Aluminum will be treated by a corrosion resistant conversion coating called Chromate Conversion.Chemical Conversion Coating is often referred to as chem film, chromate coating, or yellow chromate coating. This coating process applies chromate to the metal substrate, creating a surface that is corrosion resistant, durable, and exhibits stable electrical conductivity. It is used to protect enclosures and metal parts from corrosion, and to ensure that reliable electrical connections can be made to aluminum chassis and components. It is also an effective pretreatment for powder coat. Chemical Conversion Coating process meets a number of specifications, and qualifies as Type II, Class 1a and 3 under MIL-DTL-5541F. ‘Type II” means the process does not utilize non RoHS hexavalent chrome. Class 1a refers its ability to protect against corrosion. Class 3 refers to its ability to provide reliable electrical conductivity. Details are given below.There are three types of chromate conversion. i.e., Trivalent, Hexavalent and Chromfree.

This is a Chromate based process which shall form a passivation film on the surface and increase its corrosion resistance. It will also increase the adherence of the surface towards any organic coatings such as primers, enamels, etc.

[hexavalent] MIL-DTL-5541F, Type I, Class 1A & Class 3 coatings are intended to provide corrosion prevention when left unpainted as well as to improve adhesion of paint finish systems on aluminum alloys.

Chemical : Hexavalent Chrome Solution with specific concentration along with maintained temperature and pH.

Properties of Hexavalent Chromate Conversion :

Non-RoHS Complaint.
Process can result in Yellow as well as Clear chromate conversion.
Very high Corrosion resistant.
Very good adhesion for paints.

[trivalent-chromate] MIL-DTL-5541F, Type II, Class 3 coatings are intended for use as a corrosion preventative film for electrical and electronic applications where lower electrical resistance.

Chemical : Trivalent Chrome Solution with specific concentration along with maintained temperature and pH.

Properties of Trivalent Chromate Conversion :

Low toxicity.-RoHS complaint.
Process result in clear color.
High Corrosion resistant.
Forms a good base for painting.

Properties of Chrom-Free :

This has the similar properties as chromate based conversion with RoHS complaint except the solution used will be Titanium based.