Aluminum Chromate Conversion Coatings

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

Aluminum Chromate Conversion Coatings

Advanced Plating Technologies, a Milwaukee, Wisconsin company, offers a trivalent RoHS and ELV compliant aluminum chromate conversion coating that meets the requirements of MIL-C-5541, Type II, Class 1A as well as ASTM B449-93 Class 2 and 3.  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.

Advanced Plating Technologies provides clear, trivalent aluminum chromate conversion coatings to MIL-C-5541 (MIL-DTL-5541) and ASTM B449-93 commonly as an undercoat prior to powder coating application.  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.  Our company provides rack aluminum chromate conversion coatings across a wide range of industries including aerospace, defense, medical, electronics and plumbing to name a few.

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 Advanced Plating Technologies’ 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 CoatingsThe two most common aluminum chromate conversion coatings certified by Advanced Plating Technologies 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             Corrosion Protection
1                Yellow to Brown             0.4 to 2                    Maximum as Final Finish
2                Colorless to Yellow       0.1 to 0.4                 Moderate as Paint Base
3                Colorless                            < 0.1                        Decorative, Slight
4                Light Green to Green    0.2 to 5                    Moderate as Paint Base

Advanced Plating Technologies provides expertise in aluminum chromate conversion coatings for various applications.  From prototyping and PPAP qualification to full production processing, it is APT’s dedicated effort to exceed your expectations.  The following points highlight why Advanced Plating Technologies is a leading company in aluminum chromate conversion coatings:

  • Diversity of substrates:  Advanced Plating Technologies can work with nearly any wrought, heat-treatable or cast aluminum alloy to provide an aluminum chromate conversion coating as a final application or paint base for powder coating services.
  • Aluminum Chromate Conversion CoatingsRoHS and REACH Compliance:  Our company is at the forefront of environmental compliance and can certify aluminum chromate conversion coatings as needed to the current RoHS, ELV and REACH regulations.
  • Competitive Pricing:  Advanced Plating Technologies has been providing quality aluminum chromate conversion coatings plating services for over twenty years.  Competitive pricing in aluminum chromate conversion coatings is paramount to gaining and maintaining our customers in today’s competitive global marketplace.
  • Reliable Lead Times:  With inventories leaner than ever, consistent turn-around is a core competency that we value highly throughout the company to ensure the service our customers need in aluminum chromate conversion coatings.
  • Value Added:  Our company prides itself in meeting the customer’s needs.  Whether it’s custom engineering packaging, vacuum or nitrogen sealed packaging to enhance storage, Kanban inventory management, light subassembly or multiple finishing combinations – our company will work with you to ensure we meet your needs in aluminum chromate conversion coating services.
  • Extensive in House Analytical and Quality Lab Services:  SPC is employed in all wet-lab work surrounding our aluminum chromate conversion coating services to ensure baths are analyzed and adjusted at optimum frequencies to maintain operation between critical upper and lower control points.  Our QC lab can certify salt spray performance to ASTM B117 as well as secondary organic coating adhesion to ASTM D3359.