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Corrosion Glossary - C
coating or deposit: a layer consisting of a mixture of calcium
carbonate and magnesium hydroxide deposited on surfaces being cathodically protected
because of the increased pH adjacent
to the protected surface.
- Cadmium plating: the
electrolytic deposition of
cadmium to provide galvanic corrosion
protection. Restricted by environmental considerations.
- Cadmium ion plating: the deposition
of cadmium by a vacuum process to
provide galvanic corrosion
- Calcium: one of the principal elements
in the earth's crust. When dissolved, in
water, calcium is a factor contributing to the formation of scale and insoluble
soap curds that are a means of clearly identifying
- Calcium carbonate equivalent: a common
basis for expressing the concentration of
hardness and other salts in chemically
equivalent terms to simplify certain calculations; signifies that the concentration
of a dissolved mineral is chemically equivalent to the stated concentration
of calcium carbonate.
- Calcium hypochlorite: a chemical compound,
[Ca(ClO)2.4H2O], used as a bleach and as a source of chlorine
in water treatment for chlorination.
Specifically useful because it is stable as a dry powder and can be formed into
- Calomel electrode: an electrode
widely used as a reference electrode
of known potential in electrometric measurement of acidity and alkalinity, corrosion
studies, voltammetry, and measurement of the potentials of other electrodes.
See also electrode potential, reference electrode, and saturated calomel electrode.
- Calorie: the mean calorie is 1/100 of
the heat required to raise the temperature of 1 gram of water from 0°C to 100°C
at a constant atmospheric pressure. It is about equal to the quantity of heat
required to raise one gram of water 1°C. Another definition is, one calorie
is 4.1860 joules.
- Capillary action: a phenomenon in which
water or many other liquids will rise above the normal liquid level in a tiny
tube or capillary, due to attraction between molecules of the liquid for each
other and the walls of the tube.
- Carbide: a chemical compound formed
between carbon and a metal or metals; examples are tungsten carbide, tantalum
carbide, titanium carbide, chromium carbide.
- Carbon chloroform extract: the matter
adsorbed from a stream of water by activated carbon, and then extracted from
the activated carbon with chloroform, using a specific standardized procedure;
a measure of the organic matter in a water.
- Carbon dioxide: a gas present in the
atmosphere and formed by the decay of organic matter; the gas in carbonated
beverages; in water it forms carbonic acid.
- Carbonaceous: materials of or derived
from organic substances such as coal, lignite, peat, etc.
- Carbonaceous exchanger: ion exchange
material produced by the sulfonation of carbonaceous matter.
- Carbonate alkalinity: alkalinity due
to the presence of the carbonate ion (CO32-).
- Carbonate hardness:
hardness due to the presence of
calcium and magnesium bicarbonates and carbonates in water; the smaller of the
total hardness and the total alkalinity. (see temporary hardness.)
- Carbon dioxide: common gas molecule
consisting of one carbon atom and two oxygen atoms made by burning carbon substances
such as fossil fuels or by metabolism in the body.
- Carbonitriding: similar to Carburizing.
Diffusion of carbon and nitrogen at about 900oC (by
pack, gas, salt bath or plasma
process) into low carbon steel, followed by quenching and tempering to produce
martensitic case (typically 1 mm thick).
- Carbon steel:
steel that has properties made up mostly
of the element carbon and which relies on the carbon content for structure.
Most of the steel produced in the world is carbon steel.
- Carboxylic: an organic acidic group
(COOH) which contributes cation exchange ability to some resins.
- Carburizing (also called case hardening):
diffusion of carbon at about 900oC (by
pack, gas, salt bath or plasma
process) into low carbon steel, followed by quenching and tempering to produce
martensitic case (typically 1 mm thick).
- Carburizing flame: a nonstandard term
for reducing flame.
- Carrier gas: usually nitrogen or argon
gas that carries powder into the thermal spray process.
- Casehardening: See Carburizing.
- CASS test: see copper-accelerated
- Catalyst: substance whose presence increases
the rate of a chemical reaction. (Paint) Acid catalyst added to an epoxy
resin system to accelerate drying time.
