INTRODUCTION
In the 80's 9Cr-lMo alloys were extensively used as tubing or casing in the completion of gas wells with no H2S or in environments where the concentration of this corrosive compound was a few ppm. In many others applications 9Cr-lMo steels are used in downhole and wellhead equipment including packers, tubing hangers and safety valves. Due to the significantly lower cost relative to duplex and Corrosion Resistant Alloys (CRA's) there is considerable interest in extending the current field of application of 9Cr-lMo alloy to higher H2S contents.
This paper describes the corrosion behaviour of some 9Cr-lMo alloys for jewelry applications with particular emphasis to Sulfide Stress Cracking (SSC) resistance. In this context, engineering diagrams were drawn individuating the applicability limits in H2S containing environments. The influence of some metallurgical parameters such as austenitic gram size, yield strength and hardness on the SSC resistance was verified.
The susceptibility to localised corrosion, pitting and crevice, as well as the resistance to uniform corrosion of 9Cr-lMo alloys was also investigated in conditions simulating actual downhole environments. Long term exposure tests were performed in the range 100-150°C, in high salinity concentration solution (100 g/1 NaCI) and at elevated CO2 partial pressure (up to 1.4 MPa).
In the oil & gas industry the use of Corrosion Resistant Alloys (CRAs) has been common practice since the mid 70's, when oil companies started to exploit deeper and deeper natural gas reservoirs and CO2 injection technique was introduced to facilitate oil recovery (I-6). These events led to extremely aggressive environments for the use of carbon or low alloyed steels due to the presence of high CO2 pfll'tial pressures and H2S as a pollutant (7). In the 70', the choice of 9Cr-lMo became cost effective with respect to carbon steels plus inhibitor in a large number of situations (s). 9Cr-lMo steel was mainly utilized as tubing or casing in the completion of gas wells with no H2S or in environments where the concentration of this corrosive compound was a few ppm. Nowadays 9Cr-lMo steels are used in many other applications such as down hole and well head equipment, including packers, tubing hangers and safety valves.
Although field data on the down hole equipment are quite encouraging, the lack of information in some cases and contrasting laboratory data, has lead to an increasingly use of more alloyed materials with respect to 9Cr- 1Mo.
Concerning the corrosion behavior in the so called "sweet" environment, although tested in very aggressive conditions, 16 MPa CO2 and 80°C, 9Cr-lMo steel is immune to general corrosion. The corrosion rate is high at the initial step but after a few hours it is strongly reduced due to surface enrichment with chromium in the form of an amorphous oxide (9)
Ikeda et al. (10) studied the influence of high temperatures in 0.1 MPa CO2 environment: 9Cr-lMo and 13Cr withstand general corrosion at temperature lower than 150°C and 200°C, respectively. If the pressure of CO2 is as high as 3.0 MPa, the temperature threshold limit is 100°C for 9Cr-lMo and 150°C for 13Cr. Further Ikeda et al.(11) studied the effect of a little amount of H2S on the corrosion rate of steels with an increasing amount of chromium as a function of temperature at 3.0 MPa of CO2. 9CrlMo alloy shows an anomalous behaviour with respect to higher chromium containing alloys. The corrosion rate in the presence of 330 ppm of H2S tends to increase sharply at T>_ 95°C and then decreases with time up to values lower than those observed in the absence of H2S. Klein (12) reported that 9Cr-lMo and 13Cr can withstand SSC up t
