Electrolytic Plating » Aluminum Chromate Conversion Coatings

Aluminum Chromate Conversion Coatings

Aluminum Chromate Conversion Coatings per MIL-C-5541 (MIL-DTL-5541) and ASTM B449-93

** Advanced Plating Technologies no longer offers chromate services as a final finish. Please use the below content for reference only **

Aluminum Chromate Conversion Coatings

The chromate process provides a clear iridescent to slightly golden iridescent film of low contact resistance.  The corrosion performance of our aluminum chromate conversion coating is consistent with that of traditional hexavalent processes and is largely a function of the specific alloy that the aluminum chromate conversion coating is applied to.

Aluminum chromate conversion coatings improve the corrosion resistance of wrought, heat treatable and cast aluminum alloys while providing an excellent base for subsequent painting or powder coating services.

Aluminum Chromate Conversion Coatings – Aluminum Chromate Conversion Coatings Properties

Aluminum chromate conversion coatings, often referred to as chemical film or under the trade names Alodine or Irridite, produce a thin coating in the range of 0.00001-0.00004 inches in thickness.  Aluminum chromate conversion coatings are amorphous in structure with a gel-like composition hydrated with water.  This characteristic provides aluminum chromate conversion coatings with a unique self-healing characteristic if scratched or abraded.  Metal exposed from a scratch slowly becomes covered by soluble chromate from areas adjacent to the scratch.

Aluminum Chromate Conversion CoatingsDue to the extremely thin nature of aluminum chromate conversion coatings there is no practical way to measure the coating thickness or adhesion.  Generally the adhesion of the aluminum chromate conversion coating can be determined by evaluation of secondary paint applied to the chromated aluminum.  Standardized tests of this nature are addressed in ASTM test methods D3359 or ISO 2409.

The electrical contact resistance of aluminum chromate conversion coatings is low, with Class 3 coatings per MIL-C-5541 being the lowest.  Typical contact resistance of Class 3 aluminum chromate conversion coatings is less than 5,000 microhms per square inch as applied.  The surface roughness and flatness of the test pieces can have a significant impact on this value.

Corrosion resistance of aluminum chromate conversion coatings is highly dependent on the specific alloy employed.  As a rule, the more pure the aluminum alloy is, the better the chromated product will perform in corrosion tests.  For example, the same aluminum chromate conversion coating applied to a wrought aluminum alloy (such as 1100 series) can exceed 200-hrs in neutral salt testing per ASTM B117 as compared to less than 24-hrs for a cast aluminum alloy with silicon content exceeding 1%.

The appearance of aluminum chromate conversion coatings is also largely dependent on the specific alloy chromated.  In fact, five unique alloys processed with a consistent pretreatment and application of chromate can produce five distinct hues of chemical film.  Cast aluminum alloys typically have the widest variation and inconsistencies in appearance due to the high percentage of alloying constituents within cast aluminum grades.  ASTM B449-93 makes special note of this point in section 4.2 stating, “It should be noted that color and color uniformity will vary somewhat between one alloy and another and from a polished surface to an etched surface.”


Aluminum Chromate Conversion Coatings – Specifications

Aluminum Chromate Conversion CoatingsThe two most common aluminum chromate conversion coatings certified are MIL-C-5541 and ASTM B449.  A summary of chromating services per the ASTM and MIL specs is as follows:

Aluminum Chromate Conversion Coatings to MIL-C-5541 (MIL-DTL-5541)

Type I – Compositions containing hexavalent chromium
Type II – Compositions containing no hexavalent chromium

Class 1A – For maximum protection against corrosion, painted or unpainted
Class 3 – For protection against corrosion where low electrical resistance is required

Aluminum Chromate Conversion Coatings to ASTM B449-93

Class         Appearance    Coating Mass (g/m2)             Corrosion Protection
1                Yellow to Brown             0.4 to 2 g/m2                    Maximum as Final Finish
2                Colorless to Yellow       0.1 to 0.4g/m2                Moderate as Paint Base
3                Colorless                            < 0.1  g/m2                  Decorative, Slight
4                Light Green to Green    0.2 to 5  g/m2              Moderate as Paint Base