Abstract

The corrosion resistance of metallic materials in natural seawater is arousing more and more interest from engineering companies and Oil & Gas end-users. Indeed, hydrotesting and flooding operations are very often conducted to check the quality of the vessels and piping systems. In this case, seawater may cause pitting corrosion and crevice corrosion.

Seawater is a very complex and corrosive environment due to its high amount of chlorides, and to the presence of algae, bacteria and many other components that may lead to the formation of a biofilm on the surface of the steel. In addition, during an hydrotest, the temperature of the steel can dramatically increase under the effect of sun and reach relatively high temperatures.

This paper provides an update on the resistance of various corrosion resistant alloys in natural seawater at 30°C (86°F) and 75°C (167°F). Three materials are investigated: UNS N08904, N08825 and N06625. They are today commonly used in the Oil & Gas industry, especially in the upstream part. UNS N08825 and N06625 are often cladded to carbon steel to get a cost-efficient metallic solution for piping systems.

Introduction

Seawater is involved in many industrial applications. For instance, it can be used for heat exchangers and as a cooling medium. It is also commonly used for hydrotesting of piping systems. This environment is very aggressive, especially when considering the complexity of seawater composition combined with the process temperature and sometimes acidic conditions. Choosing an inappropriate material may induce severe corrosion, leading to the leakage of the equipment.

The risk of pitting corrosion must be taken into account for selecting the right Corrosion Resistant Alloy (CRA). Alloying elements such as chromium, molybdenum, tungsten and nitrogen have a beneficial influence on the pitting corrosion resistance of CRA materials. The Pitting Resistance Equivalent Number (PREN) is a useful empirical tool that can be used to get a ranking of stainless steels behavior in a chloride-containing environment.

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