- Cathode: the electrode of an
electrolytic cell at which
reduction is the principal reaction. Electrons flow toward the cathode in the
- Cathodic coating: coatings which become
the cathode in an electrochemical
cell with the substrate (anode). This type of coating protects the substrate
from corrosion only by being a complete barrier. If the coating allows the environment
to reach the substrate, accelerated corrosion of the substrate will occur.
- Cathodic corrosion: corrosion of a metal
when it is a cathode. (It usually happens to metals because of a rise in
pH at the cathode or as a result
of the formation of hydrides).
- Cathodic disbondment: the destruction
of adhesion between a coating and its substrate by products of a cathodic reaction.
- Cathodic inhibitor: an inhibitor that
reduces the corrosion rate by acting on the cathodic (reduction) reaction.
- Cathodic pickling: electrolytic pickling
in which the work is the cathode.
- Cathodic polarization: the change of
the electrode potential in the active (negative) direction due to current flow.
Cathodic protection: a corrosion control system in which the
metal to be protected is made to serve as a cathode, either by the deliberate
establishment of a galvanic cell or by impressed current. (see anode.)
- Cathodic reaction: electrode reaction
equivalent to a transfer of negative charge from the electronic to the ionic
conductor. A cathodic reaction is a reduction process.
- Catholyte: the electrolyte adjacent
to the cathode of an electrolytic cell.
- Cation: a positively charged ion.
- Cation exchange: ion exchange process
in which cations in solution are exchanged for other cations from an ion exchanger.
- Caustic: any substance capable of burning
or destroying animal flesh or tissue. the term is usually applied to strong
- Caustic cracking: stress corrosion cracking
of metals in caustic solutions. (See also stress
- Caustic dip: a strongly alkaline solution
into which metal is immersed for etching, for neutralizing acid, or for removing
organic materials such as greases or paints.
- Caustic embrittlement: see caustic cracking.
- Caustic soda: the common name for sodium
the formation and rapid collapse within a liquid of cavities or bubbles that
contain vapor or gas or both.
- Cavitation damage:the degradation of
a solid body resulting from its exposure to cavitation. (This may include loss
of material, surface deformation, or changes in properties or appearance).
progressive loss of original material from a solid surface due to continuing
exposure to cavitation.
- Cementation coating: a coating developed
on a metal surface by a high temperature diffusion process (as carburization,
calorizing, or chromizing).
- Cement bacillus: older term for
- Cementite: a compound of
iron and carbon, known chemically as
iron carbide and having the chemical formula Fe3C. It is characterized
by an orthorhombic crystal structure.
- Cermet: a physical mixture of ceramics
and metals; examples are alumina plus nickel and zirconia plus nickel.
- Chalking: the development of loose removable
powder at the surface of an organic
coating usually caused by weathering.
- Charge, charging: putting raw materials
into a furnace. For example, a blast furnace is charged with coke, coal, iron
or scrap to make raw steel. The charge itself is the amount of material loaded
into the furnace.
- Checking: the development of slight
breaks in a coating that do not penetrate to the underlying surface.
- Checks: numerous, very fine cracks in
a coating or at the surface of a metal part. Checks may appear during processing
or during service and are most often associated with thermal treatment or thermal
cycling. Also called check marks, checking, or heat checks.
- Chelate: a molecular structure in which
a heterocyclic ring can he formed by the unshared electrons of neighboring atoms
or a coordination compound in which a heterocyclic ring is formed by a metal
bound to two atoms of the associated ligand. See also complexation.
- Chelating agent: a chemical compound
sometimes fed to water to tie up undesirable metal ions, keep them in solution,
and eliminate or reduce the normal effects of the ion. (see sequestering agent.)
- Chelation: the process of forming complex
chemical compounds in which certain metal ions are bound into stable ring structures,
keeping the ions in solution and eliminating or reducing normal (and often undesirable)
effects of the ions. Similar to the process of sequestration.
conversion coating: a protective or decorative nonmetallic coating
produced in situ by chemical reaction of a metal with a chosen environment.
