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half cell.

An electrode immersed in a suitable electrolyte, designed for measurements of electrode potential; A pure metal in contact with a solution of known concentration of its own ion, at a specific temperature develops a potential which is characteristic and reproducible; when coupled with another half cell, an overall potential develops which is the sum of both half cells.

 

halogen.

Any of the elements of the halogen family, consisting of fluorine, chlorine, bromine, iodine, and astatine.

 

hard chromium.

Chromium plated for engineering rather than decorative applicactions.

 

hardenability.

The relative ability of a ferrous alloy to form martensite when quenched from a temperature above the upper critical temperature. Hardenability is commonly measured as the distance below a quenched surfsce at which the metal exhibits a specific hardness (50 HRC, for example) or a specific percentange of martensite in the microstructure.

 

hardfacing.

Depositing filler metal on a surfsae by welding, spraying, or braze welding to increase resistance to abrasion, erosion, wear, galling. impact, or cavitation damage.

 

hard water.

Water that contains certain salts, such as those of calcium or magnesium, which form insoluble deposits in boilers and form precipitates with soap.

 

heat-affected zone.

That portion of the base metal that was not melted during brazing, cutting, or welding, but whose microstructure and mechanical properties were altered by the heat; Refers to area adjacent to a weld where the thermal cycle has coused microstructural changes which generally affect corrosion behavior.

 

heat check.

A pattern of parallel surface cracks that are formed by alternate rapid heating and cooling of the extreme surface metal, sometimes found on forging dies and piercing punches. There may be two sets of parallel cracks one set perpendicular to the other.

 

hematite.

(1) An iron mineral crystallizing in therhombohedral system; the most important oreof iron. (2) An iron oxide, Fe,O,, corrcsponding to an iron content of approximately 70%.

 

high-temperature hydrogen attack.

A loss ofstrength and ductility of .steel by high-temperature reaction of absorhcd hydrogen with carbides in the steel resulting in dec arbwri:.alien and internal fissuring.

 

holidays.

Discontinuities in ci coating (suchasporosity, cracks, gape. and similar Bawd) that allow areas of base metal to be exposed to any corrosive environment that contacts the coated surface.

 

hot corrosion.

An accelerated corrosion of' metal surfaces that results from the combined elTect of oxidation and reactions with sulfur compounds and other contaminunts, such us chlorides, to form a molten salt on a metal iurfuce that f1uxes, destroys, or disrupts the normal protective oxide. Seealso gaseous r erosion.

 

hot cracking.

Also called solidification crackinghot cracking of weldments is caused by the segregation at grain boundaries of low-melting constituents in the weld metal. This can resultin grain-boundary tearing under thermal contraction stresses. Hot cracking can be minimized bythe use of low-impurity welding materials and proper joint design. See also cold cracl ing, lamellar tearing, and stress-relief cracl ing.

 

hot working.

Deforming metal plastically at sucha temperature and strain rate that recrystallization takes place simultaneously with the deformation, thus avoiding any strain hardening.Contrast with c old ii orking.

 

hot dip coating.

A metallic coating obtained bydipping the base metal into a molten metal.

 

hot shortness.

A tendency for some alloys to separate along grain boundaries when stressed ordeformed at temperatures near the melting point.Hot shortness is caused by a low-melting constituent, often present only in minute amounts,that is segregated at grain boundaries.

 

huey test.

Corrosion testing in a boiling solution of nitric acid. This test is mainly used to detect the susceptibilty to intergranular corrosion of stainless steel.

 

humidity test.

A corrosion test involving exposureof specimens at controlled levels of humidity and temperature. Contrast with salt-fog test.

 

hydrogen-assisted cracking (HAC).

See hydrogenembriltlement.

 

hydrogen-assisted stress-corrosion cracking (HSCC).

See hydrogen embrittlement.

 

hydrogen blistering.

The formation of blisters on or below a metal surface from excessive internal hydrogen pressure; Formation of blister-like bulges on a ductile metal surface caused by internal hydrogen pressures. Hydrogen may beformed during cleaning, plating, corrosion, and so forth.

 

hydrogen damage.

A general term for the emhrittlement, cracking, blistering. and hydride formation that can occur when hydrogen is present in some metals.

 

hydrogen disintegration

Deep internal cracks caused by hydrogen.

 

hydrogen embrittlement.

A process resulting in adecrease 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.

 

hydrogen-induced cracking (HIC).

Same as hydrogen embrittlement.

 

hydrogen overvoltage.

Overvoltage associated with the liberation of hydrogen gas.

 

hydrogen stress cracking (HSC).

See hydrogen embrittlement.

 

hydrolysis.

(1) Decomposition or alteration of a chemical substance by water. (2) In aqueous solutions of electrolytes, the reactions of cations with water to produce a weak base or of anions to produce a weak acid.

 

hydrophilic.

Having an affinity for water. Contrast with hydrophobic.

 

hydrophobic.

Lacking an affinity for, repelling, orfailing to absorb or adsorb water. Contrast with hydrophilic.

 

hygroscopic.

(1) Possessing a marked ability to accelerate the condensation of water vapor; applied to condensation nuclei composed of salts that yield aqueous solutions of a very low equilibrium vapor pressure compared with that of pure water at the same temperature. (2) Pertaining to a substance whose physical characteristics are appreciably altered by effects of water vapor. (3) Pertaining to water absorbed by dry soil minerals from the atmosphere; the amounts depend on the physicochemical character of the surfaces, and increase with rising relative humidity.

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