Plating enquiries usually arrive as a metal name: nickel, chrome, zinc, tin. That is the wrong starting point. An electroplating system has to satisfy the requirement that matters, whether that is corrosion protection, wear resistance, electrical conductivity, solderability or appearance, and the same metal behaves very differently depending on thickness, substrate and the layer beneath it. This guide sets out how to choose a system by requirement, how thickness classes are called out, what substrates change, and what to put in the request so the quote reflects the part rather than a guess.
How should a plating system be chosen?
Start from the requirement, then pick the metal.
Corrosion protection, wear resistance, conductivity, solderability and appearance each point to a different system, and several metals can serve more than one purpose at different thicknesses.
The requirement first approach avoids the most common error in plating, which is specifying a familiar metal for a job it cannot do. Zinc protects steel sacrificially and is inexpensive, but it does not resist wear. Nickel offers hardness and corrosion resistance and is a common engineering coating, but its appearance varies with the process. Chromium provides a hard, low-friction surface and a bright finish, and it is frequently applied over nickel rather than directly on the substrate. Tin supports solderability and is used on electrical components. Precious metals are specified where contact resistance has to stay low over thousands of cycles.
Once the requirement is clear, three further questions resolve the specification: how thick the coating must be, which substrate it is applied to, and whether any area must stay uncoated. Those three answers define the process route rather than just the metal, because a coating’s performance depends on the layer beneath it as much as on the layer itself.
Where a part serves more than one function, a multilayer system is often the answer. Decorative chrome is normally applied over nickel, which provides the corrosion resistance and the level surface; the chrome provides the appearance and the wear surface. Specifying the stack, not just the top layer, is what makes the result repeatable. The coating and test framework is published by ASTM committee B08 on metallic and inorganic coatings.
How do the common systems compare?
By the property each is chosen for.
Zinc protects steel, nickel provides a hard engineering surface, chrome adds wear resistance and appearance, and tin and precious metals address electrical requirements.
Zinc plating is the standard protection for steel components, particularly fasteners and brackets, because it corrodes in preference to the steel beneath and thereby protects it at a scratch. Thicker zinc, or a zinc alloy such as zinc-nickel, extends the protection in corrosive environments, and a subsequent passivation or sealing step improves appearance and resistance to white rust.
Nickel is selected where hardness and corrosion resistance matter together. Electrolytic nickel deposits evenly and takes a bright finish, while electroless nickel builds uniformly on complex geometry without the current-density effects that make electrolytic deposition uneven. That difference in deposition behaviour is often more decisive than the metal itself on parts with internal features.
Chromium is specified for wear and appearance. Hard chrome produces a thick, hard, low-friction surface used on hydraulic rods, molds and wear faces, while decorative chrome is a thin layer over nickel. Tin is chosen for solderability and for electrical connections where a non-toxic, corrosion-resistant surface is needed. Gold and silver appear on contacts and connectors, where the requirement is stable low-resistance contact over a long service life rather than protection.
| Requirement | Common systems | Consideration |
|---|---|---|
| Corrosion protection on steel | Zinc, zinc alloy, nickel | Zinc protects sacrificially; nickel protects by barrier |
| Wear resistance | Hard chrome, electroless nickel | Both build thickness; dimensional planning needed |
| Electrical conductivity | Tin, silver, gold | Contact resistance and cycle life drive the choice |
| Solderability | Tin, tin-lead where permitted | Thickness and intermetallic growth matter over time |
| Decorative appearance | Nickel over copper, then chrome | The layer stack determines the finish |
| Uniform build on complex shapes | Electroless nickel | No current-density variation |
How are thickness classes specified?
As a class or a measured range, not as a word.
Plating specifications define thickness classes, and the drawing should call out the class or a measured range rather than asking for a coating by name.
Thickness is the variable that most directly controls performance. A thin zinc coating protects for a shorter time than a thick one in the same environment; a hard chrome deposit has to be thick enough to provide a wear surface but no thicker than the dimensional allowance permits; a gold contact coating is often specified very thin because its cost and its function do not require more.
The industry handles this with classes defined in the governing specifications. Rather than asking for “zinc plating”, the drawing states the coating, the thickness class and the supplementary treatment such as a chromate conversion or a seal. That language is understood across suppliers, which makes quotes comparable and makes inspection possible: a thickness measurement can be compared with the class on the drawing rather than with an assumption.
The dimensional consequences sit alongside the class. Plating builds outward on the surface, so a coated shaft grows and a coated bore closes, and some systems require a post-plate operation such as grinding to bring a surface back to tolerance. Where the coating is thick, that operation belongs in the plan, and the allowance for it belongs on the machined part. The framework for those callouts is set out on 6CProto’s standards and tolerances page.
How does the substrate change the process?
Significantly, and sometimes decisively.
Steel, copper and brass accept plating readily, while aluminium requires a specific pretreatment and some alloys are difficult to coat at all.
Steel is the straightforward case: it is conductive, it takes a range of coatings well, and the main consideration is the pretreatment required to remove scale, oil and oxide. Copper and brass are also receptive, and both are frequently plated for appearance or to provide a base layer under nickel or chrome.
Aluminium is the difficult substrate. It carries a natural oxide that forms immediately after cleaning and prevents a sound bond, so plating aluminium requires a pretreatment sequence that replaces that oxide with a receptive layer. The details of that sequence and its consequences are covered in the companion article on plating aluminium and in the material and finish compatibility guide. The practical point for a buyer is that an aluminium part should be quoted as an aluminium part, because the pretreatment is part of the cost and part of the risk.
