ABSTRACT Supermartensitic stainless steels, recently made available as oil country tubular goods (OCTG), have been developed as a valuable cost effective alternative to duplex stainless steel for high C02, medium/high chlorides, and very low H2S environments.
Experimental tests were carried out to determine the localised corrosion and the sulphide stress cracking (SSC) resistance of supermartensitic steel UNS S41425 to be used as OCTG in slightly sour oil and gas wells and to compare its performance with standard L80 API grade UNS S42000 137. Cr steel, considered as a reference.
Supermsrtensitic steels with yield range as per L80, C95, P1 10, AP15CT, were developed and corrosion and mechanical properties were determined. The influence of different chloride contents (NaCl 10 /50 g/l), hydrogen sulphide partial pressure (0.1/100 kpa) and pH (2.7/4.5) has been investigated in order to simulate production service conditions.
Modified NACE constant load test and slow strain rate (SSR) test were performed. SSR gave the most severe evaluation for the SSC resistance. In the SSR test supermartensitic steel is deeply influenced by the chloride concentration. Supermartensitic steel corrosion resistance is by far superior to that of 13% Cr.
INTRODUCTION Martensitic 13%Cr steel is largely used in sweet oil and gas field due to its high CO2 corrosion resistance and its lower cost than duplex stainless steel , More than 2500 km of 13% Cr oil country tubular goods (OCTG)have been installed up to 1992. General corrosion, pitting, sulphide stress cracking (SSC) and anodic stress corrosion cracking are of particular concern when selecting corrosion resistant alloys (CRA) and specially martensitic steels .
In particular 13% Cr is potentially susceptible to SSC in sour environment with H2S partial pressure higher than 0.3 kPa. Recently there has been an increasing demand of (OCTG) suitable for moderately sour oil and gas field, often containing significant levels of chlorides. Superrnartensitic stainless steels have been developed in order to comply with these requirements at a competitive cost.
The SSC resistance is essential in H2S containing environments since the occurrence of this type of cracking can result in catastrophic failure with serious safety hazards especially in off shore platform operations. SSC studies based inconstant load tests have been under-taken 11-14] in order to define the application limits of the newly developed steels. Among the environmental parameters of importance for corrosion resistance (H2S partial pressure, chlorides content, temperature), increasing attention has been paid to the solution pH value . A new UNS S41425 supennartensitic steel was developed and has demonstrated promising mechanical properties and corrosion resistance.
This paper is aimed at determining the effect of environmental parameters (pH2S, chlorides concentration, solution pH) and strength of UNS S41425 superrnartensitic steel on the SSC resistance and to compare the performance of this innovative steel with the traditional 130/0Cr martensitic steel.
EXPERIMENTAL
Materials. Commercial pipes with nominal outside diameter OD 177.8 mm and wall thickness WT 10.51 mm of UNS S41425 supemtiensitic steel, UNS S42000 martensitic were used.