Corrosion Terminology - D's

DC decoupling device—a device used in electrical circuits that allows the flow of alternating current in both directions and stops or substantially reduces the flow of direct current.

deactivation—the process of prior removal of the active corrosive constituents, usually oxygen, from a corrosive liquid by controlled corrosion of expendable metal or by other chemical means, thereby making the liquid less corrosive.

dealloying—a corrosion process whereby one constituent of an alloy is preferentially removed, leaving an altered residual structure. [also known as parting,selective dissolution, or selective leaching]

decomposition potential—the potential of an electrode surface at which the electrolyte, or a component thereof, decomposes by electrolysis.

decomposition voltage—see decomposition potential.

deep ground bed—one or more anodes installed vertically at a nominal depth of 15 m (50 ft) or more below the earth's surface in a drilled hole for the purpose of supplying cathodic protection current.

delamination(1) separation of layers in a material. (2) a separation between one or more coats from another coat within a coating system. [contrast with disbondment] (3) a separation of the concrete (usually in layers) from therein forcing steel at their interface, usually as a result of corrosion.

delayed cracking(1)  cracking in a metal occurring after plating or pickling, sometimes after a considerable time delay. (2) not a preferred term for cold cracking.

depolarization—(not apreferred term) the removal of factors resisting the current flow in an electro-chemical cell. [See polarization.]

deposit corrosion—localized corrosion under or around a deposit or collection of material on a metal surface.[also called poultice corrosion] [See also crevice corrosion.]

dezincification—dealloying that results in the selective removal of zinc from copper-zinc alloys.

dielectric coating—a coating that does not conduct electricity.

dielectric shield—an electrically nonconductive material, such as a coating, sheet or pipe, that is placed between an anode and an adjacent cathode, usually on the cathode, to improve current distribution in a cathodic protection system.

differential aeration cell—a concentration cell caused by differences in oxygen concentration along the surface of a metal in an electrolyte. [See concentration cell.]

diffusion-limited current density—the current density that corresponds to the maximum transfer rate that a particular species can sustain because of the limitation of diffusion [often referred to as limiting current density]

disbondment—the loss of adhesion between a coating and the substrate.

discontinuity(1) an interruption in the normal physical structure or configuration of a coating such as cracks, laps, seams,inclusions, porosity, or holidays. (A discontinuity does not necessarily affect the usefulness of the coating.) (2) a condition in which the electrical path through a structure is interrupted by a device that acts as a dielectric or insulating fitting.

dissimilar metals—different metals that could form an anode-cathode relationship in an electrolyte when connected by an electron-conducting (usually metallic) path.

double layer—the interface between an electrode or a suspended particle and an electrolyte created by charge-charge interaction leading to an alignment of oppositely charged ions at the surface of the electrode or particle. The simplest model is represented by a parallel plate condenser.

doubler plate—an additional plate or thickness of metal used to provide extra strength or thickness to a structure locally (e.g., at the point of anode attachment to an offshore structure).

drainage—conduction of electric current from an underground or submerged metallic structure by means of a metallic conductor.

driving potential—difference in potential between the anode and the steel structure.

dry film thickness—the thickness of a dried film, coating, or membrane.

dry spray—a rough, powdery noncoherent film produced when atomized coating particles partially dry before reaching the surface.

dry to handle—stage of drying or curing of an applied coating at which time the coated object can be carefully handled without damage.

dry to recoat—stage of drying or curing of an applied coating at which time a subsequent coat can be applied satisfactorily.

dry to touch—stage of drying or curing of an applied coating at which time it no longer adheres to a finger that is lightly touched or rubbed across the surface and does not show a fingerprint at the point of contact.

drying—the process in which a liquid film is converted to a solid film by evaporation of volatile components.

drying oil—an oil capable of conversion from a liquid to a solid by slow reaction with oxygen in the air.

drying time—minimum time required for an applied coating to reach the desired stage of drying or curing.

ductile cast iron—cast iron that has been treated while molten with an element (usually magnesium or cerium) that spheroidizes the graphite. [also called nodular cast iron].

ductile fracture—fracture that occurs with appreciable plastic deformation of the material. [contrast with brittle fracture]

ductility—the ability of a material to withstand plastic deformation prior to fracture. (It is usually measured by the permanent elongation or reduction in the cross-sectional area of a fractured tensile test specimen.)

duplex stainless steel—stainless steel whose microstructure at room temperature consists primarily of a mixture of austenite and ferrite. [also called austenitic/ferritic stainless steel].