ABSTRACT
The technical suitability of the steel grades L80 and 42CrMo4 (UNS G41400) for the application in an underground gas production facility is investigated. Therefore, the materials are tested with respect to their susceptibility to embrittlement by hydrogen gas, up to a maximum H2-partial pressure of 20 bar (290 psi), by means of constant load tests (CLTs) and immersion tests with subsequent hydrogen analysis.
The tests are carried out within a specially developed and constructed autoclave setup, which enables rotation and heating of the vessels. Each autoclave holds 3 specimens: a coupon for measuring the mass loss, an immersion specimen for hydrogen analysis and a small tensile specimen under constant load (CLT).
Tests are done under realistic field conditions and hence samples are periodically wetted by the aqueous part of the test medium, which contains chloride in a concentration of 15 000 mg/L. The gaseous part of the medium consists of H2 and/or CO2. Tests are performed with or without a corrosion inhibitor and last 30 days. In general, the examined steel grades show a good resistance to hydrogen embrittlement.
INTRODUCTION
To achieve the climate goals, so-called green energy is becoming increasingly important. Changing weather conditions lead to fluctuating power outputs, which leads to the fact that the excess electricity needs to be stored. Transformation of electricity to hydrogen via electrolysis is an option, but the lack of adequate infrastructure for the storage and transport of the gas is a problem. This issue can be addressed by methanation of hydrogen and carbon dioxide to obtain natural gas:
(Equation-1)
A new approach is to use methanogenic archaea that performs the methanation process.1 Such microorganisms can produce natural gas in an underground natural gas reservoir. With H2 involved in the process, hydrogen embrittlement must inevitably be considered. This phenomenon, although known since 1874,2 is not yet fully understood.
In presence of H2 and CO2, as well as a chloride-containing, humid environment under anaerobic conditions, there are two main potential sources of absorbed hydrogen. The first one is the dissociation of gaseous H2:
(Equation)
The hydrogen molecule H2 dissociates to two adsorbed hydrogen atoms Had. The atoms can be absorbed by the material. Sieverts and Krumbhaar stated, that the concentration of hydrogen dissolved in a metal is proportional to the square root of the partial pressure of the gas in thermodynamic equilibrium.3 This is known as Sieverts’s law:
(Equation)
