D

dielectric shield
A protective covering of insulating material applied to a painted structure in the immediate vicinity of an anode to prevent the paint being stripped by alkali produced by the high current density at the steel surface close to the anode.

NOTE. This is usually applied to a ship's hull.

differential aeration
The unequal access of air to different parts of a metallic surface.

NOTE. This often results in the stimulation of corrosion at areas where access of air is restricted.

Drainages:

electric drainage
A means by which protection of an underground or underwater metallic structure against electrochemical corrosion is achieved by making an electrical connection between the structure and the negative return circuit (rail, feeder, busbar ) of a d.c. electric traction system.

NOTE. For convenience, also referred to as 'drainage'.

forced drainage
A form of drainage in which the connection between a protected structure and a traction system includes an independent source of direct current.

polarized electric drainage
A form of drainage in which the connection between a protected structure and a traction system includes a unidirectional device or devices such as a rectifier or a relay and contactor.

NOTE. For convenience, also referred to as ' polarized drainage'.

drainage tests
Tests in which current is applied for a short period, usually with temporary anodes and power sources, in order to assess the magnitude of the current needed to achieve permanent protection against electrochemical corrosion.

driving potential(of a sacrificial anode system)
The difference between the structure/electrolyte potential and the anode/electrolyte potential