Surface treatment: corrosion protection and finishing in precision mechanics

When a part is turned or milled finished, it often lacks one final step before it can be used: a surface treatment that protects against corrosion, improves appearance, or gives the surface properties it didn't have from the machine—for example, lower friction or electrical insulation.

§Anodizing of aluminum.

Anodizing is an electrochemical process that strengthens aluminum's natural oxide layer making it thicker harder and more wear-resistant. The layer can be colored in the process and also functions as an electrical insulator. Anodizing is widely used on houses handles and visible parts where both protection and finish count.

§Nickel plating and other metal coatings

Electroless nickel plating provides an even nickel layer even on internal surfaces and complex geometries where conventional galvanic nickel plating cannot reach. The coating increases corrosion resistance and wear resistance and is often used on steel and brass parts that must function in moist or chemically demanding environments.

§Passivation of stainless steel

Although stainless steel is itself corrosion-resistant the machining can leave free iron on the surface which can start rusting. Passivation — typically with nitric acid or citric acid bath according to standards such as ASTM A967 or ISO 16048 — removes the free iron and rebuilds the protective chromium oxide layer so the part lives up to its full corrosion resistance. It is a step easily overlooked but which has great importance for durability on eg medical parts.

§Polishing and lapping for finish and function

Where ordinary milling or turning typically gives a surface roughness of around Ra 1.6–3.2 µm, sealing, bearing and optical surfaces often require a much finer surface. Grinding can bring the roughness down to Ra 0.2–1.6 µm, while polishing and lapping can reach as far as Ra 0.05–0.2 µm. The finer the requirements, the more time and care it costs — so a too tight surface requirement on a non-critical surface is wasted time and money.

• 01Always select surface treatment based on function, not just appearance
• 02Check if part must be passivated or cleaned before treatment
• 03Remember that masking and threads often need to be protected during the process
• 04Ask for documentation of layer thickness or passivation testing on critical parts

Surface treatment is the last link in the chain but errors here can destroy hours of precision work — a part that is not cleaned properly before anodizing or that is not passivated after processing can rust or flake off even though the metal work itself was perfect. Treat the surface as part of the drawing's tolerance not as a cosmetic side remark.