Understanding Type 3 Hard Coat Anodizing: Thickness, Colors, Applications, Process & Comparison with Other Types

Aluminum anodizing represents a prevalent surface finishing technique employed for machined parts. With various types of anodization available, we will delve into Type III anodizing. This discussion will encompass its definition, specifications certification, achievable thickness, hardness, colors, applicable materials, process, benefits, and a comparison between Type 3 and Type 2 anodizing and hard coat anodizing vs plating.

What Is Type III Anodizing (Hardcoat Anodizing)?

Type III anodizing, also known as Hardcoat/Hard Coat Anodizing or Hard Anodic Coating, is an electrochemical process that forms a dense, hard, and durable oxide layer on the surface of colored alloys, typically aluminum alloys. This surface treatment method usually follows military standard MIL-PRF-8625 (MIL-A-8625). Hard Coat anodizing is the thickest of the three types of anodizing. This type of anodizing process can form a very thick oxide layer, which enhances the performance and properties of the alloy, including surface hardness, wear resistance, electrical insulation, and corrosion resistance.

Type III Hard Coat Anodizing Thickness

The typical thickness for Type III anodizing is 2 mils (51 µm). However, under specific process conditions, the required coating thickness can be 125 micrometers or higher. According to MIL-PRF-8625F, unless specified otherwise in the contract, purchase order, or applicable drawings, the nominal coating thickness should be 0.002 inches (2 mils). Any variations in thickness above 0.002 inches (2 mils) should not exceed ±0.0004 inches (0.4 mils). It is challenging to achieve 2 mils or higher hard anodic oxide coating on high-silicon sand castings such as 360, 380, and 383.

Type III Hard Coat Anodizing Hardness

The standard hardness for hard coat anodizing is 60-65 HRC. Some hard coat anodizing processes can produce layers with a hardness of up to 70 HRC.

Hard Coat Anodizing Colors

Can hardcoat anodized aluminum be colored? Yes, hard anodized aluminum can be colored. The coating darkens and takes on color due to the composition of the oxidation film and microstructure developed during the hard coat process. Specific aluminum alloys will develop characteristic colors in the oxidized hard coat. For example, 2000 series alloys produce a green or gray tinge from entrapped copper. 6000 series aluminum forms an almost black oxide layer. Even light gray colored coats are possible with high temperature resistant 8000 series aluminum alloys undergoing hard coat anodizing.

What Materials Can Be Hard Anodized?

While other metals, such as titanium and magnesium, can also be anodized, aluminum is more commonly treated using this technique for protection. Cast aluminum and die-cast aluminum are widely used materials for hard anodizing treatment due to their excellent surface treatment performance. Cast aluminum alloys, commonly used as aluminum-silicon alloys and aluminum-copper alloys, are popular for their good casting performance and wear resistance. Pure aluminum is also common in hard anodizing treatment.

Duration of Hard Anodized Coating

Type III anodized coatings are not permanent. They can last for several years to several decades. They are commonly used for applications requiring good wear resistance. The severity of the wear environment affects the lifespan of the protective oxide layer.

Advantages of Hard Coat Anodizing

– Increased abrasion resistance and surface hardness. Hard coat anodizing makes the surface much harder than the underlying aluminum metal, the part will be much more durable and resistant to scratches, wear, and abrasion compared to bare aluminum. This extended lifespan and durability are excellent for parts that may see heavy use or contact.

– Acts as an electrical insulator. The oxide layer formed during anodizing is dielectric in nature, meaning it does not conduct electricity very well. The thicker the coating, the higher the electrical resistivity. This makes hard coat anodizing useful for applications where electrical or thermal insulation is required, such as electronic housings or parts that may experience voltage stresses.

– Improved corrosion resistance. The hard coating also serves as a barrier to protect the underlying aluminum from corrosion by water, chemicals, saltwater, or other corrosive environments. This extends the functional lifetime of anodized parts subjected to corrosion.

– Aesthetic benefits. The hard anodizing process can produce opaque, colored finishes that can enhance the visual appearance of parts. Different colors may also be specified for identification purposes.

Applications of Hard Anodized Parts

Type III hard coat anodizing has various applications, particularly in industries requiring high wear resistance and ease of lubrication, such as piston rings and some valves, hinge mechanisms, gears, rotating joints, insulation boards, etc.

