Study Explores Effects of H2S Scavenger Byproducts on Carbon Steel

Abstract

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper NACE 2019-13401, “Effects of Triazine-Based H2S Scavenger Byproducts on the Film Composition and Cracking of Carbon Steel in Oilfield Applications,” by Leonardo Caseres, James Dante, and Florent Bocher, Southwest Research Institute, et al., prepared for the 2019 NACE International Corrosion Conference and Exposition, Nashville, Tennessee, USA, 24-28 March. This paper has not been peer reviewed. Triazines are commonly used as hydrogen sulfide (H2S) scavengers. However, stress corrosion cracking (SCC) has been reported recently when using certain triazines, making it necessary to understand the failure mechanism to control this phenomenon. In the complete paper, the authors seek to understand the effect of triazine on the electrochemical response of a carbon-steel surface in a mixed-gas system. Introduction One of the more frequently used H2S scavenger triazines is hexahydro-1,3,5-tris(hydroxyethyl)-s-triazine, or monoethanol amine (MEA)-triazine. It is a yellow, viscous liquid that can be injected into gas streams or used in a contactor tower. It removes H2S from the gas stream. It is relatively inexpensive, easy to use, and effective. The MEA byproducts that form during the scavenging process, however, may cause SCC of steel pipes, as reflected in the literature. On the basis of these findings, it is suggested that SCC is governed by corrosion processes and specifically by the transition from a passive to an active surface. This transition is a complicated function of acid gas concentration [carbon dioxide (CO2) and H2S] and amine adsorption, which are likely different for different systems. Several possible mechanisms exist by which cracking can be induced. First, a reduction in H2S content can reduce the semiprotective iron-sulfide film, thus changing the nature of the active/passive transition and making cracking more likely. Second, specific adsorption of amines onto the surface of the steel will result in a change in the active/passive transition. Therefore, characterization of the MEA/H2S reaction process, the adsorption of amines onto steel, and the electrochemical behavior of the steel under these circumstances are important factors that require study. In this paper, the electrochemistry of API X601 steel is assessed under a variety of test solutions containing H2S, CO2, and MEA triazine. Environmental Conditions The specimens were 0.25 in. in diameter and 4 in. long. Approximately 3 in. of the specimen length was exposed to the solution. All electrochemical tests were conducted at room temperature and atmospheric pressure. Three gas mixtures were employed: 20-ppm H2S balanced with dinitrogen (N2), 20-ppm H2S/2 mol% CO2, and 100% N2. In addition, three triazine-inhibitor (35% triazine concentration solution) volumetric additions were used. The actual inhibitor concentrations in solution were not determined; instead, the H2S concentration in the gas phase was measured with a sensor after the addition of the inhibitor. The supporting electrolyte [0.1-M potassium chloride (KCl)] solution was deaerated N2 gas.