Page 681 - Corrosion Engineering Principles and Practice

634 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 635

Laser Surface Alloying
This technique can be used to apply most of the same materials that
can be applied via thermal spray techniques; the powders used for both
methods are generally the same. Materials that are easily oxidized,
however, will prove difficult to deposit without recourse to inert gas
streams and envelopes. Deposition rates depend on laser power, powder
feed rates, and traverse speed. The rates are typically in the region of
2 × 10 cm for a 500-W beam. Thickness of several hundred microns
−4
3
can be laid down on each pass of the laser beam allowing thickness
of several millimeters to accumulate. If the powder density is too high,
this thermal cycling causes cracking and delamination of earlier layers,
severely limiting the attainable buildup. Research has found that easily
oxidized materials, such as aluminum, cannot be laser clad because
the brittle oxide causes cracking and delamination. Some steels may
be difficult to coat effectively. The small size of the laser’s beam limits
the size of the workpieces that can be treated cost effectively. Shapes
are restricted to those that prevent line-of-sight access to the region to
be coated.
Chemical Vapor Deposition
CVD is used mainly for purposes of corrosion resistance and wear
resistance. CVD processes are also usually applied in cases where
specific properties of materials of interest are difficult to obtain by
other means. CVD is unique because it controls the microstructure
and/or chemistry of the deposited material. The microstructure of CVD
deposits depends on chemical makeup and energy of atoms, ions, or
molecular fragments impinging on the substrate; chemical composition
and surface properties of the substrate; substrate temperature; and
presence or absence of a substrate bias voltage. The most useful CVD
coatings are nickel, tungsten, chromium, and titanium carbide. Titanium
carbide is used for coating punching and embossing tools to impart
wear resistance.

TABLE 14.7 (continued)

Thorough surface preparation usually is required because the
molten metal droplets must achieve adhesion before solidification.
The coating achieved is somewhat oxidized and porous. Common
applications are on steel surfaces exposed on offshore petroleum
production platforms, chemical tanker compartments, stack
breechings, and other structural steel locations. Metallized surfaces
frequently are top-coated (Fig. 14.17).