Modified ALLOY 28 (UNS N08028) with improved corrosion properties in sour environments
Authors: Persson Katarina Berit , Kaukas Shamus 
Published: May 2016

Alloy 29 for use in oil country tubular goods (OCTG) has been evaluated in sour environments. Stress corrosion cracking (SCC) tests were conducted in various environments containing H2S, CO2, chloride ions and elemental sulfur at temperatures between 140 and 160 °C. The results suggest that Alloy 29 is suitable for use in sour service, especially when high chloride concentrations or elemental sulfur are present.

The results show that Alloy 29 is significantly more resistant to SCC in H2S environments than the limits for type 4c materials according to NACE standard MR0175.

Introduction

The increasing global demand for oil and gas has made it profitable for the petroleum industry to extract oil and gas from more challenging fields. These fields may have deeper wells and harsher environments. The fields usually require material with high strength since the pressures and temperatures can increase with the depth of the well. In addition, the environments in these types of oil and gas fields usually contain substantial amounts of hydrogen sulfide (H2S), carbon dioxide (CO2) and chlorides (Cl-), which make it extra challenging for the material with respect to corrosion. Material selection is therefore a very important part to ensure that the oil and gas productivity is maintained at a high level. A material that contains high amounts of nickel (Ni), chromium (Cr) and molybdenum (Mo) is often considered as a good candidate for sour environments with respect to corrosion resistance [1,2]. However, there is always a balance between how highly alloyed material is needed in service and the cost of the material. The main alloying elements are expensive i.e. Ni, Mo and Cr, so substantial cost savings can be achieved by choosing a material that has lower content of these elements. Since the prices of the alloying elements can fluctuate both greatly and quickly then price differences between various materials can change rapidly and result in significant price differences. However, it is important to assure that the material will perform successfully in the specific well conditions.

Alloy 28 (UNS N08028) has been used with satisfaction in oil country tubular goods (OCTG) applications since the 80's in moderately sour wells. Since more severe wells, especially with high salinity, are extracted today a demand for slightly better materials has been raised. Alloy 28 has therefore been further developed in order to enhance the corrosion resistance in sour environments, especially with high salinity. This has been achieved with small variations of the chemical composition by increasing mainly the molybdenum and nickel concentrations. Even though the chemical composition is slightly changed the high strength of the material is maintained. The modified Alloy 28, Alloy 29, is promising for sour wells, but at the same time has a reasonable price in comparison to other more highly alloyed grades.

The presence of chlorides and hydrogen sulfide at high temperatures will expose corrosion-resistance alloys (CRAs) to risk of stress corrosion cracking (SCC) [1]. The results of Alloy 29 in these simulated sour environments are promising and are presented in this study.

Similar tests as described above were also carried out with elemental sulfur added to the brine. Elemental sulfur can be present in the wells with high partial pressures of H2S. Elemental sulfur is considered to increase the risk of corrosion and it is therefore important to evaluate [3]. The influence of elemental sulfur is therefore also discussed in this study.