Types of Metal Plating and Finishing

The part is the cathode in a solution containing the metal you want. Current drives metal ions onto it. That single fact explains the strengths and every limitation: deposit thickness follows current distribution, so it is heaviest on edges, corners and high points and lightest in recesses, bores and blind holes.

How badly that hurts depends on the bath’s throwing power. It is why alkaline zinc is specified for parts with recesses while acid chloride zinc is chosen for speed, brightness and difficult substrates such as castings and heat-treated steel. It is also why a thickness minimum should say where it applies.

Rack or barrel is decided by the part, not by price. Racked parts are individually fixtured and wired, which gives controlled thickness on named surfaces and no part-to-part contact damage — the route for anything large, delicate, cosmetic or held to a real thickness tolerance. Barrel plating tumbles small parts in bulk against each other; contact is intermittent, so the deposit is well distributed but less tightly controlled, and cosmetic faces get marked. Parts that nest, tangle or are heavy enough to damage their neighbors have to be racked whatever the quantity.

Zinc and its alloys are the volume answer for steel because zinc is anodic to steel: it corrodes instead of the steel, and keeps protecting a scratch it no longer covers. Plain zinc plating covers most indoor and general outdoor service, and black zinc handles the same job where appearance is specified. Zinc-nickel is the alloy version and is a different animal: still sacrificial to steel, far slower to corrode, better after thermal exposure, and close enough to aluminum galvanically to bolt to it. It is the standard replacement when a legacy print calls cadmium — the zinc-nickel versus cadmium comparison covers where the swap is straight and where it is not.

Nickel is the opposite mechanism and the mistake engineers make most often. Nickel is a barrier, not a sacrificial coating, and it is cathodic to steel — a scratch through nickel down to steel corrodes faster than bare steel would. Porosity and coverage matter more than thickness alone. Nickel plating splits into bright nickel, which levels the surface and is decorative but contains sulfur and is less ductile, and sulfamate nickel, a low-stress ductile deposit used for heavy build-up, salvage of undersize parts and as an underplate beneath precious metals. Sulfamate nickel runs at our McAllen plant.

Tin is chosen for solderability and for a soft, conductive, food-safe surface. Tin plating comes in two forms with different risks: bright tin is cosmetic and fine-grained, while matte tin is the low-stress deposit specified where tin whiskers are a concern, which is most of aerospace and defense electronics. Matte tin runs at Rowlett. On copper alloys, tin over a nickel barrier stops copper-tin intermetallic growth from eating the solderable layer.

Precious metals buy electrical behavior nothing else gives you. Gold plating forms no oxide, so contact resistance stays low and stable for the life of the part — connectors, contacts, bond pads. Silver plating has the highest electrical and thermal conductivity of any metal: high-current bus work, RF and waveguide surfaces, high-temperature threaded joints. It tarnishes, and the callout usually names the finish — bright silver for reflectivity, semi-bright silver as the electrical workhorse, matte silver for a non-reflective or bonding surface. Gold, bright silver and semi-bright silver run at McAllen, both over a nickel underplate to stop substrate diffusion.

Cadmium still exists because nothing else does all of its jobs at once: sacrificial protection in salt and marine air, galvanic compatibility with aluminum, solderability, and consistent torque-tension on threaded fasteners without bulky corrosion product. It is also toxic and restricted, which is why it survives mainly on legacy aerospace and defense hardware under narrow derogations. Cadmium plating runs at McAllen only, and is also served from the Dallas–Fort Worth side.

Anodizing is not a deposit. The aluminum part is the anode, and its own surface is converted into aluminum oxide. Nothing is added from the tank. Three practical consequences follow, and all three catch people out.

It grows both directions. Roughly half the coating thickness builds outward and roughly half consumes the substrate. A shaft grows by about half the coating thickness per side; a bore closes by the same. Plan for it before machining, not after.

It is an electrical insulator. Anodized aluminum will not carry a ground path or an EMI bond, and hardcoat is a better insulator than Type II. If a mating face has to conduct, that face gets chem film or a mask, not anodize — which is the usual reason a drawing carries two finishes.

Color is dye, not pigment. Color is adsorbed into the open pores of the oxide and then sealed in. That is why Type II anodizing takes the full color range while Type III hardcoat comes out naturally dark bronze to near-black and dyes poorly — its oxide is denser with less pore volume to hold dye. It is also why sealing is not optional on a dyed part. The Type II versus Type III comparison covers the trade in detail; in short, Type II is corrosion protection, paint base and appearance, Type III is a wear surface. The full color range runs at Rowlett, alongside anodizing for the Dallas area. Type III hardcoat runs at McAllen. Where a bright decorative finish is wanted underneath, a bright dip goes first.

Everything above is metal, oxide or chemical film. This family is a polymer applied on top, and it plays by different rules: much thicker, applied rather than grown, and dependent on the pretreatment underneath for adhesion and corrosion life.

Powder coating is an electrostatically applied thermoset cured in an oven, a few thousandths per surface — a tough, color-stable barrier, and the wrong answer anywhere a fit or a thread is involved. Liquid paint covers the specification work powder cannot, including CARC and mil-spec painting. Cerakote is a thin-film ceramic polymer curing low or at ambient, well under a thousandth, which puts it inside the tolerance of most threads and slip fits. PTFE and fluoropolymer coatings sit in the same family where release and chemical resistance dominate.

Dry film lubricant — resin-bonded molybdenum disulfide or PTFE — is the odd one out and sits with the plating side. It is specified to control friction: consistent torque-tension on fasteners, anti-galling on stainless threads, and lubrication where oil is not allowed. It runs at Rowlett.

Paint, powder and Cerakote all run at Gleco Paint & Powder Coating, 5020 Grisham Drive in Rowlett, which is our coatings division rather than a plating plant. Surface preparation for all of it — and for plating on castings, weldments and corroded stock — starts with media blasting. Running plating and coating in the same company is the point of keeping it under one roof: one supplier, one certificate package, one part traveling a shorter distance.

A finish is a few ten-thousandths of an inch thick. It replicates the surface it lands on, and frequently makes its defects more obvious rather than less.

A poor substrate stays poor. Tool marks, laps, seams, weld spatter and pitting show through, and a bright deposit exaggerates them. Porous castings and sintered parts trap process solution and bleed it out for weeks afterwards, staining the finish; they need sealing or a different route, not more plating. Scale and heat-treat discoloration come off before plating, not under it — that is what media blasting is for.

Geometry that will not drain will not finish. Blind holes, undrained pockets and capillary crevices hold solution through the rinses, then release it into the next tank or onto the finished surface. Add a drain path, break the corner, or accept staining and a masked area.

Tolerances with no room left are a design problem. A plated diameter grows by twice the deposit thickness; a bore closes by twice. On a thread the pitch diameter is consumed from both flanks, so the standard rule is about four times the plating thickness on pitch diameter — enough to make a fastener that gauged fine bare refuse to assemble. Anodize eats into the substrate as well as building outward. Decide before the drawing is released whether the dimension is before or after finish, and say so on the print. Our design guide covers what else belongs there.

And plating cannot make high-strength steel safe by itself. Above roughly 31 HRC, hydrogen from the plating process is an embrittlement risk that has to be managed with stress relief before and a bake after, on a clock. Put the substrate and hardness on the RFQ.