In production we treat sheet metal plating options as a process decision, not a design preference. When the order hits the shop floor, CAM and process engineers first lock the base metal, then match zinc, nickel or chrome to the required corrosion class, conductivity path and final appearance. The plating line is set to a specific thickness window and the parts are racked or barrel-plated accordingly. If the drawing only says "plate" without specifying type and thickness, we stop the job and request clarification before any chemistry is mixed.
Why zinc still dominates when corrosion is the main risk
Most mild-steel brackets, chassis and enclosures that leave our factory carry zinc. The reason is straightforward: zinc gives sacrificial protection at the lowest cost per square meter. In the plating tank we run alkaline or acid zinc baths at 8–15 µm for indoor use and 15–25 µm for outdoor or humid environments. After plating we add a chromate or trivalent passivation layer so white rust does not form during storage. From a fabrication standpoint this combination is forgiving—minor scratches still protect the steel underneath because zinc corrodes first.
Nickel enters the picture when the part needs both corrosion resistance and a harder, more conductive surface. We typically specify 5–12 µm electrolytic nickel or electroless nickel-phosphorus. Electroless is preferred on complex geometries because it deposits evenly without current-density variations. Chrome is almost never used alone; it is applied as a thin decorative or hard-chrome top coat (0.2–0.5 µm decorative, 10–50 µm hard) over a nickel underlayer. Pure chrome plating on sheet metal is rare because adhesion and ductility problems appear during forming or vibration.
What shifts when plating thickness and tolerance are ignored
If the drawing calls for "zinc plate" without a thickness range, the line can easily run 5 µm on one rack and 20 µm on the next. That variation shows up later as assembly interference on tight-fitting covers or as early red rust on thin areas. Nickel that is deposited too thick on threaded features can raise the pitch diameter enough to lock fasteners. Chrome that is applied without a proper nickel barrier develops micro-cracks; those cracks become corrosion paths within months in salt-spray conditions.
In real production the consequences are concrete: panels that no longer slide into extruded rails, grounding points that lose electrical continuity after a few thermal cycles, and cosmetic rejects when the customer expects a bright chrome look but receives a dull, uneven surface. Scrap rates climb, rework means stripping and re-plating, and shipment dates slip while the plating house waits for a revised specification.
How the plating line is actually controlled on the shop floor
We start with the material certificate. Cold-rolled steel, stainless or aluminum each requires different surface activation. For zinc we set the bath temperature, current density and immersion time so that the target thickness lands inside ±3 µm on critical faces. Barrel plating is used for small parts; rack plating is reserved for larger panels that cannot tumble. After zinc we measure with a magnetic thickness gauge at three locations per part and record the values.
Nickel jobs receive a strike layer first, then the main deposit. Electroless nickel is controlled by bath loading factor and phosphorus content (mid-phos for general use, high-phos for better corrosion). Chrome is applied only after the nickel has been verified; the chrome tank is kept at a tight temperature window because even 2 °C drift changes brightness and hardness. Final inspection includes adhesion bend tests, salt-spray coupons run in parallel with the production lot, and visual checks under standardized lighting for decorative finishes.
When conductivity is required—grounding straps, EMI shields—we specify a minimum nickel thickness and prohibit organic sealers that would raise contact resistance. For pure decorative chrome we accept a thinner deposit and focus on surface preparation so that the final appearance matches the sample board approved by the customer.
Where we relax the rules without risking the job
Prototype or low-volume runs sometimes arrive with only a note for "zinc or nickel." In those cases we default to 10–12 µm zinc with clear passivation if the part is indoor and non-critical. Decorative chrome can be replaced by bright nickel when the customer accepts a slightly different color and the quantity is under fifty pieces. Hard-chrome is never skipped on wear surfaces, but the thickness window can be widened from ±5 µm to ±10 µm if the mating part has clearance.
Once the volume exceeds a few hundred pieces or the application moves outdoors, the relaxed defaults disappear. At that point the plating specification is locked on the traveler and any deviation requires a signed process change. That keeps the sheet metal plating options predictable for both the fabricator and the customer.