(It is often used to prepare the surface prior to the application of an organic
- Chemical potential: in a
thermodynamic system of several constituents,
the rate of change of the Gibbs function of the system with respect to the change
in the number of moles of a particular constituent.
- Chemical stability: resistance to attach
by chemical action.
- Chemical vapor deposition: a
coating process, similar to
gas carburizing and carbonitriding, whereby a reactant atmosphere gas is fed
into a processing chamber where it decomposes at the surface of the workpiece,
liberating one material for either absorption by, or accumulation on the workpiece.
A second material is liberated in gas form and is removed from the processing
chamber, along with excess atmosphere gas.
- Chemisorption: a process related to
adsorption in which atoms or molecules of reacting substances are held to the
surface atoms of a catalyst by electrostatic forces having about the same strength
as chemical bonds. Chemisorption differs from physical adsorption chiefly in
the strength of bonding, which is much greater in chemisorption than in adsorption.
- Chloramination: the process of treating
drinking water by applying chlorine before or after ammonia. This creates a
persistent disinfectant residual.
- Chlorides: salts of chloride are generally
soluble. High concentrations contribute to corrosion problems.
the treatment process in which chlorine gas or a chlorine solution is added
to water for disinfection and control of microorganisms.
Chlorination is also
used in the oxidation of dissolved iron, manganese, and hydrogen sulfide impurities.
- Chlorinator: a device designed to feed
chlorine gas or solutions of its compounds, such as hypochlorite, into a water
- Chlorine: a widely used gas used in
the disinfection of water and an oxidizing agent for organic matter. (see
- Chlorinated rubber: a non-convertible
binder used to produce single
pack paints, with good resistance to acids, alkalies, humidity.
- Chlorine demand: a measure of the amount
of chlorine which will be consumed by organic matter and other oxidizable substances
in a water before a chlorine residual will be found. Chlorine demand represents
the difference between the total chlorine fed and the chlorine residual. (see
the total halogen ion (chloride, bromide, etc.) content as titrated by the addition
of silver nitrate, expressed in parts per thousand, i.e. o/oo.
- Chlorofluorocarbon (CFC): gas containing
carbon and fluorine in various combinations. CFCs are generally volatile and
are alleged to decrease the earth's protective ozone layer.
- Chroma:a measure of color. The degree
of saturation of a hue. A color at its full intensity has maximum chroma.
- Chromadizing: improving paint adhesion
on aluminum or aluminum alloys, mainly aircraft skins, by treatment with a solution
of' chromic acid. Also called chromodizing or chromatizing. Not to be confused
with chromating or chromizing.
treatment: a treatment of metal in a solution of a hexavalent
chromium compound to produce a conversion
coating consisting of trivalent and hexavalent chromium compounds.
chromate conversion is a process which completely degreases and removes all
traces of the oxide film, replacing it by immersion with a chromate coating
which can then be painted. It is used as a post-treatment for cadmium, zinc
and aluminum coatings.
- Chromium: a steel-gray,
lustrous, hard and brittle metallic element that takes its name from the Greek
word for color (chrome) because of the brilliant colors of its compounds. It
is found primarily in chromite. Resistant to tarnish and corrosion, it is a
primary component of
stainless steel and is used to harden steel alloys.
- Chromizing: a surface treatment at elevated
temperature, generally carried out in
pack, vapor, or salt bath, in
which an alloy is formed by the inward diffusion of
chromium into the base metal.
- Clad metal: a composite
metal containing, two or more layers that have been bonded together. The bonding
may have been accomplished by co-rolling, co-extrusion, welding, diffusion bonding,
casting, heavy chemical deposition, or heavy electroplating.
- Cladding: a surfacing
variation that deposits or applies surfacing material, usually to improve corrosion
or heat resistance.
- Clear coating: a transparent protective
and/or decorative film, generally the final coat of sealer applied to automotive
- Cleavage: splitting (fracture) of a
crystal on a crystallographic plane of' low index.
