Damage Control: High-Temperature Hydrogen Attack Detection

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

High-temperature hydrogen attack (HTHA) is a complex, time-dependent damage mechanism that deleteriously affects pressure retaining equipment and piping components subject to hydrogen gas at elevated operating temperatures and internal pressures. Dissociated hydrogen atoms react with carbon and carbides in the steel to form methane. The formation of methane can be damaging to pressure equipment when this methane pressure builds up in the pressure boundary, initially forming bubbles or cavities, then micro-fissures, and eventually, these fissures can coalesce to ultimately form macro-cracks [1, 2]. A general schematic that shows the underlying hydrogen-carbon reaction (decarburization process) and subsequent fissure formation in steel from rising methane pressure is shown in Figure 1. Worrisomely, over time, these crack-like flaws can grow in service and eventually reach a critical (unstable) size, leading to rupture/loss of containment [3].

While the mechanism of HTHA can lead to surface or internal decarburization (that, in extreme cases, could reduce material strength and possibly lead to gross plastic collapse) or even visual blistering (due to either molecular hydrogen or methane accumulation in existing steel laminations or other inclusions), the initiation and propagation of crack-like flaws usually presents the most significant process safety risk. This installment of Damage Control reflects the first of a three-part series on HTHA, and the intent of the content conferred herein is to provide a fundamental level of understanding of HTHA, including critical parameters influencing damage proclivity and commonly afflicted steels and equipment. Focused commentary on C-0.5Mo material is also given. Furthermore, in this article, a historical overview of HTHA will be offered, and a summary of the damage mechanics associated with HTHA will be provided with relevant examples of damage. Additionally, a synopsis of a notable, catastrophic industry HTHA failure will be rendered herein, and commentary on pragmatic inspection methods for accurately detecting and characterizing HTHA damage will be generated [4].

Historical Perspective on HTHA

It has been recognized for decades that, over time, HTHA can progressively degrade the load-carrying capacity of certain pressure equipment in the oil refining, fertilizer, and related industries [5]. Hydrogen attack was first recognized in the early 1900s when investigators reported that internal decarburization and cracking were found in plain carbon steel vessels used for ammonia synthesis [6]. In 1948, investigators of ammonia converters, heat exchangers, and piping from a DuPont process plant concluded that carbon steels containing 0.10-0.35 percent carbon were potentially susceptible to HTHA at a hydrogen partial pressure of 350 psi (2.4 MPa) and temperatures above 570°F (300°C) [7].

In general, the resistance of pressure equipment steels to attack by hydrogen (and subsequently, the rate at which damage occurs) at elevated temperatures can vary significantly, depending on the material of construction and the process operating conditions (specifically, metal temperatures and hydrogen partial pressure). As discussed in API 941, “Steels for Hydrogen Service at Elevated Temperatures and Pressures in Petroleum Refineries and Petrochemical Plants,” HTHA of steel, in general, can result in surface decarburization, internal decarburization, fissuring, methane bubble formation, delamination, or cracking of base metal and weld heat affected zones (HAZs)/weld fusion lines [3]. Furthermore, HTHA is an irreversible phenomenon resulting in the degradation of mechanical strength, fracture toughness, and material ductility. Pressure boundary failure may manifest as leaks, fires, or even catastrophic ruptures/explosions [8].