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Corrosion Glossary - P
- Pack rust:
this particular form of corrosion is often used in relation to bridge
inspection to describe built-up members) of steel bridges which are already
showing signs of rust packing between steel plates.
- Paint: a
resin, a solvent, additives, pigments
and, in some products, diluents. Paints are generally opaque, and commonly represent
the portion of the industry known as "architectural coatings."
- Paint remover: a chemical that softens
old paint or varnish and permits
it to be easily scraped off. Also called "stripper."
- Paint thinner: see mineral spirits.
- P alkalinity: phenolphthalein alkalinity
of a water as determined by titration with standard acid solution to the phenolphthalein
endpoint (pH approx 8.3). Includes
carbonate and hydroxide alkalinity.
- Particle size: a) a measure of dust
size, expressed in microns or per cent passing through a standard mesh screen;
b) the size of a particle suspended in water as determined by its smallest dimension,
- Parting: the
selective corrosion of one or more
components of a solid solution alloy.
- Parting limit: the minimum concentration
of a more noble component in an alloy, above which parting does not occur in
a specific environment.
- Parts per billion (ppb): a measure of
proportion by weight, equivalent to one unit weight of a material per billion
(10^9) unit weights of compound. One part per billion is equivalent to 1 mg/kg.
- Parts per million (ppm): a measure of
proportion by weight, equivalent to one unit weight of a material per million
(10^6) unit weights of compound. One part per million is equivalent to 1 mg/Mg
- Pass: a confined passageway, containing
heating surface, through which a fluid flows in essentially one direction.
- Passivation: the process in metal corrosion
by which metals become passive. (See passive).
- Passivator: a type of inhibitor which
appreciably changes the potential of a metal to a more noble (positive) value.
- Passive: the state of the metal surface
characterized by low corrosion rates in a potential region that is strongly
oxidizing for the metal.
- Passive-active cell: a corrosion cell
in which the anode is a metal in the active state and the cathode is the same
metal in the passive state.
- Passivity: a condition in which a piece
of metal, because of an impervious covering of oxide or other compound, has
a potential much more positive than that at the metal in the active state.
- Patina: the coating, usually green,
that forms on the surface of metals such as copper and copper alloys exposed
to the atmosphere. also used to describe the appearance of a weathered surface
of any metal.
- Pearlite: a metastable lamellar aggregate
of ferrite and cementite resulting from the transformation of austenite at temperatures
above the bainite range.
- Peening: blasting process using spherical
shaped beads or shot for cleaning and/or modifying surface properties.
- Percent solids: the percentage mass
of non-liquid components in paint.
- Permeability: a property measured as
a rate of passage of a liquid or gas through a coating.
- Perfect combustion: the complete oxidation
of all the combustible constituents of a fuel, utilizing all the oxygen supplied.
- Periodic chart: a chart containing all
the nuclides, i.e., all elements and their family of isotopes. Older charts
showed only the elements.
- Permanent hardness: water
hardness due to the presence of
the chlorides and sulfates of calcium and magnesium, which will not precipitate
by boiling. This term is largely replaced by ‘noncarbonate hardness’.
- Permanganate: Generally refers to potassium
permanganate, a chemical compound used in water treatment.
- Permeability (magnetic): a relative
measure of the ability of a material to serve as a path for magnetic times of
force based on air=1. Permeability is the magnetic induction B in testa (gauss)
divided by the magnetizing force H in ampere per meter (oersteds).
- Petroleum: naturally occurring mineral
oil consisting predominately of hydrocarbons.
a measure of the acidity or alkalinity of a solution; the negative logarithm
of the hydrogen-ion activity; it denotes the degree of acidity or basicity of
a solution. at 25ºC, 7.0 is the neutral value. Decreasing values below 7.0 indicate
increasing acidity; increasing values above 7.0, increasing basicity.
