ABSTRACT
Corrosion Research...................................................................................... 277
8.4.1 Galvanized Steels .............................................................................. 278
8.4.1.1 ZnNi Electroplated Steels................................................... 278
8.4.1.2 Galvanneal.......................................................................... 278
8.4.2 Hot Rolled Steels.............................................................................. 279
8.4.3 TiN and Other Nonmetallic Coatings on Steels
and Hard Materials .......................................................................... 279
8.4.4 Passivation Layers on Stainless Steels .............................................. 280
8.4.5 Polymer Coatings ............................................................................. 280
References ............................................................................................................ 282
8.1 GLOW DISCHARGE DEVICES — AN INTRODUCTION
Glow discharge (GD) is a general term for low-power electrical discharges in reduced-
pressure systems. A GD device consists of a vacuum vessel, with two physically
separated surfaces inside that form a cathode and an anode. The geometrical shape
of the vessel and electrodes can be varied almost infinitely. The vacuum vessel is
evacuated and filled with a discharge gas to a pressure ranging from about 10 to
1000 Pa. Noble gases, most commonly argon, are used in analytical GDs in order to
avoid chemical reactions inside the device. The current is low to moderate, varying
between approximately 0.1 mA to a few amperes. The GD plasma is physically
divided into more or less distinct regions. In the cathode dark space, which typically
extends about a millimeter from the cathode, the electrons lose most of their kinetic
energy through collisions with gas atoms, and practically the entire potential drop
takes place here. This region is comparatively dark due to the fact that the cross-
section for electron excitation of the atoms is low at high electron energies. The cross-
section for ionization is also low at high electron energies, and relatively little
ionization occurs in the dark space. Beyond this region, one finds the intensely
luminous negative glow, which varies from several millimeters to a few centimeters
in length depending on the design of the device. Here, the average electron energy is
typically 20 to 30 eV, where the cross-sections for ionization and excitation are at a maximum. The actual degree of ionization varies, but very seldom exceeds a few
tenths of a percent. If the distance between the electrodes is sufficiently long (as in,
e.g., a neon tube), the negative glow is followed by a slightly less luminous region
known as the positive column, which extends all the way up to the anode. In this entire
region, the electrons have essentially thermal energies and the plasma is nearly on
anode potential.