- Cleavage fracture: a fracture, usually
of polycrystalline metal, in which most of the grains have failed by cleavage,
resulting in bright reflecting facets. It is associated with low-energy brittle
- Coagulant: a material, such as alum,
which will form a gelatinous precipitate in water and cause the agglomeration
of finely divided particles into larger particles which can then be removed
by settling and/or filtration.
- Coagulant aid: a material which is not
a coagulant, but which improves the effectiveness of a coagulant by forming
larger or heavier particles, speeding the reactions, or by permitting reduced
- Coagulation: the clumping together of
very fine colloidal (less than 0.1 micron in size) and dispersed (0.1 to 100
microns in size) particles into larger visible agglomerates of these particles
(usually between 100 and 1,000 microns in size) caused by the use of chemicals
(coagulants). The chemicals neutralize the electrical charges of the fine particles
and cause destabilization of the particles. This clumping together makes it
easier to separate the solids from the water by settling, skimming, draining,
- Coalescence: the union or growing together
of colloidal particles into a group or larger unit as a result of molecular
attraction on the surfaces of the particles.
- Coalescent aid: the small amount of
solvent contained in latex coatings. Not a true solvent since it does not actually
dissolve the latex resins, the coalescent aid helps the latex resins flow together,
aiding in film formation.
- Coal tar epoxy: a
combination of epoxy, curing agent,
and tar products which give a very water resistant film.
a paint, varnish, lacquer or other finish used to create a protective and/or
decorative layer. Generally used to refer to paints and coatings applied in
an industrial setting as part of the original equipment manufacturer (OEM) process.
- Coating strength: a) a measure of the
cohesive bond within a coating, as opposed to coating-to-substrate bond (adhesive
strength), b) the tensile strength of a coating, usually expressed in kpa (psi).
- Coating stress: the stresses in a
coating resulting from rapid
cooling of molten material or semimolten particles as they impact the substrate.
coating stresses are a combination of body and textural stresses.
- COD: the abbreviation for "chemical
- Cohesion: a bonding together of a single
substance to itself. Internal adhesion.
- Cold cracking: a type of weld cracking
that usually occurs below 203ºC. Cracking may occur during or after cooling
to room temperature, sometimes with a considerable time delay. Three factors
combine to produce cold cracks: stress (for example, from thermal expansion
and contraction), hydrogen (from hydrogen-containing welding consumables), and
a susceptible microstructure (plate martensite is most susceptible to cracking,
ferritic and bainitic structures least susceptible). (see also
stress corrosion cracking)
- Cold-rolling (CR): rolling steel without
first reheating it. This process reduces thickness of the steel, produces a
smoother surface and makes it easier to machine.
- Cold welding: cohesion between two surfaces
of a metal, generally under the influence of externally applied pressure at
- Cold working: deforming metal plastically
under conditions of temperature and strain rote that induce strain hardening.
Usually, hut not necessarily, conducted at room temperature. Contrast with hot
- Coliform bacteria: a group of microorganisms
used as indicators of water contamination, and the possible presence of pathogenic
(disease producing) bacteria.
- Colloid: very finely divided solid particles
which do not settle out of a solution; intermediate between a true dissolved
particle and a suspended solid which will settle out of solution. The removal
of colloidal particles usually requires coagulation.
- Colorant: concentrated color (dyes or
pigments) that can be added to paints to make specific colors.
- Colorfast: non-fading in prolonged exposure
- Color Retention: the ability of paint
to keep its original color. Major threats to color retention are exposure to
ultraviolet radiation and abrasion by weather or repeated cleaning.
- Combined available chlorine: the chlorine
present as chloramine or other chlorine derivatives in a water, but still available
for disinfection and the oxidation of organic matter. Combined chlorine compounds
are more stable than free chlorine forms, but are somewhat slower in disinfection
- Combustion: The rapid chemical combination
of oxygen with the combustible elements of a fuel resulting in the release of
- Combustion efficiency: The effectiveness
of the burner to completely burn the fuel. A well designed burner will operate
with as little as 10 to 20% excess air, while converting all combustibles in
the fuel to useful energy.
