coatings offer a
high level of corrosion protection
due to their good
adhesion propri-eties for paint, e-coat and powder
coatings topcoats. The sacrificial zinc layer in
the thermal diffusion basecoat is another reason for good
performance against corrosion.
In most automotive applications, there is a need for a
top coat over the Greenkote process. Currently the best
topcoat combination is a black electro-coat topcoat, commonly called “e-coat,” or “KTL” in Europe. The Greenkote
PM- 10 coating paired with e-coat offers several synergies
and is a solid replacement for zinc nickel plating with e-coat. Greenkote duplex coatings protect all sorts of lock
parts including base plates, strikers and latches (see
Figure 1 above).
Figure 1
THE THERMAL DIFFUSION
PROCESS AND HOW IT WORKS
Greenkote TD patented PM coatings work by thermo-chemical surface modification. Carbon steel, cast iron
and sintered metal parts can be coated with zinc TD coatings. The coating temperatures range between 360°C and
410°C (680°F and 770°F).
The coating layer forms in steps:
1. Zinc in the coating diffuses into aluminum in the coating
and into the iron in the substrate (see Figure 2 at right).
2. Iron in the substrate forms intermetallics of Zn-Fe-Al
(see Figure 3 at right).
3. Aluminum inclusions are formed on the coating surface
(see Figure 4 at right).
The thermo-chemical process forms a protective layer
that partially diffuses into the substrate and cannot be
separated by physical or environmental testing. The aluminum inclusions on the surface are a key in healing any
micro cracks or porosity in the coating while the zinc provides a sacrificial layer.
Preparation before the zinc TD coatings process varies
depending upon the condition of the part itself. While
some parts may be ready for coating as received, other
parts such as those with heavy oils or lubricants on the
surfaces need to be cleaned in a parts washer. There is no
need to rinse after alkaline cleaning—simply dry the
parts. If the parts arrive with significant rust or scale,
usually from high temperature heat treating, the parts
must be mechanically blast cleaned.
The heart of the zinc TD coating process is the heating
time in a steel retort. These closed steel cylinders come in
varying sizes depending on the size and volume of the
parts. One end opens for easy loading and unloading. The
parts, ceramic media and zinc TD coating powder are
then added based upon weight from a predetermined
Figure 2
Figure 3
Figure 4
loading menu for each part.
The heating cycle consists of the time it takes to ramp
up, the time it takes to hold the temperature for one hour,
and then cool-down time for unloading. The cycle time is
typically three hours with convection heat, but can be can
be less than two hours with IR (infrared) heat.
Finally, the cooled parts are rinsed with ambient city
water and then passivated with a zinc phosphating rinse
before drying and packing. The rinse step removes a light
amount of coating dust left from the retort operation.
