In parts procurement within the manufacturing industry, “aluminum castings,” which excel in lightweightness and workability, are an indispensable material. However, aluminum easily oxidizes in the air, and depending on the usage environment, it has the weakness of being prone to corrosion and wear. This is where “surface treatment technology” becomes crucial. By applying appropriate surface treatment, the product’s lifespan is dramatically extended, and its visual appeal and functionality are improved.
In this article, based on the expert knowledge of Daiwa Aluminum Vietnam, we will thoroughly explain the types and characteristics of major surface treatment technologies for aluminum castings, as well as the “optimal selection method” to achieve both quality improvement and cost reduction. We provide practical information that will serve as the next step for management and procurement managers considering overseas procurement or supply chain reviews.
The Importance of Surface Treatment in Aluminum Castings
Why is Surface Treatment Necessary for Aluminum Castings?
Aluminum combines with oxygen in the air to form a natural oxide film on its surface. While this film has a certain rust-prevention effect, its thickness is extremely thin, only about 0.001 μm (micrometers) to 0.01 μm, and it cannot provide sufficient protection in harsh environments or applications involving physical friction.
Furthermore, aluminum has a specific gravity of approximately 2.7 g/cm³, which offers the major advantage of a 65% weight reduction compared to iron (approximately 7.8 g/cm³). On the other hand, the material itself has low hardness and is easily scratched. Therefore, artificial surface treatment is essential to supplement its corrosion resistance (rust resistance), wear resistance (abrasion resistance), and decorative properties (visual appeal), thereby enhancing the product’s added value.
Balancing Quality Improvement and Cost Reduction
In the field of procurement, surface treatment is often perceived as a “cost-increasing factor.” In reality, adding a surface treatment after the casting process typically increases the lead time by 3 to 7 days and adds processing costs. However, selecting a surface treatment that is not suited for the part’s usage environment will lead to early corrosion and complaints due to defects, resulting in worsened yields and increased lifecycle costs.
Conversely, by selecting the “optimal surface treatment” at a reasonable cost that is not over-specified for the requirements, and by procuring from a reliable supplier (for example, an overseas base like Vietnam capable of handling everything up to surface treatment consistently), it is entirely possible to reduce overall costs by 15% to 30%.
Types and Characteristics of Major Surface Treatment Technologies for Aluminum Castings
There are various surface treatments applied to aluminum castings, ranging from chemical to physical approaches. Here, we explain five representative technologies.
Anodizing Treatment (Anodic Oxidation)
Anodizing is a typical surface treatment unique to aluminum. An electric current is passed through an electrolytic solution with the aluminum acting as the anode (positive electrode) to form an artificial, thick aluminum oxide film on the surface.
- Standard Anodizing (Clear/White Anodizing, Color Anodizing) Intended for general rust prevention and decoration. The film thickness is typically formed between 5 μm and 25 μm. The film has countless microscopic pores, and by permeating them with dye, vivid coloring such as red, blue, and black is possible. It has a dielectric strength of approximately 30V per 1 μm.
- Hard Anodizing Used when wear resistance needs to be maximized. By treating the aluminum slowly in a low-temperature electrolytic solution, an extremely thick and hard film of 50 μm to 100 μm is formed. Its surface hardness reaches HV (Vickers hardness) 400 to 500, boasting a hardness comparable to quenched steel materials. It is ideal for automotive engine parts and sliding parts of industrial machinery.
Chemical Conversion Treatment (Chromate / Non-Chromate Treatment)
Chemical conversion treatment involves immersing aluminum in a chemical solution to form a rust-preventive film on the surface through a chemical reaction. It is also widely used as an undercoat treatment for painting and plating.
- Characteristics and Environmental Compliance In the past, chromate treatment using “hexavalent chromium,” which has excellent corrosion resistance, was the mainstream. However, due to concerns over its environmental impact and adverse effects on the human body, “trivalent chromium treatment” and “non-chromate (such as zirconium-based) treatment” compliant with the RoHS directive (European restriction of hazardous substances directive) are now standardized.
