Glossary of Corrosion Related Terms

H

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 applications.

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 surface at which the metal exhibits a specific hardness (50 HRC, for example) or a specific percentage of martensite in the microstructure.

hardfacing

Depositing filler metal on a surface 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 caused 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 are of iron. (2) An iron oxide, Fe,O,, corresponding to an iron content of approximately 70%.

high-temperature hydrogen attack

A loss of strength and ductility of .steel by high-temperature reaction of absorbed hydrogen with carbides in the steel resulting in decarburization and internal fissuring.

holidays

Discontinuities in a coating (such as porosity, cracks, etc.) 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 effect of oxidation and reactions with sulfur compounds and other contaminants, such us chlorides, to form a molten salt on a metal surface that f1uxes, destroys, or disrupts the normal protective oxide. See also gaseous erosion.

hot cracking

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

hot working

Deforming metal plastically at such a temperature and strain rate that recrystallization takes place simultaneously with the deformation, thus avoiding any strain hardening. Contrast with cold working.

hot dip coating

A metallic coating obtained by dipping the base metal into a molten metal.

hot shortness

A tendency for some alloys to separate along grain boundaries when stressed or deformed 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 susceptibility to intergranular corrosion of stainless steel.

humidity test

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

hydrogen-assisted cracking (HAC)

See hydrogen embrittlement.

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 be formed during cleaning, plating, corrosion, and so forth.

hydrogen damage

A general term for the embrittlement, 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 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 crack growth 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 presence 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, or failing 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 physiochemical character of the surfaces, and increase with rising relative humidity.

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