Anodizing vs. Chromate on Aluminum Sheet

Aluminum does not "rust", but it will oxidize.

The chromate coated parts still retain their electrical conductive, but do not stand up ti high temperatures like anodizing. Type I is a very caustic application process, whereas Type II is not. Chromate is generally much thinner than anodizing, but limited to a yellow green color.

Anodizing can be done as Type II or a hard coat called Type III that is not only thicker, but much harder to scratch or mar. Anodizing also produces an electrical barrier and can be done in a variety of colors. I think anodizing is a little more robust, but you should check with your plater.

Type I & Ib are chromatic acid anodizing, conventional coatings produced by chromatic acid baths using standard and low voltage processes.

Type Ic is a non-chromatic acid anodizing

Type II is a sulfuric acid anodizing

Type IIb is a thinner sulfuric acid anodizing

Type III is Hard coat

with these types of anodize you also have a class

Class I is an anodize without dye or pigment

Class II is and anodize with dye or pigment and a color must be specified

with these coatings, dimensional changes occur

Types I Ib & II are intended to improve surface corrosion protection under sever service conditions or as a base for paint to resist abrasion.

Types Ic & IIb provide a non chromate alternative when corrosion resistance, paint adhesion, and fatigue resistance is required

Type III is strictly intended to provide wear resistance and abrasion resistance to surfaces as well as improved corrosion resistance.

All anodize processes reduce the amount of electrical conductivity.

The surface build-up (amount of coating) is generally as follows,..

chromatic anodize .00005"-.0003" per surface

sulfuric anodize .00005"-.0010" persurface

hard coat .0005"-.0045" per surface (can be specified with some platters)

Type I and Type III anodize coatings also include a dichromatic seal which further stabilizes the abrasion resistance as well as the reduced electrical conductivity.

The information contained here is derived from the Military Standard Mil-A-8625f.

For more information please consult this document and or a plating house.

Hope this helps.

Regards

Bruce K. Crabtree