(phosphating or phosphate conversion coating): phosphatizing
is a metal pretreatment primarily used to prepare steel for paint or coatings
and to prevent corrosion. Phosphatizing is termed a conversion coating because,
unlike paint or traditional coatings, it does not lie on the surface of the
metal, but rather the surface of the metal is chemically changed to a new substance.
The coating will, therefore, not chip- or scratch-off exposing the base material
to corrosion and wear.
- Photo-thermal NDT: an NDT technique
for spayed coatings. A repeated pulse of heat, from a laser source, flows through
the coating and substrate. The thermal signature is detected and related to
the input signal thereby indicating coating thickness.
vapor deposition: a coating process whereby the cleaned and masked
component to be coated is heated and rotated on a spindle above the streaming
vapor generated by melting and evaporating a coating material source bar with
a focused electron beam in an evacuated chamber.
- Physisorption: the binding of an adsorbate
to the surface of a solid by forces whose energy levels approximate those of
condensation. contrast with chemisorption.
- Pickle: a solution or process used to
loosen or remove corrosion products such as scale or tarnish.
- Pickling: removing surface oxides from
metals by chemical or electrochemical
reaction. Treating the surface of iron
or steel with acid to remove scale,
rust and dirt, preparatory to further processing such as cold rolling, tinning,
galvanizing, or polishing.
- Pig iron: the product of the blast furnace,
when cast in a pig bed or in a pig-casting machine. It derives its name from
the fact that the channel or runner leading from the furnace branched out into
side channels called sows, and then into smaller channels called pigs. Pig iron
today is sold on chemical analysis.
insoluble, finely ground materials that give paint its properties of color and
hide. Titanium dioxide is the most important pigment used to provide hiding
in paint. Other pigments include anatase titanium, barium metaborate, barium
sulfate, burnt sienna, burnt umber, carbon black, China clay, chromium oxide,
iron oxide, lead carbonate, strontium chromate, Tuscan red, zinc oxide, zinc
phosphate and zinc sulfide.
Pit Gauge or US Pit Gage:
a device to measure the depth of corrosion. A pit gauge can take
various forms, in the most basic as a simple lever and pointer or
more accurate units using a dial or digital indicator to provide the
measurement displacement. Pit depth gauge, depth gauge, bridging
corrosion of a metal surface, confined to a point or small area, that takes
the form of cavities.
factor: ratio of the depth of the deepest pit resulting from
corrosion divided by the average penetration as calculated from weight loss.
resistance equivalent number: an empirical relationship to predict
the pitting resistance of austenitic and duplex stainless steels. It is expressed
as PREN = Cr + 3.3 (Mo + 0.5 W) + 16N.
- pK: the reciprocal of the logarithm
of the ionization constant of a chemical compound.
- Plane strain: the stress condition in
linear elastic fracture mechanics in which there is zero strain in a direction
normal to both the axis of applied tensile stress and the direction of crack
growth (that is, parallel to the crack front); most nearly achieved in loading
thick plates along a direction parallel to the plate surface. Under plane-strain
conditions, the plane of fracture instability is normal to the axis of the principal
- Plasma: a gas so hot that all electrons
are stripped away from the atoms. As such, the gas has a positive charge and
can be confined in a magnetic field. High-temperature plasma is used in controlled
- Plasma jet or plasma flame: a jet of
highly ionised gas usually produced from a plasma torch. An electric arc is
struck between a cathode and anode and is then blown through a nozzle to form
the flame or jet.
- Plasma spraying:
a thermal spraying process in which the coating material is melted with heat
from a plasma torch that generates a non-transferred arc. The molten
coating material is propelled against the base metal by the hot, ionized
gas issuing from the torch.
- Plastic deformation: the permanent (inelastic)
distortion of metals under applied
stresses that strain the material beyond its elastic limit.
- Plasticity: the property that enables
a material to undergo permanent deformation without rupture.
