How to choose the right surface treatment process for your aluminum components

Hansheng Automation focuses on providing component services for precision machinery. We have seen many cases where improper surface treatment resulted in parts not meeting tolerance standards. Such as smaller aperture causing assembly jamming, blindly pursuing hardness leading to brittle fracture of thin-walled parts, or beautiful anodized layer fading after two weeks of outdoor exposure, etc.

Surface treatment is not only the last step in the manufacturing process, but also related to the accuracy and functionality of the parts. For precision aluminum components, especially core parts used in aviation, medical, and robot joints, surface treatment is also very important. Next, Hansheng will take you from another perspective to explain the decision-making logic of aluminum surface treatment and some surface treatment traps.

No surface treatment process is omnipotent, we need to choose the surface treatment process that can achieve the best performance according to the actual situation. Hansheng has listed four different categories for you based on his actual processing experience.

Mechanical performance category

Prioritizing the mechanical performance of parts is primarily aimed at achieving extremely high wear resistance, surface hardness, and low friction coefficient. Typical scenarios include robot joints, gears, slides, and pistons.
Suggestion: Hard anodizing (Type III), electroless nickel plating (ENP), DLC coating.

Chemical/Environmental Performance Category

When prioritizing the chemical/environmental performance of the parts, it is generally because the parts require extremely high corrosion resistance (salt spray test>1000h), weather resistance, and insulation. Typical scenarios include marine equipment, outdoor communication base stations, and medical disinfection equipment.
Suggestion: anodizing, powder coating, and micro arc oxidation (MAO) with good hole sealing.

Physical/aesthetic performance category

Components that prioritize physical/aesthetic performance typically require specific colors, textures (matte/high gloss), and tactile sensations. Typical scenarios include consumer electronics casings and instrument panels.
Suggestion: Decorative anodizing (Type II), liquid spraying, sandblasting+oxidation.

Special function category

Priority should be given to components with special functions that generally require good conductivity, biocompatibility, and ultra-high true air conditioning efficiency.
Suggestion: Chemical conversion coating (conductive), electrolytic polishing (clean), PTFE impregnation (anti sticking).

 

Hard Anodizing/Type III

The advantage of this process is that its hardness can reach HV400-600 and its insulation is excellent.

But let's analyze the hidden risk traps:
Brittle risk: The hard oxide layer is alumina with ceramic characteristics, which is very brittle. If applied to thin-walled components subjected to high impact or bending deformation, the coating will crack like glass and even cause fatigue fracture of the substrate.
Material sensitivity: Extremely unfriendly to 7075 (high copper) or cast aluminum (high silicon). Copper and silicon can hinder the growth of oxide films, leading to burning or loosening of the film layer. This requires extremely special electrolyte formulations and pulse current control.

Standard anodizing and staining (Type II)

The advantages are low cost, rich color selection, and corrosion resistance.

Risk Trap:
Fading: Many bright organic dyes are very fragile under ultraviolet (UV) light. If you use it for outdoor equipment, you must specify inorganic salt coloring or electrolytic coloring, otherwise "Deep Space Black" will turn into "Eggplant Purple" after three months.
Corrosion channel: The anodized layer is essentially porous. If the sealing process does not meet the standards (such as insufficient temperature or time), salt spray molecules will pass through the pores and directly reach the substrate.

Powder/Liquid Coating

The advantages are excellent weather resistance, coverage of substrate defects, and good color consistency.

Risk Trap:
Influence tolerance: The thickness of powder coating is usually between 60-120 μ m. For the fit tolerance of H7/g6 level, this thickness has a significant impact on the tolerance.
Irreversibility: Once the spraying fails or there are local scratches, it is extremely difficult to repair. Stripping coatings typically requires strong alkali or sandblasting, which can further damage the dimensions of precision substrates.

Electropolishing/Chemical Polishing

The advantage is that it can reduce micro roughness (Ra), improve cleanliness, and increase gloss.

Risk Trap:
This is a process of reducing materials. It will preferentially corrode grain boundaries and impurities. If the crystal structure of the raw material (such as aluminum rod) is uneven, orange peel or pitting may appear on the surface after polishing. It has extremely high requirements for substrate quality.

Chemical conversion film (Chromate/Alodine)

The advantages are conductivity, corrosion resistance, extremely thin (<1 μ m), and do not affect size.

Risk Trap:
Many people mistakenly believe that it is very durable. In fact, it is very soft and not wear-resistant. During the assembly process, acidic sweat or slight friction from the fingers may damage it. It can usually only be used as a primer or for the conductive surface inside the chassis, and cannot be used as the final working surface for precision moving parts.

If you are designing high-precision fittings, based on our experience, we provide two suggestions for you.

Calculation of size for anodizing

Physical mechanism: The growth of oxide film follows the empirical rule of "50% penetration+50% growth" (the specific ratio may vary slightly depending on the type of alloy). This means that if you need to generate a 50 μ m film layer, approximately 25 μ m of thickness will erode towards the interior of the substrate, while the other 25 μ m will grow outward.

GD&T Engineering Suggestions:
For non mating surfaces, this can usually be ignored. But for precision holes or shafts of H7/g6 grade, this will cause a deviation in the size system.

Calculation example: If the design requires a final aperture of ϕ 20.00 ± 0.01mm and specifies 40 μ m hard anodizing. So in the CNC machining stage, the target size for turning should not be ϕ 20.00, but should be approximately ϕ 20.04 (calculated from 20.00+0.02 × 2).

Drawing annotation standard: It is strongly recommended to clearly label the "Dimensions before plating" and "Dimensions after plating" on the drawing to eliminate any misunderstandings between the processing parties.

Oxidation induced brittleness and the "volume effect" of thin-walled components

Physical mechanism: The molar volume of alumina is greater than that of aluminum (Al). When aluminum is converted to alumina, its volume expands. This microscopic volume expansion will generate significant residual compressive stress within the membrane layer.

Risk: For thin-walled parts with a wall thickness less than 1-2mm, if this compressive stress is not controlled, it can cause macroscopic warping or elliptical deformation of the workpiece. In addition, the elongation of hard oxide film is extremely low (<0.5%), and when the substrate undergoes elastic deformation, the film layer is prone to microcracks, significantly reducing the fatigue life of the parts.

Solution: For such parts, it is necessary to balance "film thickness" and "deformation" during process design. It is usually recommended to limit the film thickness (such as not exceeding 25-30 μ m) or use specific low stress oxidation process parameters, and if necessary, introduce specialized internal support fixtures to resist the deformation force caused by volume expansion.

Attention: Calculation and data are for reference only. It is recommended to confirm with the surface treatment supplier in actual production.

Surface treatment covers complex disciplines such as chemistry, physics, and mechanics. At Hansheng Automation, we adapt surface treatment to our precision manufacturing process, making it an important part of our precision manufacturing process.If you are interested in our surface treatment solutions, please click here – Aluminum Surface Treatment Services.

ASTM B117-19: Standard Practice for Operating Salt Spray (Fog) Apparatus.

ISO 10074:2017: Anodizing of aluminium and its alloys - Specification for hard anodic oxidation coatings on aluminium and its alloys.

Wernick, S., Pinner, R., & Sheasby, P.G. (1987). The Surface Treatment and Finishing of Aluminium and Its Alloys. ASM International.

ASM Handbook, Volume 5: Surface Engineering. ASM International.