Environmental Engineering Reference
In-Depth Information
ture, and high-temperature corrosion, which mostly involves reactive gases. The
former is known as aqueous corrosion, which claims the major share in loss
of metallic materials due to environmental degradation. In aqueous corrosion,
localized types of attacks, rather than the general attack, are a matter of greater
concern because of their unpredictability and lack of immediate visibility. Pitting,
intergranular corrosion, and corrosion-assisted cracking among them account for
many a premature failure of components.
High-temperature corrosion involving the attack by reactive gases is also pop-
ularly known as oxidation of metals, although the electrochemical term ''oxida-
tion'' applies to aqueous corrosion as well. The formation of scales and their
instability account for the degradation due to oxidation by gases, and this behav-
ior varies widely in single-component systems (pure metals) and in multicompo-
nent systems (alloys). Another important type of high-temperature corrosion is
hot corrosion, which is the attack of metals by molten salts in conjunction with
gases. High-temperature corrosion is of particular concern in power-generating
equipment that undergoes prolonged exposure to high-temperature contamination
of fuel as aggravating the situation in some cases.
Hydrogen damage refers to the results of the action of hydrogen in reducing
the physical and mechanical properties of metals to a degree that renders them
unreliable or useless. Hydrogen entry in the metal is possible from many sources,
starting from its melting stage, during welding, or while undergoing corrosion
in acid environments. The manifestation of damage is most generally a loss in
ductility or the formation of cracks, external or internal. There are three main
forms of hydrogen damage. One form results in internal pores, cracks, or fissures
or external blister formation arising from hydrogen entrapment and subsequent
pressure buildup in the flaws inside the metal. The second form of damage is
embrittlement due to hydride formation. The third form of damage results despite
the absence of a known chemical reaction or hydride formation; nevertheless,
the hydrogen causes crack formation and growth, particularly in the presence of
sustained loads. This form of damage has been described as ''hydrogen stress
cracking'' or ''hydrogen-assisted cracking,'' and high-strength steels have been
found to be particularly susceptible to this type of attack.
Metallic components may come in contact with liquid metal during operations
like brazing, soldering, or galvanizing and in some applications like the use of
molten sodium as coolant in fast-breeder nuclear reactors. Liquid metal may cor-
rode the solid metallic component or there may be diffusion-controlled intergran-
ular penetration of liquid metal in the solid metal. However, the most drastic
form of liquid metal attack is the instantaneous fracture of the solid metal in the
presence of stress, a phenomenon described as ''liquid metal embrittlement.''
The flow behavior of the solid metal is not significantly affected, but a significant
reduction in fracture stress or strain is encountered.
With the advent of nuclear power generation, the concern about the radiation
