Are you looking to increase the performance and durability of aluminum? Hard coat anodized aluminum is a surface treatment that increases the properties of metal. Hard coat anodized aluminum is broadly used in different industries from aerospace to construction.

This complete guide to hard coat anodizing will discuss its process, application and advantages. We will also cover its definition, limitations, and factors that impact the finish.

What Is Hard Coat Anodizing?

Hard coat anodizing is an electrochemical process that makes a thick and durable oxide layer on aluminum surfaces. This layer is thicker ( 50-70 μm) than regular anodizing.

The main function of hard coat anodizing is to improve surface properties of aluminum. It also gives better corrosion protection, hardness and wear resistance. These upgrades are important for parts in aerospace and industrial areas.

Hard Coat Anodizing Process

The hard coat anodizing process has many important steps. We will cover these steps in detail:

Pre-treatment

It involves thorough cleaning and degreasing of the aluminum surface. This step prepares the metal for anodizing using solvents, alkaline cleaning, solvent degreasing or acid etching. It also removes contaminants which results in appropriate surface conditions.

Anodizing

During hard anodizing, the aluminium part is soaked in an electrolyte bath (sulfuric acid). Then, an electric current is applied. It causes oxygen ions to move from the electrolyte solution to the aluminium surface where they form a hard, porous oxide layer.

Hard Coat Formation

The formation of hard coat continues with the anodizing process. This stage usually uses higher current densities and lower temperatures (0-5°C) to acquire the desired thickness and hardness.

Coloring

Coloring is usually limited in hard coat anodizing because of the dense oxide layer. Dyes can also be used through organic dye soaking or electrolytic coloring according to project needs. However, color choices are mostly limited than regular anodizing.

Sealing

The final stage includes sealing the porous oxide layer. This is mostly done by using chemical sealants or hot water. Sealing closes the pores which increases corrosion resistance and cements the applied color. This step is important for the longevity of anodized surfaces.

Hard Coat vs Regular Anodized Aluminum

Hard coat and regular anodized aluminum have different characteristics. Let’s discuss these differences in detail.

Thickness

Hard coat anodizing makes thicker layers from 25 to 100 μm (1-4 mils). Whereas regular anodizing makes thinner layers of 5 to 30 μm (0.2-1.2 mils). The greater thickness of hard coat anodizing gives better durability and wear resistance. This makes it appropriate for demanding machining and finishing operations.

Seal

The thick oxide layer of hard anodizing needs more robust sealing methods. It usually uses chemical sealing with nickel acetate or hot water sealing at higher temperatures (95-100°C). While regular anodizing uses lower temperature sealing (80-90°C) or room temperature sealants. Seal quality has a direct impact on finish durability and corrosion resistance in machining and finishing processes.

Color

Regular anodizing provides many vibrant hues through dye absorption. But hard coat anodizing has limited color choices due to its dense oxide layer. It usually creates darker shades, mainly in gray to black spectrum.

Applications

Hard coating aluminium is usually best for high-wear operations in industrial, aerospace, and automotive areas. It is preferred for components with severe abrasion like valve bodies and hydraulic cylinders. On the other hand, regular anodizing is appropriate for moderate-wear and decorative applications. It is mostly used in architectural elements and consumer electronics.

Table 1: Comparison of Hard Coat Anodizing vs Regular Anodizing

Advantages of Hard Coat Anodizing

Hard coat anodizing provides different advantages than regular anodizing. We will discuss how this process improves metal properties.

Increased Wear Resistance

The hard coat anodize process makes a 100μm deep thick oxide layer on aluminum surfaces. This layer is very hard and has a hardness of 350–400 on the Knoop hardness scale. It also makes hard-anodized aluminium surfaces resistant to wear and abrasion.

Improved Corrosion Protection

The thick oxide layer formed after the anodization process of aluminum provides a strong hindrance against corrosive elements. This hard anodized aluminum surface can handle harsh environments like chemical exposure and salt spray better than regular anodized aluminum.

Increased Durability

Hard coat anodizing forms a hard thick oxide layer that is inherent to the base metal. This treatment allows aluminium parts to survive in harsh conditions like high mechanical stress and heat. The improved durability also increases the lifespan of components.

Electrical Resitivity

The anodization process converts the oxide layer into an outstanding electrical insulator. Hard-anodized surfaces according to their coating thicknesses can reach resistivity values up to 10,000 megohms. This characteristic makes hard anodized aluminium appropriate for applications that need electrical separation.

Limitations of Hard Coat Anodizing

Hard coat anodizing has many advantages. But it is important to consider its limitations too. We will discuss two main limitations.

Color Variability

Hard coating aluminium process usually makes less vibrant, darker shades than regular anodizing. Process settings and alloy composition can cause color variations between batches. This restriction may be a challenge for operations where precise color matching is important.

Lower Fatigue Strength

Hard coat anodizing can decrease aluminum’s fatigue strength up to 30%. It also makes tiny surface cracks that act as stress concentration points. These cracks may spread under repeated loading which can result in premature fatigue failure. This decrease in fatigue life should be considered carefully for components subjected to repeated stress.

