Aluminum anodizing is characterized into three types based on the intensity of coating. While Type I and Type II are used in light-frictional applications, Type III anodizing (hard-coat anodizing) is used where the application requires sufficient wear resistance, corrosion resistance and surface hardness. Type I is chromic acid anodize and has a thickness of 0.0001”. What is often referred to as sulfuric acid anodize is Type II in North America, as named by MIL-A-8625; it has a moderate thickness of 1.8 μm to 25 μm (0.00007″ to 0.001″).
The anodic layer for Type III can be made between 13 and 150 μm (0.0005″ to 0.006″) thick. The thicker the anodic layer, the better the resistance to corrosion, abrasion, and wear. These properties may not be provided by bare aluminum. Hard anodizing adds a sufficient layer of aluminum oxide to the aluminum surface. However, the process of enhancing the coating involves a series of methods including post-process treatment. This guide discusses the process, properties and design considerations for hard coat anodizing.
What Is Hard Coat Anodizing?
Hardcoat anodizing is sometimes called Type III anodizing or simply hard anodizing. It is an electrochemical conversion process, usually performed on aluminum that generates a hard, thick aluminum oxide layer on aluminum components.
During the hard anodizing process, the aluminum component is connected to the DC supply as the anode and dipped in an electrolytic cell. The part to be anodized is connected to the negative terminal of the DC supply, serving as the cathode. Once the DC power is switched on, an ionic dissociation occurs around the aluminum component, releasing oxygen ions. These ions react with the aluminum component of the electrolyte to form an aluminum oxide layer. The reaction is continuous, where the electrolyte simultaneously dissolves part of the oxide. The final reaction leads to an equilibrium between oxide formation and chemical dissolution.
This reaction is typical of the normal anodizing process. To achieve hard anodizing, the temperature is regulated to lower levels, while the current density is controlled to prevent excessive chemical dissolution. Effectively, the process leads to more oxide layer formation. Hard anodizing basically differs from the other types of anodizing because of the relatively higher chemical concentration. Further, the anodizing time for hard anodizing is higher than for Type I and Type II. Type III anodizing can be summarized as;
- Aluminum component
- Surface preparation
- Acidic electrolyte
- Controlled DC current
- Oxide growth
- Rinçage
- Optional dye/sealing
- L'inspection
How Does Type III Anodizing Differ From Type II Anodizing?
It is important to understand the differences among the three types of anodizing to avoid manufacturing errors. While Type I uses chromic acid as an electrolyte, both Type II and hard anodizing use sulfuric acid. The classification and differentiation between Type II and hard anodizing help to determine the concentration of the acidic electrolyte during the electrochemical process. The following table summarizes the differences between the two anodizing processes.
| Characteristic | Type II Anodizing | Type III Hard Coat |
|---|---|---|
| Primary purpose | Appearance, corrosion protection, general surface protection | Wear, abrasion, durability, engineering performance |
| Typical coating thickness | Approximately 1.8–25.4 µm in AAC reference data | Approximately 12.7–115 µm in AAC reference data |
| Surface hardness | Higher than bare aluminum | Much higher than bare aluminum |
| Wear resistance | Modéré | Haut |
| Typical appearance | Clear, dyed, metallic colors | Gray, dark gray, bronze, brown, or near-black depending on alloy/process |
| Dimensional impact | Usually relatively small | Must be incorporated into design |
| Dyeing | Common | Possible, but color range is more limited |
| Scellement | Common for corrosion/color stability | Application-dependent; extreme wear applications may avoid sealing |
| Typical applications | Consumer products, architectural components, housings | Sliding components, cylinders, pistons, gears, tooling, aerospace hardware |
The performance difference between Type II and hard anodizing comes from the nature and structure of the anodic oxide. Aluminum oxide is a ceramic composed of aluminum and oxygen. The oxide layer is thick and has pores, which is relatively harder than naturally untreated aluminum. However, engineers are not just interested in hardness. Rather, other parameters like wear resistance, chemical corrosion resistance and stress and fatigue resistance are factored in during standard coat anodizing. These parameters are achieved through proper selection of aluminum alloy materials, appropriate load distribution calculations, lubrication applications and surface roughness control.
What Happens During the Hard Coat Anodizing Process?
Hard anodizing is a multi-stage process involving numerous activities from material preparation to sealing where necessary. Let’s have a stepwise discussion of these processes.
Complete hard anodizing workflow covering pre-finish preparation, cleaning and degreasing, etching, desmutting, hard anodizing, and rinsing. It produces a uniform, dense, wear-resistant hard oxide layer with controlled thickness.
- Pre-finish preparation
Before the anodizing process, the aluminum component must have been manufactured. Irrespective of the manufacturing process (including CNC machining) used to manufacture the part, it may have defects that require cleaning and engineering treatment before hard anodizing. Defects like tool marks, machining marks, burrs and contaminants can persist even after anodizing. Anodizing is not meant to eliminate these defects. Some of the pre-finish preparation approaches include establishing edge and corner conditions, determining required anodizing thickness, defining final dimension tolerances and finding thread requirements.
