ABSTRACT

The objective of the present study was to evaluate the effect of alloying elements (Cr, Mo and Cu) on the corrosion behavior of low carbon steel in CO2 environments. Six samples were prepared with varying Cr content from 0 to 2 wt.% and with added 0.5 wt.% of Mo and Cu; the specimens had ferritic/pearlitic microstructures. Steel samples were exposed to a CO2-saturated 1 wt.% NaCl solution with different combinations of pH and temperature (pH 4.0 at 25°C, pH 6.6 at 80°C, and pH 5.9 at 70°C). Changes in corrosion rate with time were determined by linear polarization resistance (LPR) measurements. The surface morphology and the composition of the corrosion product layers were analyzed by surface analysis techniques (SEM and EDS). Results showed that the addition of Cr and Cu showed a slight positive effect on the corrosion resistance at pH 4.0 and 25°C. At pH 6.6 and 80°C, regardless of the alloying elements, the trend of corrosion rate with time was similar, i.e., the corrosion rate of all specimens decreased with time due to the formation of protective FeCO3. A beneficial effect of Cr addition was clearly seen at pH 5.9 and 70°C, where steel sample without Cr showed no decrease in corrosion rate with time. The addition of Cr promotes the formation of protective FeCO3 and it decreases the corrosion rate.

INTRODUCTION

Internal environments encountered in the oil and gas transportation pipelines can cause severe corrosion of mild steel mainly due to the presence of CO2, H2S, organic acids, and water. In CO2 environments, low carbon steel has often been used in combination with corrosion inhibitor since it is considered the most cost effective option, compared with utilizing expensive Corrosion Resistance Alloys (CRA).1 The corrosion protection of low carbon steel depends on either the spontaneous formation of protective corrosion product layers which is influenced by environmental parameters (CO2 content, solution chemistry, pH, temperature, fluid velocity, etc.) and material parameters (microstructure and chemical composition)2 or the addition of corrosion inhibitors. To date, many studies3-8 have been conducted to understand the effect of both the environmental and metallurgical conditions on corrosion behavior of steels in CO2 environments.

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