The Corrosion Process of an API 5L X80 Welded Joint in a System with Different pH and H2S Concentration

M B L Carvalho,A B Forero,J A C Ponciano,I S Bott

doi:10.1590/1980-5373-mr-2021-0443

Abstract

The corrosion evaluation of steels in the oil and gas industry environments is a crucial issue because corrosion can cause economic and human losses. It is well known that H2S can be originated from different processes in the oil and gas industry, accelerating the corrosion process. The objective of this work was to evaluate the H2S corrosion resistance of an API 5L X80 steel and its welded joint obtained by submerged arc welding process (SAW). All tests were performed in an aerated 5% wt NaCl and thiosulphate aqueous solution. The H2S concentration, pH, weight loss, electrochemical tests, and microstructure were considered. The results obtained showed an increase of the corrosion rate values, with decreasing pH and increasing concentration of H2S generated by the thiosulphate. For the lowest H2S concentrations, the corrosion process was inhibited, due to the formation of a partially protective film on the samples’ surfaces. The heat affected zone (HAZ) showed severe localized corrosion attack which was attributed to the microstructural characteristics of this region.

Highlights

The deterioration of a buried metal that has been submerged or exposed to industrial environments represents a real problem for the economy and the environment
The stable form of H2S depends upon the solution acidity, H2S is stable as a dissolved species in aqueous solutions only at low pH levels; whereas at higher pH levels the predominant form is HS, and for alkaline pH values the stable form is S2
The corrosion rate valuesincrease with increasing concentration of H2S generated by the thiosulphate

Summary

Introduction

The deterioration of a buried metal that has been submerged or exposed to industrial environments represents a real problem for the economy and the environment. It is well known that H2S originating from different processes in the oil and gas industry can accelerate anodic and cathodic reactions of the corrosion process[1,2]. The welding procedures adopted during the production of the tubes and for construction of the pipelines can modify its microstructure and, the mechanical and corrosion properties of the base metal in the heat affected zone (HAZ). These changes can increase the corrosion attack in these specific zones[11,12,13,14,15,16,17]. Corrosion rates of low alloy steels in H2S containing environment tend to increase with increasing H2S concentration and decreasing pH of the aqueous solution[18]

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Conclusion