– Aerospace and Aviation: Anodizing treatment forms a hard, relatively inert alumina (Al2O3) layer, enhancing the material’s hardness and wear resistance. This is crucial for aircraft and aerospace parts subjected to long-term mechanical wear. Additionally, good electrical insulation prevents electrical short-circuit issues, ensuring the normal operation of spacecraft.

– Medical Devices: Type III anodic oxide coatings are primarily used for medical devices, such as bone screws and steel plates. These devices require good corrosion resistance and easy identifiability, which Type III anodizing can provide.

– Automotive and Heavy Machinery: Commonly used in automotive and heavy machinery components’ surface treatment. For example, heavy-duty truck filters, video conferencing device frames and support parts, excavator hydraulic and pneumatic blocks, etc., utilize this coating.

– Semiconductors and Electronic Devices: Applicable for various electronic connectors, valve housings, etc.

Type III Hard Coat Anodizing Specifications and Certifications

• MIL-PRF-8625 (Mil-A-8625)

• MIL-STD 171 7.5.1 & MIL-STD 171 7.5.2

• AMS 2468 & AMS 2469

• AMS-A-8625

• BS 5599 & BS EN 2536

• ASTM B 580 & ASTM 2482

• BAC 5821

• PS 13208, PS 13201 & PS 13021.1

• HP 4-79

How to Remove Hard Anodized Coating?

1. Chemical

– Acid Washing: This is a commonly used method. By immersing the metal in an acid solution, such as HF-HCl or HF-HNO3 acid washing baths, the surface reaction layer can be quickly and completely removed without introducing other elemental contaminants.

– Alkaline Washing: Alkaline solutions can also be used, but specific details are less frequently mentioned.

2. Electrochemical

– These methods involve immersing the metal in an acidic or alkaline solution and removing the oxide coating through chemical reactions.

3. Other methods

When removing all thick anodic coatings, one of the best practices is to immerse the parts or load in a strong acid bath, such as a deoxidizer, or even an anodizing bath for 45 minutes, followed by rinsing and removing the coating.

Impact of Hard Anodized Coating Removal on Part Size

Removing the hard anodized coating will reduce the dimensions of the parts. This is because, as the anodized coating is forming, it actually penetrates the aluminum. Some of it permeates, while some accumulate. Consequently, if the coating thickness is 0.0050 inches, upon removal, the areas with the anodic coating may be approximately 0.0025 inches or larger than the areas without the coating. However, during the removal process, uncoated areas will start to etch before coated areas.

Process of Aluminum Hard Coat Anodizing

There are some established fundamental and common hard anodizing processes that have been in use for a long time. These, along with many variants, form the basis of the processes still in use today.

1. Martin Hard Coat (MHC) Process

– Using a 15% (165 g/l) sulfuric acid electrolyte solution

– Bath temperature: 32°F±3°C (0°C±2°C)

– Current density: 20-25 A/sq.ft (2.2-3.2 A/sq.dm)

– Produces approximately 1.0 mil (25 µm) coating thickness in 30-35 minutes

2. Alumilite 225/226 Process

– Using a “mixed acid electrolyte” of 12% (132 g/l) sulfuric acid and 1% (40-45 g/l) oxalic acid

– Bath temperature: 48°F to 52°F (9°C to 11°C)

– Current density: 36 A/sq.ft (3.9 A/sq.dm)

– Produces approximately 1.0 mil of coating thickness every 20 minutes

3. Basic Steps and Procedures:

1) Pretreatment:

– Degreasing: Immersion in a degreasing solution, followed by rinsing to remove the degreaser. Degreasing is usually performed using solvents based on tetrachloroethylene.

– Alkaline etching: Immersion in a sodium hydroxide solution to remove Al2O3, followed by rinsing with water.

– Chemical bright dipping: Remove dirt and natural oxide film using a phosphoric and nitric acid mixture.

– Desmutting: Immersion in a de-smutting solution, usually a mixture of phosphoric and nitric acids, to remove any remaining smut or metal particles.

2) Anodizing:

– Hanging Parts: Suspending aluminum parts on hooks.

– Electrolyte Preparation: Preparing a 10% sulfuric acid solution and ensuring temperature control within ±25°C.

– Anodizing: Immersing aluminum parts in a sulfuric acid electrolyte as the anode, while a permanent cathode, typically made of graphite or lead, is placed in the bath and completes the electrical circuit. Anodizing occurs on the aluminum surface, forming an oxide film. The typical current density is 3.0 A/dm², with an anodizing time of 70 minutes and a sulfuric acid concentration of 240 g/L.

– Rinsing: Rinsing with water to remove residual electrolyte after anodizing.

