External Hydrogen Embrittlement

Virtually any metal or alloy is subjected to the hydrogen attack. However, high strength steels and special alloys are the most susceptible to hydrogen corrosion. Besides heat treatment sensitization to corrosion, see Sect. 14.2, it is precisely hydrogen embrittlement the major mechanism of corrosion attack under static 

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It is somewhat less corrosion resistant than tantalum, and like tantalum suffers from hydrogen embrittlement if it is made cathodic by a galvanic couple or an external e.m.f., or is exposed to hot hydrogen gas. The metal anodises in acid electrolytes to form an anodic oxide film which has a high dielectric constant, and a high anodic breakdown potential. This latter property coupled with good electrical conductivity has led to the use of niobium as a substrate for platinum-group metals in impressed-current cathodic-protection anodes. 

Standard Test Method for Evaluating the Influence of Thermal Insulations on External Stress Corrosion Cracking Tendency of Austenitic Stainless Steel Standard Test Method for Electronic Measurement for Hydrogen Embrittlement fi-om Cadmium-Electroplating Processes... 

In general, cathodic protection can be applied to any material susceptible to corrode, but this method is commonly used to protect carbon steel stmctures in diluted or alkaline electrolytes, such as seawater and soil The corrosion mechanism of iron or carbon steel was introduced in Section 13, Chapter one and Chapter four. Nevertheless, the corrosion product may be an unstable ferrous hydroxide [Fe OH) solid compound, which reacts in the environment to form ferric hydroxide compound [Fe (OH) ] or hydrated ferric oxide (FezOs-SH ) known as mst. The formation of this corrosion product is avoidable using cathodic protection. However, careful application of an external potential to a structure must be considered because hydrogen evolution may be induced leading to destmction of any coating and Hydrogen Embrittlement [1]. 

All forms of degradation that could reasonably be expected to affect a vessel in any particular service. Examples of degradation mechanisms include internal or external corrosion or erosion, all forms of cracking (both internal and external), fatigue, embrittlement, creep, high temperature, and hydrogen attack. 

Hydrogen gas pipelines are subject to corrosion on the external surface. While corrosion damage has created leaks in hydrogen gas pipelines, - interactions between corrosion and hydrogen gas embrittlement have not been cited as concerns for pipelines. 

Stress corrosion cracking also involves localized breakdown of the protective film. The corrosion is narrowly confined within the metal due to stress factors which may arise from either residual Internal stress or applied external stress. In some cases the stress failure can be accelerated by chemical factors, such as surface adsorption or hydrogen dissolution from cathodic hydrogen leading to embrittlement. 

Non-carbonated and chloride-free concrete. In concrete that is not carbonated and does not contain chlorides, and in the absence of external cathodic polarization, hydrogen evolution, and thus consequent embrittlement, cannot take place. In this type of concrete, characterized by a pH above 12, hydrogen evolution can only occur at potentials below about —900 mV SCE. Passive steel under free corrosion conditions has much less negative potentials (Chapter 7) in the case of atmospherically exposed structures, the potential is between 0 and —200 mV (zone A of Figure 10.9). 

Fracture of massive brittle and ductile pieces are rather well understood. By taking proper account of the microstructure as well as the micro- and macro-defects, most catastrophic and fatigue failures find a satisfactory explanation within the scope of the linear elastic fracture mechanics or the elasto-plastic fracture mechanics. Metallic filaments are particular and in many respects deserve a treatment of their own. Particular fabrication methods, such as drawing, melt spinning or crystallization from the vapor phase for whiskers are needed to obtain their small lateral dimensions. These processes may give rise to particular textures, intrinsic and extrinsic defects. Thermal treatments may modify or eliminate such defects but in many cases fracture is initiated by defects that stem from the fabrication process. Moreover, the small lateral dimensions, especially in micro-wires, make metallic filaments prone to external influences. Corrosive attacks may rapidly affect an important fraction of their cross-section. Hydrogen, for instance, which usually results in a severe embrittlement, may diffuse up to the core in a rather short time. 

Hydrogen stress cracking of embrittled metal is caused by static external stresses, transformation stresses (for example, as a resnlt of welding), internal stresses, cold working, and hardening. As a rule, cracking does not occur in dnctile steels or in steels that have received a proper post-weld heat treatment. 

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