Applications of Hard Coat Anodizing

Hard coat anodizing increases material properties which makes it valuable across multiple industries. Let’s look at five important uses:

Aerospace Parts

Aerospace relies heavily on hard coat anodizing. It’s applied to precision-driven components like:

• Engine parts

• Aircraft landing gear
• Hydraulic actuators

By promoting corrosion protection and wear resistance, hard coat anodizing increases part life. For example, treated aluminum pistons can handle high pressures and temperatures in aircraft engines which maximizes reliability.

Marine Hardware

In marine environments, hard coat anodizing is important. It’s usually used on:

• Underwater camera housings

• Deck hardware
• Boat propellers

The treatment makes a protective layer up to 100 microns thick. This coating protects aluminum parts from saltwater which greatly increases lifespan in harsh marine conditions.

Automotive Manufacturing

Hard coat anodizing performs an important role in auto manufacturing. It’s used on:

• Valve bodies

• Engine pistons
• Brake calipers

The resulting wear-resistant surface handles high pressures and temperatures. This maximizes the vehicle’s overall performance and increases component durability.

Industrial Machinery

Many industrial machine parts benefit from hard coat anodizing like:

• Hydraulic cylinders

• Pump shafts
• Valve bodies

The process produces surface hardness up to 70 Rockwell C which greatly reduces wear. This process permits aluminum to replace steel in many applications which facilitate lighter and more energy-proficient industrial machinery.

Specifications of Hard Coat Anodizing

Hard coat anodizing must meet a variety of industry standards. Here are three important specifications:

MIL-A-8625 Type III

This military specification for hard coat anodizing requires:

• Surface hardness: 65 Rockwell C

• Minimum coating thickness: 50 μm
• Particular wear and salt spray resistance testing procedures

MIL-A-8625 Type III is broadly used in defense and aerospace to guarantee of corrosion protection and maximum durability in their components.

ASTM B580 Type A

ASTM B580 Type A sets standards for anodic oxide coatings on aluminum. For hard coat anodizing, it specifies:

• Minimum coating thickness: 25 μm
• Quality control measures
• Testing procedures for hardness, wear resistance, and corrosion protection

This standard is mainly applied in industrial operations.

ISO 1007

ISO 1007 is an international standard for anodizing aluminum as well as its alloys. For hard anodizing, it mandates:

• Surface hardness: 300 HV0.05
• Minimum coating thickness: 25 μm

ISO 1007 is adopted globally to guarantee stable quality in hard coat anodizing processes.

Factors That Affect Hardcoat Anodized Aluminum

The durability and quality of hard coat anodized aluminum depend on many important things. Let’s look at four precision-driven factors: alloy makeup, surface prep, bath chemistry and anodizing duration. Each factor has an important role in the final product’s traits.

Alloy Composition

Not all aluminum alloys work well with hard coat anodizing. The 6061 aluminum alloy greatly responds to the process while others require special attention. For example, the 7075 alloy needs special care due to its high zinc content. Alloys with lots of copper or silicon usually make coatings that are less wear-resistant and is softer.

Surface Preparation

Properly prepping the surface is a must for the best quality hard coat anodizing. This involves cleaning, degreasing and etching the aluminum. Good preparation increases coating adhesion by 30% and can guarantee even oxide growth. Inadequately performing or skipping this step may lead to issues like pitting or uneven coating thickness. These problems can possibly decrease component lifespan and performance as well.

Bath Chemistry

The electrolyte bath chemistry is important in hard coat anodizing. Usually, sulfuric acid is used. Its composition affects coating properties and quality. It’s very important to maintain the acid temperature at 0 to 5°C and the concentration between 15 to 20%. Addition of substances like oxalic acid can increase coating hardness by up to 15%. Proper Control of the bath makes sure of consistent coating hardness and thickness across different batches.

Conclusion

Hard coat anodizing makes aluminum parts more corrosion-resistant, wear-resistant and durable. It’s used in marine, aerospace, automotive and industrial fields. To get the best results, it is very important that you completely know about the anodizing process, its specifications and factors.

For best-quality hard coat anodizing, Yijin Hardware is a great option. Our skilled workers having more than 20 years of experience and top-of-the-line machines guarantee hard-anodized and highly accurate finishes according to industry standards. So contact Yijin today if you want to maximize the functioning  of your aluminum parts.

Frequently Asked Questions

Q1. What is hard anodizing used for?

Answer: Hard coat anodizing is used when great durability, wear resistance and corrosion protection are required. It’s common for marine, automotive, aerospace and industrial machinery components to increase their lifespan and surface properties.

Q2. How long will hard anodized aluminum last?

Answer: Hard anodized aluminum can last multiple decades under normal conditions. Its lifespan depends on multiple factors like environmental exposure, thickness of the coating and mechanical stress. In tough applications, hard anodized parts can last 5 to 10 times longer as compared to untreated aluminum.