ABSTRACT

The rapid process at the initial stage of sour corrosion on carbon steels and the relatively low uniform corrosion rate show the change of corrosion mechanism and the effect of corrosion products over time. In this paper, the scaling mode at the early stage of carbon steel exposed to H2S environments with short exposure time were investigated. Corrosion exposure tests and linear polarization resistance (LPR) techniques were performed. The corrosion products of pipeline steel formed under sour corrosion conditions were analyzed by scanning electron microscopy (SEM), and X-ray diffraction (XRD). Based on corrosion rate and the parameters of water chemistry, solubility limits of iron sulfide (FeS) were calculated. A preferred orientation of mackinawite appeared after 30 min of immersion. After exposure for one hour, the solution supersaturated and the corrosion product began to precipitate on the steel surface. Assuming the corrosion film covered the matrix layer by layer, the thickness of FeS were calculated based on water chemistry input. There was a significant difference between the calculated result and the film thickness of the cross-section. The corrosion product layers were loose and covered the matrix incompletely at the early stage of H2S corrosion. The rapidly scaling mode derived by the experiment and theoretical works is expected to improve the understanding of the mechanism of H2S corrosion at early stage and the possibility of the nucleation of the pitting under sour conditions.

INTRODUCTION

Sour corrosion has become one of the most important topics during the recent decade due to demands of oil and gas industry1-3. The rapid process at the initial stage of sour corrosion on carbon steels and the relatively low uniform corrosion rate show the change of corrosion mechanism and the effect of corrosion products over time. Numerous studies have investigated the initial stage of corrosion and the sensitivity of localized corrosion under sour conditions. Shoesmith4 reported that there are two types of scaling mode of mackinawite at the initial stage, one is solid-state reaction and the other is precipitation processes. Smith5, Sun6 and Cancio7 have also provided data that support the direct reaction between the H2S and the steel surface. Sun proposed a plausible mechanism for H2S corrosion of mild steel and suggested that a very thin (<1 µm) but dense and protective non-stoichiometric iron sulfide film forms rapidly. Smith8 found that the dimensions for a tetragonal mackinawite crystal is very close to the dimensions of ferrite which means a bare ferrite surface is almost a perfect template for the nucleation of mackinawite. Bai9 observed that H2S corrosion begins at the grain boundary as the preferential site of dissolution. However, the understanding of the initial stage of sour corrosion and the key factors inducing pitting are still poorly understood because of the high toxicity and the rapid dissolving properties of H2S. This paper investigated the scale formation of iron sulfide at the early stage of sour corrosion. The property and structure of corrosion scales and the water chemistry of precipitated mackinawite are discussed in details.

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