HARD COAT ANODIZING (Type III)
Type III aluminum anodizing, also called hard coat anodizing or hard anodizing, is used to increase the wear and corrosion resistance, increase the surface hardness, and improve the thermal and dialectical properties of treated aluminum objects. The ability to retain lubricants, PTFE coatings (like Sanford Hardlube™), also improves with Type III anodizing.
Hard coat anodizing increases the thickness of the naturally occurring oxide layer on aluminum surfaces. Natural oxide layers are generally 2 to 3 nanometers thick—with Type III anodizing, this layer can be increased to or above 50 μm* (0.002”). Properly applied hard anodic coatings create a very regular, uniform layer across the entire surface of the treated object.
Type III anodizing is an abbreviation derived from the military standard Mil-A-8625 Type III category.
* μm = micrometer (1000x the length of a nanometer)
Conventional Type III aluminum anodizing uses direct current (DC) passed through an electrolytic solution of diluted sulfuric acid to create the oxide layer. The aluminum object itself serves as the positive electrode (or anode, from which the term “anodizing” originates) in the process. The formation of the oxide film take place by the migration of Al3+ ions away from the metal towards the electrolyte interface, while simultaneously O2- ions from the electrolyte move in the opposite directions.
The acid solution used in hard anodizing is concentrated at around 200g/liter. The voltage used starts at 25V DC and increases to 60-100V as the process continues.
Conventional Type III coatings can be sealed by hydrothermal or precipitation means, or left unsealed. Choice of seal depends on the object’s intended purpose and use.
An alternative Hard Coat method that achieves superior performance properties is the Sanford Low Voltage rectification process where an AC current overlays DC current and leads to lower resistance at the anode surface. This produces a more uniform coating with a number of performance improvements. Sanford Low Voltage Processes include: PLUS®, QUANTUM®, SALVAGE™, and MICRALOX®.
Conventional Aluminum Hard Coat is used for applications requiring:
- High corrosion resistance is required.
- High abrasion resistance is required
- Dielectric barrier usually > 1000 < 2000 Volt DC.
- High bond strength with polymeric coatings and adhesives.
- Mil-A-8625 Type III, Class 1 & 2
- MIL-STD 171 7.5.1
- MIL-STD 171 7.5.2
- AMS 2468 & 2469
- BS 5599
- BS EN 2536
- ASTM B 580 Type A
- BAC 5821
- PS 13208
- PS 13201
- PS 13021.1
- HP 4-79
Note that each specification has specific processing requirements.
- Can Aluminum Hard Coat Be Colored?
- High voltage hard coat oxides darken due to an excess of aluminum atoms in the film, lower porosity compared to Low Voltage Hard Coat, and the less uniform cell structures. Further, alloying elements in the aluminum become entrapped, often causing the oxide to shade: 2000 series: green and gray tones, 5000-dark gray, 6000-almost black, 7000-blue-gray, 8000-high temperature alloy, can be light gray color.
- Typically, coloring is for identification of the part only – dark burgundy, dark blue, and/or Black are all possible for conventional high voltage coatings at 2 Mils.
- What Seals Can Be Used for Aluminum Hard Coat?
- Both hydrothermal (steam, hot water, Nickel Acetate, Magnesium Acetate, etc.) and precipitating seals (nickel fluoride, di-chromates, etc.)
- How To Get Maximum Adhesion for Aluminum Hard Coat
- Unsealed coatings and di-chromate sealed coatings provide the best adhesion characteristics.
- How to Get Easy Release for Aluminum Hard Coat.
- Hydrothermal sealed anodic coatings will have better release characteristics than unsealed or precipitated seal coatings. For enhanced release, specify HARDLUBE™ PTFE.
- What Is the Coating Thickness for Aluminum Hard Coat?
- Depends on the customer’s requirements. Mil-A-8625 requires 2 mils +/- .0004. Note that the coating both penetrates the surface and builds from the surface in roughly 50% proportions.
- What Is the Penetration vs. Build Up for Aluminum Hard Coat?
- Half of the coating is penetration. Half is Build Up.
- What Are the Applicable Coating Thickness Tolerances for Aluminum Hard Coat?
- While an engineered solution can provide tighter tolerances, the general rule of thumb is + / - 20% coating thickness or 0.4 mils on a .002” coating.
- How Are Parts Racked for Aluminum Hard Coat?
- High voltage rectification normally requires > 30 Amps per square foot current density. This in turn requires a rack that can hold the current density, and hold the part securely to prevent arching. Typically conventional hard coat will result in a more visible rack mark than rack marks after Type II anodizing, where half the current density is used.
- What is the maximum coating thickness for Aluminum High Voltage Hard Coat?
- Typically for Conventional Hard Coat, the increasing electrical resistance of the oxide as it builds, coupled with the polarization of the cathode and anode, and the dissolution of the coating left in the electrolyte over time, restricts the formation of a coating much beyond 3 mils.
- Can Aluminum Cast Materials be Hard Coated?
- Only aluminum can form an anodic coating, and silicon in the cast is an impurity. Some cast materials low in silicon can receive a hard coat after selective elimination of surface impurities. However, the higher the silicon content, the less likely a good quality hard coat will ensue.
- Can parts processed using Conventional Hard Coat process receive a high gloss or highly matt finish?
- Primary factors resulting in a high gloss or high matt finish for a hard coat are not directly related to the aluminum hard coat process. The mechanical finish (mechanical polishing or mechanical bead sanding / blasting) determines macro smoothness or roughness of the substrate. The hard coat anodizing process has secondary factors such as chemical polishing steps and or surface etching. The only factor directly related to the hard coat anodizing process is coating thickness where imperfections in the substrate are “grown” and are thus dependent on coating thickness. To specify gloss and/or matt finish, please conduct an engineering review prior to parts being run.
- Is Aluminum Hard Anodizing a Plating technique?
- No. The coating is formed through penetration and build up. A plated coating of 2 mils will add 4 mills to the diameter of a rod where as a hard coat anodizing of the same coating thickness will add 2 mils.
- What are Edge Defects?
- The aluminum oxide coating grows perpendicular to the surface. At a sharp edge, there is a gap in the coating. This gap becomes more pronounced as the anodic coating thickness increases. To eliminate the edge defect, corners should be radiused.
- Can threads be hard anodized?
- Yes. Note the pitch diameter decreases by 4 times the coating thickness on internal threads and increases 4 times on external threads.
- Do final tolerances need to allow for hard anodizing?
- Yes – the final specification needs to allow for 1/ 2 the coating thickness