The effect of an inert solid deposit on uniform CO2 corrosion of mild steel is modeled based on a mechanistic electrochemical CO2 corrosion model. Laboratory testing has shown that the dominant factors introduced by the inert solids deposit are related to surface coverage, where both anodic and cathodic reaction rates are decreased because of less active surface area being exposed. The inert solid deposits also create a mass transfer barrier for corrosive species which limits the rate of the cathodic reactions. An existing mechanistic electrochemical model was modified to account for these effects and was capable of capturing the main features of uniform CO2 corrosion of mild steel under inert solid deposits.
Severe crevice and pitting corrosion problems can be found under solid deposits in oil and gas pipelines[1]. Localized corrosion can occur under these deposits as they can provide an environment which is chemically and physically different than the areas which are uncovered. Such heterogeneities may lead to formation of galvanic corrosion cells, affect inhibitor performance or harbor bacterial growth leading to MIC,[2],[3]. Underdeposit corrosion is more prevalent at the bottom of horizontal lines and where flow rates are lowest. However, there are very few studies to be found in the open literature related to the mechanisms of underdeposit CO2 corrosion[4]. Most of the available literature refers to the effect of deposits on corrosion inhibitor performance.[5] -[8] Since deposits have been reported as an important factor which may lead to severe CO2 corrosion, it's very important to understand first the mechanisms of uniform corrosion under solid deposits, before focusing on the effect they have on corrosion inhibitor performance. Real-life scenarios for under deposit CO2 corrosion found in oil/gas pipelines are very complex. An insitu deposit is likely to be neither pure nor inert. Rather it has complex composition and even some reactivity. Typical deposits consist of combinations of inorganic solids such as sand, scale and corrosion products, and organic matter such as wax and inhibitor residues. In addition, oxygen (O2), acetic acid (CH3CO2H), hydrogen sulfide (H2S) and bacteria were found in some deposits.
Experiments were conducted at atmospheric pressure in a three-electrode glass cell, Figure 1. The cell was filled with 2 liters of 1 wt% NaCl solution. CO2 was continuously bubbled through the cell. API# 5LX65 mild steel was used as the working electrode (WE) for electrochemical measurements. Platinum wire was used as a counter electrode (CE) and a KCl saturated silver-silver chloride (Ag/AgCl) reference electrode (RE) was connected to the cell externally via a Luggin capillary. A glass pH electrode was immersed in the electrolyte to monitor the pH during the experiment. Hydrochloric acid (HCl) or sodium bicarbonate (NaHCO3) was added to adjust the pH at the beginning of the test to desired value, which didn't change much throughout the duration of the test. The temperature was maintained within ± 1°C using a hot plate and a thermocouple with feedback control.
