Surface Finishes for Metal Parts

A finish protects metal from corrosion and wear, changes how it looks and feels, and can change how it conducts electricity. Every finish also changes the part: some add thickness, some need areas masked off, and some only work on certain metals. Choose the finish when you design the part, not after, because it can affect tolerances, threads and cost.

As-machined and bead blasted

An as-machined finish shows light tool marks, typically around Ra 3.2 µm, and costs nothing extra. Bead blasting sprays fine glass beads at the surface, giving an even matte texture that hides machining marks and handling scuffs. It's often done before anodizing for a matte anodized look. Blasting slightly rounds sharp edges and can affect threads and fits, so those are masked.

Anodizing

Anodizing is an electrochemical process that grows a hard layer of aluminum oxide out of the aluminum itself. The layer is porous when it forms, so it can be dyed black, gold, red, blue and many other colors, and then sealed. Standard sulfuric acid anodizing, called Type II under MIL-PRF-8625, is typically 5 to 25 µm thick.

• It changes dimensions. As a rule of thumb, about half the coating grows outward and half grows into the metal, so each surface grows by about half the coating thickness. Tight bores and threads need an allowance or masking.
• The alloy affects the color. 6061 anodizes evenly. 7075 and cast alloys can come out darker or less even, and color can vary between batches, so order parts that must match together.
• The layer doesn't conduct electricity. Grounding points and electrical contacts need to be masked.

Hard anodizing (Type III)

Hard anodizing runs in a colder bath at a higher current, building a thicker, denser oxide layer, commonly 25 to 75 µm and often about 50 µm. It's much harder and more wear resistant than standard anodizing, and its natural color is dark gray to bronze; it can be dyed black. It's used on sliding surfaces, pistons, gears and other aluminum parts that see wear. The thicker coating changes dimensions more and can reduce fatigue strength, so it's specified only where the wear resistance is needed.

Chem film (chromate conversion)

Chem film, also known by trade names such as Alodine, is a very thin conversion coating on aluminum, under 1 µm, specified under MIL-DTL-5541. It protects against corrosion, makes a good base for paint and, unlike anodizing, keeps the surface electrically conductive, which is why it's common on electronics enclosures. It comes in gold-colored versions made with hexavalent chromium and clear trivalent versions that meet RoHS restrictions.

Powder coating

Dry powder is sprayed onto the part with an electrostatic charge, then baked at around 180 to 200°C so it flows and cures into a tough film, typically 50 to 100 µm thick. Powder coating comes in almost any color, gloss and texture, and stands up to chips and weather better than most paints. It suits steel and aluminum frames, brackets, guards and enclosures. It's too thick for precision fits, so threads, bores and mating faces are masked, and the part has to tolerate the oven temperature.

Paint and e-coat

Wet paint is used for color matching, very large parts and parts that can't go through a curing oven. E-coat (electrophoretic coating) deposits a thin, even primer by dipping the part in a paint bath with an electric charge, reaching inside cavities that spray can't. It's the primer on most car bodies.

Zinc plating and galvanizing

Zinc protects steel by corroding first, so small scratches don't rust. Zinc electroplating, typically 5 to 25 µm, is finished with a clear, yellow or black passivate and is common on fasteners and brackets. Hot-dip galvanizing dips the steel in molten zinc for a much thicker coating, often 50 µm or more, for structural steel that lives outdoors. High-strength steels are baked after plating to drive out hydrogen, which can otherwise make them brittle.

Electroless nickel

Electroless nickel deposits a nickel-phosphorus alloy chemically rather than with an electric current, so the coating is the same thickness everywhere, including inside bores and blind holes. Coatings are typically 5 to 25 µm, hard and corrosion resistant, and can be heat treated to be harder still. It's used on molds, hydraulic parts, and aluminum and steel parts that need wear resistance with tight dimensional control.

Hard chrome

Hard chrome plating gives a very hard, low-friction surface for hydraulic cylinder rods, rollers and other wear surfaces, and can be ground to size afterward. It uses hexavalent chromium, which is tightly regulated, so alternatives such as electroless nickel and thermal spray coatings are increasingly used.

Black oxide

Black oxide is a conversion coating on steel and stainless steel, around 1 µm thick, so it doesn't change dimensions. It gives a matte black look and cuts glare, but on its own it protects very little; most of its corrosion resistance comes from the oil or wax applied afterward. It's common on tools, fasteners and machine parts that stay indoors.

Passivation

Stainless steel resists corrosion because of a thin chromium oxide film on its surface. Machining can leave free iron on the surface, which rusts and breaks up that film. Passivation removes it in a nitric or citric acid bath, to standards such as ASTM A967 or AMS 2700, so the protective film reforms evenly. It doesn't change the look or the size of the part, and it's standard for medical, food and marine parts.

Titanium finishes

Titanium can be anodized too, but the color doesn't come from dye. A thin, transparent oxide layer grows according to the voltage applied, and light reflecting through it produces blue, purple, gold, green and other colors, with no meaningful change in size. A gray anodize under AMS 2488 Type 2 is used to reduce galling between sliding titanium parts. Titanium is also commonly bead blasted, brushed or polished, or coated with PVD for durable color.

PVD and DLC coatings

Physical vapor deposition (PVD) applies a very thin, very hard coating in a vacuum chamber, typically 1 to 5 µm. Titanium nitride gives a gold color and diamond-like carbon (DLC) a black one. These coatings are used on cutting tools, watches and premium hardware where the color has to survive wear.

Masking, tolerances and threads

• Say on the drawing whether dimensions apply before or after finishing, so the machinist knows how much to allow.
• Mark the surfaces to mask: threads, precision bores, bearing seats and electrical contact points.
• Electroplated coatings build up on edges and corners and thin out in holes. Electroless nickel and anodizing are much more even.
• Order parts that must match in color in one batch, from the same alloy.

Quick comparison

Tell us the finish you need when you ask for a quote, and we'll flag threads and fits that need masking. See CNC machining and metal fabrication. For an example, our 7075 aluminum control arm is CNC machined and gold anodized.

Related reading

• CNC machining processes
• How to choose a metal for a machined part
• Metal fabrication processes
• Why carbon fiber yellows, and how clear coat stops it

Terms are defined in the glossary, and every guide is listed in the knowledge base.