Engineering / Mechanical Engineering
Finishing
A machined part is rarely finished when it leaves the machine. A surface treatment — plated, anodised, galvanised, painted or coated — protects it from corrosion and wear, sets its appearance, and sometimes restores its size. Each works differently, and the choice follows the purpose.
- Reading time · 6 min
- 7 sections
- Plating thickness, charted
- Faraday's law worked
§1Why finish a surface
Surface finishing adds a thin layer or altered skin to a component after it is made, to do what the base metal alone cannot — resist corrosion, resist wear, look right, or come back to size.
Most engineering metals corrode, wear or simply look unfinished, so a treatment is applied to the surface to serve one or more of four purposes: corrosion protection (a barrier or a sacrificial metal that keeps rust off steel), wear resistance (a hard skin on a soft or hardenable part), appearance (colour, gloss or a decorative metal), and dimension (building a worn or undersize surface back up with plating). The treatments fall into a few families — depositing a metal electrically (§2–3), growing a protective oxide (§4), coating with a sacrificial metal (§5), or applying an organic layer (§6). Which to use is decided by the purpose and the base metal, and the last section draws that choice together. Finishing is the final, and often decisive, step in making a part fit for service.
Contents§2Electroplating
Electroplating deposits a thin layer of one metal onto another using electricity: the part is made the cathode in a solution of the plating metal's ions, which the current reduces onto its surface.
The part to be plated is hung as the cathode (negative) in an electrolyte containing dissolved ions of the plating metal, with an anode (often of the same metal) completing the circuit. Passing a direct current drives the metal ions onto the part, building an even layer atom by atom, while the anode dissolves to replenish the solution. Common plates each serve a purpose: chromium for a hard, bright, wear- and corrosion-resistant finish (and decoration); nickel for corrosion resistance and as an undercoat; zinc for cheap sacrificial rust protection of steel; copper as an undercoat and for electrical work; gold and silver for contacts and appearance. Because the layer grows with the current and the time, its thickness can be controlled precisely and calculated exactly (§3) — which is what makes plating suitable both for protection and for restoring a worn surface to size.
Contents§3Faraday's law and thickness
How much metal plates onto a part is governed exactly by Faraday's law: the mass deposited is proportional to the electric charge passed — the current multiplied by the time.
Plate nickel (molar mass 58.7 g/mol, two electrons per ion) onto a 1 dm² surface at 5 A. In 20 minutes the charge passed is 5 × 1200 = 6000 coulombs, so the mass deposited is (58.7 × 6000)/(2 × 96 485) = 1.825 g; spread over 100 cm² of nickel (density 8.9 g/cm³) that is a layer about 20.5 µm thick. Because mass — and so thickness — grows in direct proportion to current × time (the hero charts it), the plater sets the thickness simply by how long the current runs: double the time and the layer doubles. This is why plating is used to restore worn or undersize parts to exact dimension as well as to protect them — the deposit can be dialled to a specified thickness. The relationship is one of the most exact in manufacturing: charge in, metal out, in fixed proportion.
§4Anodising
Anodising is not a coating added on but a controlled oxide grown out of the metal itself — chiefly aluminium — making its natural oxide skin far thicker, harder and more protective.
Aluminium already forms a thin, tenacious oxide that protects it (the materials pages), and anodising deliberately thickens that oxide by making the part the anode (positive — the reverse of plating) in an acid electrolyte and passing a current, which grows a hard, porous aluminium-oxide layer integral with the metal. The layer is hard and wear-resistant, improves corrosion resistance, and — because it is porous before sealing — can be dyed a range of colours, then sealed to lock the colour and close the pores. This is the origin of the coloured, durable finish on aluminium consumer goods, architectural sections and cookware. Unlike plating, anodising adds no foreign metal: it converts the surface of the aluminium into a thicker version of its own protective oxide. It suits aluminium (and a few other metals like titanium); it does not apply to steel, which is protected by other means (§5).
Contents§5Galvanising and sacrificial protection
Steel is most cheaply protected from rust by coating it with zinc — galvanising — which guards the steel even where the coating is scratched, because the zinc corrodes in the steel's place.
