In today post we want to talk about the hydrogen embrittlement. This chemical-physical process affects different metals, including right steels, more specifically those with high-resistance.
How does the hydrogen embrittlement take place?
This type of process is the result of the “undesirable” introduction of hydrogen within the steel which can take place during
- forming processes
- finish machining
This mechanism is triggered when hydrogen atoms diffuse into metal blocks. What conditions should be there?
Hydrogen diffuses into the metal in different ways as
- high temperature – the spread is due to the high solubility of hydrogen
- low temperature – requiring the presence of a concentration gradient
The hydrogen atoms recombine into mini-cavities the metal structure, forming hydrogen diatomic molecules. The latter are able to generate a pressure from inside the cavity that gives the metal both ductility and a reduced tensile strength. thus originating cracks.
What are the processes that can lead to embrittlement?
The hydrogen embrittlement can occur as a result of various processes such as
- cathodic protection
- phosphating
- chemical pickling
- galvanisation
- galvanic corrosion
- chemical reaction of metals with acids or with other reagents
What problems do arise?
As mentioned above, hydrogen embrittlement leads to increased formation of cracks in the metal, in this case steel, making the latter brittle and susceptible to fracture. High-strength steels and low alloy ones (no elements above 5%) are those most susceptible to this phenomenon; Those with a tensile strength less than 1000 MPa or with an hardness less than 30 HRC (Hardness Rockwell Cone) instead are not. The results of hardness tests carried out by Jewett on stainless steels have shown that those austenitics have been free from hydrogen embrittlement. Another problem that can occur is that of “attack of hydrogen“.
But how does it work? If the steel is exposed to hydrogen at high temperatures, the latter will spread in the league, combining with the carbon. Just along the inner surfaces (as grain and cavity walls), then, mini-pockets of methane will form that will settle into the cavities, helping the formation of cracks. This process of “selective corrosion” may cause decarbonization of steel, which will lose its ductility and resistance.
How to remedy the hydrogen embrittlment?
If there are no cracks, the process of embrittlement can be retrieved. How? Hydrogen source must be removed, so that the latter can spread out from the metal. If we talk about welding of high-resistance and low-alloys steels, instead, it is necessary to preheat and post heat the metal to allow the escape of hydrogen before it can affect the material.
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