One of the common corrosion products of iron or steel in a neutral or acidic medium is atomic hydrogen. Normally, these highly reactive species combine with other hydrogen atoms to produce a relatively harmless molecule, hydrogen gas (H2). In the presence of some catalysts such as sulfur, arsenic, or tin, the hydrogen atoms dissolve into the steel to a significant degree. (reference)
This generation of atomic hydrogen can be used for corrosion monitoring purposes in both intrusive or non-intrusive forms. In the latter, hydrogen monitoring sensors are often attached to the outside walls of vessels and piping. It is the diffusion of atomic hydrogen into the metallic substrate that is of most concern, as it can lead to problems such as hydrogen induced cracking. This "uptake" of hydrogen occurs when the recombination of hydrogen atoms and subsequent release as molecular hydrogen into the environment is impeded. Hydrogen monitoring is highly applicable to the oil refining and petrochemical industries with hydrocarbon process streams. The presence of hydrogen sulfide in these industries promotes the uptake of hydrogen into plant items. Hydrogen monitoring probes are based on either of the following three principles:
Pressure increase with time in a controlled chamber, as hydrogen passes through the material into the probe chamber
An electrochemical current resulting from the oxidation of hydrogen under an applied potential
Current flow in an external circuit, based on a fuel cell principle, whereby hydrogen entering the miniature fuel cell causes the current flow.