1 Department of Manufacturing Engineering, Technical University of Denmark2 Department of Management Engineering, Technical University of Denmark
Fundamental studies on the Electrochemical Behaviour of Carbon Steel Exposed in Sulphide and Sulphate-reducing Environments
The aim of the report is to give a fundamental understanding of the response of different electrochemical techniques on carbon steel in a sulphide environment as well as in a biologically active sulphate-reducing environment (SRB). This will form the basis for further studies and for recommendations in regards to electrochemical monitoring of MIC. The work presented here and further studies are also planned to lead to a Ph.D. thesis on "MIC monitoring based on mechanisms of corrosion".The results of laboratory experiments conducted in the period 1995 to 1997 are summarised. Conclusions will be based on results from the entire 3 year period, but only selected experimental data primarily from the latest experiments will be presented in detail here.Microbial corrosion of carbon steel under influence of sulphate-reducing bacteria (SRB) is characterised by the formation of both biofilm and corrosion products (ferrous sulphides) on the metal surface. Experiments have been conducted on carbon steel exposed in near neutral (pH 6 to 8.5) saline hydrogen sulphide environment (0 to 100 mg/l total dissolved sulphide) for a period of 14 days. Furthermore coupons have been exposed in a bioreactor for a period of up to 120 days in sulphide-producing environment controlled by biological activity of (SRB).Electrochemical studies have been conducted in order to characterise the electrochemical response of the biofilm / ferrous sulphide / metal interface and clarify whether the tested electrochemical techniques (LPR, Tafel, EIS, stepped LPR) are actually applicable to this system.Exposure of carbon steel in the bioreactor environment leads to:· formation of very porous sulphur rich surface films· mild grain boundary corrosion attacks Electrochemical studies from the bioreactor can be summarised to:· apparent corrosion rates are very high ( Rp appr. 100 Ohm*cm2 ~ 3 mm/yr) which is highly in contradiction with the visual inspection indicating little corrosion· large interfacial capacitances (up to 300 mF/cm2) are obtained· hysteresis dominates DC polarisations· for purely capacitative hysteresis a corrected corrosion rate can be estimatedExposure of carbon steel in hydrogen sulphide atmosphere results in:· formation of sulphur rich surface films with varying degree of porosity· indications that high film sulphur content correlate with low polarisation resistanceElectrochemical studies in hydrogen sulphide atmosphere can be summarised to:· Polarisation resistance estimated with four different techniques show a good correlation in solutions with a sulphide concentration below ~10 mg/l sulphide· Polarisation resistance increases with increasing sulphide concentration and time· Corrosion rate estimation is in the range of 3 to 30 kOhm*cm2 (0.1 to 0.01 mm/yr) which does not contradict with visual inspection and weightloss measurements.· A threshold value can be found at 5 to 10 mg/l sulphide above which interfacial capacitance increases almost two decades· Large interfacial capacitances (up to 50 mF/cm2) are obtained· Stepwise Tafel polarisations indicate that hysteresis is not only caused by high capacitative current contributions but probably highly dominated by diffusionThree different electrochemical characteristics can be suggested:· Formation of a uniform, dense and not electrochemically active film. The polarisation resistance increases with the film resistance and an small underestimation of corrosion rate is possible, if film resistance is large.· An electrochemically reactive film (ferrous sulphides) results in current contributions that will be added to the metal dissolution reaction current. A very reactive film will therefore increase the apparent corrosion rate, even if the corrosion rate of steel is low. In this case corrosion rate estimation will fail.· Porous film formation gives an increased surface area and induces large effects of diffusion. The interfacial capacitance and the polarisation resistance are distributed over a huge area. The result is an increased apparent corrosion rate and capacitance. Corrosion rate estimation fails - especially if porosity is combined with a reactive film.The conclusion is therefore that corrosion rate estimation by electrochemical techniques is possible. However, a very large risk exists that the true metal dissolution rate is totally masked and that the operator will not know when to trust data and when to discard them. It does not seem likely that corrosion rate can be estimated for long term exposure in a biological environment. Whether the corrosion rate estimations in the hydrogen sulphide environment can be trusted is still not settled, but possibly corrosion rate estimation by electrochemical techniques is possible below a sulphide threshold value.Identification of a high interfacial capacitance determined by EIS is a very strong indication of sulphide. The high capacitance is primarily caused by the formation of a reactive porous layer with a rapid enhancement of surface area. The effect is increased in the biological environment as compared to the hydrogen sulphide solutions.· Tafel polarisations are only relevant for mechanistic studies. The technique is, however, destructive, time consuming and fails, when things become complicated.· LPR and stepped LPR are applicable only in simple cases and gives little information on when and why the measurements may be unreliable.· EIS gives the opportunity to recognise characteristic changes such as increasing capacitance and indications of porous behaviour, but analysis is somewhat complicated.To conclude electrochemical measuring techniques must be used with caution and a direct corrosion rate measurement (ER, weightloss) applied in order to obtain actual corrosion rate. However, electrochemical techniques still represent a diagnostic tools and e.g. the identification of high interfacial capacitance might be a way of recognising the initiation of conditions leading to MIC. Future studies will cover other electrochemical techniques (electrochemical noise, galvanic coupling) and direct corrosion rate estimation for mechanistic studies.