Introduction
Aluminum is used in a wide range of fields, including automotive parts, electronic devices, and building materials, due to its light weight and excellent corrosion resistance. However, in its raw state, the surface is easily damaged, and there are challenges regarding durability, aesthetics, and functionality. This is where “surface treatment” becomes important. By applying a surface treatment, it is possible not only to dramatically improve corrosion and wear resistance but also to achieve a variety of designs, such as colored anodizing and mirror finishes.
In recent years, due to fluctuations in global supply and demand balances and strengthening environmental regulations, the aluminum surface treatment market has grown to a scale of approximately 50 billion yen annually (domestic estimate). In particular, the need for weight reduction accompanying the electrification of automobiles and the demand for higher quality for precision electronic devices like smartphones are driving advanced treatment technologies such as hard anodizing and electroless nickel plating. On the other hand, the increase in treatment costs and lead times, as well as the burden of waste liquid treatment for chemical substances, are major concerns for many companies.
This article will introduce a wide range of representative surface treatment technologies for aluminum, comparing their respective features and effects, as well as providing detailed comparisons of unit prices and delivery time estimates. We hope to provide a helpful guide for manufacturing executives and procurement managers in selecting the optimal treatment method by organizing the key points for achieving both cost reduction and quality improvement.
Basics of Aluminum Surface Treatment
Purpose and Benefits of Surface Treatment
While aluminum is lightweight and has excellent machinability, its raw surface is easily scratched, and the naturally formed thin oxide film (approx. 0.002 μm) provides insufficient corrosion and wear resistance. By applying a surface treatment, the following functional and performance improvements are possible:
- Providing Corrosion and Rust Resistance
An artificially formed oxide film (anodizing) or chemical conversion coating secures a film thickness of several micrometers or more on the base material, preventing corrosion. Regular anodizing provides a film of about 6 μm or more, while hard anodizing allows for even thicker and harder films.
- Improving Wear Resistance and Hardness
By increasing the surface hardness to HV200–800 or more with hard anodizing or electroless nickel plating, wear and scratches can be suppressed.
- Enhancing Decoration and Aesthetics
Colored anodizing (dyeing, secondary electrolytic coloring) and various types of plating can achieve a wide range of colors and mirror/matte textures. This can enhance corporate logos and design aesthetics while maintaining functionality.
- Adding Functionality
Additional functions can be provided according to the application, such as low friction with PTFE-containing films, solder wettability with electroless nickel plating, and enhanced electromagnetic shielding with copper plating.
Criteria for Selecting a Surface Treatment
When choosing a surface treatment, it is necessary to comprehensively consider not only the required performance of the product but also cost, delivery time, shape compatibility, and environmental/regulatory aspects.
- Required Properties and Film Thickness
Clarify the required film thickness and hardness according to the purpose (corrosion resistance, wear resistance, design, functionality, etc.). For example, for wear resistance applications, select a hardness of HV400 or more, and for decorative purposes, a thin film of about 5–10 μm.
- Shape and Dimensional Tolerance
Consider the presence of irregularities and pores, and the effect on dimensional tolerances (the increase due to film thickness). Electroless plating is good for uniform coating on complex shapes, but anodizing may require jig design.
- Cost and Lead Time
The unit price and processing time vary greatly depending on the treatment method and batch size. Generally, anodizing takes several days including pre- and post-treatment, plating takes 1–2 days, and painting takes from the same day to several days (details in a later section).
- Environmental and Regulatory Compliance
Consider waste liquid treatment costs and compliance with regulations such as RoHS/REACH. Chromate-based chemical conversion treatments and heavy metal plating have particularly strict waste liquid management, and the consideration of alternative technologies is necessary.
Based on these points, the next section will explain the features and application points of representative treatment methods in detail.
Main Types and Characteristics of Surface Treatments
Anodizing
This is a method of generating an artificial oxide film by applying electrolytic oxidation to the aluminum base material.
- Regular Anodizing: With a film thickness of about 5–15 μm, it is optimal for improving corrosion and decorative properties. It is basic and has a lower cost burden.
- Hard/Super-Hard Anodizing: Achieves a film thickness of 20 μm or more and a hardness of HV300 or more. It is resistant to wear and scratches and is often used for tool parts and sliding components.
