Question

Why hydrogen is active in high temperature

Answer 1

High temperature hydrogen attack (HTHA), also called hot hydrogen attack, is a problem which concerns steels operating at elevated temperatures (typically above 400°C) in hydrogen environments, in refinery, petrochemical and other chemical facilities and, possibly, high pressure steam boilers. It is not to be confused with hydrogen embrittlement or other forms of low temperature hydrogen damage.

HTHA is the result of hydrogen dissociating and dissolving in the steel, and then reacting with the carbon in solution in the steel to form methane. This can result in either surface decarburisation, when the reaction mostly occurs at the surface and draws carbon from the material, or internal decarburisation when atomic hydrogen penetrates the material and reacts with carbon to form methane, which accumulates at grain boundaries and/or precipitate interfaces, and cannot diffuse out of the steel. This causes the fissures and cracking which are typical of HTHA.

Surface decarburisation results in a decrease in hardness and increase in ductility of the material near the surface. This is usually only a minor concern for these types of application. However, internal decarburisation, and in particular the formation of methane and consequent development of voids, can lead to substantial deterioration of mechanical properties due to loss of carbides and formation of voids, and catastrophic failure.

The main factors influencing HTHA are the hydrogen partial pressure, the temperature of the steel and the duration of the exposure. Damage usually occurs after an incubation period, which can vary from a few hours to many years depending on the severity of the environment. High temperatures and low hydrogen partial pressures favour surface decarburisation while the opposite conditions (lower temperature, high hydrogen partial pressure) favour fissuring. In addition, the composition of the steel influences the resistance to HTHA; in particular elements that tie-up carbon in stable precipitates such as Cr, Mo and V are very important. Increasing content of such elements increases the resistance to HTHA, and Cr-Mo steels with more than 5% Cr, and austenitic stainless steels, are not susceptible to HTHA.

In 1949, Nelson gathered and rationalised a number of experimental observations on different steels. In the Nelson diagram, boundaries are placed in a temperature/hydrogen partial pressure graph, which delineates the region of safe use for carbon steels, 1.25Cr-0.5Mo steels, etc. This diagram has been updated a number of times by the American Petroleum Institute (API) and published in the API recommended practice 941. More recently, analytical models have been used to predict the kinetics of HTHA with some success (Shih, 1982 and Parthasarathy, 1985).