Galvanising coats steel with zinc, usually by dipping it in a bath of molten zinc (hot-dip), leaving a metallurgically bonded zinc layer. Its power is that the protection is sacrificial, and this follows directly from the galvanic series of the materials pages: zinc is less noble than iron, so when the coating is scratched and both metals are exposed, the zinc — not the steel — corrodes preferentially, protecting the exposed steel until the surrounding zinc is consumed. A barrier coating like paint fails the moment it is scratched, but a galvanised coating keeps protecting a small bare spot because the zinc sacrifices itself to it — the same principle as the sacrificial anodes bolted to ships and tanks. This is why galvanised steel is the standard for fences, structures, roofing and outdoor hardware: cheap, tough, and self-protecting at scratches. The trade is appearance (a matt grey) and that the protection lasts only as long as the zinc.
Contents§6Coatings and choosing a finish
Beyond the electro-chemical treatments lie the organic coatings — paint and powder — and the whole choice comes down to matching the finish to its purpose and the base metal.
Paint and powder coating apply an organic layer for corrosion protection and colour: powder coating sprays a dry polymer powder, electrostatically charged to cling, then bakes it to a tough, even film — more durable and cleaner than wet paint, the standard for appliances, furniture and architectural metalwork. These are barrier coatings — they protect only while intact. Choosing among all the finishes is a matter of purpose: for rust protection of steel, galvanising (self-protecting, structural) or zinc plating (cheaper, decorative), or paint/powder for a coloured barrier; for a hard, wearing, bright surface, hard chrome plate or, on aluminium, anodising; for appearance, decorative chrome, anodising in colour, or powder coat; for restoring size, plating built to thickness (§3). And the base metal narrows it — anodising for aluminium, galvanising for steel. The rule is to name the purpose first, then pick the finish that serves it on that metal.
Contents§7Quick reference
The working core of the page on one card rack.
Purposes
corrosion · wear · appearance · size
Electroplating
part = cathode · Cr/Ni/Zn/Cu
m = M·I·t/(n·F)
Anodising
grows hard oxide on aluminium
dyeable, sealed
Galvanising
zinc on steel · sacrificial
protects scratches too
Coatings
powder / paint = barrier
choose by purpose + metal
Handbook application: from concept to controlled practice
Purpose. This expanded section turns the original page into a practical handbook. It preserves the supplied material and adds a repeatable way to apply, check and review Finishing. It does not replace a contract, legislation, a controlled standard, competent engineering judgement or specialist advice.
The operating aim is to carry the subject from function and assumptions through design evidence, verification and controlled release. Read the original explanation first, then use the workflow and checks below to convert knowledge into evidence.
Apply Finishing by beginning with the duty, not the component or software command. Convert the key ideas—finish, surface, electroplating, faraday's, anodising—into measurable requirements and interfaces. Record operating and non-operating environments, duty cycle, expected life, loads, energy sources, human interaction and reasonably foreseeable abnormal conditions. When a value is not a project requirement or verified supplier datum, identify it as an assumption or illustrative value.
Create a calculation and evidence trail that another competent person can audit. Every input should carry a source, unit, revision and uncertainty or tolerance where relevant. Every model should state its boundary conditions and limitations. Keep nominal capacity separate from design capacity, and keep verification margin separate from an arbitrary safety factor. If a code or standard governs the work, confirm the applicable edition and contractual status rather than copying a number from a secondary summary.
Design for manufacture, assembly, inspection, operation and maintenance at the same time. A technically valid geometry can still fail because it cannot be fixtured, measured, cleaned, guarded, reached or replaced. Review process capability, datum or reference strategy, tolerance accumulation, access, error-proofing and changeover. Where people interact with plant, apply the hierarchy of controls and consult those who will operate, clean, maintain and recover the equipment.
Plan verification before release. Define the characteristic, method, equipment, sample or test condition, acceptance criterion, record and responsible person. Validation then asks a different question: whether the resulting system is effective and suitable in the intended use context. A passed drawing check or analysis does not by itself validate usability, maintainability or production performance.
Step-by-step operating method
- Define the duty. Capture the required function, interfaces, operating environment, life, loads and unacceptable outcomes.
