ABSTRACT

The use of corrosion resistant alloys (CRAs) incurs costs and only when these costs are justified will such materials be used in industrial service. If these materials can be coated by an industrial method onto a cheaper substrate from which the component is made then their industrial uptake can possibly be increased. To understand the behaviour of CRA coatings of UNS N10276, UNS N06625 and UNS S31603 on steel, several steel coupons were sprayed using high velocity oxy-fuel (HVOF). These were then tested in de-aerated 3.5 wt.% NaCl solution for 30 days, purged with 10 MPa and 50 MPa CO2. Tests under 10 MPa CO2 were conducted at two different temperatures, 40°C and 80°C. Test in 10 MPa CO2 was performed at 40°C. Microstructural characterization revealed that the coatings protected the steel substrate from CO2 corrosion when undamaged. The bare steel in the exposed region formed a siderite scale, while no such scale was seen in the case of undamaged CRA-coated steel. The substrate close to the coating showed accelerated corrosion due to galvanic effects.

It was concluded that thermally sprayed CRA coatings can provide a cost-effective corrosion mitigation method for infrastructure likely to be in contact with wet supercritical CO2 at 40°C and 80°C. The scales formed on the steel protected it from further corrosion in 10 MPa and 50 MPa CO2. However, it was evident that care must be taken to ensure that the thermally sprayed CRA layer does not have any through porosity or defect; else, such coatings may accelerate corrosion of the underlying steel substrate due to galvanic interactions.

INTRODUCTION

Oill and gas exploration and production operations are increasingly encountering high concentrations of CO2 in the so-called extreme fields. Operations handling CO2-containing fluids at elevated pressures and often, elevated temperatures require understanding of materials behavior in such environments. Although, testing and performance evaluation including field application of materials and welds have been carried out in low pressure, gaseous CO2 and gas mixtures containing CO2 for a number of years, information at higher pressures is limited. Performance data are required if materials are to be used with confidence in high pressure CO2 environments. In addition to high pressure CO2-containing wells, this will have a significant impact on the industrial uptake of technologies such as carbon dioxide capture and storage (CCS).1,2

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