Chromate conversion coating is a type of conversion coating used to passivate aluminum, zinc,
cadmium, copper, silver, magnesium, and tin alloys. It is primarily used as a corrosion
inhibitor, primer, decorative finish, or to retain electrical conductivity. The process
is named after the chromate found in chromic acid, also known as hexavalent chromium, the
chemical most widely used in the immersion bath process whereby the coating is applied.
However, hexavalent chromium is toxic, thus, highly regulated, so new, non-hexavalent chromium-based
processes are becoming more readily available at a commercial level. One alternative contains
trivalent chromium. The RoHS Directive is commonly referred to regarding elimination
of hexavalent chromium. Chromate conversion coatings are commonly
applied to everyday items such as hardware and tools, and can usually be recognized by
their distinctively iridescent, greenish-yellow color.
Substrates Aluminium
Chromate conversion coatings on an aluminum substrate are known by the following terms:
chemical film, yellow iridite, and the brand names Iridite and Alodine. It is also commonly
used on aluminum alloy parts in the aircraft industry.
Iridite NCP is a non-chromium type of conversion coating for aluminum substrates.
The most commonly referred-to standard for applying chromate conversion coating to aluminum
is MIL-DTL-5541. Magnesium
Alodine may also refer to chromate-coating magnesium alloys.
Phosphate coatings Chromate conversion coatings can be applied
over phosphate conversion coatings used on ferrous substrates. The process is used to
enhance the phosphate coating. Zinc
Chromating is often performed on galvanized parts to make them more durable. The chromate
coating acts as paint does, protecting the zinc from white corrosion, thus making the
part considerably more durable, depending on the chromate layer's thickness. Steel and
iron cannot be chromated directly. Chromated of zinc plated steel does not enhance zinc's
cathodic protection of the underlying steel from rust.
The protective effect of chromate coatings on zinc is indicated by color, progressing
from clear/blue to yellow, gold, olive drab and black. Darker coatings generally provide
more corrosion resistance. However, the coating color can also be changed with dyes, so color
is not a complete indicator of the process used.
ISO 4520 specifies chromate conversion coatings on electroplated zinc and cadmium coatings.
ASTM B633 Type II and III specify zinc plating plus chromate conversion on iron and steel
parts. Composition
The composition of chromate conversion solutions varies greatly, depending on the material
to be coated and the desired effect. Most solution formulae are proprietary.
The widely used Cronak process for zinc and cadmium consists of 5–10 seconds of immersion
in a room-temperature solution of 182 g/l sodium dichromate crystals and 6 ml/l concentrated
sulfuric acid. Iridite 14-2, a chromate conversion coating
for aluminum, contains chromium(IV) oxide, barium nitrate, sodium silicofluoride and
ferricyanide. Chromate coatings are soft and gelatinous
when first applied, but harden and become hydrophobic as they age. Coating thickness
vary from a few nanometers to a few micrometers. Curing can be accelerated by heating up to
70 °C, but higher temperatures will gradually damage the coating. Some chromate conversion
processes use brief de-gassing treatments at temperatures of up to 200 °C, to prevent
hydrogen embrittlement of the electro-deposited substrate underlying the chromate layer. It
is typically specified for electroplated steels > 180 ksi tensile strength, and is normally
performed prior to conversion coating. It is the pre-conversion plating that requires
embrittlement relief, not the chromate-conversion coating itself.
References
External links yellow and green chromating chemistry on aluminum
MIL-C-17711B, Chromate conversion coating specification for zinc and zinc alloy casting
and hot dipped galvanized parts