- Colored Anodizing (Dyeing, Secondary Electrolytic Coloring): After anodizing, the part is immersed in dyes or metal salts to impart various colors. It is effective for design parts and for promoting a brand image.
Chemical Conversion Coating (Alodine/Alsurf/Direct Chromate)
A film is formed by immersing the part in a chemical solution without electrolysis. It is characterized by its thin film (1–2 μm) that improves corrosion resistance and adhesion, and has high compatibility with subsequent painting and bonding processes. Chromium-free products are also available, contributing to compliance with environmental regulations.
Various Types of Plating
This is a method of depositing metal on the surface by electrolysis or chemical reaction.
- Electrolytic Nickel Plating / Electroless Nickel Plating: It is easy to obtain a uniform film thickness, has a hardness of HV400–600, and excellent corrosion resistance. It can also be applied to complex shapes.
- Nickel-Chrome, Copper, Tin Plating: Chrome plating is used for decorative purposes due to its high mirror-like finish, copper plating for improving electrical and thermal conductivity, and tin plating for providing solder wettability.
Painting
By applying a resin coating, not only corrosion resistance but also a wide range of design possibilities are expanded.
- Electrodeposition Coating: Charged powder paint is sprayed and applied uniformly. It allows for high adhesion and thick films and can be applied to thin-walled products.
- Conventional Painting: Solvent-based paint or powder coating is sprayed and then baked. While it offers a high degree of color freedom, skill is required for film thickness management.
Polishing and Deburring
This also serves a pre-treatment role by removing fine surface irregularities and burrs to create a smooth surface.
- Blasting: Roughens or cleans the surface by impacting it with sand or beads. It is essential for pre-treatment before anodizing.
- Barrel Polishing: Polishes many parts to a mirror finish with abrasive media while rotating them. It is suitable for continuous batch processing.
- Buff Polishing: Achieves a high mirror finish with a soft buff and polishing compound. It is highly effective for design parts.
- Electrolytic Polishing / Chemical Polishing: Dissolves and smooths the metal surface with electric current or chemical solutions. It is particularly effective for finely machined parts and parts with micropores.
Comparison of Effects of Each Treatment
- Corrosion and Rust Resistance
Anodizing, chemical conversion coating, and plating all strengthen the natural oxide film of the base material, significantly reducing the corrosion rate. In particular, anodizing forms an artificial oxide film of several micrometers or more, and even regular anodizing dramatically improves corrosion resistance.
- Wear Resistance and Hardness Improvement
Hard anodizing can achieve a surface hardness of HV400 or more, and electroless nickel plating or nickel-chrome plating can achieve HV800 or more. This significantly improves resistance to wear and scratches.
- Aesthetics and Design (Mirror/Matte/Color)
In addition to achieving a wide range of colors with dyed anodizing or secondary electrolytic coloring, chrome plating provides a high mirror-like finish. In combination with buff polishing, the texture of mirror and matte finishes can be freely controlled.
- Electrical and Thermal Properties (Conductivity/Heat Dissipation)
Copper or silver plating improves electrical conductivity, while an anodized film with a PTFE-containing layer improves heat dissipation (emissivity). For electromagnetic shielding applications, electroless nickel plating is also effective.
- Environmental and Regulatory Compliance (RoHS, REACH, etc.)
The spread of chromium-free chemical conversion treatments and electroless plating reduces the use of heavy metals. It is possible to comply with international environmental regulations while reducing waste liquid treatment costs and environmental impact.
Cost and Lead Time Comparison
Comparison of Execution Unit Prices
The typical unit price ranges for each treatment method are as follows (varies depending on part size and lot quantity).
| Treatment Method | Unit Price Guideline (JPY/m²) | Features |
| Chemical Conversion Coating (Alodine, Alsurf, etc.) | 100–300 | No electrolysis required, short process, lowest cost |
| Regular Anodizing | 400–1,200 | Film thickness 5–15 μm. Balances corrosion resistance and decoration |
| Hard/Super-Hard Anodizing | 1,000–2,000 | Film thickness 20 μm or more. High hardness of HV300+ |
| Plating (Electroless Nickel, etc.) | 1,500–3,000 | Uniform film thickness, good corrosion/wear resistance. Handles complex shapes |
| Painting (Electrodeposition, Powder Coating, etc.) | 800–1,500 | Film thickness 10–50 μm. High designability but includes pre/post-treatment costs |
| Polishing (Barrel, Buff, Electrolytic, etc.) | 300–1,000 | Mainly serves as a pre-treatment. Buff polishing is essential for a mirror finish |
Cost Impact of Film Thickness Increase and Dimensional Change
The increase in treatment film thickness directly affects dimensional tolerance management and assembly costs in subsequent processes.
