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What is Hydrogen Embrittlement? - Causes, Effects and Prevention

   

Hydrogen Embrittlement is a process whereby metals become brittle and fracture as a result of the introduction and diffusion of hydrogen into the material.

Also known as hydrogen induced cracking, embrittlement is often caused by the introduction of hydrogen during forming, coating, cleaning, finishing and plating processes. This often called internal embrittlement. External embrittlement can also occur as a result of environment exposure to soils and chemicals (including water), corrosion, cathodic protection and from hydrogen created through coating corrosion.

Three factors are needed for a metal to be susceptible to hydrogen damage, these are the presence and diffusion of hydrogen atoms, the use of a susceptible material, and stress. Stress corrosion cracking occurs when hydrogen is introduced into the metal and sufficient tensile stress is applied, this stress factor is called the threshold stress or Ki_SCC.

Not all metals are vulnerable to hydrogen embrittlement as it affects hydride-forming metals such as niobium, tantalum, titanium, vanadium and zirconium. The most vulnerable metals are high strength steels and titanium and aluminium alloys. However, when it does occur, hydrogen embrittlement can cause reduced ductility and a lessening of load-bearing capacity, which can lead to hydrogen induced cracking and brittle failures below the yield stress of the susceptible materials themselves.

How to Prevent Hydrogen Embrittlement

Hydrogen embrittlement can be prevented by minimising contact between the metal and any sources of hydrogen gas. This is particularly important during fabrication and the electrolysis of water and involves the avoidance of procedures, such as acid pickling, which can lead to embrittlement.

In addition increased contact with elements like sulphur and phosphate should be avoided. Hydrogen embrittlement can also be prevented through the use of correct electroplating solutions and procedures. Heat treatment is another commonly used method to prevent or even reverse embrittlement by removing the hydrogen source and causing the hydrogen within the metal to diffuse from the material. This technique, known as baking, is especially effective with high strength and low alloy steels.

Another method for preventing the problem of embrittlement is through materials selection. Quite simply, using materials that are not vulnerable to hydrogen embrittlement. For example, steel with a tensile strength of less than 1000 mpa or a hardness of under 23 HRC are not generally considered to be susceptible to hydrogen embrittlement.

Hydrogen embrittlement is not a permanent condition as, if the environmental conditions are changed before cracking occurs so that no hydrogen is generated on the surface of the metal, the hydrogen can rediffuse from the material so that ductile properties are restored.

Hydrogen Embrittlement Testing at TWI

TWI has years of experience assisting industry with resisting the effects of hydrogen on materials. This includes identifying metals that are resistant to embrittlement, the use of cathodic polarisation, determining the optimal operating conditions for duplex and supermartensitic stainless steels in subsea environments, and developing welding parameters to avoid fabrication hydrogen cracking.

As a world leader in this field, TWI has the necessary facilities to study the impact of hydrogen on materials, as well as various processes to mitigate against embrittlement.

For more information please email:


contactus@twi.co.uk