- Performance Indicators The thickness of the chemical conversion coating is extremely thin, at 1 μm or less, but in a salt spray test (a test to evaluate corrosion resistance), it shows high rust prevention performance with no white rust appearing even after 120 to 168 hours. The cost is also relatively low, making it suitable for mass-produced parts.
Plating Treatment (Electroless Nickel, etc.)
This is a technology that coats the surface of aluminum with a film of other metals (such as nickel, chromium, gold, or silver). Although aluminum easily conducts electricity, the natural oxide film on its surface interferes, making it a material with a high difficulty level for plating. However, thanks to technological improvements such as zincate treatment (zinc substitution), stable plating is now possible.
- Electroless Nickel Plating Forms a nickel film through a chemical reduction reaction without using electricity. Its greatest strength is the ability to form a film of uniform thickness between 10 μm and 30 μm, even on castings with complex shapes. The hardness is high at HV 500 to 700, and it is possible to further increase the hardness (to HV 900 or more) by applying heat treatment. Because it can simultaneously impart wear resistance and corrosion resistance, it is highly valued for precision machinery parts and the like.
Painting Treatment (Baking Finish / Powder Coating)
This is a treatment that covers the surface of aluminum with paint to improve its appearance and protect it from the environment. Because aluminum castings have a casting surface (characteristic unevenness), putty filling and base treatment must be performed carefully.
- Baking Finish After spraying paint such as melamine resin or acrylic resin, it is baked in a high-temperature hot air drying oven at 150°C to 200°C to cure the paint film. The film thickness is generally about 20 μm to 40 μm, with a surface hardness of about F to 2H in pencil hardness.
- Powder Coating An environmentally friendly painting method in which powdered paint made of pigments and resins is attached using static electricity, then melted and baked with heat. It is eco-friendly as it does not use volatile organic compounds (VOCs), and a single coating can form a very thick film of 50 μm to 150 μm. Because it is highly resistant to flying stones and impacts, it is ideal for outdoor building materials and automotive suspension parts.
Physical Treatment (Shot Blasting, etc.)
This is a type of surface preparation that uses physical impact to smooth the surface without using chemicals or electricity. Countless fine particles (abrasive media) are blasted onto the surface of the aluminum casting at high speed.
- Effects and Roles It is used to remove burrs (unnecessary protrusions) generated during casting and to strip away surface dirt and mill scale. Additionally, by intentionally roughening the surface, the surface roughness is adjusted to a range of Ra (arithmetic mean roughness) 3.2 μm to 12.5 μm, creating an anchor effect that dramatically improves the adhesion of subsequent painting or adhesives.
Key Data: Statistics and Performance Indicators Regarding Aluminum and Surface Treatments
- Indicator 1: Specific gravity of aluminum: approx. 2.7 g/cm³ (approx. 65% lighter compared to iron’s approx. 7.8 g/cm³) (Source: Japan Aluminium Association)
- Indicator 2: Surface hardness of hard anodizing: HV 400 to 500 (Comparable to quenched steel parts) (Source: JIS H8603)
- Indicator 3: Standard film thickness of powder coating: 50 μm to 150 μm (High impact resistance) (Source: Association of Powder Process Industry and Engineering, Japan)
- Indicator 4: Corrosion resistance of non-chromate chemical conversion treatment (Salt spray test): No white rust for 120 to 168 hours (Source: Ministry of the Environment, Chemical Substance Countermeasures Data)
- Indicator 5: Global aluminum die casting market size: Estimated at approximately 75 billion USD in 2023 (Source: Grand View Research)
Reference: Japan Aluminium Association Basic Knowledge
By Purpose: How to Select the Optimal Surface Treatment
To avoid failures in procurement, it is important to clarify “what kind of environment the part will ultimately be used in and what is primarily required of it” before selecting a treatment.