- Establish the model. Identify governing principles, units, material or process data, assumptions and uncertainty.
- Develop alternatives. Compare feasible concepts against performance, manufacturability, safety, maintainability and cost.
- Verify the design. Use analysis, test, inspection or demonstration with acceptance criteria defined before execution.
- Release and learn. Baseline the design, control changes, retain evidence and feed operating results into the next revision.
Illustrative design review record
Illustrative values only. Build a one-page record with the required function, input sources, assumptions, governing load or process condition, failure consequences, selected concept, verification method and acceptance criterion. Mark every numerical input as project requirement, verified supplier data, measured value, calculation output or assumption. Review the weakest evidence first. If an assumption can change safety, compliance, interchangeability or capacity, it must be resolved before release rather than buried in a calculation note.
| Evidence class | Question | Release expectation |
|---|---|---|
| Requirement | What must the design do and under which conditions? | Approved and traceable |
| Input | Where did the load, property, tolerance or process limit come from? | Source, unit and revision recorded |
| Analysis | Which model and assumptions connect input to result? | Checkable calculation or simulation |
| Verification | How will conformity be demonstrated? | Method and acceptance criterion agreed |
| Validation | Will the solution work for intended users and conditions? | Representative use evidence |
Common failure modes and recovery actions
1. Watch for
Starting detailed design before interfaces and operating limits are agreed.
Recovery: Return to the governing definition or requirement and restate the decision in one sentence.
2. Watch for
Using catalogue or typical values as though they were certified project inputs.
Recovery: Separate evidence from assumption, assign an owner and set a date for validation.
3. Watch for
Checking nominal performance while ignoring tolerances, degradation and foreseeable misuse.
Recovery: Run a small counterexample, boundary test, pilot or independent check before proceeding.
4. Watch for
Confusing verification of requirements with validation of user need.
Recovery: Record the consequence, decision and rationale, then update the controlled baseline.
5. Watch for
Releasing drawings or procedures without configuration, inspection and change controls.
Recovery: Escalate when the issue affects safety, compliance, acceptance, material value or an agreed tolerance.
Review checklist
- What function and failure consequence govern this decision?
- Which inputs are measured, specified, assumed or illustrative?
- How will conformity be demonstrated and recorded?
- What change would invalidate the current evidence?
- Are mandatory requirements distinguished from recommendations and illustrative values?
- Are sources, assumptions, units, dates and versions recorded closely enough to reproduce the decision?
- Have safety, legal, ethical, stakeholder and operational consequences been considered at the appropriate level?
- Is there a named owner and a trigger for review, escalation, change or retirement?
Questions for deeper application
What is the most important distinction a practitioner must preserve when applying Finishing?
Answer with a fact or cited source where available. Where evidence is incomplete, record the assumption, consequence, responsible owner and next validation action.
Which assumption about finish would change the result most if it proved false?
Answer with a fact or cited source where available. Where evidence is incomplete, record the assumption, consequence, responsible owner and next validation action.
What evidence would allow an independent reviewer to reproduce or challenge the conclusion?
Answer with a fact or cited source where available. Where evidence is incomplete, record the assumption, consequence, responsible owner and next validation action.
Which boundary, exception or failure case has not yet been tested?
Answer with a fact or cited source where available. Where evidence is incomplete, record the assumption, consequence, responsible owner and next validation action.
What must be handed over, monitored or reviewed after the immediate work is complete?
Answer with a fact or cited source where available. Where evidence is incomplete, record the assumption, consequence, responsible owner and next validation action.
Authoritative references and use notes
The sources below were selected as institutional or primary guidance for the broader practice. They support the handbook method; they do not imply that every statement or clause in a source applies to every project. Confirm the current edition, jurisdiction, contract and application before treating any requirement as mandatory.
- NIST Manufacturing Extension Partnership — National Institute of Standards and Technology. Used for manufacturing productivity, quality, cost and capability improvement. Accessed 2026-08-13.
- Identify, assess and control hazards — Safe Work Australia. Used for hazard identification, risk assessment, controls and review. Accessed 2026-08-13.