- Compatibility: the ability of two or
more substances to mix with each other in a wet or dry state to form a homogeneous
- Compensated hardness: a calculated value
based on the total hardness, the
magnesium to calcium ratio and the sodium concentration of a water. It is used
to correct for the reductions in hardness removal capacity caused by these factors
in zeolite exchange water softeners.
- Complexation: the formation of complex
chemical species by the coordination of groups of atoms termed ligands to a
central ion, commonly a metal ion. Generally, the ligand coordinates by providing
a pair of electrons that forms an ionic or covalent bond to the central ion.
See also chelate, coordination compound, and ligand.
- Composite: mixture of two or more materials.
Nearly all have a reinforcing material (wood, glass, etc.), called filler, and
a natural or artificial resin, called matrix to achieve specific characteristics
and required properties.
- Composite coating: mixture of two or
more materials. Many thermal spray coatings could be considered as composites.
- Composite powder: a powder in which
each particle consists of two or more distinct materials joined together.( Not
the same as a powder blend.).
- Computerized maintenance management system:
a computerized system to assist with the effective and efficient management
of maintenance activities through the application of computer technology.
cell: an electrolytic
cell, the emf of which is caused by a difference in concentration of some
component in the electrolyte. (This difference leads to the formation of discrete
cathode and anode regions).
polarization: that portion of the polarization of a cell produced
by concentration changes resulting from passage of' current through the electrolyte.
- Condensate: Condensed water resulting
from the removal of latent heat from steam.
- Condition based maintenance: an equipment
maintenance strategy based on measuring the condition of equipment in order
to assess whether it will fail during some future period, and then taking appropriate
action to avoid the consequences of that failure.
- Condition monitoring: the use of specialist
equipment to measure the condition of equipment. Vibration Analysis, Tribology
and Thermography are all examples of Condition Monitoring techniques.
- Conditional probability of failure:
the probability that an item will fail during a particular age interval, given
that it survives to enter that age.
- Conductance: a measure of the ability
of a material to conduct electrical current. The reciprocal of the resistance
of the material, expressed in siemens (mhos).
a) a material property relating heat flux (heat transferred per unit area per
unit time) to a temperature difference. b) The property of a water or soil sample
to transmit electric current (inverse of resistivity) under a set of standard
conditions. Usually expressed as microhms
- Connate water: water deposited simultaneously
with rock and held with essentially no flow; usually occurs deep in the earth,
and usually is high in minerals due to long contact.
- Contact resistance: the resistance in
ohms between two objects in contact with each other.
- Continuity bond: a metallic connection
that provides electrical continuity between metal structures.
- Continuous blowdown: the uninterrupted
removal of concentrated boiler water from a boiler to control total solids concentration
in the remaining water.
- Convection: the transmission of heat
by the circulation of a liquid or gas. It may be natural, with the circulation
caused by buoyancy affects due to temperature differences, or forced with circulation
caused by a mechanical device such as a fan or pump.
- Conversion coating: a coating consisting
of' a compound of the surface metal, produced by chemical or electrochemical
treatments of the metal. Examples include chromate coatings on zinc, cadmium,
magnesium, and aluminum and oxide and phosphate coatings on steel. See also
chromate treatment and phosphating.
structure in a power plant used to remove heat from cooling
water from the condenser. The cooling
tower prevents thermal pollution of lakes and rivers.
- Copper cake: a by-product of electolytic
zinc refining, usually containing a fair amount of cobalt.
- Copper accelerated salt spray (CASS) test:
an accelerated corrosion test
for some electrodeposits for anodic
coatings on aluminum.
- Copper plating: the
electrolytic deposition of
copper to provide either a corrosion barrier (often as an undercoat for hard
chrome plate) or for reclamation of worn parts.
- Copper strip corrosion: a standard for
the evaluation of an oil's tendency to corrode copper or copper alloys (see
ASTM D130). Test results
are based on the matching of corrosion stains. Non corrosiveness is not to be
confused with rust inhibiting, which deals with the protection of a surface
from some contaminant, such as water, rather than the oil itself.
