Erosion-corrosion mechanism and comparison of erosion-corrosion performance of API steels Md. Aminul Islam a,b , Zoheir Farhat b a National Research Council Canada, Energy, Mining and Environment, Vancouver, BC, Canada V6T 1W5 b Department of Process Engineering and Applied Science, Materials Engineering Program, Dalhousie University, Halifax, NS, Canada B3J 2X4 article info Article history: Received 2 September 2016 Received in revised form 2 December 2016 Accepted 5 December 2016 Key words: Erosion-corrosion API steels Synergy Sweet corrosion Degradation mechanism abstract During the transportation of oil and gas, pipes are exposed to flowing corrosive environment which causes erosion-corrosion. The high degradation rates attributed to this mechanism can create increased challenges to project economy and operation where material integrity, accurate erosion-corrosion rate prediction and long term performance are key concerns. Although the problem caused by the interaction of erosion and corrosion is severe, the mechanism of synergy is still not thoroughly understood because of its complexity. This research focuses on understanding the degradation processes of pipeline steels (API X42, API X70 and API X100) in CO 2 containing salt water. The application of cyclic erosion-corrosion allowed the individual contribution of erosion and corrosion components of mass loss to be quantified and mechanisms by which erosion affects corrosion and vice-versa to be identified. The present research also compares the erosion-corrosion performance of different API steels. & 2017 Elsevier B.V. All rights reserved. 1. Introduction Erosion–corrosion is common in oil and gas processing plants and pipelines where there is interaction between solid particles, corrosive fluid and target material [1–6]. During erosion-corrosion, the observed mass loss is higher than the summation of mass loss due to pure erosion and pure corrosion acting separately. The in- teraction between these two processes has been referred to by different researchers as a ‘synergistic’ effect [5–14]. Corrosion of carbon steel in CO 2 environment has been a con- tinuing problem in oil and gas industries and several electro- chemical investigations [15–20] have been reported to study the corrosion phenomena involved in these systems. The presence of CO 2 leads to the formation of weak carbonic acid (H 2 CO 3 ), which drives the carbonate/bicarbonate (CO 3 /HCO 3 ) reactions. The sub- sequent corrosion process is controlled by three cathodic reactions and one anodic reaction [21]. Cathodic reactions include the re- duction of carbonic acid into bicarbonate ions, reduction of bi- carbonate ions into carbonate ions and the reduction of hydrogen ions to hydrogen gas. In carbonate/bicarbonate media, anodic reaction involves oxidation of iron to ferrous (Fe 2 þ ) ions. These reactions provide a chemical environment that promotes the for- mation of iron carbonate (FeCO 3 ), where ferrous ions react directly with carbonate ions. It can also form by a two-step process when ferrous ions react with bicarbonate ions, iron bicarbonate forms, which subsequently dissociate into iron carbonate along with carbon dioxide and water [22]. The significance of FeCO 3 formation is that it drops out of solution as a precipitate due to its limited solubility and acts as a protective barrier to prevent the corrosion of the steel surface [22]. Once the scale has grown to a certain thickness, it becomes considerably brittle and is easily removed by the mechanical forces of the flow [16]. Thus, the newly exposed areas become highly susceptible to the corrosion. Erosion-corrosion synergism normally takes place in pipe bends (elbows), tube constrictions, and other structures that alter flow direction or velocity. During erosion-corrosion, corrosion products are first deposited on the internal pipeline surface in the form of scale. Several investigations [23–26] have been reported to study the erosion-corrosion phenomena involved. It is believed that erosion affects corrosion by removal of surface deposits, increase in local turbulence and surface roughness. The action of mechanical wear results in damage of the passive film, leading to exposure of fresh, bare surfaces to the corrosive medium. This accelerates the corrosion process which, in turn, leads to the development of a new passive film which will eventually be damaged by further mechanical action. In addition, high speed abrasive particles deform the metal sig- nificantly, and forms work hardened layer. The work hardened layer has high chemical activities and can form primary micro-cells be- cause of strain differences with adjacent low-strain domains, thus accelerating the metal dissolution process [27–29]. It has also been reported in previous investigation [30] that corrosion removes the work hardened layer and exposes fresh unhardened surfaces for further erosion. Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/wear Wear http://dx.doi.org/10.1016/j.wear.2016.12.058 0043-1648/& 2017 Elsevier B.V. All rights reserved. E-mail address: Md.Aminul.Islam@nrc-cnrc.gc.ca (Md.A. Islam). Wear 376-377 (2017) 533–541