Surface Treatment Basics (Plating, Anodizing, Black Oxide)

Surface treatment forms a film or layer on a part's surface, adding properties like corrosion resistance, wear resistance, appearance, and conductivity. It's often specified on drawings and affects dimensions. This article compiles the methods, thickness, and cautions.

Representative treatments

Treatment	Main materials	Purpose
Electroless nickel plating	Steel, aluminum, etc.	Corrosion/wear resistance, uniform thickness
Chrome plating	Steel, etc.	Wear resistance, hardness, appearance
Zinc plating	Steel	Corrosion resistance (cheap)
Anodizing	Aluminum	Corrosion resistance, insulation, appearance (hard type = wear)
Black oxide	Steel	Light corrosion resistance, appearance (thin film)
Painting / powder coating	Various	Appearance, corrosion resistance

Dimensional effect (coating thickness)

Plating and anodizing increase the dimension by the coating thickness (anodizing partly penetrates the base, so the increase is roughly half the layer thickness).

• For fits, allow for the coating thickness when setting the machining dimension
• Check whether the drawing gives the "after-treatment" or "before-treatment" size

Key points for choosing

• Mainly corrosion resistance → zinc plating, electroless nickel
• Wear resistance / hardness → chrome plating, hard anodizing
• Aluminum corrosion resistance / insulation → anodizing
• Light corrosion resistance for steel, cost-first → black oxide

Cautions

• Hydrogen embrittlement: high-strength steel can crack after plating → bake (dehydrogenate) to prevent
• Masking: specify areas you don't want coated, such as threads and fits
• Rack/contact marks: plating uses current, so rack contact marks may remain

FAQ

Q. How do anodizing and plating differ? A. Anodizing is an oxide layer grown from the aluminum surface itself (part of the base); plating deposits a different metal on top. Different materials and principles.

Q. How thick is the coating? A. It varies by method and spec, from a few µm to tens of µm. Always account for thickness on fits.

Q. What if dimensional change after treatment is a problem? A. Clearly state "before/after-treatment size" on the drawing and allow for thickness in machining. Masking also helps.

Conclusion

Choose surface treatment by "function (corrosion/wear/appearance) × material × the dimensional effect of thickness." For fits, don't forget the coating thickness.

Sontis Management centralizes machining/treatment info and quotes. Try it free now.