ABSTRACT

Corrosion behavior of 3Cr steel in CO2-O2 aqueous environment containing silty sand with different sizes was investigated by immersion tests and electrochemical measurements. The results showed that corrosion rate and corrosion morphology of 3Cr steel were affected by the size of silty sand. Silty sand could form an adsorbed layer on steel surface. The adsorbed layer in 5000 mesh condition had more pores for O2 transportation and O2 could promote cathodic reaction. Cr(OH)3 deposited on steel surface and 5000 mesh silty sand could not separate the Cr-rich corrosion product. Therefore, a Cr-rich corrosion film formed on 3Cr steel surface, resulting in uniform corrosion and low corrosion rate. However, in 1000 mesh condition, the sand adsorbed layer with less pores blocked O2 transportation to steel surface. The cathodic current decreased, but the integrity of the Cr-rich corrosion film was destroyed by 1000 mesh silty sand. Consequently, the tendency of local matrix dissolution increased, therefore 3Cr steel exhibited localized corrosion and high corrosion rate in 1000 mesh condition.

INTRODUCTION

CO2 corrosion has been a serious problem in oil and gas exploration, production and transportation due to the high general corrosion rate and severe localized corrosion 1, 2. One of the most common anticorrosion measures is through alloying to improve CO2 corrosion resistance of steel. In recent years, 3 wt.% Cr steel has been widely used in oil and gas industries because of its effective corrosion resistance and price advantage 3. Due to the dissolution of chromium, amorphous Cr(OH)3 deposits rapidly on steel matrix, forming Cr-rich layer together with corrosion product FeCO34-7.The Cr-rich layer has been reported to be compact and increases the protective effect of corrosion film, inhibiting localized corrosion and improving CO2 corrosion resistance of 3Cr steel.

Silty sand, the size of which is below 50 µm, could pass through the screen and enter the practical production system together with CO2-conatining produced water when most of sand with large size has been screened 8. Previous research indicated that CO2 corrosion rate and morphology of X70 pipeline steel and 3Cr steel were affected by the size of silty sand 9-11. Silty sand could absorb on steel surface and mix with corrosion product to form a mixture film. Corrosion film of X70 steel in sand-containing condition was more compact than that in sand-free condition, exhibiting low corrosion rate 9, 10. For 3Cr steel, the integrity of the Cr-rich corrosion film was affected by the size of silty sand 11. O2 is inevitably introduced into the oil and gas production through chemical treatments, the reinjection of CO2, inhibitor and produced water. When 3Cr steel was used in CO2 aqueous environment containing oxygen, oxygen could promote the formation of Cr(OH)3 and Fe(OH)3. But the distribution of corrosion product and the protective properties were affected by the oxygen content 12-15. In the above mentioned oxygen- containing CO2 aqueous environment, the corrosion effect of silty sand on 3Cr steel would be more complicated.

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