Industrial Dogma and You – How, When, and Why to Perform Hydrogen Bake-out

Introduction

Across the widely varied industrial landscape, the effects of hydrogen diffusion into and through steels continues to present owners with challenging situations regarding the need for degassing steels before maintenance or repairs and the associated headaches if not done properly. The type of service, alloy of fabrication, planned maintenance activities, and even thermal history during shutdowns can influence the degree to which a material is susceptible to the damaging effects of dissolved hydrogen. This article seeks to shed some light on the realities of hydrogen diffusion in the context of repair planning and equipment management and help answer the age-old question, “Do I have to bake this out?”

Service Environment

There are two primary hydrogen service environments, albeit with a somewhat fuzzy line between them: (1) services in which vapor/gas exposure to hydrogen is the predominant means of entry into steel, and (2) acidic services, which promote increased hydrogen adsorption by delaying or preventing hydrogen from returning to the service fluid. Experts in the field can (and will) argue for decades about how many distinct classifications there should be, but for this engineer, only one question really matters: what is the effective hydrogen pressure driving diffusion? The answer most often relies on a prediction of the partial pressure which is largely influenced by which environment one is operating in.

Gaseous Hydrogen Charging of Steels

Units such as hydrogen reforming and handling, ammonia production, portions of the diesel sulfur hydrotreating process, and others typically see hydrogen exposures that are dominated by hydrogen gas decomposing to H⁺ ions and free electrons at the surface of the steel. Each ion diffuses through or exits the steel, where it recombines with another H⁺ ion to form H₂ gas. The majority of hydrogen diffusion studies use the mixture’s partial pressure of hydrogen (P_H2) in the benchmarking process to identify the severity of hydrogen charging. Of critical importance to this article, however, is that the P_H2 for gaseous services is usually less than or equal to the service pressure.

Aqueous/Acidic Hydrogen Charging of Steels

In contrast to gaseous hydrogen charging, hydrogen charging due to the corrosion of ferritic steels in aqueous environments can be much more aggressive. Owing to the nature of surface reactions, hydrogen ions liberated during the oxidation reactions are essentially already adsorbed into the metal matrix, and will begin diffusing out to a free surface, where one will combine with another H⁺ ion to form gaseous H₂. The relatively long diffusion path through the bulk of the steel, compared to the short path back to the corroding fluid, means that most of the evolved hydrogen will recombine in the fluid and not contribute to large concentrations of solubilized hydrogen in the steel.