Hard Coat Anodizing

H&W Global offers both hard-coat anodizing (MIL-A-8625F Type III) and soft-coat anodizing
(MIL-A-8625F Type II).

What is Anodizing?

The anodizing process is essentially controlled oxidation (rusting) of a metal. Anodizing is most often performed on aluminum and aluminum alloys, but the process can also be applied to magnesium, titanium, and other metals. Anodizing causes the metal itself to "grow" a highly durable, uniform oxide layer that is remarkably hard and uniform. The process can be customized for the specific metal and desired coating type.

The Anodizing Process

Anodizing is an electrochemical process that involves the use of electricity in DC (Direct Current) form. During this process, we place the part to be anodized (the "anode") on a specialized rack, and lower it into a tank of acid, where it is submersed. The tank has metal cathodes (made from aluminum, or in some cases, 316 stainless steel) on the side walls.

Anodizing involves these steps:

• Racking
• Cleaning
• Etching
• Deoxidizing (desmutting)
• Anodizing
• Dyeing
• Sealing
• Unracking

All immersions are followed by rinsing. At H&W, we frequently utilize the sulfuric acid anodizing process to increase surface hardness and resistance to abrasion and corrosion. Anodizing solutions can, however, be made up of different acids, such as chromic, phosphoric, boric, oxalic, and combinations of these.

When an aluminum part is submersed in the anodizing solution, and DC amperage applied, the metal at first forms a thin barrier layer, an oxide that comes from the part itself. After a few minutes, another layer begins to grow through the barrier layer; this is called the porous layer. The part is now developing a uniform oxide (the controlled oxidation mentioned earlier).

The coating achieved through anodizing may be decorative (soft-coat), or it may be applied for mechanical strength (hard-coat). Careful consideration should be given when using the hard-coat anodizing process on highly-stressed parts, because of the marked lowering of fatigue performance, and on parts with sharp corners and edges where chipping may result.

Typically, a soft-coat (MIL-A-8625F Type II) process takes place at 72 degrees F in a sulfuric acid solution; coating thicknesses range from 0.2 mils to 0.7 mils for interior coatings, and 0.8 mils to 3.0 mils for exterior coatings. DC power needed may range from 12 to 18 ASF (amps per square foot).

The standard hard-coat procedure (MIL-A-8625F Type III) has the same tank setup, but is run at a much colder temperature - 32 degrees F - because the DC power applied is significantly higher (34 to 38 ASF), and thus generates more heat. Generally, these coatings range from 1.0 mils to 4.0 mils.

The final anodized coating is porous, and made up of many tube-shaped, hexagonal structures called cells. Each cell has a hole in the center, called a pore. After the anodizing acid is removed via a thorough rinse, the coating can be colored/dyed and sealed. Color choices are unlimited, and can be customized. If no coloring is desired, the coating may also simply be sealed, in which case it will appear silver (referred to as "clear anodizing").

Anodizing Aluminum and Aluminum Alloy Parts

Aluminum is the most abundant metal on earth, and a common choice for anodizing. Aluminum is used for various industries, including aerospace, military, medical, foil, coil, automotive (for example, engine blocks and pistons), and the construction of doors, windows, and skylights.

Anodizing brings value to an aluminum part by creating an aluminum oxide, which is the hardest of all surface finishes. The anodizing process actually changes the surface of the aluminum to protect it against further oxidation, while also making it wear-resistant. Anodizing prevents aluminum from pitting, and is excellent for both indoor and outdoor applications.

Anodizing aluminum and aluminum alloy parts not only increases their corrosion resistance, it also provides surfaces which will ensure satisfactory adherence of paint and organic finishes. (All alloys of aluminum can be anodized, including casted, extruded, and special proprietary alloys.)

As with all of our processes, anodizing at H&W undergoes proven process-control inspections. This allows us to deliver the highest levels of quality and consistency. H&W Global Industries will work with you to meet all of your anodizing needs. Contact us today via our contact form or call 724.465.6177.