What is Hard Anodizing and its Properties

Hard anodizing, or hard coat anodizing, type III anodizing, is a critical electrochemical surface treatment that transforms aluminum alloys to achieve better hardness and abrasion resistance for engineering applications. This article will provide a comprehensive review of this process.

What is Hard Anodizing?

Hard anodizing is an electrochemical process defined by the primary objective of achieving superior hardness and abrasion resistance in the anodic oxide film. These films are designed for engineering, industrial, or military applications, and their thickness routinely exceeds 25μm. To achieve the required hardness and wear characteristics, the hardcoat process uses lower electrolyte temperatures, higher current densities, and specialized electrolyte compositions.

Hard anodizing technology applies to both wrought aluminum alloys and cast aluminum alloys. Typical hardcoat film thicknesses range from 25μm to 150μm. Films less than 25μm are less common but may be used in specific applications such as keyways or threads. For components subjected to dynamic friction and requiring maximum wear and electrical insulation—such as pistons, cylinders, and other moving mechanical parts—the most common and effective thickness range is 50μm to 180 μm.

Hard Anodizing Material Selection

The performance of the hard anodic oxide film is profoundly influenced by the specific aluminum alloy composition and its manufacturing process. This includes the alloy series, temper, and form (e.g., sheet, plate, extrusion, forging, or casting).

A brief overview of how different aluminum alloy series respond to the hard anodizing process is provided below:

1000 Series Alloys

The hardcoat films formed on these commercially pure alloys are primarily used for electrical insulation applications. Special conductive aluminum alloys are recommended when a combination of high electrical conductivity and moderate strength is required.

2000 Series Alloys (Al-Cu)

The main technical challenge is the preferential dissolution of copper-rich intermetallic phases during processing, which can lead to the formation of voids within the anodic film. This defect can be mitigated by careful control of the current ramp-up rate and by employing lower initial current densities, thereby minimizing the localized dissolution of copper-rich phases at the film’s genesis.

5000 Series Alloys (Al-Mg)

Hard anodizing these alloys is generally manageable, but if constant current density control is inadequate, there is a risk of “burning” or localized overheating, leading to excessively thick or damaged films. This risk escalates with increasing magnesium content in the alloy.

6000 Series Alloys (Al-Mg-Si)

The 6063 alloy typically poses no significant issues. However, alloys like 6061 or 6082 can present metallurgical challenges. For example, 6013 alloy, used in certain aerospace applications and containing 0.90% copper, exhibits behavior similar to 6061, resulting in lower film formation efficiency and relatively poor Taber abrasion resistance.

7000 Series Alloys (Al-Zn)

While these alloys may exhibit “pinhole” or “void” formation, the issues are typically not severe. The hardness and abrasion resistance of the resulting oxide films are generally lower compared to the 6000 eries. Furthermore, the voltage required to sustain a given current density is lower than for the 2000 or 5000 series. The ultimate film thickness and properties are highly dependent on the strict control of electrolyte composition, temperature, current density, current type, and oxidation time.

Sulfuric Acid Electrolyte in Hard Anodizing

The most common hard anodizing process employs direct current (DC) technology within a sulfuric acid electrolyte, primarily due to cost efficiency.

DC process is the MHC (Martin Hard Coating) process, developed by the Glenn L. Martin Company. This process uses a 15% sulfuric acid solution at a low temperature of 0 ℃ with a DC density of 2 to 2.5 A/dm². To maintain a constant current density, the voltage typically increases from an initial 20 to 25V up to 40 to 60V.

A significant limitation of conventional DC anodizing is the propensity for “burning,” particularly in high-copper alloys, unless electrical contacts and agitation are highly effective. Industrial standard sulfuric acid hard anodizing processes often include minor additions of oxalic and/or other organic acids. Operating temperatures are generally maintained below 10 ℃, with current densities ranging from 2 to 5 A/dm².

A disadvantage of using dilute solutions at low operating temperatures is the risk of the electrolyte freezing, and it need effective solution circulation to maintain operational fluidity. Additionally, films produced in dilute solutions tend to have rougher surfaces compared to those from concentrated solutions, often requiring subsequent mechanical finishing.

Non-Sulfuric Acid Electrolyte in Hard Anodizing

While sulfuric acid is favored for its low cost, its relatively high corrosive action on the anodic film has motivated the development of alternative non-sulfuric electrolytes to meet specialized performance requirements and broaden the range of hard anodizable alloys.

Organic Acids and Sulfates: Early hardcoat development explored oxalic acid, but its requirement for high applied voltages limited its widespread use. Later non-sulfuric formulations included:

• 80 g/L oxalic acid with 55 g/L formic acid, using a current density of 6 A/dm² and voltage ramped from 25V to 60V, yielding gray or black films.
• 240 g/L sodium bisulfate with 100 g/L citric acid, with voltage ramped up to 50 to 100V, yielding brown or black films.

