Chromate conversion alternatives
One of the largest uses of chromates is for chromate
conversion. Chromate conversion coatings are thin layers of
chromate-containing compounds formed by modifying the
chemistry (converting) the surfaces of metals. The coating
is put down by treating with strontium chromate, zinc
dichromate or sodium dichromate.
The
primary purpose of chromate conversion is to create a
self-healing layer at the surface of the alloy that will
continue to provide corrosion resistance when the surface is
scratched or damaged. In addition conversion layers can be
made different colors (depending on thickness), which can be
used for identification (as well as to show that the
conversion layer has been applied).
Chromate
conversion is used on a large number of products, such as
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Aluminum
alloys and coatings
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Zinc
alloys and diecast products and galvanized coatings
-
Galvanized steel sheet and other product forms
-
Magnesium alloys
There are
now a large number of chromate conversion alternatives
available. The most common alternatives use trivalent in
place of hexavalent chrome. How well these alternatives
work depends on the application, but in general Cr3+-based
alternatives do not have as good corrosion performance as Cr6+
materials. This does not mean that they are inadequate –
they are likely to work adequately for many applications,
but may not be good options for extreme corrosion
environments. There are various other chemistries, based on
Zr, Mo or Mn, some of which work well.
NEEDS
MORE – how can we find out what really works industrially?
Sounds like we should do some interviews with users.
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Chemistry |
Comments |
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Trivalent Cr conversion coating |
There are quite a large number of these on the
market. Their performance is generally not as good
as hexavalent chromates, but some Cd alternative
coating systems that employ trivalent conversion
actually perform better than hexavalent chromated Cd.
The standard chromate conversion colors are not
available – most trivalent conversion coatings are
iridescent light blue. |
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Molybdates |
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Permanganates |
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Zr-based conversion coating |
Enhance adhesion, poor performance on bare metals.
Light tan iridescent color. |
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Organic paint adhesion promoters |
These are not conversion coatings, but enhance
paint adhesion to improve the performance of paint
systems. They have poor bare corrosion resistance.
Manual application required. Minimal color change. |
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Sol-gel coatings based on organosiloxanes and
zirconates |
Minimal bare corrosion resistance. Clear and
colorless. |
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Anodized layers |
Used for aluminum, magnesium and titanium alloys.
Provides lubricity, wear and abrasion resistance,
but will not provide corrosion resistance if
scratched or damaged. |
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Plasma electrolytic oxidation (fused ceramic
coating containing alumina, silica or magnesia) |
Used for aluminum and magnesium alloys. High voltage
anodization using benign solutions. Used in
automotive and aerospace industries. Provides wear
and abrasion resistance, but will not provide
corrosion resistance if scratched or damaged. |
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