ABSTRACT

ABSTRACT

The commonly used Corrosion Resistant Alloy UNS N08028 has been further developed in order to enhance corrosion resistance in sour environments with high salinity. Testing in oxidizing laboratory environment has shown that by increasing the PRE value from 39 to 42, the critical pitting temperature in chloride bearing solutions is increased. Furthermore, the modifications made to the alloy also increase the resistance to sulphide induced stress corrosion cracking and thereby enlargening the application window for the material. The modified UNS N08028 has been installed and is successfully under operation in very harsh environments within the Caspian Sea region. The upgraded properties of modified UNS N08028 make it a candidate for sour environments with high salinity. This study summarizes testing on mechanical, micro structural and corrosion properties.

INTRODUCTION

In the past mainly carbon steel has been used within oil and gas applications. Stainless steels and nickel base alloys have been used to a lower extent, and mainly for specific parts, for example valves or instrumentation. Today the need for high strength Corrosion Resistant Alloys (CRA) is increasing, since oil and gas are being extracted from deeper and sourer wells. Deeper wells mean higher temperatures, higher pressures and very often also need for greater corrosion resistance, especially with regard to sour service. For offshore applications weight reduction is also a major consideration. Economic considerations will often justify the use of a CRA instead of carbon steel.1

UNS N08028 has been used within various oil and gas applications since 1981, and there is large experience from using UNS N08028 in applications such as production tubing, liners, casing, wire lines and heat exchanger tubing.

CRA's can be divided into different types. According to NACE MR0175/ISO 15156-3, Table A.12, cold-worked solid-solution nickel-based alloys are divided into three types, presented in Table 1.2UNS N08028 belongs to the type 4c, with a Cr content of at least 19,5%,Ni+Co content of at least 29,5% and Mo content of at least 2,5%. The step to the following two types, 4d and 4e, is rather big. A significant increase in Ni+Co is necessary to reach either one of these types. Since the price for alloying elements, especially for Nickel, today is high, this gap is not only large with regards to material but also cost.

This wide gap between the types could be narrowed by adding a new material within the type 4c. Such a material could probably cover some of the new deeper and sour applications where today materials from type 4c are not suitable. Alloy 29 has been developed with the purpose to decrease this gap and this study presents results from this development.

MATERIAL

The material used within this study is commercially produced; cold worked seamless tubing of grade Sandvik Sanicro 29, 110 ksi, referred to as Alloy 29 in this study. No UNS number exists for Alloy 29 today. Nominal chemical composition and min PRE value(1) for Sandvik Sanicro 28 (UNS N08028) and Alloy 29 are found in Table 2.

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