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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.
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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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