- Plate: wide, flat-rolled
steel. It is now generally accepted
that steel more than 3 mm (1/8 inch) thick is plate and less than 3 mm is sheet
- Plowing: the formation of grooves by
plastic deformation of the softer of two surfaces in relative motion.
- pOH: the negative logarithm of the hydroxyl
ion concentration. The pOH is related to pH by the expression:
pH + pOH = 14.
- Polarization: the change from the open-circuit
electrode potential as the result of the passage of current.
- Polarization admittance: the reciprocal
of polarization resistance (di/dE).
- Polarization curve: a plot of current
density versus electrode potential for a specific electrode-electrolyte combination.
- Polarization resistance: the slope (dE/di)
at the corrosion potential of a potential (E) current density (i) curve. (It
is inversely proportional to the corrosion current density when the polarization
resistance technique is applicable.)
- Polishing: the smoothing of a material
surface by means of the action of abrasive particles attached usually to a fabric
cloth. The final mechanical step in metallographic preparation.
- Polyester: a condensation polymer formed
by the interaction of polyhydric alcohols and polybasic acids. They are used
in some coatings and the manufacture of glass-fibres products. See Alkyd resin.
- Polymer: a chain of organic molecules
produced by the joining of primary units called monomers.
- Polymerization: the interlocking of
molecules by chemical reaction to produce very large molecules. The process
of making plastics and plastic-based
- Polyphosphate: A form of phosphate polymer
consisting of a series of condensed phosphoric acids containing more than one
atom of phosphorous. Polyphosphate is used as a sequestering agent to control
iron and hardness, and as a coating agent that forms a thin passivating film
on metal surfaces to control corrosion.
- Polyurethane: a type of paint or resin
known for its toughness, flexibility, weather resistance, chemical resistance,
and abrasion resistance. The resin or vehicle commonly used to make topcoats.
- Polyvinyl chloride: a synthetic resin
used in the binders of coatings.
Tends to discolor under exposure to ultraviolet radiation. Commonly called "vinyl."
- Porosity: a measure of the volume of
internal pores, or voids, in ion exchangers and filter media; sometimes expressed
as a ratio to the total volume of the medium. (See void volume.)
- Portland cement:
is a closely controlled chemical combination of calcium, silicon, aluminum,
iron and small amounts of other compounds, to which gypsum is added in the final
grinding process to regulate the setting time of the concrete.
- Positive charge: the net electrical
charge on an electrode or ion in solution due to the removal of electrons.
- Postchlorination: The application of
chlorine to a water following other water treatment processes.
diagram: see Pourbaix diagram.
refers to a technique wherein the potential of an electrode with respect to
a reference electrode is varied at a selected rate by application of a current
through the electrolyte.
- Potentiodynamic (potentiokinetic): the
technique for varying the potential of an electrode in a continuous manner at
a preset rate.
- Potentiostaircase: refers to a potentiostep
technique for polarizing an electrode in a series of constant potential steps
wherein the time duration and potential increments or decrements are equal for
- Potentiostat: an instrument for automatically
maintaining an electrode in an electrolyte at a constant potential or controlled
potentials with respect to a suitable reference electrode.
- Potentiostatic: the technique for maintaining
a constant electrode potential.
- Potentiostep: refers to a technique
in which an electrode is polarized in a series of potential increments or decrements.
- Pot life: the useable or sprayable life
of a coating after mixing has occurred. As mixed material reacts in the pot,
the viscosity always increases.
- Poultice corrosion: a term used in the
automotive industry to describe the corrosion of vehicle body parts due to the
collection of road salts and debris on ledges and in pockets that are kept moist
by weather and washing. Also called deposit corrosion or attack.
- Pourbaix diagram
(electrode potential-pH diagram): a graphical representation showing
regions of thermodynamic stability of species in metal-water electrolyte systems.
- Powder coating: a
polymeric coating deposited via electrostatic
attraction and applied to the surface as a dry, finely ground powder and then
heated above its melting point so the powder particles flow together or cure.
