The effect of aerobic corrosion on anaerobically-formed sulfide layers on carbon steel in dilute near-neutral pH saline solutions
Introduction
An integrity system based on coatings and cathodic protection (CP) protects buried gas transmission pipelines. If coatings disbond, exposing the steel to groundwater, the effectiveness of the CP system becomes inhibited and the underlying steel becomes susceptible to corrosion [1], [2]. Based primarily on field inspections of coating failure sites, TransCanada PipeLines Ltd. (TCPL, Calgary, Alberta, Canada) has proposed six corrosion scenarios that lead to pipeline damage [1], [2]. One particularly damaging scenario involves anaerobic corrosion with microbial effects which subsequently turn aerobic, and accounts for 17% of all reported coating failures [1], [2]. Reported corrosion rates for this scenario can be as high as 2–5 mm yr−1 [1], [2]. While anaerobic microbial activity may decrease as conditions become more oxidizing, the anaerobically-formed Fe(II) sulfides can oxidize to Fe(III)-containing oxides and elemental sulfur, a process that leads to further pipeline damage, including localized corrosion [2], [3].
Recent studies have addressed the Fe/S/H2O/O2 system [4], [5], [6], [7]. Bourdoiseau et al. [4] monitored the oxidation of Fe(II)-mackinawite () under wet oxidizing conditions. The first oxidation stage leads to the formation of an Fe(III)-containing mackinawite;As oxidation proceeds, transformation to Fe(III) oxy/hydroxides and elemental sulfur occursUnreacted Fe(III)-containing mackinawite can also react to form greigite () [7], which may further convert to form Fe(III) (oxyhydr)oxides and pyrite, FeS2, in air [4],These studies indicate that films formed microbiologically on pipelines will be reactive under dry out conditions when air becomes available. These anaerobic to aerobic transformations may be very important in determining the high corrosion rates observed following a transition from microbiologically induced corrosion (MIC) under anaerobic conditions to aerobic conditions.
In an attempt to develop corrosion mechanisms that reflect external pipeline conditions, we have previously investigated the effect of inorganic sulfide on carbon steel corrosion in solutions containing chloride, bicarbonate, and sulfate (pH 8.9) under both anaerobic [8] and aerobic [6] conditions. The experiments generated three distinct observations:
- (1)
On a steel surface covered with anaerobically-formed magnetite (Fe3O4)/siderite (FeCO3), sulfide causes an increase in corrosion rate initially within pores in the film. Sulfide initiates a conversion from magnetite to mackinawite but the sulfide deposit formed does not passivate the surface and leads to a steady on-going increase in corrosion rate [8].
- (2)
When sulfide is present from the beginning of steel exposure to an anaerobic dilute simulated groundwater, a “passive” layer of mackinawite is formed. The corrosion rate is independent of [HS−], indicating that it is controlled by the properties of the sulfide film [8].
- (3)
If the steel surface is covered by aerobically-formed goethite-covered tubercles, sulfide addition causes surges in corrosion rate, followed by an apparent partial suppression of corrosion; although and ECORR values indicate the corrosion rate is higher than those measured during the aerobic corrosion (without sulfide) period. On-going exposure to aqueous sulfide leads to a slow corrosion process as the goethite-covered surface surrounding the tubercles is converted to mackinawite [6].
In this study we have investigated the influence of aerobic corrosion on an anaerobically-formed mackinawite film using the methodologies previously developed [6], [8], [9], [10], [11], [12]. Corrosion was monitored over a 106 day period by following the corrosion potential (ECORR) and periodically measuring the polarization resistance (RP) using linear polarization resistance (LPR) measurements. Subsequently, the morphology and composition of the corrosion product deposits were determined using scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX), and Raman spectroscopy.
Section snippets
Materials and electrode preparation
Experiments were performed on X65 carbon steel (0.07 C; 1.36 Mn; 0.013 P; 0.002 S; 0.26 Si; 0.01 Ni; 0.2 Cr; 0.011 Al (wt.%)) supplied by TCPL. For corrosion measurements, a cubic coupon, 1.0 cm × 1.0 cm × 1.0 cm, was cut from metal plates and fitted with a carbon steel welding rod (4 mm diameter) to facilitate connection to external equipment. The electrode was then encased in a high performance epoxy resin (Ameron pearl grey resin and 90HS cure) so that only a single face was exposed to the solution.
Results and discussion
Fig. 1 shows the ECORR, and values (data points) recorded as a function of exposure time. A detailed description of the corrosion behavior and surface analysis under anaerobic conditions (for times <day 78) was described previously [8]. Over this period both ECORR (∼ −900 m VSCE) and the ([4.5 ± 0.6] × 10−5 ohm s−1 cm−2) remain fairly constant, despite the incremental increase in sulfide concentration up to 0.9 mmol L−1 [8]; the incremental increases are indicated in Fig. 1. The corrosion rate
Conclusions
- (1)
The influence of switching from anaerobic to aerobic conditions on the corrosion of steel in sulfide solutions has been studied using electrochemical techniques, SEM/EDX, and optical microscopy/Raman spectroscopy.
- (2)
The anaerobically-formed mackinawite is sufficiently porous to allow direct access of dissolved O2 to the substrate steel. This leads to a corrosion process beneath the iron sulfide film, yielding maghemite and causes blistering and spalling of the mackinawite layer.
- (3)
Chemical conversion
Acknowledgements
This research was carried out for NOVA Research & Technology Centre (NRTC, Calgary, AB, Canada) and TCPL through an Industrial Postgraduate Scholarship Agreement with the University of Western Ontario and the Canadian Natural Sciences and Engineering Research Council (NSERC, Ottawa, ON). Fraser King, Integrity Corrosion Consulting Ltd., and Robert (Bob) G. Worthingham, Worthingham Professional Services Inc., are gratefully acknowledged by the authors for their guidance and continued support of
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