- Chemical Polishing / Buff Polishing: Dimensional decrease (by the amount of polishing)
- Chemical Conversion Coating: No dimensional change (ZERO)
- Anodizing: Dimensional increase by 1/3 to 1/2 of the film thickness
- Plating / Painting: Increases directly by the film thickness
Designing without considering the increase in film thickness can lead to assembly defects and rework costs, so it is important to reflect this in the specifications beforehand.
Lead Time and Delivery Time Estimates
Please use the following as a guideline for general lead times (including pre- and post-treatment). It may vary depending on the order volume and equipment availability.
- Chemical Conversion Coating: 1–2 business days
- Polishing: 1–3 business days
- Plating: 2–4 business days
- Painting: 2–4 business days
- Anodizing: 3–7 business days
In particular, for hard anodizing and thick-film treatments, the electrolysis time is longer, so a schedule with ample delivery time is desirable. Also, if environmental regulation compliance or waste liquid treatment procedures are necessary, it is safe to allow for an additional buffer of 1–2 business days.
Optimal Surface Treatment Case Studies by Application
Automotive Parts
In the automotive field, in addition to corrosion and wear resistance, design aesthetics and weight reduction needs are important. For example, hard anodizing (Type III) is often used for aluminum die-cast gearbox casings and cylinder head covers. Hard anodizing achieves a film thickness of 20 μm or more and a hardness of HV300 or more, significantly improving wear and corrosion resistance, thus demonstrating excellent durability even in the harsh engine compartment environment. Also, Type II (sulfuric anodizing) is increasingly used to apply colored anodizing to exterior parts, achieving both branding and rust prevention (Evident Scientific).
Electronic and Precision Equipment
For complex-shaped parts such as heat sinks and casings, anodizing or chemical conversion coating is basic. In particular, for heat sinks aiming to improve heat dissipation (emissivity), hard anodizing that is blackened by the oxide film is used. The emissivity is reported to increase from around 0.1 for untreated aluminum to 0.8 or more, improving cooling performance by about 20%. Also, electroless nickel plating is chosen for the printed circuit board fittings in casings to provide solder wettability and electromagnetic shielding properties.
Building Materials and Interiors
For architectural applications such as exterior panels, handrails, and sashes, long-term weather resistance and aesthetics are required. In environments exposed to acid rain or coastal salt damage, thick-film (over 10 μm) anodizing is standard, and by further combining it with powder coating, weather resistance of 30 years or more can be achieved. As a case study, there is a track record of over 20 years of maintenance-free performance for the exterior panels of a high-rise building using chemical conversion coating plus powder coating (chemetallna.com).
Other Industrial Applications
For various fields such as medical devices, food machinery, and aircraft parts, the addition of special functions is key.
- Parts for Soldering: Electroless nickel plating + tin plating ensures good solder wettability and corrosion resistance.
- Electromagnetic Shielding Parts: Copper plating or electroless nickel plating enhances shielding properties and is used for communication equipment casings and sensor parts.
- Chemical Plant Equipment: Emphasizing chemical resistance, the corrosion life is extended by combining fluorine-based coatings and electrolytic polishing.
As shown above, it is important to select the optimal surface treatment according to the application and required functionality.
Environmental Load and Waste Liquid Treatment Cost
Overview of Environmental Regulations
The chemical substances used in surface treatment may include those with a high environmental impact, such as hexavalent chromium, heavy metal ions, and organic solvents.
- Hexavalent Chromium Regulation: Hexavalent chromium has long been widely used for its strong rust-preventing properties, but it has been pointed out to be carcinogenic, and the standard concentration in wastewater has been strictly regulated to 0.5 mg/L or less (itic.pref.ibaraki.jp). As an alternative, the development of trivalent chromium treatment and geomet (chromate-free) chemical conversion treatment technologies is progressing (daiwakk-vn.com).
- RoHS/REACH/ELV Directives: For electrical and electronic equipment, there are restrictions on the use of lead, cadmium, hexavalent chromium, etc., and compliance with these regulations is also essential for aluminum surface treatment (J-STAGE).