- Powder metallurgy: the art of producing
metal powders and utilizing metal powders for production of' massive materials
and shaped objects.
- Prechlorination: The application of
chlorine to a water prior to other water treatment processes.
- Precious metal: one of the relatively
scarce and valuable metals: gold, silver, and the platinum-group metals. also
called noble metal(s).
- Precipitate: to separate materials from
a solution by the formation of insoluble matter by chemical reaction. the material
which is removed.
- Precipitation: the removal of solid
or liquid particles from a fluid.
- Precipitation hardening: hardening caused
by the precipitation of a constituent from a supersaturated solid solution.
see also age hardening and aging.
- Precipitation heat treatment: artificial
aging in which a constituent precipitated from a supersaturated solid solution.
- Precracked specimen: a specimen that
is notched and subjected to alternating stresses until a crack has developed
at the root of the notch.
- Predictive maintenance: a type of
condition-based maintenance focused
on early prediction of failure using non-destructive techniques such as wear
- Preheated air: air at a temperature
exceeding that of the ambient air.
- Pressure: force per unit of area.
- Pressure differential: a difference
or change in pressure detected between two points in a system due to differences
in elevation and/or pressure drop due to flow.
- Pressure drop: the difference in pressure
between two points in a system, caused by resistance to flow.
- Pressure vessel: a closed vessel or
container designed to confine a fluid at a pressure above atmospheric.
- Pressurized water reactor (PWR): nuclear
reactor in which water is kept under pressure in a vessel to prevent boiling.
Steam is made in a second vessel.
- Preventive maintenance: maintenance
performed according to a fixed schedule involving the routine repair and replacement
of machine parts and components.
- Primary current distribution: the current
distribution in an electrolytic
cell that is free of polarization.
- Primary passive potential (passivation potential):
the potential corresponding to the maximum active current density (critical
anodic current density) of an electrode that exhibits active-passive corrosion
- Primer (prime
coat): the first coat of paint applied to a surface. formulated
to have good bonding and wetting characteristics; may or may not contain inhibiting
- Priming: the discharge of steam containing
excessive quantities of water in suspension from a boiler, due to violent ebullition.
- Principal stress (normal): the maximum
or minimum value at the normal stress at a point in a plane considered with
respect to all possible orientations of the considered plane. on such principal
planes the shear stress is zero. There are three principal stresses on three
mutually perpendicular planes. the state of stress at a point may be:
- uniaxial, a state of stress in which two of the three principal stresses
- biaxial, a state of stress in which only one of the three principal
stresses is zero
- triaxial, a state of stress in which none of the principal stresses
is zero. multiaxial stress refers to either biaxial or triaxial stress.
- Proactive maintenance: a type of condition-based
maintenance focused on the routine detection and correction of root cause conditions
that would otherwise lead to failure. Such root causes as high lubricant contaminant,
alignment and balance are among the most critical.
- Process anneal: annealing close to,
but below, the eutectoid temperature.
- Process steam: steam used for industrial
purposes other than for producing power.
- Products of combustion: the gases, vapors,
and solids resulting form the combustion of fuel.
- Profile: anchor pattern on a surface
produced by abrasive blasting or acid treatment.
- Protection potential: the most noble
potential where pitting and crevice corrosion will not propagate.
- Protective potential: the threshold
value of the corrosion potential
that has to be reached to enter a protective potential range. The term used
in cathodic protection to refer to the minimum potential required to control
- Protective potential range: a range
of corrosion potential values in which unacceptable corrosion resistance is
achieved for a particular purpose.
- Pull-out: pull-out occurs when particles
are plucked from the coating
during machining or grinding. Also occurs during metallographic preparation.
It is sometimes confused with porosity.
- PVD: see
Physical vapor deposition.
- Purge: to introduce air into a vessel
flue passages in such volume and manner as to completely replace the air or
gas-air mixture contained therein.