The effect of H2S concentration on the corrosion behavior of carbon steel at 90 °C
Introduction
The evaluation of metallic materials corrosion in oil refinery environments is a very important issue, especially in distillation plants, as this phenomenon is responsible for costly economic and human losses. The most important corrosive agents in primary distillation plants are chlorides and H2S [1]. It is very useful to know the mechanism of action of different corrosive agents on metallic materials in oil refinery environments. In this regard, the electrochemical behaviors of both iron and steel in solutions with chlorides and H2S have been investigated through the years by many researchers [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14]. In the literature, there are studies of the effect of H2S concentration on steel [2], iron [3], [4], [5], [6], chromium [7] and nickel [8], the effect of Cl− on steel [9], the effect of Cl− and H2S on steel weld [10], [11], [12], [13], and the effect of buffered acetic acid solutions with chlorides and H2S on steel [1], [14]. The results demonstrated that H2S could accelerate both the anodic iron dissolution and the cathodic hydrogen evolution in most cases [3], [4], [5], [6], but some results showed that H2S could also inhibit the corrosion of iron under certain special conditions [4], [5]. Though much effort has been put into H2S or Cl− corrosion, little research have been carried out on the temperature of the condensation system in primary distillation plants, which usually is greater than 70 °C. Therefore, it is of great interest to study carbon steel corrosion process in H2S–HCl–H2O system by simulation of oil refinery conditions at a temperature greater than 70 °C.
In the present work, weight loss test, potentiodynamic polarization curves, electrochemical impedance spectroscopy (EIS), X-ray diffraction (XRD) and scanning electron microscopy (SEM) were applied to investigate the effect of H2S concentration on the corrosion of SAE-1020 carbon steel at 90 °C.
Section snippets
Experimental setup
Experiments were conducted at atmospheric pressure in a glass cell (Fig. 1). A typical three-electrode setup was used with a saturated calomel electrode (SCE) (GD-II, BRICEM, China) as the reference electrode, a large piece of platinum with a surface area of over 4 cm2 as the counter electrode, and a SAE-1020 carbon steel specimen as the working electrode. The temperature of electrolyte solutions was controlled with a bath at 90 ± 1 °C. Residual H2S was absorbed by gas absorbent.
Material and specimen preparation
The material
Microstructure
The result of the microstructure observation of SAE-1020 carbon steel is shown in Fig. 2. The phase of carbon steel mainly consisted of ferrite (F). A mass of carbide (Fe3C) deposited in the grain boundary especially in the tri-angle grain boundary was also observed.
Corrosion rates of carbon steel
With respect to the determination of corrosion rate, the most accurate and precise method is probably that of weight loss [15]. The average corrosion rates of carbon steel in the H2S-containing solutions obtained from weight loss
Conclusion
The corrosion rate of SAE-1020 carbon steel at 90 °C increased with the increase of H2S concentrations from 58.91 to 408.44 mg L−1. H2S showed strong acceleration effect on the cathodic hydrogen evolution of carbon steel, causing carbon steel to be seriously corroded. The corrosion products formed on carbon steel surfaces in the solutions containing H2S at 90 °C were composed of mackinawite, which was loose and did not show any protective property in the solution containing H2S at 90 °C. Severe
Acknowledgement
The authors acknowledge support of The Academe of Lanzhou Petrochemic Company, CNPC.
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