Damage Control: Sulfidation and High-Temperature H2/H2S Corrosion Mitigation

Editor’s Note: This regular column offers practical insights into various damage mechanisms affecting equipment in the O&G, petrochemical, chemical, power generation, and related industries. Readers are encouraged to send us suggestions for future topics, comments on the current article, and raise issues of concern. All submissions will be reviewed and used to pick topics and guide the direction of this column. We will treat all submissions as strictly confidential. Only Inspectioneering and the author will know the names and identities of those who submit. Please send your inputs to the author at damagecontrol@inspectioneering.com.

Introduction

The previous installment of Damage Control offered a practical perspective on fitness-for-service (FFS) methods commonly employed to evaluate the detrimental effects of wall loss on pressure equipment due to sulfidation and high-temperature H₂/H₂S corrosion. Specifically, an overview of the Level 1, Level 2, and Level 3 FFS procedures in API 579-1/ASME FFS-1, Fitness-For-Service (API 579) was provided, including a review of the design-by-analysis fundamentals that form the basis of the Level 3 FFS methods [1]. Furthermore, these advanced stress analysis techniques can be used to qualify general or local metal loss in pressure components and ensure protection against plastic collapse, local failure, and buckling (for cases where compressive stresses may govern). Leveraging FFS technology can be beneficial in extending useful equipment operating life and deferring costly repairs and/or replacement. In general, any FFS assessment requires accurate inspection data and characterization of damage (e.g., meaningful thickness readings capturing the extent and attributes of wall loss profiles due to in-service corrosion). Part 3 of this Damage Control series focuses on practical damage mitigation and life cycle management techniques to avoid sulfidation failures and high-temperature H₂/H₂S corrosion in refinery pressure equipment. As discussed herein, positive material identification (PMI) strategies, integrity operating windows (IOWs), and special emphasis mechanical integrity (SEMI) programs represent useful tools for managing the risk associated with operating components in sulfidation-prone environments. To this end, a holistic approach to equipment life cycle management in sulfidation service environments, including specifying favorable engineering design/fabrication practices, inspection techniques, and leveraging advanced analysis, reflects the best strategy to maximize plant safety in an economical and pragmatic manner.

Understanding the Root Cause of Sulfidation Failures

In addition to advantageous design approaches, proper materials selection, and planning/implementing appropriate inspection procedures, understanding sulfidation fundamentals is a critical aspect of managing the risk associated with sulfidation corrosion-related failures. Part 1 of this Damage Control series provided observations on typical sulfidation and high-temperature H₂/H₂S corrosion damage morphology and the refining units most susceptible to related in-service failures. To offer further insights into the root cause of documented industry sulfidation incidents, a summary of 45 documented industry failures, primarily on piping components, is offered in Figure 1 [2,3]. As inferred from this figure, low silicon (Si) content-related scenarios are the most common root-cause failure mechanism reported based on this sample size of occurrences (15 out of 45). It is noted that modern-generation refinery seamless carbon steel piping material specifications, commonly available in North America, are often triple-stamped ASTM A-106/ASTM A-53/API 5L and contain greater than 0.10% (weight percentage) Si [2]. Vintage (pre-1985) ASTM A-53 seamless piping and in certain cases, contemporary welded (e.g., electric resistance welded) ASTM A-53 piping, may have lower Si content than the 0.10% threshold. Furthermore, large diameter piping and fittings made from rolled plate may not necessarily exhibit adequate Si content, depending on the specification (e.g., modern A-516 would generally be satisfactory but A-285 could potentially be problematic).

The overall influence of Si content on sulfidation damage proclivity creates a notable inspection challenge for owner-users, because small piping sections (e.g., seemingly inconsequential pup pieces) or fittings with relatively low Si content may corrode at rates of two, and in some cases up to 10 times faster than surrounding higher-Si piping components. To this end, even very small components (e.g., threaded plugs, small bore piping components, blind flanges, etc.) with insufficient Si content or with improper metallurgy can lead to loss of containment/catastrophic failures. Even though the corrosion rate may not be substantial over short periods of time, many refining units contain piping components that are 30-40+ years old. Given this, relatively minimal corrosion rates can result in exhaustion of the original design corrosion allowance, with further thinning continuing (often unrecognized) until loss of containment/eventual rupture occurs after long periods of operation [2].