Page 673 - Corrosion Engineering Principles and Practice

626 C h a p t e r 1 4 P r o t e c t i v e C o a t i n g s 627

steam, strong alkalies, cyanides, or a great number of other chemicals.
The films must be very thin and continuous to have the mechanical
and physical properties desired.
Austenitic stainless steels and hardenable stainless steels such as
martensitic, precipitation hardening, and maraging stainless steels
are seldom coated, but their corrosion resistance depends on the
formation of naturally occurring transparent oxide films. These films
may be impaired by surface contaminants such as organic compounds
or metallic or inorganic materials. Treatments are available to clean
and degrease the surface of these materials to produce uniform
protective oxide films. These usually involve immersion in an aqueous
solution of nitric acid and a dichromate solution.

14.7.8 Pack Cementation
Certain materials can be diffused into the surface of a metal to produce
a surface product entirely different from the metal. Powders of
carbonaceous materials, aluminides, chromium salts, borides, or
silicides are packed around the object in a container. The mass is taken
to high temperature for sufficient time for the carbon, aluminum,
chromium, or boron to diffuse into the metal surface. A hard, abrasion-
resistant surface of some 20 to 50 m m in thickness can be thus produced.
These coatings are used to impart abrasion and/or high-temperature
resistance to gas turbine blades and vanes, among other similar uses.
Aluminum coatings are frequently used on heat exchanger surfaces
subject to attack by sulfur gases (H S, SO , and SO ).
3
2
2
The sherardizing process involves diffusion of a zinc coating into
steel by tumbling steel parts with zinc dust at high temperatures.
14.8 Metallic Coatings
For many rugged services, the choice of a metal coating is preferred.
Where severe impact, abrasion, or high temperatures are a part of the
materials selection consideration, the use of metallic coatings should
be considered. However, for service in presence of an electrolyte it is
imperative to examine if there are any galvanic incompatibility
between the substrate and the metallic coating being applied. If there
is a breach in the protective coating and a contact established between
the two different metals by the presence of some aqueous electrolyte,
corrosion of either may be accelerated or accentuated, depending on
the existing conditions (Fig. 14.11).

14.8.1 Electroplating
Electroplating is achieved by passing an electrical current through a
solution containing dissolved metal ions and the metal object to be
plated. The metal object serves as the cathode in an electrochemical
cell, attracting metal ions from the solution. Ferrous and nonferrous