- Corona charge: an electrostatic charge
induced on powder particles by passing them through an electrostatic field generated
by a high-voltage device.
- Corrective maintenance: any maintenance
activity which is required to correct a failure that has occurred or is in the
process of occurring. This activity may consist of repair, restoration or replacement
- Corrodkote test: an accelerated
corrosion test for electrodeposits.
- Corrosion: the chemical or
electrochemical reaction between a material,
usually a metal, and its environment
that produces a deterioration of the material and its properties.
fatigue: the process in which a metal fractures prematurely under
conditions of simultaneous corrosion and repeated cyclic loading at lower stress
levels or fewer cycles than would be required in the absence of the corrosive
- Corrosion fatigue strength: the maximum
repeated stress that can be endured by a metal without failure under definite
conditions of corrosion and fatigue and for a specific number of stress cycles
and a specified period of time.
inhibitive: a type of metal
paint or primer that prevents
rust by preventing moisture from reaching the metal. Zinc phosphate, barium
metaborate and strontium chromate (all pigments) are common ingredients in corrosion-inhibitive
coatings. These pigments absorb any moisture that enters the paint film.
potential: the potential of a corroding surface in an electrolyte
relative to a reference electrode measured under open-circuit conditions.
- Corrosion product: substance formed
as a result of corrosion.
- Corrosion rate: the amount
of corrosion occurring in unit time. (For example, mass change per unit area
per unit time; penetration per unit time).
resistance: ability of a metal to withstand corrosion in a given
- Corrosive wear: wear in which chemical
or electrochemical reaction with the environment is significant.
- Corrosivity: tendency of an environment
to cause corrosion in a given corrosion system.
- Counter electrode: See auxiliary electrode.
- Cratering: the formation of small bowl-shaped
depressions in a coating film.
These depressions frequently have drops or bands of material at their centers
and raised circular edges. Some common causes of cratering are gel particles,
oil droplets from air lines, and substrate contamination.
- Crawling and dewetting: the tendency
of a wet paint film to recede
from certain areas of a painted surface. A frequent cause of dewetting is a
dirty substrate that may result from poor wetting, contamination, fingerprints,
or cutting oils.
- Crazing: a network of checks or cracks
appearing on a coated surface.
- Creep: time-dependent strain occurring
under stress. The creep strain occurring at a diminishing rate is called primary
creep; that occurring at a minimum and almost constant rate, secondary creep;
and that occurring at an accelerating rate, tertiary creep.
- Crenothrix polyspora: a genus of filamentous
bacteria which utilize iron in their metabolism, and cause staining, plugging
and taste and odor problems in water systems. (see iron bacteria.)
- Creosote: a liquid coating made from
coal tar once used as a wood preservative. It has been banned for consumer use
because of potential health risks.
corrosion: localized corrosion of a metal surface at, or immediately
adjacent to, an area that is shielded from full exposure to the environment
because of close proximity between the metal and the surface of another material.
- Critical anodic current density: the
maximum anodic current density observed in the active region for a metal or
alloy electrode that exhibits active-passive behavior in an environment.
- Critical flaw size: the size of a flaw
(defect) in a structure that will cause failure at a particular stress level.
- Critical humidity: the relative humidity
above which the atmospheric corrosion rate of some metals increases sharply.
- Criticality: the priority rank of a
failure mode based on some assessment criteria.
- Critical pitting potential: the least
noble potential where pitting corrosion
will initiate and propagate. (See breakdown potential ).
- Cross-sectional area: the area of a
plane at a right angle to the direction of flow through a tank or vessel; often
expressed in square feet, and related to the flow rate. (example: 5 gallons
per minute per square foot of ion exchanger bed area.)
- Cure, curing: the process whereby a
liquid coating becomes a hard
film. Enamels cure. Lacquers do
density: the electric current to or from a unit area of an electrode
- Current efficiency: the ratio of the
electrochemical equivalent current
density for a specific reaction to the total applied current density.
- CVD: See
chemical vapor deposition.