Electrochemical study of carbon steel corrosion in buffered acetic acid solutions with chlorides and H₂S

Abstract

In this work, the corrosion behavior of SAE 1018 carbon steel in buffered acetic acid (HAc) solutions containing chlorides, with and without H₂S, was studied. Polarization curves obtained by different electrochemical techniques, indicate negligible modification of anodic slopes when adding H₂S; however, the cathodic branch is more sensitive showing an accelerated reduction reaction in the presence of H₂S. Interface characterization was performed by electrochemical impedance technique (EIS) in the absence and presence of H₂S and near to the corrosion potential (E_corr). Analysis of results shows no film of corrosion products, since the impedance spectra characteristics indicate a great activity of steel in the solutions studied, with differences only at low frequencies. The adsorbed complexes formed in the solution containing HAc, acetate and chlorides control the corrosion process and prevent passive film formation, even in the presence of H₂S.

Introduction

Evaluation of metallic materials' resistance to corrosion in oil refinery environments is a very important issue, especially in recent times since process optimization and durability of plant equipment are emphasized. One of the most frequently used methods for evaluating metals and alloys subject to corrosion by stress and/or blistering is NACE TM 0177 [1]. Exposure to a highly corrosive evaluation environment proves to be appropriate for quick reliable results. The most important corrosive agents in primary distillation plants are acetic acid (HAc), chlorides, H₂S (NACE solution TM 0177). However, when a corrosive is employed to evaluate the behavior of corrosion inhibitors, it is useful to know the mechanism of action of different agents. In this regard, in the literature there are studies of the effect of HAc on iron [2], [3], [4], [5], [6] and steel [7], [8], [9], [10], the effect of Cl⁻ on iron [11], [12], [13] and steel [14], [15]; the effect of H₂S on iron [16], [17]; and the combined effect of HAc and sulfides [18] and that of Cl⁻ and H₂S on the same [19]. However, little has been written about the corrosion mechanism of carbon steel in the presence of HAc, chlorides and H₂S.

In this work, the corrosion process of SAE 1018 carbon steel in a NACE TM 0177 solution (CH₃COOH, CH₃COONa, NaCl, pH 3.5, with and without the saturation of H₂S) is studied by electrochemical techniques. This study will provide a corrosion mechanism that may be used later in the evaluation of corrosion inhibitors in such environments. A buffered solution is used to prevent, as much as possible, the pH variation and the consequential change of experimental conditions.