When Emphasizing Corrosion Resistance and Weather Resistance
For communication equipment housings used outdoors or equipment parts along the coast where salt damage is a concern, the combination of “Powder Coating” or “Chemical Conversion Treatment + Acrylic Resin Baking Finish” is optimal. Powder coating, in particular, completely blocks moisture and salt from the outside with a thick coating film of 100 μm or more. Because the melting point of aluminum is high at 660.3°C, the risk of thermal deformation of the main body is kept low even during a baking process of around 200°C.
When Emphasizing Wear Resistance and Hardness
For sliding components and machine elements where parts frequently come into contact, “Hard Anodizing” or “Electroless Nickel Plating” should be selected. Hard anodizing has a hardness of HV 400 or more and excellent lubricity, but it is unsuitable for composite parts made of dissimilar metals because the treatment solution will react if metals other than aluminum (such as iron pins) are press-fitted. In such cases, electroless nickel plating, which can uniformly form a film of HV 500 or more even on complex shapes, is advantageous.
When Emphasizing Decorativeness and Appearance Quality
For consumer home appliance parts and high-end audio housings, “Color Anodizing,” which utilizes the unique texture of metal, is popular. By applying shot blasting or hairline finishing (a process of creating fine linear scratches) beforehand and then performing anodizing treatment, a premium, matte finish with reduced gloss can be achieved.
When Emphasizing Cost and Lead Time
For internal structural parts where appearance does not matter and only a minimum level of rust prevention capability is required, “Trivalent Chromium Treatment” or “Non-Chromate Chemical Conversion Treatment” is overwhelmingly low-cost and quick to deliver. Compared to plating or painting, the process is simple, and the cost benefits during mass production are maximized.
Advantages of Surface Treatment in Vietnam Procurement
In recent years, due to the “China Plus One” movement and demands for cost reduction, aluminum casting procurement in Southeast Asian regions such as Vietnam has rapidly increased. However, a fragmented supply chain where “casting is done overseas, and surface treatment is done back in Japan” creates waste in transportation costs and lead times, and carries the risk of blurring the point of responsibility (which process caused the defect) when an issue occurs.
The One-Stop Solution Provided by Daiwa Aluminum Vietnam
At Daiwa Aluminum Vietnam, we have built a system to consistently handle everything from high-quality aluminum casting to machining, and the various surface treatments introduced this time (including our partner factory network). Through this, we provide the following benefits to procurement managers:
- Shortened Lead Times through Integrated Production: Eliminates cross-border transverse transportation, significantly reducing the time until delivery.
- Unified Quality Assurance: Even for defects that are easily discovered after surface treatment, such as plating failures or paint blistering caused by casting-related “shrinkage cavities (internal voids),” centralized management enables rapid root cause analysis and a feedback loop, keeping the market outflow defect rate below 1%.
- Total Cost Reduction: By utilizing Vietnam’s competitive manufacturing costs while applying surface treatments that meet Japanese quality standards (JIS standard compliant), we can propose reductions in total procurement costs of 20% or more.
Conclusion
The surface treatment of aluminum castings is not merely “coloring” or “rust prevention,” but extremely important engineering that determines the added value of a product. Correctly understanding the characteristics of each method, such as anodizing, chemical conversion treatment, plating, and painting (differences in film thickness measured in μm, hardness expressed in HV, etc.), and making the optimal selection according to the required specifications is the shortest route to quality improvement and cost optimization.
At Daiwa Aluminum Vietnam, we offer comprehensive VA/VE (Value Analysis / Value Engineering) proposals, including optimal material selection and surface treatment, to address the three challenges of “cost,” “quality,” and “stable procurement” faced by Japanese management and procurement departments. If you are reconsidering aluminum casting procurement or are troubled by surface treatment in the development of new parts, please consult our expert team. Let’s build the optimal manufacturing process together.