INTRODUCTION

ABSTRACT

The corrosion behaviour of alloy 31 (UNS N08031 - 31 Ni - 27Cr - 6.5Mo - 1.2Cu - 0.2 N - bal. Fe) and other corrosion resistant materials (CRAs) was tested in a wide variety of corrosion tests comprising laboratory and field tests in seawater with and without additions of CO2 and/or H2S, slow strain rate tests, and SSC (sulphide stress corrosion)-tests according to NACE MR0175.

Because of its high chromium and molybdenum concentration the critical pitting temperature and crevice corrosion temperature of alloy 31 are higher than that of UNS N08028 (alloy 28) and UNS N08825 (alloy 825). CCT and CPT temperatures are in the same range as those of nickel base alloys like UNS N06625 (alloy 625). It is not sensitive to chloride-induced stress corrosion cracking, sulphide- induced stress corrosion cracking or sulphide stress cracking even at high strength levels. UNS N08031 was tested in the cold-worked condition (HRC<35) for sour gas applications according to NACE TM 0177-96 A and has been approved in NACE MR0175 up to LEVEL VI.

The combination of properties makes UNS N08031 a suitable alloy for sour service especially under conditions, where high strengths, resistance to seawater and resistance to H2S is required. Typical examples would be casings, wirelines, downhole tubing and topside facilities.

Depletion of shallow water reservoirs in combination with predicted growth of hydrocarbon demand suggest an global increase in deepwater activity, resulting in more corrosive conditions and - as a consequence - in a higher demand for Corrosion Resistant Materials (CRAs) in oil & gas production 1.

Depending on the process conditions, corrosion by CO2 and H2S, chloride-induced localised pitting or crevice corrosion, stress corrosion cracking and/or H2S-induced sulphide stress cracking may be encountered by metallic materials used for wires, casings, tubing and piping in oil and gas field service.

The chemical composition of some Corrosion Resistant Alloys (CRAs) typically used in oil & gas production are listed in Table 1. The corresponding mechanical properties of these alloys are listed in Table 2. Alloy 316L, like the 13Cr steel, shows reasonable resistance to chloride-induced corrosion and COz-corrosion but it is not suitable for application in severe sour gas (H2S) conditions 2. The same applies to the Duplex (2205) and Superduplex (2507) stainless steels, which show limited resistance to sulphide stress cracking (SSC) in the presence of H2S 2. Alloy 825 (UNS N08825) is widely used in oil & gas production systems for a variety of applications because of its excellent resistance to CO2- and HaS- corrosion and its high resistance to stress corrosion cracking. However, it has only limited resistance to chloride-induced crevice and pitting corrosion. Hence if both chlorides and H2S are encountered in oil & gas production, oRen nickel base alloy like alloy 625 must be used.

Alloy 31 (UNS N08031) is a high alloyed stainless steel containing 31% Ni, 27 % Cr and 6.5 % Mo, which has found wide application in chemical and petrochemical process industry due to its high resistance in a variety of corrosive environments 3'4. However, its high resistance to localised corrosion in both acid solutions of chlorides and in seawater suggested that it may be a candidate material for service in the oil and gas industry.

To check the potential of UNS N08031 for oil and gas applications, a wide variety of corrosion tests comprising tests in natural seawater and in various chloride-containing solutions, slow strain rate tests, and SSC-tests according to NACE MR0175 both in the cold-worked and in the solution-annealed condition have been carried out.

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