed directly from mixing chamber 1 (although further oxygen scavenger was added as required directly to tubing supplying loop 1). The balance of water from mixing chamber 1 was then passed to mixing chamber 2, into which a source of inoculum was fed from a rockpile (this was a nutrient-fed glass cylinder containing granitic chips upon which a biofilm containing the Statfjord A enriched culture was growing). The other 3 test loops were fed from this chamber. Of these, loop 2 had no further additions (control with nutrients). Loops 3 and 4 were dosed with biocide as detailed below. The test loops consisted of 1cm diameter mild steel tubing, mounted vertically, with the water exiting the top of the tubes. Glass vessels containing the corrosion monitoring electrodes, were placed upstream from the sample sections thus minimising the chance
