Surface Finishing Should Be Considered Before the Part Is Finished
After stamping, CNC machining, forging, or sheet-metal fabrication, metal components often require plating, coating, or another finishing process.
Surface finishing may be used to:
- Improve corrosion resistance
- Enhance appearance
- Increase wear resistance
- Meet color requirements
- Modify surface characteristics
- Meet customer specifications
At the same time, coatings and plating add thickness to the finished component and can affect holes, threads, sliding fits, and assembly dimensions.
For products with tighter mating tolerances, the surface finish should therefore be considered during product and drawing development rather than after machining is complete.
When Is Zinc Plating Used?
Zinc plating is one of the most common corrosion-protection treatments for carbon-steel hardware.
A zinc layer is applied to the steel surface to improve resistance to corrosion in general operating environments.
Common applications include:
- Buckles
- Brackets
- Retaining plates
- Bolts and fasteners
- Automotive hardware
- Industrial equipment components
- Furniture hardware
Key Advantages of Zinc Plating
Zinc plating is a mature and widely available process with relatively manageable cost, making it suitable for many high-volume industrial components.
Depending on the specification, requirements may include:
- Coating thickness
- Color
- Trivalent chromium treatment
- Salt-spray performance
- RoHS or other compliance requirements
For general indoor or ordinary industrial environments, zinc plating is often one of the first finishes worth evaluating.
When Is Zinc-Nickel Plating Considered?
When higher corrosion resistance is required than conventional zinc plating can provide, zinc-nickel plating may be considered.
Common applications include:
- Automotive components
- High-corrosion-resistance hardware
- Brackets used in demanding environments
- Fastening and retaining components
- Parts with customer-specified corrosion requirements
Is Zinc-Nickel Always Better Than Zinc Plating?
Zinc-nickel can provide stronger corrosion performance in many applications, but that does not mean every product requires it.
The process and specification are generally more demanding than standard zinc plating and may also increase cost.
If a component is used in a normal indoor environment, selecting a higher-specification coating may provide little practical benefit.
The required finish should therefore be based on the application and actual test requirements.
What Is E-Coating?
E-coating, also known as electrophoretic coating, applies paint to a conductive metal surface using an electrochemical process.
One of its advantages is good coverage on components with recesses, corners, and more complex geometry.
Common applications may include:
- Automotive hardware
- Brackets
- Hinges
- Industrial structural components
- Metal parts requiring overall coating coverage
What Should Be Considered with E-Coating?
The final coating condition depends on the base material, pretreatment, coating thickness, curing conditions, and part geometry.
Special attention may be needed when components include:
- Precision mating areas
- Threads
- Moving hinge surfaces
- Tight assemblies
- Designated grounding areas
Certain areas may require masking or secondary processing depending on the functional requirements.
When Is Powder Coating Suitable?
Powder coating applies a dry powder to the metal surface and then cures it through heat to form a protective coating.
It is commonly used for:
- Industrial brackets
- Furniture hardware
- Equipment enclosures
- Sheet-metal components
- Racks and structural parts
- Outdoor-equipment components
Powder coating can provide a wide range of colors and appearances and is often used where a relatively substantial protective coating is required.
Is a Thicker Powder Coating Always Better?
Not necessarily.
A thicker coating may offer different appearance or protection characteristics, but it also changes the finished dimensions of the part.
For holes, insertion features, snap fits, sliding interfaces, or tight assemblies, excessive coating thickness can create assembly problems.
Coating thickness should therefore be defined according to the product design and customer requirements.
Anodizing Is Common for Aluminum Components
Anodizing is frequently used for aluminum-alloy parts.
Applications may include:
- CNC-machined aluminum parts
- Solar mounting hardware
- AI server brackets
- Drone metal components
- Precision-equipment parts
Anodizing can modify the surface characteristics of aluminum and can also provide different colors and finishes.
However, different aluminum grades, machining conditions, and surface states may result in visible color variation.
If appearance consistency is critical, acceptable color and surface standards should be defined early in development.
Salt-Spray Hours Do Not Directly Equal Product Service Life
Customers often specify salt-spray testing when selecting a surface finish.
Salt-spray testing can be useful for comparing corrosion performance under controlled test conditions, but the number of test hours should not be interpreted directly as the number of years a product will last in actual service.
Real-world durability can also be affected by:
- Installation environment
- Temperature and humidity
- Salt exposure
- Chemicals
- Friction and wear
- Component geometry
- Damage to the coating
If a specific corrosion test is required, the applicable standard, duration, and acceptance criteria should be clearly stated in the drawing or technical specification.
Can Surface Finishing Affect Dimensions and Assembly?
Yes.
This is an important consideration in custom hardware development.
Examples include:
- Shaft-to-hole fits
- Sliding interfaces
- Threaded connections
- Snap-fit buckles
- Moving hinge components
If plating or coating thickness is not considered during design, a component that fits before finishing may become too tight or impossible to assemble afterward.
For functional dimensions, drawings should clearly identify whether the specified tolerance applies before or after surface finishing.
How Should a Surface Finish Be Selected?
The following factors should be reviewed together.
1. Base Material
Carbon steel, stainless steel, aluminum alloys, and zinc alloys may require different finishing approaches.
2. Operating Environment
Indoor, outdoor, coastal, humid, and chemically aggressive environments require different levels of corrosion protection.
3. Appearance
Specified colors, black or silver finishes, matte surfaces, and gloss levels may limit the available finishing options.
4. Coating Thickness and Tolerances
Tightly fitted components must be evaluated for dimensional changes caused by plating or coating.
5. Testing Requirements
Salt-spray, coating-thickness, adhesion, or other testing requirements should be provided before quotation whenever possible.
6. Compliance Requirements
RoHS, REACH, trivalent chromium, or other customer-specific requirements should also be identified before the finish is selected.
7. Cost
Higher corrosion performance is not automatically the better choice.
Selecting a finish that matches the actual service conditions usually provides a better balance between cost and performance.
Surface Finishing Should Be Evaluated Together with Product Design
The same finish may perform differently depending on the material, component geometry, and operating environment.
For custom metal products, material selection, manufacturing process, tolerances, surface finishing, and service conditions should therefore be evaluated together.
Homer Hardware can support zinc plating, zinc-nickel plating, e-coating, powder coating, anodizing, and other surface-finishing requirements according to customer drawings and specifications, including coating thickness, color, corrosion performance, and related quality requirements.
Defining the surface finish early in development can reduce the risk of later dimensional, appearance, or assembly problems.