Brittle materials fracture without significant deformation (strain) when subjected to stress. Ductile materials are able to absorb considerably more energy before failure. Most commercially useful metals and alloys, including steel, are ductile. A common example of a brittle alloy is cast iron.
Hydrogen embrittlement is a phenomenon by which hydrogen atoms diffusing into the micro-structure of a metal cause it to become more brittle, resulting in sudden and unpredictable fracture (hydrogen induced cracking).
There is no consensus amongst materials scientists on the exact mechanism which causes this effect. Theories relate to the diffusion and entrapment of hydrogen atoms through the metal matrix. A particular metal’s micro-structure plays a key role in the movement of atoms through the metal and explains why certain metals are susceptible and not others.
Where do the hydrogen atoms come from?
When parts made from susceptible metals are exposed to recently produced hydrogen, either in service or during their production, hydrogen atoms can diffuse through the micro-structure of the material, leading to hydrogen embrittlement. It is a random effect, so may only affect a percentage of components. It is vital to take steps to minimise risk, in order to avoid catastrophic failures from occurring. De-embrittlement operations similarly have a chance of being ineffective but can be considered to greatly reduce the occurrence of hydrogen induced cracking. International standards have been developed to reflect effective practices.
Certain chemical or electrochemical processes expose the treated metal to hydrogen production. Electroplating is one such treatment as hydrogen gas is evolved at the cathode (ie the item being plated) during electrolysis. Acid cleaning, which is used to strip unwanted coatings or prepare surfaces for plating, also involves the production of hydrogen gas. Acid pre-treatment and electroplating therefore put susceptible base materials at risk of hydrogen embrittlement.
Processes which can introduce hydrogen:
- Acid pickling
- Electroplating, including zinc plating and zinc-nickel plating.
- Electroless plating
- Fabrication operations prior to cleaning and plating
What metals are at risk?
High strength steels (greater than 1000 MPa ultimate tensile strength) are at risk of hydrogen embrittlement, along with some other alloys.
We rely on customers to inform us of the base material properties and design considerations of items sent to us for metal treatments.
What can be done to prevent hydrogen embrittlement?
It may not be practical within the design requirements of a component to use a lower strength steel or a different alloy in order to avoid the risk of hydrogen embrittlement. Stress relief baking or shot peening of components prior to the finishing operation may be required to prevent potential hydrogen damage. Heat treatment (relief baking) after hydrogen exposure can be used to reduce the risk of hydrogen embrittlement. Heat treatment does not guarantee freedom from hydrogen embrittlement.
The only way to avoid hydrogen embrittlement of susceptible materials is to avoid sources of hydrogen by specifying only mechanical (as opposed to chemical or electrochemical) cleaning, descaling or plating operations.
Relief baking needs to be specific to the requirements of the item. Parts must be held at a certain temperature (and to have achieved this temperature throughout) for a specified period of time (8 – 22 hours). It is vital that relief baking commences within 3 hours, preferably within 1 hour, of exposure to hydrogen risk so that hydrogen can diffuse out before it causes any damage. Requirements for heat treatment are laid out in international standards.
Relevant standards for post-plating heat treatment:
- ISO 9588
- ASTM B580
- Def Stan 03-4
Stress-relief and de-embrittlement operations increase the time and cost for processing parts, as well as imposing a requirement for record-keeping and calibrated, certified equipment. The costs involved are minimal when compared with the risks posed as a structural failure could have potentially fatal consequences. Suppliers offering this service should be audited to ensure compliance with standards due to the risks involved.
When sending parts to us that require de-embrittlement or stress relief. It is essential that they are clearly identified as requiring these pre/post-treatments and that the material specification and tensile strength is flowed down to us.
More Info and Specialist Advice
Please email us if you would like advice on hydrogen embrittlement risks. This page just gives general suggestions and our specialists will be able to provide more in-depth information.
This page is provided for information only, it should not be considered advice and we cannot accept any responsibility or liability for your use of the information on this page. The information on this page is used and relied on at your own risk and you bear the sole responsibility for any outcomes. E&OE.