These formulations can produce hardcoat films up to 200μm.

Sulfonic Acid Solutions

Developed in Germany, sulfonic acid partially replaces sulfuric acid to mitigate the film’s chemical dissolution, allowing for the formation of wear-resistant hardcoats even at room temperature. Although these solutions are now more widely adopted for integral color anodizing, they are recognized for producing relatively dense hard anodic films.

Oxalic Acid and Glycolic Acid Solutions

Hard anodizing can also be performed in solutions based on these two dicarboxylic acids:

• A concentration of 1 to 100 g/L of oxalic or glycolic acid, or
• Various concentrations (from 10 g/L to saturation) of dicarboxylic and polycarboxylic acids, including malonic, tartaric, citric, malic, and glycolic acid.

Tartaric Acid-Based Solutions

Formulations based on 1 mol/L tartaric, malic, or malonic acid, with the addition of 0.15 to 0.2 mol/L oxalic acid, can operate at higher temperatures (40 to 50 ℃) and applied voltages (40 to 60V). They can maintain a stable current density of 5 A/dm² without powdering and achieve Vickers microhardness of 300 to 470 HV. Since this process operates significantly above room temperature, it offers a distinct cost advantage by reducing the substantial energy consumption typically required for cryogenic cooling.

Power Supply for Hard Anodizing

The high current densities required for hard anodizing fundamentally introduce the problem of effective heat dissipation. Beyond conventional measures like refrigeration and agitation, a key technological advancement in recent years has been the adoption of complex power supply waveforms, including biased voltage, pulsed voltage, interrupted current, and periodic reverse current. Currently, the most effective power supply is the DC unipolar pulse technology.

Conventional DC Power Supplies

Conventional DC power supplies remain the most common due to their simplicity and low cost. Advances in silicon-controlled rectifiers (SCR) have improved their reliability. However, issues related to localized overheating are compounded at high current densities. Consequently, many facilities prefer to operate at 2.5 A/dm² or less, even when equipment is rated for 5 A/dm², especially when processing susceptible alloys like the 2000 series.

A critical operational step in DC hard anodizing is controlling the initial current. If the current ramp-up rate is too fast, the component risks “burning,” where localized excessive current causes rapid dissolution or damage. Two conventional methods mitigate this:

• Controlling the current ramp rate.
• Starting at a standard anodizing current density of 1.0 to 1.5 A/dm² until a 2μm to 3μm film is established, then gradually increasing the current.

While somewhat effective, both methods increase the anodizing time, making them non-optimal for high-efficiency industrial production.

Pulsed DC Power Supplies

The most extensively adopted modern power technology is the DC unipolar pulse system. Since the late 1980s, pulse rectifiers have been increasingly used, pioneered in Japan and subsequently adopted in Italy and the U.S. The primary advantage is the ability to operate at significantly higher average current densities(often 3 A/dm² or more for many alloys) without the risk of burning. This enables stable production of high-quality oxide films with substantially increased efficiency.

AC/DC Superposition

AC/DC superposition technology has been explored since the 1950s. Generally, the AC voltage component is kept below the base DC voltage, with the AC waveform being rectified (unipolar). This allows the anodizing temperature to be run higher than with conventional DC hard anodizing, and the process has found industrial application. While other complex waveforms, such as interrupted current and periodic reverse current, have been studied extensively, unipolar pulsed DC remains the most widely accepted and effective industrial solution.

Properties of Hard Anodizing

Hard anodic films are engineered for high hardness, superior wear resistance, and excellent electrical insulation due to their high relative density and low porosity. They also generally exhibit improved corrosion resistance.

Appearance and Uniformity

Generally, higher applied voltage leads to increased surface roughness and decreased film uniformity. The film color is dependent on both the alloy and the film thickness. For die-cast aluminum, the color transitions from grayish-yellow to dark gray with increasing silicon content. Pure aluminum (99.99% Al) shows no color at 25μm but turns a light brown at 125μm. Compared to conventional anodizing, hard anodic films have noticeably poorer image clarity. Furthermore, the thick hardcoat films often contain microcracks.

Hardness and Abrasion Resistance

Hardness and wear resistance are fundamental properties of the hardcoat film. The alloy influences Microhardness, the specific anodizing process parameters, the load used in testing, and the position within the film cross-section (hardness typically decreases from the surface towards the aluminum substrate). For instance, the text{Hardas film on 6061-T6 alloy achieves approximately 500 HV, while the MHC film can reach 530 HV.

It is important to note that while high hardness is often correlated with good wear resistance, they are distinct physical properties. Hard anodic films (400 to 500 HV) are technically softer than high-speed tool steel or hard chrome plating (950 to 1100 HV). However, MHC hardcoats often exhibit wear resistance comparable to hard chrome and, in some cases, superior to high-speed tool steel.

