The effect of electrode surface area on corrosion initiation monitoring of X65 steel in soil
Introduction
The effect of metal surface size on localised corrosion is a rather fundamental issue that has attracted considerable attention in the past, because it affects not only the reliability of corrosion monitoring using metal electrodes, but also the strategy of protecting coated metallic structures with coating defects of varied sizes. Examples of such studies can be found in many research reports in the historical literature [[1], [2], [3], [4], [5], [6], [7], [8]]. The initiation of localised corrosion on passive metals has often been considered as a stochastic process that is attributed to the randomness in local properties of the passive film [7]. The probable initiation sites of localised corrosion are often considered to be related to ‘active’ spots in the passive film, such as sulphide inclusions and grain boundaries [[9], [10], [11], [12]]. Since the amount of these probable initiation sites is proportional to the exposed metal surface area, naturally the chance for localised corrosion initiation on a larger metal surface would be higher than that on a smaller one [[9], [10], [11], [12]]. Therefore, a general conclusion from these investigations is that the chances for localised corrosion initiation on a larger metal surface are higher than that on a smaller one [[9], [10], [11], [12]]. For instance, Aziz et al. [10] found that the pitting probability of an aluminium specimen in tap water increased with the increase of exposed specimen area until it reached 60 cm2. Burstein et al. found that in acidic chloride solutions the increase of 316 stainless steel specimen size led to the decrease in the pitting potential, indicating higher localised corrosion vulnerability [11]. Likewise, Li presented results showing that the average pitting potential decreased significantly with increasing specimen size [12]. Similar behaviour was found in concrete environment where a high pH usually keeps steel reinforcements passive. Angst el al. have observed that the threshold chloride concentration decreases, which suggests higher localised corrosion susceptibility in concrete structures, with the increase in the exposed steel surface area [13]. Based on the strong influence of specimen size on experimental results, Angst et al. suggested that results measured on certain sized samples can only be used to perform service life calculations and predictions of the same sized structures [13]. These results suggest that in localised corrosion measurements the choice of electrode surface area would be an important factor affecting the experimental results. The notion that localised corrosion measurement results could be affected by electrode size causes major concerns since laboratory corrosion tests usually use only relatively small sized specimen, and the results could be misrepresentative when they are used to predict localised corrosion on larger structures.
Although the effect of electrode size on localised corrosion initiation has been investigated in relatively homogeneous aqueous conditions [[9], [10], [11], [12]], little study has been done in more complex environments such as underground pipeline conditions. Buried carbon steel and low alloy steel structures are commonly protected by cathodic protection (CP) and organic coatings. Unfortunately, coatings usually contain defects of various sizes due to various reasons such as manufacturing flaws, construction damages and numerous forms of cracks and scratches. It has been recognised through experimental studies and mathematical modelling that the coating defect size affects the distributions of CP potential and currents [[14], [15], [16], [17], [18]] and the local chemical environment over a buried pipeline [19,20]. For instance, Orazem et al. demonstrated that larger coating holiday showed a less negative potential under impressed current CP [[14], [15], [16], [17], [18]]. Kodym et al. found that the pH distribution in the vicinity of the coating holiday was influenced by the water saturation, soil texture as well as coating defect size [19,20]. However, the main aims of these prior works were only to understand the CP characteristics such as CP current, potential and pH distributions affected by various factors affecting general corrosion. They did not intend to consider corrosion initiation on buried pipelines when CP system is disrupted by certain operational reasons [21] or interfered by stray currents [[22], [23], [24], [25]]. Unfortunately it has been found that although pipeline steel can passivate under a CP induced high pH environment, it could lose passivity and localised corrosion occurred when CP system is disrupted [21] or interfered by stray currents [[22], [23], [24], [25]]. This needs to be better understood, especially for coated steel structures buried in complex soil environments with coating defects of varied sizes.
Currently the effects of electrode surface area on corrosion initiation on buried X65 steel and its monitoring have not been understood. Therefore, this work particularly focuses on the effect of electrode size on the initiation of localised corrosion occurring on buried steel surfaces under the scenario that CP was disrupted. The main aim is to understand the extent that steel electrode size could affect the localised corrosion initiation and susceptibility. This knowledge is important because it could affect the CP and maintenance strategies for underground pipelines as well as the design of corrosion probes for pipeline corrosion monitoring and predicting.
Section snippets
Materials
X65 pipeline steel with chemical composition (wt%) of 0.04 C, 0.2 Si, 1.5 Mn, 0.011 P, 0.003 S, 0.02 Mo, and Fe balance was employed throughout this work. Coupon electrode specimens of four different sizes (5 mm × 5 mm × 3 mm; 10 mm × 10 mm × 3 mm; 20 mm × 20 mm × 3 mm; 30 mm × 30 mm × 3 mm) were made from the same steel sheet, with no further heat treatment. These samples were soldered with copper wires for electrical connection. Subsequently, they were mounted by epoxy resin, leaving 0.25 cm2
Probing corrosion initiation processes
In order to simulate and probe the behaviour of corrosion initiation on X65 steel electrodes of various sizes under simulated underground pipeline conditions, as shown in Fig. 1, the coupon electrode (or WBE) specimens were buried in the soil box. A prior CP potential of −950 mVCSE was applied on the working electrode surface for 24 h in order to establish a stable CP condition on steel surfaces. CP was then disrupted after 24 h, to simulate situations that CP rectifier stopped working or the
Conclusions
Current mapping by means of the electrode array method, in conjunction with electrode potential monitoring, cyclic potentiodynamic polarisation and surface pH measurements, has been successfully utilised to visualise and evaluate the dynamic corrosion initiation processes occurring on X65 steel electrodes of different surface area buried in unsaturated sandy soil under the scenario that CP was disrupted. Prior-CP was applied on buried X65 steel electrodes for 24 h under −950mVCSE, mimicking a
Data availability statement
The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study.
Acknowledgements
This work was funded by the Energy Pipelines Cooperative Research Centre (Energy Pipelines CRC), supported through the Australian Government’s Cooperative Research Centres Program. The funding and in-kind support from the Australian Pipelines and Gas Association Research and Standards Committee (APGA-RSC) is gratefully acknowledged.
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