High Temperature Hydrogen Attack (HTHA) is a complex damage mechanism that continues to defy investigators trying to make predictions on the anticipated degree of damage or service life. This article provides some background on HTHA, discusses some current developments in HTHA inspection and mitigation, and describes how one Refiner is instituting an HTHA risk management plan for its refineries and the challenges and pitfalls they have encountered. The article also describes a new innovative screening methodology developed by the author, who served as the Corrosion & Materials Engineering expert assisting the Refiner in characterizing over 75 equipment items operating in high temperature hydrogen services that needed individualized risk management plans. Materials included in the evaluations are carbon steel (CS), C-0.5Mo, 1Cr, and 1.25Cr equipment ranging in service exposure of 10 to over 50+ years. Several examples are provided.
What is HTHA?
Mechanistically, HTHA can be thought of as hydrogen promoted creep damage at temperatures below the typical creep regime. Atomic hydrogen present in the process environment must first enter the surface of the equipment and then can diffuse into the subsurface. Atomic hydrogen combines with carbon present in unstable carbides to form methane. At first this creates very small bubbles, and then as the pressure in the bubbles builds, they begin to combine to form fissures, and these fissures then combine to form cracks.
The historical method by which the industry has managed HTHA has been to use experienced-based curves (API 941 Nelson Curves) that were drawn below the lowest reported case of HTHA attack. Generally speaking, these curves have been used for material selection and for evaluating the integrity of existing equipment, and have served the industry well. The Nelson Curves show a temperature/pp H2 (hydrogen partial pressure) relationship for each material.
Although the shape of the Nelson Curve for CS is experientially based, the API TR 941 Technical Basis Document (TBD) describes the theoretical basis for the Nelson Curves based on first principles and confirms that their general shape and position in P/T/Material (pressure/temperature/Material) space is fundamentally sound. Thermodynamically, the methane formation reaction is favored at the lower the temperature, but the reaction rate becomes limiting at lower temperatures. Therefore HTHA is more likely to occur at intermediate temperatures which coincides with engineering use of the common materials of construction. It has long been known that alloy additions of Cr (chromium) and Mo (molybdenum) create more stable carbides than the iron carbides in carbon steel and will not form methane as readily.
In reality, HTHA is much more complicated and there are many other factors such as age, upsets, stress, PWHT, etc. that determine an actual materials’ particular susceptibility or resistance to HTHA.