Page 679 - Corrosion Engineering Principles and Practice

632 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 633

Combustion Torch/Flame Spraying
Flame spraying is noted for its relatively high as-deposited porosity,
significant oxidation of the metallic components, low resistance to
impact or point loading, and limited thickness (typically 0.5 to 3.5 mm).
Advantages include the low capital cost of the equipment, its simplicity, and
the relative ease of training the operators. In addition, the technique uses
materials efficiently and has low associated maintenance costs.
Combustion Torch/High-Velocity Oxy-Fuel (HVOF)
This technique has very high-velocity impact, and coatings exhibit little or
no porosity. Deposition rates are relatively high, and the coatings have
acceptable bond strength. Coating thickness range from 0.000013 to
3 mm. Some oxidation of metallics or reduction of some oxides may occur,
altering the coating’s properties.
Combustion Torch/Detonation Gun
This technique produces the densest of the thermal coatings. Almost any
metallic, ceramic, or cement materials that melt without decomposing can
be used to produce a coating. Typical coating thickness range from 0.05 to
0.5 mm, but both thinner and thicker coatings are used. Because of the
high velocities, the properties of the coatings are much less sensitive to
the angle of deposition than most other thermal spray coatings.
Electric Arc Spraying
Coating thickness can range from a few hundredths of a mm to almost
unlimited thickness, depending on the end use. Electric arc spraying can be
used for simple metallic coatings, such as copper and zinc, and for some
ferrous alloys. The coatings have high porosity and low bond strength.
Plasma Spraying
Plasma spraying can be used to achieve thickness from 0.3 to 6 mm,
depending on the coating and the substrate materials. Sprayed materials
include aluminum, zinc, copper alloys, tin, molybdenum, some steels, and
numerous ceramic materials. With proper process controls, this technique
can produce coatings with a wide range of selected physical properties,
such as coatings with porosity ranging from essentially zero to high porosity.
Ion Plating/Plasma Based
This technique produces coatings that typically range from 0.008 to
0.025 mm. Advantages include a wide variety of processes as sources
of the depositing material; in-situ cleaning of the substrate prior to film
deposition; excellent surface covering ability; good adhesion; flexibility
in tailoring film properties such as morphology, density, and residual film
stress; and equipment requirements and costs equivalent to sputter
deposition. Disadvantages include many processing parameters must be
controlled; contamination may be released and activated in the plasma; and
bombarding gas species may be incorporated in the substrate and coating.

TABLE 14.7 Limits and Applicability of the Main Techniques for Producing Thermal
Spray Coatings