– Dyeing or electrolytic coloring: Performed as needed for aesthetic purposes. Dyeing before sealing helps the dye penetrate deeper into the oxide layer.

– Sealing: Using a low-temperature sealing process makes the oxide layer denser and more durable.

Process Parameters:

– Current density: 3.0 A/dm²

– Anodizing time: about 70 minutes

– Sulfuric acid concentration: 240 g/L

– Temperature control: Maintaining a constant temperature is necessary during the hard anodizing process.

– Voltage control: Type III anodizing requires higher voltage than Type II anodizing. A power source capable of providing sufficient current and up to 120 VDC voltage is necessary for the anodizing cycle. The initial voltage applied is around 25 VDC, increasing to 120 VDC by the end of the process. As the anodic layer thickens, its resistance to current increases, which may hinder the oxide deposition process. Increasing voltage compensates for the decrease in the electrical conductivity of the workpiece. Be cautious with voltage control to prevent the anodic layer from burning due to a sudden increase in voltage.

Differences Between Type II and Type III Anodizing – Hard Coat Anodizing vs Regular Anodizing

Type II anodizing is also known as the standard or regular anodizing.

1. Electrolyte Composition

– Type II anodizing typically uses sulfuric acid as the electrolyte, with a concentration between 10%-20%.

– Type III anodizing employs a sulfuric acid solution as well but can use other acids or acid combinations.

2. Coating Thickness

– Type II anodizing generates a thin oxide layer, usually 5-8 µm thick, while Type III anodizing forms a thicker hard coating, typically requiring 50µm in thickness.

3. Conditions

– Type II anodizing has a current density of DC 1-2 A/dm².

– Type III anodizing requires higher voltage or current density and is performed in extremely cold conditions to produce a thicker, harder anodic layer.

4. Applications

– Type II anodizing is primarily used to enhance the corrosion resistance and surface quality of aluminum alloys, suitable for protecting automotive and machinery parts, medical equipment, household appliances, and industrial components.

– Type III anodizing is mainly used to increase wear resistance and corrosion resistance, is applicable for high wear resistance applications.

5. Appearance

– Type II anodizing has a bright, shiny appearance. Due to its thicker multi-pore layer, Type III anodizing has a duller, less glossy surface.

6. Porosity

– Type II anodizing has lower porosity, with smaller and denser pores. Type III anodizing has a highly porous honeycomb structure, allowing dye penetration.

7. Hardness

– Due to its thicker layer, Type III anodizing provides greater wear resistance. It is harder and more durable than Type II anodizing.

8. Corrosion Resistance

– Type III anodizing, with its greater thickness, offers better corrosion resistance, especially in harsh environments. Type II anodizing provides less protection.

Hard Coat Anodizing vs Plating

Hard coat anodizing and plating differ in their processes and final results. Plating involves adding a coating layer on the material surface, while hard coat anodizing forms a coating through penetration and external accumulation. Half of the coating penetrates, and the other half accumulates on the outer surface. As a result, even with the same coating thickness, hard coat anodizing does not add as much dimensional change to the material as plating would.

1. Principles

– Hard Coat Anodizing: Through contact between the metal surface and an oxidizing solution, a dense, hard oxide layer is formed on the metal surface to enhance its performance. This is a natural growth process and the oxide layer is formed on the aluminum surface without altering the shape or size of the original material.

– Plating: A layer of metal or alloy coating is electrodeposited onto the metal surface to change its appearance and performance. Plating involves electrolysis, where the material to be plated serves as the cathode, and the plating metal of the same material as the anode (or a non-soluble anode) is immersed in an electrolyte solution containing the plating metal ions; a certain current is then applied.

2. Coating Properties

– Hard Coat Anodizing: Typically generates a rough surface oxide layer with a silver-white or gray appearance, providing a protective oxide layer without altering the product’s appearance.

– Plating: Can produce a bright, mirror-like, or plating metal appearance, often used to enhance the product’s visual appeal.

3. Usge

– Hard Coat Anodizing: Suitable for the surface treatment of light metals for corrosion resistance and wear resistance.

– Plating: Applicable for the surface treatment of heavy metals, allowing for various appearances and colors.

4. Efficiency

– Hard Coat Anodizing: Due to its natural growth characteristic, the process efficiency is relatively low, but it provides long-term protective effects.

– Plating: Due to electrolytic deposition, the process efficiency is high, but regular replenishment of metal ions in the electrolyte solution is required.

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