Hydrogen flux and hydrogen permeation
A flux is simply a flow per unit area. In the case of hydrogen permeation through a metal, hydrogen flux relates to the flux of atomic hydrogen diffusing through the metal lattice, but flux is usually measured in the lab as a rate at which atomic hydrogen is stripped from the metal surface, or in the field as a flux of hydrogen exiting, that is, coming out from an area of the metal’s surface as molecular hydrogen. The flux through the steel and the flux exiting the steel are only equal if none of the hydrogen entering the steel is trapped in the metal, and when the permeation reaches a steady state: time must be allowed for a hydrogen flux to reach a steady state through the metal. A schematic of a general through wall corrosion scenario is shopwn below.
The fact that hydrogen flows through a metal such as steel at all is quite remarkable. The chemical state of mobile hydrogen within the metal is effectively metallic in that its valency electron is completely delocalised within the metal conduction band.
There are several causes of hydrogen flux, although, unless you are at a reading this at a petrochemical facility, steel plant or major site of welding operations, there is unlikely to be a significant hydrogen flux through steel anywhere near you! In the lab, a very reliable source of low flux is a high pressure hydrogen gas cylinder: about 15 pL/cm2/s is measured from a full cylinder containing 240 bar hydrogen behind 7.8 mm wall steel at 20 oC (F.W.H.Dean, S.J.Powell, A.Witty, Corrosion 2014, Paper 144089, Conference Series, NACE, Houston 2014). Note how small such a flux is: it will take a few years for a full hydrogen cylinder to lose 1% of its hydrogen.
Direct dissolution of hydrogen into steel in the above case, is usually benign. Of far greater industrial concern is hydrogen in steel at a much higher activity than provided by 240 bar hydrogen gas. This can come about in two ways: either by corrosion with the agency of a hydrogen ‘promoter’ or ‘occluder’, typically sour gas or HF acid, or by hydrogen being introduced to a metal at an elevated temperature, which then cools, as in welding.
The activity of hydrogen introduced by hydrogen occluders can a million bar molecular hydrogen equivalent (F.W.H.Dean, ‘A review of hydrogen flux promoters’, Corrosion 2010, Paper 10182, Conference series, NACE, Houston, 2010). Not surprisingly, the hydrogen atoms in steel at such an activity are prone to be segregated in the steel, either as discrete hydrides, or diatomic gas. The latter is the cause of hydrogen induced cracking of steel (HIC), and measurement of flux through a steel provides a direct indicator the severity of the environment to cause HIC. It is, in certain circumstances to use this flux as a direct indicator of corrosion activity.
High hydrogen activity in steel caused by cooling arises as follows. Hydrogen is much more diffusible metals at increased temperatures. If steel at 300 oC is suddenly cooled (quenched) to 30 oC, so the hydrogen in the steel has no time to escape, the activity, which is inversely proportional to the solubility, increases dramatically.
For example, a weld pool in the presence of any hydrogen source (eg moisture, rust, detergents or hydrogen trapped in adjoin cool steel) can readily dissolve 1 ppm of hydrogen – the equivalent of just a teaspoon of water in a ton of steel – which is sufficient at ambient temperatures to reach activities equivalent to about a hundred thousand bar of hydrogen.
Diffusivity as well as solubility of hydrogen in steel also increases with temperature. The permeability of hydrogen through steel is the product of both, increasing about 3000-fold at 300 vs 30 oC. The much higher permeability of hydrogen in steel at such elevated temperatures provides an opportunity to use flux as an indicator of any high temperature corrosion, as well as to confirm the successful ‘bake out’ of hydrogen trapped in steel as a result of hydrogen service, prior to welding.
Flux measurements have also been made on welds. The value of the measurements is somewhat compromised by rapid rate of temperature change on a weld, attended as it is by a dramatic decrease in hydrogen permeability of the test steel, not to mention complex thermal gradients and geometries in the weld cross section. None the less, it provides a very good ‘red/green’ indicator of a maximum possible hydrogen content of welds as they cool, and may find application in multi-pass welding.
The illustration below depicts in cross section the permeation of hydrogen through steel. Note that it is really the hydrogen activity gradient that informs hydrogen movement, not concentration, which is much enhanced at sites of hydrogen segregation. In the illustration, this enhanced concentration is shown at the centreline of the steel where impurities in the steel, which trap hydrogen, are often concentrated.