Hydrogen embrittlement/sulfide stress cracking (H2S) 6

[tbeckeratvarco]

Have given all my ready references away to a needy school. Have been asked "Is hydrogen embrittlement/sulfide stress cracking time based?" I believe that the effect is nearly instantaneous. Is my memory correct? It's been a long time since I've had to answer questions like this one.

 

[mcguire]

No, it is delayed. The delay is involved with the diffusion of hydrogen to the initiation site of fracture.
In stress corrosion cracking, its first cousin, the corrosion process, generally pitting, must start to get the hydrogen generation started. Sulfide stress cracking is also environmentally produced so you have a delay until the sulfide environment causes hydrogen entry into the metal. In HE as most people use the term, hydrogen is already present in the metal, eg from melting, pickling, etc.

 

[grampi1]

It can occur very quickly when conditions are right, or it can take weeks, or it can be eposodic if the enviroment is changing with time.

 

[mcguire]

I agree with grampi1's additional comments. Delay in this case only means not exactly when the stress is applied.

 

[Carburize]

Hydrogen embrittlement.
A process resulting in a decrease of the toughness or ductility of a metal due to the presence of atomic hydrogen. Hydrogen embrittlement has been recognized classically as being of two types. The first known as internal hydrogen embrittlement, occurs when the hydrogen enters molten metal which becomes supersaturated with hydrogen immediately after solidification. The second type, environmental hydrogen embrittlement, results from hydrogen being absorbed by solid metals. This can occur during elevated-temperature thermal treatments and in service during electroplating, contact with maintenance chemicals, corrosion reactions, cathodic protection, and operating in high-pressure hydrogen. In the absence of residual stress or external loading, environmental hydrogen embrittlement is manifested in various forms, such as blistering, internal cracking, hydride formation, and reduced ductility. With a tensile stress or stress-intensity factor exceeding a specific threshold, the atomic hydrogen interacts with the metal to induce subcritical crackgrowth leading to fracture. In the absence of a corrosion reaction (polarized cathodically), the usual term used is hydrogen-assisted cracking(HAC) or hydrogen stress cracking (HSC). In the prcsence of active corrosion, usually as pits or crevices (polarized anodically), the cracking is generally called stress-corrosion cracking(SCC), but should more properly be called hydrogen-assisted stress-corrosion cracking (HSCC). Thus HSC and electrochemically anodic SCC can operate separately or in combination(HSCC). In some metals, such as high-strength steels, the mechanism is believed to be all, or nearly all, HSC. The participating mechanism of HSC is not always recognized and may be evaluated under the generic heading of SCC.

 

[mcguire]

Carburize's message should be required reading for all materials engineers.