- Cleaning and degreasing
The aluminum component is likely to contain dirt, oils and fingerprints after manufacturing processes. The component is cleaned before anodizing to remove these contaminants. Without proper cleaning, contaminants can cause adhesion problems, non-uniform oxide growth, and discoloration.
- Gravure
The aluminum component is subjected to alkaline or other related chemical treatment to achieve a particular surface finish before anodizing. Etching changes surface texture while removing other possible materials.
- Desmutting
During etching, chemical reactions with aluminum alloys such as zinc, magnesium, copper and silicon can produce insoluble residues on the surface. Desmutting removes these residues, creating a clean surface for anodizing.
- Hard anodizing
Once prepared, the component is immersed in an electrolyte bath and connected to a DC power supply as the anode. The electrochemical process is controlled by a process controller managing chemical concentration, temperature, current density, agitation and anodizing time. Unlike conventional anodizing, which raises temperature, hard anodizing operates at a lower temperature while increasing anodizing time.
- Rinçage
After anodizing, rinsing follows to remove residual reaction products and electrolyte. Rinsing helps to avoid staining and corrosion-related defects.
Where necessary, manufacturers can apply dyeing and sealing the components.
What Properties Does Hard Coat Anodizing Provide?
Hard coat anodizing provides wear resistance, corrosion resistance, electrical insulation and surface durability.
Résistance à l'usure
Most engineering applications involving machines and moving parts are characterized by wear resulting from mating of contact parts. A good engineering design must consider wear resistance. Aluminum, a widely used metal for its weight advantage, requires hard anodizing to enhance its wear resistance. This makes the aluminum components more abrasive, provides better sliding contact and allows them to rub against other components. This wear-resistance property makes anodized aluminum applicable in diverse areas, including valve components, cylinders, and guides.
Résistance à la corrosion
The corrosion of aluminum can be prevented by avoiding direct contact of the aluminum surface with the working environment. Aluminum oxide layers effectively serve this purpose, providing a protective barrier that meets the rigorous testing requirements defined in ASTM B580 [1]. However, the effectiveness of aluminum corrosion resistance depends on coating thickness, sealing conditions, the type of aluminum alloy used, and possible defects on the aluminum surface.
Isolation électrique
Beyond wear and corrosion resistance, aluminum oxide enhances electrical insulation. Aluminum oxide is an insulator. This oxide layer prevents electrical transfer that would occur with bare aluminum. However, the aluminum oxide must be thick enough to prevent breakdown voltage from bypassing the coating layer. Thus, the thick oxide layer formed by hard-coat anodizing is an effective electrical insulator.
Surface Durability
Hard coat anodizing produces a thick surface coating that reduces scratching and other surface damage during component operation and handling. However, this thick coating does not guarantee the component from mechanical damage. Very high contact stress or impact can fracture the component’s oxide layer, leading to destruction.
What Are the Most Common Hard Anodizing Defects?
Common defects experienced during hard anodizing include burning, uneven thickness, color variation, surface defects, and cracking or brittle damage.
Excessive local heat or abnormal oxide growth can occur, especially during high-temperature or current-density hard-coat anodizing. This process is called burning. Burning can also result from insufficient electrolyte circulation and geometry-related current concentration.
Uneven thickness usually results from part geometry, electrolyte circulation, current distribution, and local heat generation. Sometimes these effects can result in surface defects like machining marks, inclusions and scratches.
While thick anodizing is recommended for wear and corrosion resistance, the resulting thick aluminum oxide layer is brittle, especially under heavy mechanical loading. This requires controlled design-manufacturing applications.
When Should Engineers Choose Hard Coat Anodizing Instead of Type II?
Generally, engineers choose hard-coat anodizing when surface durability is the primary engineering requirement. Other requirements include higher abrasion resistance, greater surface hardness, electrical insulation and sliding wear resistance. Further, Type III is preferred when engineers seek to achieve a relatively thick protective oxide to improve corrosion resistance. Effectively, they seek a component with a long service life in an operational environment with repeated contact.
On the other hand, engineers would choose Type II when primary considerations include color, appearance, lower dimensional impact and general corrosion protection. For instance, consider aluminum housing exposed to moisture and fingerprints. This component does not require heavy-duty design considerations. Effectively, engineers would opt for Type II anodizing. If the same component is exposed to a working environment with a sliding steel mechanism running against it, it requires protective enhancement. In this case, engineers would opt for hard coat anodizing.
Référence
[1] ASTM International. (2019). ASTM B580-79(2019) Standard Specification for Anodic Oxide Coatings on Aluminum. ASTM Standards Database. https://www.astm.org/b0580-79r19.html
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