Chromate conversion coating is a type of conversion coating applied to passivate aluminium, zinc, cadmium, copper, silver, magnesium, tin and their alloys to slow corrosion. Chromating is commonly used on zinc-plated parts to make them more durable. The chromate coating acts like a paint, protecting the zinc from white corrosion, this can make the part several times more durable depending on chromate layer thickness. It cannot be applied directly to steel or iron, and does not enhance zinc's cathodic protection of the underlying steel from brown corrosion.
It is also commonly used on aluminium alloy parts in the aircraft industry where it is often called chemical film, or the well known brand name Alodine. It has additional value as a primer for subsequent organic coatings, as untreated metal, especially aluminium, is difficult to paint or glue. Chromated parts retain their electrical conductivity to varying degrees, depending on coating thickness. The process may be used to add color for decorative or identification purposes.
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. Chromate conversion coatings are common on everyday items such as hardware and tools and usually have a distinctive yellow color.
Why would anyone want to alodine aluminum?
It provides good corrosion protection. It even protects when scratched. Alodined 2024 aluminum withstands salt spray 150-600 hours before forming white corrosion. Untreated 2024 corrodes in less than 24 hours.
It provides an excellent electrically conductive surface. This helps to provide good electrical bonding in an airframe.
Paint sticks to it extremely well. In some cases, it can substitute for primer.
Advantages of alodining compared to other coatings such as primer or anodizing:
Adds no measurable weight.
A more cost effective process than anodizing.
Does not alter the dimensions of parts (does not make holes smaller).
Requires essentially no cleanup after application. Encourages treatment of all small parts as they are fabricated and installed.
Requires no electricity or skill to apply.