Hard anodic films offer significantly better wear resistance than conventional anodic films. Direct quantitative comparison of wear data is difficult due to wide variations in testing methods and equipment across different laboratories.

Comparative data (Table 6-8 in the source material) shows that hard anodizing MHC and Alumilite 226 films increases the specific wear resistance (mass of abrasive required to penetrate per mm of film thickness by 100% to 200% for most alloys compared to conventional films (Alumilite 204), except for 2024 alloy, which saw only a 20% increase.

Grade	Film Type	Film Thickness/μm
1200	Alumilite204	11.9
	Alumilite226	56.9
	MHC	57.6
3103 – H18	Alumilite204	13.5
	Alumilite226	39.2
6061 – T6	Alumilite204	11.7
	Alumilite226	54.6
	MHC	58.7
7075 – T6	Alumilite204	11.4
	Alumilite226	54.1
2024 – T3	Alumilite204	10.4
	Alumilite226	53.3
	MHC	63.0

A notable observation is that the wear resistance measured immediately after hard anodizing can degrade after a period of atmospheric exposure. This degradation is prominent in Al-Cu-Mg-Mn alloys after six months but less significant in text{6061 alloy, possibly due to the influence of atmospheric humidity.

Corrosion Resistance

In general, hard anodic films exhibit better corrosion resistance than conventional anodic films, attributable to their greater thickness and lower porosity. Hard-anodized components have successfully passed 5%neutral salt spray tests and can be comparable to stainless steel in many environments. However, the hardcoat on 2024 alloy is an exception, showing no significant improvement in either wear or corrosion resistance over a conventional film.

Sealing with potassium dichromate can improve corrosion resistance but negatively impacts wear resistance. Therefore, hard anodic films are typically left unsealed or, if sealing is required, they are impregnated with materials like paraffin wax, mineral oil, or silanes. Preventing microcracks in thick hardcoat films is essential, as these fissures can significantly reduce corrosion resistance. Impregnation with Polytetrafluorofluoroethylene(PTFE) is highly effective, improving corrosion resistance without sacrificing wear performance. PTFE filling can also reduce the film’s coefficient of friction to as low as 0.05, a highly effective anti-friction treatment used on the internal surfaces of cylinders.

Thermal Properties and Heat Resistance

Anhydrous aluminum oxide melts at 2100 ℃, while hydrated aluminum oxide begins to lose its water of crystallization around 500℃. The thermal conductivity of the anodic film is only 1/10 to 1/13 that of aluminum, and its coefficient of linear expansion is 1/5 that of the aluminum. Conversely, the thermal emissivity of the aluminum surface rapidly increases with the growth of the oxide film, a 10μm film increases emissivity by approximately 80%. Thus, thick hardcoat films function as excellent thermal dissipators or “black bodies,” eliminating hot spots on heated components, a property exploited in the manufacturing of cookware.

Electrical and Dielectric Properties

The anodic film is non-conductive, with the breakdown voltage of hardcoat films potentially exceeding 2000V. Masking is often employed to maintain electrical contact areas during hard anodizing. Film on 5054A alloy show that boiling water sealing and paraffin wax impregnation significantly improve dielectric strength. The dielectric constant is high, and combined with good thermal conductivity, hard-anodized aluminum is superior to other insulating materials for specific electronic components. For instance, the film can be used up to 480℃, with a dielectric strength of 26 V/μm and a thermal conductivity of 3.1 W/（m·℃).

Mechanical Performance

The application of a hard anodic film has little effect on the ultimate tensile strength of the aluminum substrate but causes a marked reduction in both ductility (elongation) and fatigue strength.

Grade	Film Thickness / μm	Ultimate Strength / MN/m²	Elongation（%）
6061 – T6	—	329	12.0
	13	339	12.5
	25	336	11.5
	75	313	8.0
	125	311	5.5
2024 – T3	—	467	18.0
	13	459	17.5
	25	463	15.0
	75	434	11.0
	125	402	—
7075 – T6	—	552	8.5
	13	556	7.5
	25	550	7.5
	75	538	7.0
	125	503	6.5

The impact of the alloy composition on hard anodic film properties is significant, particularly affecting hardness and wear resistance. Because hard anodizing affects alloys more severely than conventional anodizing, mixing different alloy batches during processing should be avoided. Alloys susceptible to “burning,” such as those with high copper content, require strict control of current density, especially during the initial phase. High-zinc or high-magnesium alloys form films with poorer adhesion than pure aluminum and are therefore not recommended for applications involving impact loading.

About Getzshape

At Getzshape, we are experienced manufacturers of CNC machined aluminum parts. We have produced thousands of aluminum machined parts made of grade 6061, 7075, 5052 and so on, and most of them finished with anodizing or hard anodizing. If you are looking for custom hard coat anodized aluminum parts, feel free to contact us.