ABSTRACT

Corrosion Resistant Alloys (CRAs) are routinely utilized to mitigate against the complex damage mechanisms encountered in refining operations that carbon and low alloy steels are highly susceptible to. However, CRA materials can suffer similar corrosion damage when improperly manufactured or exposed to aggressive environments. In this paper, three modes of CRA failure observed at a client’s site were analyzed in a lab and mitigation strategies proposed.

Tower trays near the top of a crude tower made of UNS S41008 martensitic stainless steel (SS) failed as a result of localized under-salt corrosion due to formation of amine hydrochloride salts. Appropriate crude pre-treatment was implemented to mitigate this corrosion mechanism.

UNS N06625 flexible hoses located at the inlet of a reformer in a hydrogen plant failed upon start-up during a turnaround. It was found that these materials were heavily sensitized with embrittling phases present at the austenite grain boundaries. Improper annealing processes at the manufacturing plant likely caused the sensitization of the microstructure.

Downstream of the reformers, UNS S30403 austenitic SS tube ends of the boiler feed water heat exchanger underwent a failure. The tube to fixed tube sheet seal weld failed as a result of fatigue cracking originating at a lack of weld deposit location. Ensuring a proper weld profile in compliance with the weld procedure would reduce such stress riser concentrations.

INTRODUCTION

Carbon steel has been the most common structural material used for oil and gas applications since its development around 1870. However, high susceptibility to corrosion has limited the use of carbon steel under aggressive operating conditions.1 Corrosion resistant alloys (CRAs) were developed in the 1980s as alternative materials for the extreme service conditions and corrosive produced fluids typically encountered in the oil and gas industry.1 CRAs generally refer to martensitic stainless steel (SS), duplex SS, austenitic SS, and nickel-based alloys that form a passive film on the metal surface which protects it against corrosive environments.2-4 CRAs were initially designed to prevent CO2 corrosion in pipelines but recent advances have been primarily oriented towards increasing the corrosion resistance of these materials to other corrosion mechanisms such as sulfide stress corrosion (SSC) cracking, chloride stress corrosion cracking (Cl-SCC), and hydrogen embrittlement.1 Common examples of CRAs used for oil and gas applications include austenitic SSs such as UNS S30400 and UNS S31603, martensitic SSs such as UNS S41000, duplex SSs such as UNS S31803 and UNS S32750, super austenitic SSs such as UNS N08904, and Ni-based alloys such as UNS N06625.1 Although these CRAs are expected to provide long-term corrosion resistance in oil and gas environments, they can still suffer from different corrosion issues, depending on metallurgical factors such as chemical composition, heat treatment, microstructure, and strength, and environmental conditions including temperature, chloride concentration, CO2 partial pressure, H2S partial pressure, pH of the solution, and presence of elemental sulfur.4-7 These factors can deteriorate the passive film stability, and increase the susceptibility to pitting corrosion and the likelihood of any form of environmentally assisted cracking. Thus, significant efforts have been made to implement proper material selection procedures, considering mechanical properties and corrosion resistance, to avoid premature failure of CRAs under specific service conditions.4, 6 In this study, failure of three different CRAs (UNS S41008, UNS N06625, UNS S30403) in refining operations will be investigated. A brief description of these materials and corrosion mechanisms associated with their failure are also presented.

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