Evaluating the Passivation of Corrosion of API-X100 Steel with Cyclic Voltammetry

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EXTERNAL corrosion of buried oil pipelines is a common problem.[1] Electrochemically, active species including bicarbonate and carbonate in oxygen-free, hydrated media attack the surface and cause it to deteriorate.[2] They get involved in the complex processes of corrosion, driving together dissolution, the cathodic reduction and the rust formation.[3–5] Experimentally, there was a need to achieve better understanding of the effects of bicarbonate and carbonate when they coexist at low concentrations—in a conveniently comprehensive way. Cyclic voltammetry allows investigating corrosion with parameters that can discern separately the significances of the multiple processes of corrosion.[6–8] It involves varying the potentials at ranges with different scan rates in ‘forward’ and ‘backward’ directions, measuring the electrochemical currents of the oxidation and reduction reactions. In this study, the roles of bicarbonate and carbonate with anodic dissolution and passivation were examined with cyclic voltammetry.

In literature, the electrochemical relationship between the anodic dissolution of a polarized surface and the passive films in low bicarbonate/carbonate solutions was not well understood. Some studies correlated the passive currents to the activity of the surface, and others to the activity of the passive films.[8,9] The effects of transpassivation on the passive films are difficult to analyze physically because of the lack of suitable in situ setups. This makes it difficult to interpret the effects of transpassivation on the passive films with traditional electrochemical methods. This was the case with the effects of the cathodic reduction during voltammetry, as at low potentials the anodic dissolution takes place together with the cathodic reduction of water, during when the measured currents were negative. The original voltammetric behavior was studied in this paper, and specific modifications to the voltammetric scans were considered to study the significance of the voltammetric cycling, transpassivation, and cathodic reduction on the electrochemical passive response.

II.

EXPERIMENTAL DETAILS

A. Corrosion Test Setup FAYSAL FAYEZ ELIYAN is with the Mechanical Engineering, American University of the Middle East, P.O. Box 220, Dasman 15453, Kuwait. Contact e-mail: [email protected] AKRAM ALFANTAZI is with the Department of Chemical Engineering, The Petroleum Institute, Abu Dhabi, United Arab Emirates and also with the The University of British Columbia, Vancouver, BC, Canada. Manuscript submitted August 2, 2016.

METALLURGICAL AND MATERIALS TRANSACTIONS B

The tests were carried out in a standard 600 mL, three-electrode, multi-port glass jacketed cell. The working electrodes, made of API-X100 steel, were fitted into a TefzelÒ sample holder that maintained a standard exposure to the test solutions. The potentials were measured against the saturated calomel electrode, of +0.240 V vs SHE. The counter electrode was a graphite rod. A Versastat 4 potentiostat was used to run and

monitor the experiments, synchroniz