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Chromate Conversion Coating - SMLease Design

Chromate conversion coating is a type of conversion coating used to passivate zinc-plated Steel, aluminium, copper, silver, magnesium and tin alloys parts to provide protection from white rust or red rust.

Chromate conversion coating is primarily used as a corrosion inhibitor, primer, decorative finish or to retain electrical conductivity. Chromates can come in many different colour variations: clear, blue, yellow, or black. 



Chromate conversion Coating Specifications:

MIL-C-5541C 

Class 1A - (Yellow) Maximum Protection against corrosion, Painted or unpainted

Class 3 - (Clear or Yellow) For protection against corrosion where low electrical resistance is required


MIL-DTL-5541F / MIL-DTL-81706B

 

Type I (Hexavalent Chromium)

Class 1A- (Yellow) For maximum protection against corrosion, painted or unpainted.

Class 3- (Clear or Yellow) For protection against corrosion where low electrical resistance is required.


Type II- (Hexavalent chromium)

Class 1A- (Clear) For maximum protection against corrosion, painted or unpainted.

Class 3- (Clear) For protection against corrosion where low electrical resistance is required.


ASTM B 449-93

Class 1- Yellow to Brown, Maximum corrosion resistance generally used as final finish.
Class 2- Colorless to yellow, Moderate corrosion resistance, used as a paint base and for bonding to rubber
Class 3- Colorless, Decorative, slight corrosion resistance, low electrical contact resistance
Class 4- light green to green, Moderate corrosion resistance, used as a paint base and for bonding to rubber (Not done at AST)



Chromate Conversion Coating Process

 

Degreasing --> Water Rinsing --> Caustic Treatment --> Water Rinsing --> Nitric Acid Treatment -->

-Water Rinsing --> Chromating --> Water Rinsing --> Drying



Common Attributes of Chromate conversion Coating

  • Chromate Conversion Coating (commonly referred to as Chemical Film, Chem Film, Alodine or Iridite) converts the surface properties of the substrate (typically aluminum or magnesium) as compared to plating which applies a coating onto the surface of the substrate.
  • Chromate provides excellent corrosion resistance, is conductive and results in no measurable buildup
  • Chromate coatings are the most widely used coating for corrosion protection of Aluminum and Aluminum alloys minimizing surface oxidation.
  • Chromate Conversion coating is also used as a decorative finish
  • Commonly used for an undercoat for paint or adhesive applications due to the excellent bonding properties
  • Both Hexavalent and Trivalent films at gives approximate 336 hour salt spray testing and are subject to monthly 168 hour salt spray testing as well as paint adhesion testing.

 



Difference between trivalent chromates and hexavalent chromates?

Now a days metal finishing processes are moving towards trivalent chromates from hexavalent chromates to meet European Union initiatives End of Life of Vehicle (ELV) and RoHS.

The usage of hexavalent chromates has been limited, resulting in trivalent chromates now being used more predominantly. Trivalent chromates produce a harder, scratch-resistant coating with lower water content providing more heat resistance.

Trivalent chromates address some of the major issues when switching over from hexavalent chromates:

  • Color Variability : Ultra Blue and an Ultra Yellow chromate. Both produce a true conversion coating on plated alkaline non-cyanide and chloride zinc deposits.
  • Corrosion Resistance : Most metal finishers are worried of getting poor results when transitioning to trivalent chromates fearing a lower corrosion resistance. However, both the E-Chrome Ultra Blue and E-Chrome Ultra Yellow trivalent chromates offer high salt spray resistance on zincplated surfaces of up to 150 hours on white rust, per ASTM B-117. Resistance to red rust is typically 240 hours, depending on zinc plate thickness. No additional top coat is needed to obtain this high degree of corrosion resistance.
  • Easy to Control – Utilizes Only a Single Dip Ultra Blue and Ultra Yellow trivalent chromates can be used with automatic feeder equipment, which boosts productivity.


 

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