Corrosion Cyclic Voltammetry of Two Types of Heat-Affected Zones (HAZs) of API-X100 Steel in Bicarbonate Solutions
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INTRODUCTION
CORROSION in bicarbonate-based environments is one of the chemical failures the oil and gas pipelines are commonly prone to, externally and internally.[1,2] Bicarbonate is a main constituent in the hydrated soils where the external surface of a buried pipeline becomes exposed to after an accidental coating disbondment,[3] for instance. In association with carbonate, chloride, and other electroactive species, bicarbonate drives a series of complex corrosion reactions, in aerated or deoxygenated mildly alkaline environments.[4–7] Internally, it accelerates the corrosion rates and hydrogen generation after it results from the aqueous equilibrium of CO2 hydration, at extents depending on the pressure and temperature.[8] In these conditions, bicarbonaterisen (or -rising) alkalinity[9,10] of the multiphase could be dependent on external factors, such as an excessive inhibitor injection. Apart from alkalinity, bicarbonate that results from the reduction of carbonic acid in lowpH, CO2-saturated environments in the vicinity of the surface gets involved in the anodic reactions with special mechanisms.[11] In fact, bicarbonate plays many and contradictory mechanistic roles that are difficult to determine in part because they are essentially associated to its concentration.[12,13] In a few concentrations, it is an aggressive species that prevents the filming process; complexing the surface to be mainly active onto which localized attacks and pits occur and increase in number and size to deteriorate the surface.[14,15] Thick and complex corrosion products (rust) form as a result in most cases, but in literature their effectiveness in suppressing corrosion in FAYSAL FAYEZ ELIYAN, Researcher, and AKRAM ALFANTAZI, Professor, are with the Corrosion Group, Department of Materials Engineering, The University of British Columbia, Vancouver, BC V6T 1Z4, Canada. Contact e-mails: faysalfayez@ yahoo.ie; [email protected] Manuscript submitted March 17, 2014. Article published online August 8, 2014. 2464—VOLUME 45B, DECEMBER 2014
relation to their properties and environmental conditions,[16–19] in the short and long runs, is not fully established—to consider the corrosion products reliable in protecting the underlying surfaces. In higher concentrations, bicarbonate initiates different kinetics, driven by carbon-carrying intermediate species, which is counteractive to dissolution and precipitates carbonate-based films.[20–22] And accordingly, it gets involved in the cathodic reactions to increase the rate of hydrogen generation, destabilizing the passive films (during the cathodic scan), or making the corrosion reactions more under the cathodic control, especially if other aselectroactive species, such as chloride, exist in the corrosive environment.[23–25] For a pipeline, bicarbonate is therefore involved in the anodic, cathodic, and passivation reactions in manners that depend on the environmental factors,[26] and also on the metallurgy of pipeline steel.[27] For the metallurgy, it is necessary to design the pipelines to
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