- Water Pollution Control Act: When discharging wastewater containing heavy metals as factory effluent, investment in pre-treatment (neutralization, precipitation, adsorption) and advanced treatment facilities to meet water quality regulation standards is required.
To comply with these regulations, surface treatment companies are reviewing their chemical compositions and updating or sharing their wastewater treatment facilities, and the cost of environmental regulation compliance is increasing year by year.
Practical Examples of Waste Liquid Treatment and Recycling
- Metal Recovery from Plating Wastewater
Plating wastewater contains metal ions such as nickel, copper, and zinc, and is treated as industrial waste as a hazardous substance. Generally, the metal is recovered through a process of neutralization with chemical additives → precipitation → filtration, and the residue is incinerated or landfilled. For example, in zinc plating wastewater, there are cases where caustic soda and sodium hypochlorite are added to adjust the zinc concentration to below the standard, allowing for safe discharge (Marusho Co., Ltd.).
- Recycling of Anodizing Baths
In sulfuric acid anodizing baths, the concentration of dissolved aluminum increases during treatment, and to maintain quality, the bath solution was periodically renewed. However, in recent years, there have been examples where the amount of waste liquid has been significantly reduced by introducing ion exchange membranes or sedimentation devices to regenerate and recycle part of the bath solution (jilm.or.jp). This has made it possible to reduce the cost of renewing the waste liquid by 30% or more and to suppress the consumption of sulfuric acid.
- Waste-Liquid-Suppressing Equipment
Factories have also appeared that have virtually eliminated wastewater discharge by creating a closed-loop system for the processes from substrate cleaning → chemical conversion treatment → anodizing, and combining it with evaporative concentration or reverse osmosis (RO) treatment. Although the initial investment is large, it is attracting attention as an approach to reduce long-term wastewater treatment costs and legal compliance risks.
By introducing these measures, companies are achieving both a reduction in wastewater treatment costs and legal compliance, and are promoting the construction of a sustainable production system.
Conclusion
Aluminum surface treatment is an important technology that significantly improves corrosion resistance, wear resistance, aesthetics, and functionality while leveraging the material’s light weight and machinability. The methods compared in this article—anodizing, chemical conversion coating, plating, painting, and polishing/deburring—each have clear and distinct advantages and disadvantages. For example, hard anodizing achieves high hardness and wear resistance but tends to increase lead time and cost, while chemical conversion coating contributes to low cost and shorter lead times but requires selection according to the purpose due to its thin film. Also, when viewed by application, the required performance, shape, and design aesthetics differ greatly for automotive parts, electronic devices, and building materials, making it difficult to select a single optimal surface treatment.
When companies select a surface treatment supplier, they should emphasize the following points:
- Clarification of Required Performance: Specifically define the purpose, such as corrosion resistance, wear resistance, aesthetics, or special functions, and determine the necessary film thickness and hardness at the design stage.
- Balance of Cost and Delivery Time: Understand the correlation between unit treatment price and lead time, and optimize batch size and production schedule.
- Dimensional Change and Tolerance Management: Reflect dimensional changes due to film thickness increase or polishing in the design specifications to prevent assembly defects and rework costs.
- Ability to Comply with Environmental Regulations: Evaluate the environmental impact, including international regulations like RoHS/REACH and wastewater treatment costs, and choose a supplier that has alternative technologies or wastewater recycling facilities.
By comprehensively considering these points, it is possible to achieve both cost reduction and quality improvement, and to realize a sustainable manufacturing system. In the next step, let’s proceed with building a partnership with the optimal surface treatment supplier through actual quotation acquisition and sample testing.
Sources
- Ministry of Economy, Trade and Industry “Dynamic Statistics of Manufacturing” https://www.meti.go.jp/statistics/monodzukuri/
- JIS Standard “Anodizing” https://www.jisc.go.jp/app/jis/general/GnrJISSearch.html
- Silvec: Proposal for Surface Treatment to Enhance the Value of Aluminum https://www.mekkikakou.com/aluminum-increase-value/
- Nihon Shinkan: List of Various Surface Treatments for Aluminum https://www.nihonshinkan.co.jp/blog/2017/12/26/99
- Aluminum Machining Cost Down Center: Surface Treatment Column https://aluminium-costdown-center.com/column/category/tag4/
