Surface finishing

Surface finishing, also called surface treatment, is an umbrella term for processes that change the surface of a part without significantly changing its shape. The goal is usually corrosion protection, wear resistance, a required appearance and color, lower friction, electrical insulation or, conversely, conductivity, easy cleaning, or better adhesion of paint or adhesive.

By principle, finishes fall into several groups: mechanical (blasting, grinding, polishing, brushing, tumbling), chemical and electrochemical (anodizing of aluminum, zinc electroplating, nickel and chrome plating, electroless nickel, phosphating, black oxide, pickling and passivation of stainless steel), paints and coatings (powder coating and liquid painting, e-coating, hot-dip galvanizing) and thin vacuum coatings, PVD and CVD, used mainly on tools. Closely related are thermochemical heat treatment processes such as nitriding, which change the properties of the surface layer of the material itself.

Layer thicknesses differ by orders of magnitude. Passivation and black oxide have negligible thickness, zinc plating is usually 5 to 25 µm, standard anodizing 5 to 25 µm, powder coating typically 60 to 120 µm and hot-dip galvanizing from tens to over a hundred micrometers. On threads, engineering fits and precision surfaces, the layer therefore affects function and has to be accounted for as early as the design stage.

The choice depends on the material, the environment and the required service life. Aluminum is most often anodized or powder coated, structural steel is galvanized, painted or both, and stainless steel is pickled and passivated, or polished. Corrosion resistance is verified, for example, with a salt spray test to ISO 9227 (ASTM B117 is the common US equivalent), and environments for paint systems are classified into the corrosivity categories C1 to C5 and CX under ISO 12944.

Plan surface finishing for every part that comes into contact with moisture, chemicals, food or skin, for sliding and wearing surfaces and for all cosmetic parts. On prototypes, use the same finish as in production if you are testing corrosion, friction, assembly or appearance, because the finish changes dimensions, roughness and surface behavior.

Combinations of finishes are used where a single one is not enough: for example, hot-dip galvanized steel with a powder coat (a duplex system) for outdoor structures, or bead-blasted and anodized aluminum for a uniform matte look.

The most common mistake is a finish called out in a single word, such as "zinc plate" or "black anodize". On the drawing or in the request for quote, state the type of finish, the layer thickness, the color and gloss (for example a RAL shade), the surfaces that must not be finished (threads, fits, grounding contacts) and whether dimensions apply before or after finishing. Also define where rack marks from hanging the part are allowed.

High-strength steels (for example bolts of property class 10.9 and higher) are at risk of hydrogen embrittlement after electroplating. Require a hydrogen embrittlement relief bake, or choose a finish without this risk, such as a zinc flake coating. Closed sections and weldments need holes for draining the process baths and for venting. Hexavalent chromium is heavily restricted in the EU, so zinc plating commonly uses trivalent passivation. Confirm corrosion resistance and appearance with the supplier on samples.