Hydrogen embrittlement
Once absorbed, hydrogen lowers the stress required for cracks in the metal to initiate and propagate, resulting in embrittlement.
The most common causes of failure in practice are poorly controlled electroplating or damp welding rods.
[12] The mechanisms (there are many) by which hydrogen causes embrittlement in steels are not comprehensively understood and continue to be explored and studied.
[1][13][14] Hydrogen embrittlement is a complex process involving a number of distinct contributing micro-mechanisms, not all of which need to be present.
[14] There is a great variety of mechanisms that have been proposed[14] and investigated as to the cause of brittleness once diffusible hydrogen has been dissolved into the metal.
[6] In recent years, it has become widely accepted that HE is a complex process dependent on material and environment so that no single mechanism applies exclusively.
[15] Hydrogen embrittles a variety of metals including steel,[19][20] aluminium (at high temperatures only[21]), and titanium.
During manufacture, hydrogen can be dissolved into the component by processes such as phosphating, pickling, electroplating, casting, carbonizing, surface cleaning, electrochemical machining, welding, hot roll forming, and heat treatments.
[2] In one case of failure during construction of the San Francisco–Oakland Bay Bridge galvanized (i.e. zinc-plated) rods were left wet for 5 years before being tensioned.
Embrittling procedures such as acid pickling should be avoided, as should increased contact with elements such as sulfur and phosphate.
In the case of welding, often pre-heating and post-heating the metal is applied to allow the hydrogen to diffuse out before it can cause any damage.
This will build an inherent resistance to this process and reduce the need for post-processing or constant monitoring for failure.
Certain metals or alloys are highly susceptible to this issue, so choosing a material that is minimally affected while retaining the desired properties would also provide an optimal solution.
Similar tests can also be used during quality control to more effectively qualify materials being produced in a rapid and comparable manner.
Coatings act as a barrier between the metal substrate and the surrounding environment, hindering the ingress of hydrogen atoms.
The choice of coating depends on factors such as the type of metal, the operating environment, and the specific requirements of the application.
Electroplating can provide an excellent protective layer that enhances corrosion resistance and reduces the susceptibility to hydrogen embrittlement.
The conversion coating chemically reacts with the metal surface, resulting in a thin, tightly adhering protective layer.
Thermally sprayed coatings offer several advantages in the context of hydrogen embrittlement prevention.
Ideally, specimens should be made of the final material or the nearest possible representative, as fabrication can have a profound impact on resistance to hydrogen-assisted cracking.
