Embrittlement by hydrogen

Other special additions are used to deoxidize copper. Such alloys may be preferred in appHcations where embrittlement by hydrogen through reaction with internally dispersed copper oxide particles is a concern, such as in CllO. The most common deoxidized copper is C122, in which phosphoms reacts with copper oxide to form phosphoms pentoxide that can be slagged from the copper while molten. 

Niobium like tantalum relies for its corrosion resistance on a highly adherent passive oxide film it is however not as resistant as tantalum in the more aggressive media. In no case reported in the literature is niobium inert to corrosives that attack tantalum. Niobium has not therefore been used extensively for corrosion resistant applications and little information is available on its performance in service conditions. It is more susceptible than tantalum to embrittlement by hydrogen and to corrosion by many aqueous corrodants. Although it is possible to prevent hydrogen embrittlement of niobium under some conditions by contacting it with platinum the method does not seem to be broadly effective. Niobium is attacked at room temperature by hydrofluoric acid and at 100°C by concentrated hydrochloric, sulphuric and phosphoric acids. It is embrittled by sodium hydroxide presumably as the result of hydrogen absorption and it is not suited for use with sodium sulphide. 

Tritium and its decay product, helium, change the structural properties of stainless steels and make them more susceptible to cracking. Tritium embrittlement is an enhanced form of hydrogen embrittlement because of the presence of He from tritium decay which nucleates as nanometer-sized bubbles on dislocations, grain boundaries, and other microstructural defects. Steels with decay helium bubble microstructures are hardened and less able to deform plastically and become more susceptible to embrittlement by hydrogen and its isotopes (1-7). 

Having confirmed the susceptibility of the material to direct embrittlement by hydrogen, it then becomes worthwhile considering the contributions made by hydrogen embrittlement to the loss in ductility in other environments, particularly where aggressive solutions are involved that may cause significant amounts of corrosion. 

Figure 7.1. Schematic illustration of the sequential processes for environmental enhancement of crack growth by gaseous (a) and aqueous (b) environments. Embrittlement by hydrogen is assumed, and is schematically depicted by the metal-hydrogen-metal bond.

A form of cracking which can occur in a wide range of materials caused by the combined action of stress and a chemically aggressive environment. In ferritic steels, certain forms of stress corrosion cracking are the result of embrittlement by hydrogen introduced chemically from the environment and residual stresses resulting from welding. Restrictions on the hardness of steel and weldments are made to avoid the problem. 

Tantalum is attacked by alkalies and by hydrofluoric acid. It is readily embrittled by hydrogen at room temperature when the metal is cathodically polarized. 

In the case of metallic systems, there were early indications that metallic glasses and disordered alloys may be more corrosion resistant, less susceptible to embrittlement by hydrogen and have a higher hydrogen mobility than ordered metals or intermetallics. All of these properties are desirable for hydrogen storage. Subsequent research has shown that thermodynamic instability is a severe problem in many amorphous metal hydrides. The present ASI has provided an appropriate forum to focus on these issues. 

An effective surface treatment requires a clean surface. Metal surfaces are cleaned with an alkaline, neutral, or weakly acidic cleaner, an organic solvent, or by pickling with molten-salt baths [5,87]. Fluorinated surfactants in a pickling and descaling bath disperse scum, speed runoff of acid when metal is removed from the bath, and increase bath life [206,207]. The fluorinated surfactant inhibits nascent hydrogen formation and, therefore, prevents embrittlement by hydrogen [208]. 

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