The Anodizing Process

Aluminum anodizing is the electrochemical process by which aluminum is converted into aluminum oxide on the surface of a part. This coating is desirable in specific applications due to the following properties:

1. Increased corrosion resistance

2. Increased durability / wear resistance

3. Ability to be colored through dying

4. Electrical insulation

5. Excellent base or primer for secondary coatings

Types of Anodizing

Over the last several decades a variety of anodizing processes have been developed. However, there have been three main variations of aluminum anodizing: chromic anodizing, sulfuric anodizing, and hardcoat anodizing. Each of these has advantages and disadvantages depending on the application.

Chromic Anodizing

 Chromic anodize, commonly referred to as Type 1 anodizing, is formed by using an electrolytic solution of chromic acid that is about 100° F and a density of 1.5 to 4.5 A/ft2. The process will run for 40 to 60 minutes. This will produce a clear to gray coating, depending on sealing and alloy used, that is about 2 µm. One third of the coating thickness will build up per surface and 2/3 will be penetration.

Advantages

Chromic anodize, like the Type I and Type II coatings, offers a minimum of 336 hours 5% salt spray resistance per ASTM B117. It is not as durable as Type II or Type II, and does not accept dyes as well as Type II.

Due to the low thickness, it can be an advantage of parts with tight tolerances. Also, since chromic acid is less aggressive towards aluminum than the sulfuric acid used in Type II and Type III coatings, it should be used in parts that are difficult to rinse such as welded and riveted assemblies.

Specifications

There are many governmental, industrial, and commercial anodizing specifications in use, each with their own method of calling out coatings, seals, dyes, etc. Any anodizer should recognize the Type I designation to specify a chromic anodize, and Class I to indicate natural color or Class 2 to indicate a dye.

Materials

Most alloys are suitable for chromic anodizing. Exceptions would be high-silicone die-cast alloys and high-copper alloys. Basically, an alloy with more than 5% Cu, 7% Si, or 7.5% of alloying elements should not be used.

Sulfuric Anodizing

Sulfuric anodize, commonly referred to as Type II anodizing, is formed by using an electrolytic solution of sulfuric acid at room temperature and a current density of 15 to 22 Amps per square foot. The process will run for 30 to 60 minutes depending on the alloy used. This will produce a generally clear coating, depending on sealing, a minimum of 8µm thick. One third of the coating thickness will build up per surface and 2/3 will be penetration.

Properties

Type II sulfuric anodizing provides for several desirable qualities such as:

• Corrosion Resistance (336+ hours salt spray resistance per ASTM B117)

• Moderate Durability

• Excellent Dyability (yielding deep, rich colors)

• Electrical Insulation

• Sulfuric anodize coatings are often sealed to enhance corrosion resistance, lock in dyes, or both. Hot water seals produce the clearest sulfuric anodize while sodium dichromate yields a yellow-green appearance but is generally a better seal.

Specifications

There are hundreds of governmental, industrial, and commercial anodizing specifications in use, each with their own method of calling out coatings, seals, dyes, etc. Any anodizer should recognize the Type II designation to indicate a sulfuric anodize, Class 1 to specify natural color or no dye, and Class 2 to indicate a dye.

Materials

Sulfuric anodizing is rather tolerant of aluminum alloys for anodizing with the exception of high-silicon die-cast alloys such as 380. The less alloying elements there are the higher the clarity and depth of color of the anodize coating.

Hardcoat Anodizing

Hardcoat anodize, commonly referred to as Type III anodizing, is formed by using an electrolytic solution of sulfuric acid at approximately 32° F and a current density of 23 to 37 Amps per square foot. The process will run for 20 to 120 minutes depending on the alloy used and desired coating thickness. This will produce a generally gray coating 10µm to 50µm thick with 50% buildup and 50% penetration.

Properties

Hardcoat anodizing provides for several desirable qualities such as:

• Corrosion Resistance (336+ Hours salt spray resistance per ASTM B117)

• High Durability (file hard, 60-70 on Rockwell C-scale)

• Electrical Insulation (800 V / mil thickness)

• Hardcoat anodize coatings may be dyed a variety of colors. However, due to its naturally darker appearance, dyeing does not produce the vibrant colors that a Type II sulfuric anodize will. Additionally, the required sealing process after dyeing does slightly reduce hardness.

Specifications

There are hundreds of governmental, industrial, and commercial anodizing specifications in use, each with their own method of calling out coatings, seals, dyes, etc. Any anodizer should recognize the Type III designation to indicate a hardcoat anodize, Class 1 to specify natural color or no dye, and Class 2 to indicate a dye.

Materials

Hardcoat anodizing works very well with 6000, 5000, and 7000 series alloys. High-copper 2000 series alloys can present some difficulties and should be avoided if possible. As with all forms of anodizing high-silicon die castings are not recommended.