Protective Finishes on Aluminum

Parts Manufactured from Aluminum offer many benefits. Aluminum is abundant, economical, lightweight, and machines easily. Additionally, the surface hardness, wear resistance and corrosion resistance of Aluminum can be greatly enhanced by chemical and electrochemical processes.

These processes are generally divided into “ANODIC COATINGS OF ALUMINUM” and “CONVERSION COATING OF ALUMINUM.” These are discussed in further detail below.

Anodic Coating: Anodize and Hardcoat

Anodic Coatings are formed in an Electro-Chemical process. The part being processed is immersed in an oxygen-rich electrolyte and becomes the ‘Anode’ of a DC Electrical circuit. Under these controlled conditions surface molecules of Aluminum are transformed into molecules of Aluminum Oxide. This process is highly flexible, applicable to singular prototypes or large runs of products.

Process VERIFICATION is assured by use of routine testing of Process Solutions (Chemistry and Temperature) and uniform Process Sequences.

Process VALIDATION, assures our coatings meet established PERFORMANCE CHARACTERISTICS including Coating Weight, Corrosion Resistance, Wear Resistance.

We offer each of the three common types of aluminum anodizing:

Type I – Chromic Anodize:
Widely used in military and aircraft industries

Type II – Sulfuric Anodize:
Most commonly prescribed surface finish on aluminum

Type III – Hardcoat:
Most durable of aluminum finishes which produces a measurable build

Specifications & Certifications

MIL-PRF-8625 is considered the Controlling Document for Anodic Coatings. It defines the Performance Requirements for each TYPE and CLASS of Anodize. All other Commercial and Customer Specifications follow the format and guidelines of this document.

These are designated as follows:

MIL-PRF-8625 Type I: Chromic Anodize (*due to the use of Chromium, Type I is not ROHS Compliant)
MIL-PRF-8625 Type II: Sulfuric Anodize
MIL-PRF-8625 Type III: Hardcoat

Each of these “Types” is followed by a “Class” designation for the type of Seal used as follows:
Class I: Non-dyed seals (Hot Water, Nickel Acetate, Teflon or Dichromate)
Class II: Dyed seals (Colors- black, blue, gold, green, red or purple)

Details on the Three Common Types of Anodizing

Chromic Anodizing (Type I)

Produced in dilute Chromic Acid solution, this anodize provides a thin film approx .0001 to .0004 thickness. Since only 1/3 of the thickness is "growth" Chromic Anodize is an excellent choice for close fitting assemblies.

• Good Corrosion Resistance
• Excellent Pre-treatment for Paints
• Good Electrical Resistance
• Minimum Metal Fatigue
• Anti-Galling for Close Fitting Assemblies

Sulfuric Anodizing (Type II)

The most commonly prescribed surface finish on aluminum. Conventional anodize is formed by an Electro-Chemical Process that produces a layer of aluminum oxide. Time cycles vary from 30 to 60 minutes depending on Alloy and Desired Appearance. Sulfuric anodize offers:

• Good Corrosion Resistance
• Improved Wear and Anti-Galling Properties
• Electrical Insulation Properties
• Anodic Film commonly produced from 0.0003 / 0.0011
• Most Commonly DYED Anodize. Can be dyed to a variety of colors.
• Class II - Dyed seals ( RCI Colors- black, blue, gold, green, red or purple)

Anodized surfaces are normally "sealed" to improve corrosion resistance, and/or lock in dyes. RCI offers the following Seal Options: Hot Water, Teflon, Nickel Acetate, and Sodium Dichromate.

Hardcoat (Type III)

Most durable of Aluminum Finishes, Hardcoat can be produced in thickness ranges of .0002" to .002" on most alloys. The "File Hard" Aluminum Oxide formed by this process is Grey-to-Charcoal-to-Black depending on Base Alloy and Coating Thickness. Hardcoat Offers:

• Superior Corrosion Resistance
• Superior Wear and Anti-Galling Properties
• Electrical Insulation Properties
• Customer Designated Coating Thickness
• Unique "Half growth / Half penetration" simplifies engineering for close tolerance parts
• Subsequent option to Grind or Lap surfaces to exacting tolerances
• TEFLON treatments to improve lubricity

The most commonly used alloys, Series 5000, 6000, and 7000 all produce excellent finishes. Alloys with high concentrations of Copper, Silicone and/or Magnesium, however, produce less desirable results. The 2000 series (2024 for example), with its high copper content, is often prescribed because of its superior strength characteristics even though there are challenges involved in processing. Therefore it is extremely important for us to know the exact alloy when performing the “Hardcoat” finish on your product.

Chromate Conversion (Alodine – Iridite)

Chromate Conversion Coating (also commonly referred to as Chem Film, Alodine or Iridite) forms a microscopically thin layer with
A. Enhanced Corrosion Resistance, and
B. Improved Adhesion Characteristics

We offer both a traditional Chem Film process as well as a RoHS compliant hex-free process.

Specifications & Certifications

The formation and properties of Chem Film finishes are described in the controlling document MIL-DTL-5541. All other customer specifications follow the format and guidelines of this document.

The common specification call-outs are:

MIL-DTL-5541 Type I, Class 1A
Gold with Standard Coating for Maximum Protection

MIL-DTL-5541 Type I, Class 3
Gold with Minimum Thickness for Reduced Electrical Resistance
(*due to the use of Chromium, Type I is not RoHS Compliant)

MIL-DTL-5541 Type II, Class 1A
Clear with Standard Coating for Maximum Protection

MIL-DTL-5541 Type II, Class 3
Clear with Minimum Thickness for Reduced Electrical Resistance
(*Type II is RoHS Compliant)