Abstract
The electrochemical behavior of mild steel in bicarbonate solution at different dissolved oxygen (DO) concentrations and immersion times has been studied under dynamic conditions using electrochemical techniques. The results show that both DO and immersion times influence the morphology of the corrosion products. In comparative tests, the corrosion rate was systematically found to be lower in solutions with lower DO, lower HCO3− concentrations and longer immersion time. The SEM analyses reveal that the iron dissolution rate was more severe in solutions containing higher DO. The decrease in corrosion rate can be attributed to the formation of a passive layer containing mainly -FeO (OH) and (-Fe2O3/Fe3O4) as confirmed by the X-ray diffractometry (XRD) and X-ray photoelectron spectroscopy (XPS). Passivation of mild steel is evident in electrochemical test at ≈ −600 mVSCE at pH 8 in dearated (0.8 ppm DO) chloride bicarbonate solution under dynamic conditions.
Highlights
Bicarbonate plays a major role in the dissolution reactions of steel pipelines [1]
The anodic polarization current was greater with 5 g/L bicarbonate solution at corrosion potential ≈−600 mVSCE
The result is consistent with that the diameter of the semi-circle increases with immersion time, which implies that the steel surface more corrosion product being deposited on the the steel surface, and forming a relatively inert surface resistance improves with immersion time
Summary
Bicarbonate plays a major role in the dissolution reactions of steel pipelines [1]. Studies indicate that increasing HCO3 − concentration exacerbates iron dissolution [2,3,4]. The formation and stability of such passive films depends to a large extent on environmental conditions, such as; applied potential, pH, oxygen, solution chemistry, temperature, fluid flow rate, immersion time and metallurgy of the steel [14,15,16]. In the absence of lepidocrosite in the scale, the corrosion reaction still proceeds in deaerated water as reported elsewhere [26], indicating that in a near-neutral pH environment, the Kuch mechanism is not the only mechanism of metal loss in deaerated conditions. The influence of dissolved oxygen and immersion time in the corrosion process of mild steel in a near-neutral pH in corrosive media remains controversial. We present an investigation of the corrosion behaviour of mild steel in chloride-containing bicarbonate solutions of varied DO and immersion time in near-neutral pH, under dynamic conditions at moderate temperature (23 ± 1 ◦ C). By incorporating a range of surface analysis techniques, including X-ray diffraction (XRD) and scanning electron microscopy (SEM), we were able to characterize the chemistry and morphology of the corrosion products
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