Corrosion Terminology - P's

paint—a pigmented liquid or resin applied to a substrate as a thin layer that is converted to a solid film after application. (It is commonly used for decoration or protection.) 

paint system—see coating system.

parting—see dealloying.

parting limit—the minimum concentration of a more noble component in an alloy above which dealloying does not occur in a specific environment.

passivation—the process in metal corrosion by which metals become passive. [See passive.]

passivation potential—see primary passive potential.

passivator—a corrosion inhibitor that reduces the corrosion rate of a metal by changing reactions at the metal surface to cause the formation of a protective corrosion product, resulting in a positive shift in corrosion potential.

passive(1) the state of a metal surface characterized by low corrosion rates in a potential region that is strongly oxidizing for the metal. (2) the positive direction of electrode potential.

passivity—the state of being passive.

patina(1) the corrosion product film, usually green, that forms on the surface of copper and copper alloys exposed to the atmosphere. (2)  a corrosion product film on the weathered surface of any metal.

peeling—detachment or partial detachment of a coating from the substrate or undercoat in sheets or strips. [contrast with chipping]

pH—the negative logarithm of the hydrogen ion activity written as:






hydrogen ion activity = the molar concentration of hydrogen ions multiplied by the mean ion-activity coefficient.


phosphating—treatment of steel or other metals with an aqueous phosphate, phosphoric acid solution, orboth to form an adherent phosphate surface layer that can serve as a good base for subsequent coating application. [also known as phosphatizing ]

phosphatizing—See phosphating.

pickling(1) treating a metal or alloy in a chemical bath to remove scale and oxides (e.g., rust) from the surface. (2) complete removal of rust and mill scale by acid pickling, duplex pickling, or electrolytic pickling. [See SSPC-SP8.]

pickling solution—a chemical bath, usually an acid solution, used for pickling.

pigment—a solid substance, generally in fine powder form, that is insoluble in the vehicle of a formulated coating material. It is used to impart color or other specific physical or chemical properties to the coating.

pinhole—a minute hole through a coat or coats that exposes an underlying coat or the substrate.

pipe-to-electrolyte potential—see structure-to-electrolyte potential.

pipe-to-soil potential—see structure-to-electrolyte potential.

pit—a surface cavity with depth equal to or greater than the minimum dimension at the opening. [contrast with crater]

pitting—localized corrosion of a metal surface that is confined to a small area and takes the form of cavities called pits.

pitting factor—the ratio of the depth of the deepest pit resulting from corrosion divided by the average penetration as calculated from mass loss.

pitting resistance equivalent number—A number calculated using a weighted formula typically based on the chromium (Cr), molybdenum (Mo), nitrogen (N), and sometimes tungsten (W) content of an alloy, developed to rank the pitting and crevice corrosion resistance of stainless steels and some nickel-based alloys.(Larger numbers indicate increased resistance to pitting and crevice corrosion in seawater and other halide-containing aqueous environments.) (For example,NACE MR0175/ISO 15156 uses PREN = Cr + 3.3 {Mo + 0.5W} + 16N, where each element symbol represents the mass percent of that element in the alloy.)

plastic deformation—permanent deformation caused by stressing beyond the elastic limit.

plasticity—the ability of a material to deform permanently (nonelastically) without fracturing.

polarization—the change from the corrosion potential as a result of current flow across the electrode/electrolyte interface.

polarization admittance—the reciprocal of polarization resistance.

polarization cell—a DC decoupling device consisting of two or more pairs of inert metallic plates immersed in an aqueous electrolyte. The electrical characteristics of the polarization cell are high resistance to DC potentials and low AC impedance.

polarization curve—a plot of current density versus electrode potential for a specific electrode/electrolyte combination.

polarization decay—the change in electrode potential with time resulting from the interruption of applied current.

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.)

polarized potential (1) (general use) the potential across the electrode/electrolyte interface that is the sum of the corrosion potential and the applied polarization. (2)(cathodic protection use) the potential across the structure/electrolyte interface that is the sum of the corrosion potential and the cathodic polarization.

polyester—type of resin formed by the condensation of poly basic and mono basic acids with polyhydric alcohols.

polyurethane—a polymer formed by reaction of an isocyanate with a polyol (hydroxyl-containing material). (When used as a coating binder, it generally produces a tough,durable, glossy protective coating with good chemical and ultraviolet light resistance.)

post weld heat treatment—heating and cooling a weldment in such a way as to obtain desired properties.

potential-pH diagram—a graphical method of representing the regions of thermodynamic stability of species for metal/electrolyte systems. [also known as Pourbaix diagram]

potentio dynamic—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.

potentio kinetic—see potentio dynamic .

potentio staircase—refers to a potentio step technique for polarizing an electrode in a series of constant potential steps wherein the time duration and potential increments or decrements are equal for each step.

potentio stat—an instrument for automatically maintaining an electrode in an electrolyte at a constant potential or controlled potentials with respect to a suitable reference electrode.

potentio static—refers to a technique for maintaining a constant electrode potential.

potentio step—refers to a technique wherein an electrode is polarized in a series of potential increments or decrements.

pot life—the maximum elapsed time during which a coating can be effectively applied after all components of the coating have been thoroughly mixed.

poultice corrosion—see deposit corrosion.

Pourbaix diagram—see potential-pH diagram.

power tool cleaning—removal of loose rust, loose mill scale, and loose coating by power tool chipping, descaling, sanding, wire brushing, and grinding. [SeeSSPC-SP 3.]

precipitation hardening—hardening caused by the precipitation of a constituent from a supersaturated solid solution.

primary passive potential—the potential corresponding to the maximum active current density (critical anodic current density) of an electrode that exhibits active-passive corrosion behavior.

prime coat—see primer.

primer—a coating material intended to be applied as the first coat on an uncoated surface. The coating is specifically formulated to adhere to and protect the surface as well as to produce a suitable surface for subsequent coats. [also referred to as prime coat]

profilometer—an instrument for measuring and recording the topographical profile of a surface.

protection potential—the most noble potential at which pitting or crevice corrosion, or both, will not propagate in a specific environment.

protective coating—a coating applied to a surface to protect the substrate from corrosion.