Other substrates introduce their own constraints. Castings may outgas or contain porosity that affects the deposit, and hardened steels behave differently from mild steels in the pretreatment line. Where a part is a mixed assembly of two metals, the galvanic relationship between them matters, and plating one component while leaving another bare can accelerate corrosion at the joint. The metallurgical background for those substrate differences is documented by ASM International.
What do racking and contact marks mean for cosmetics?
Every plated part needs a contact, and it leaves a mark.
Parts are held on racks or in barrels to carry the plating current, and the contact point is not coated, so the position of those marks is a specification decision on cosmetic parts.
Rack plating is used for parts that must be held in a fixed orientation, and it allows control over where the contact sits. Barrel plating tumbles many small parts together in a rotating drum, which is efficient and inexpensive but leaves contact marks distributed unpredictably. The choice between the two is therefore an appearance decision as much as a cost one.
Where a part is cosmetic, the contact point should be placed where it will not be seen, which is usually a concealed face, an internal surface or an area covered by another component. That decision has to be made before the parts are shipped, because the rack is built around it. On parts where appearance is not a consideration, the marks are irrelevant and the more economical process can be used.
Masking interacts with the same planning. Any area that must stay uncoated, such as a thread, a press fit or an electrical contact surface, has to be masked, and each masked area is hand labour. Specifying a limited masking list keeps the cost down and reduces the chance of an error.
What drives cost and lead time?
Handling, masking and finishing steps.
Plating prices follow the number of process steps and the amount of hand work, so a multilayer coating on a masked part costs more than a single-layer coating on a plain one.
The process route sets the floor. Each layer in a multilayer system is a separate tank and a separate step, and each adds time and cost. Racking or barrelling follows, and the choice of method affects both the price and the appearance of the parts. Masking adds a hand operation at the start and another at the end, and its cost scales with the number of protected features rather than with the part’s size.
Thickness and finish requirements then modulate the price. A thicker deposit takes longer in the tank; a bright or specular finish may require additional polishing or a different chemistry; and a requirement for post-plate grinding adds a machining operation. Where a part needs a specific appearance, that requirement is worth stating precisely, because a general instruction to make it look good leads to a process choice the buyer did not intend.
Lead time follows the same structure, with one addition: bath scheduling. Some systems run continuously and some are scheduled in batches around a particular chemistry, so a specialty coating may wait for its slot. Where a program has a fixed date, that is worth confirming rather than assuming. Requests submitted through the quote flow receive a review so the process route, the masking and the dimensional consequences are identified before the parts are made.
What should be inspected and documented?
Thickness, adhesion and the features that must function.
Inspection covers coating thickness measured on the part, adhesion assessed by a standard method, appearance against a reference, and verification that masked features still work.
Thickness measurement is the primary check because it correlates with performance. A gauge reading on the finished part is compared with the class on the drawing, and on a coated assembly the measurement is taken at several points because deposition varies with geometry and with current density. Adhesion is assessed by a method such as the tape test described in ASTM D3359, which is destructive and therefore performed on samples.
Appearance is judged against an approved reference under a defined condition, which is the only way to make a brightness or colour decision repeatable. Functional checks then cover the features the process could compromise: threads verified with their fasteners, press fits checked with their mating components, and contact surfaces checked for the conductivity requirement where that is the purpose of the coating.
6CProto provides quality inspection reports on request and assigns a dedicated project manager to each order, so the inspection scope can be agreed with the plating specification. The quality practices behind those checks are described by NIST MEP, the materials engineering context by ASM International, and plating waste, which is a regulated stream, is handled under the framework published by the US Environmental Protection Agency.
Specifying plating by requirement
An electroplating specification works when it states the requirement, the coating system including any underlayer, the thickness class, the substrate, the areas that must stay bare and the acceptance method. A request that names only a metal leaves five decisions to the supplier, and the result is a part that may meet the drawing and still fail in service.
The practical habit worth adopting is to write the requirement in a sentence before choosing the coating: what the part must resist, what it must conduct, or how it must look. That sentence resolves most of the specification, and it makes the conversation with the plating shop about the part rather than about the metal. The routes available alongside plating are described in the surface finish guides.
FAQ
How much does electroplating cost?
Price follows the number of process steps and the amount of hand work rather than the metal alone. A single-layer coating on parts that can be barrel plated is the least expensive; a multilayer system on racked parts with several masked features costs considerably more, because each layer is a separate tank and each masked area is a hand operation. Stating the masking list and the thickness class makes quotes comparable.
What metals cannot be electroplated?
The process requires a conductive substrate, so non-metals cannot be plated by the standard route without a conductive coating first. Among metals, the practical difficulties are aluminium, which carries an oxide layer that must be replaced before plating, and some refractory metals that do not accept a deposit readily. Castings can also be problematic because porosity and outgassing interfere with the deposit.
Can electroplating be done at home?
Small setups exist for hobby and jewellery work, but they cannot deliver the process control that an engineering part requires. Thickness, adhesion and appearance all depend on controlled chemistry, current density and time, and the waste streams from plating are regulated. For a production part, the practical route is a shop with documented process control and the ability to verify thickness on the finished component.
What are the downsides of electroplating?
Three are worth planning for. The coating builds outward, so dimensions change and thick deposits may require post-plate grinding. Deposition is uneven on complex geometry, because current density varies, which is why electroless processes exist for parts with internal features. And the process generates regulated waste, so the environmental controls add cost that is part of the price rather than an optional extra.
If a part needs a coating chosen by requirement rather than by name, send the model with the environment, the wear or conductivity requirement and the areas that must stay bare. 6CProto reviews the part together with the finishing route and returns a DFM report with the quote. Upload the file at the 6CProto quote page or send it to projects@6cproto.com.