Into the Grain - Advancements in HTHA Detection Methods - Part 1

Introduction

Hydrogen-induced damage in refineries is nothing new. Hydrogen-induced cracking (HIC), hydrogen stress corrosion cracking (HSCC), wet H₂S cracking, and many others have been well documented for decades. Hydrogen-induced damage mechanisms have been studied endlessly, leading to changes in equipment design, including construction practices and metallurgical specifications. In addition, new inspection tools have been developed with continued improvement in their ability to detect and size existing damage in the early stages.

However, one hydrogen-induced damage mechanism whose inspection methods lag behind—due to its highly localized nature and difficulty of detection—is high temperature hydrogen attack (HTHA). HTHA is an intergranular and volumetric damage mechanism that can occur in process equipment that is exposed to hydrogen at elevated temperatures (at least 400ºF or 204ºC). Under dry conditions, hydrogen dissociates into nascent (atomic) hydrogen, which then diffuses into the steel driven by the temperature and pressure of the environment. Next, atomic hydrogen reacts with unstable carbides in steel to form methane gas, resulting in the formation of non-spherical gas pockets that lead to material degradation. The damage that results from HTHA ranges from superficial surface decarburization of the inservice contact to severe through wall material embrittlement, loss of mechanical properties, and cracking. Particularly susceptible are high pressure boiler tubes, hydroprocessing units, and hydrogen, methanol, and ammonia producing units.

HTHA is a time-temperature-pressure-stress function. The longer a piece of equipment is exposed to these variables above its resistance limit in a certain hydro-process environment, the more damage to the steel will accumulate; the higher the temperature rises above the limit of the steel, the more rapidly the damage will occur.

HTHA damage extends beyond just fixed equipment materials in the hydrocarbon process industry. It is present in high pressure boiler tubes, hydrogen producing units, synthetic gas units, ammonia plants, and other equipment where hydrocarbons may not be present but high temperatures exist. HTHA affects carbon and low alloy steels, but is most commonly found in carbon steel and carbon-1/2 Mo steel that is operating above its corresponding Nelson Curve limits (refer to Figure 1 to see the Nelson Curve depicted in API RI 941). Areas that are hotter, often near the outlet nozzle of catalytic equipment or the inlet nozzle including all weld seams and HAZ of attachment welds and of an exchanger in a cooling process, are areas of concern for HTHA.