Metal Surface Finishing in China: A Specification Guide for Engineers and Buyers

Last updated: 16 September 2026

1. What a finish is expected to do

Finishing is not decoration applied after the engineering is finished. On most exported metal parts it carries three separate jobs, and a drawing that does not say which of them matters will be finished to the lowest common denominator.

1. Corrosion protection. A coating can work as a barrier (powder coating, paint), as a sacrificial layer (zinc plating, hot-dip galvanising) or by restoring an existing passive film (passivation of stainless steel). These mechanisms behave differently when the surface is scratched, so the choice is an engineering decision, not a cosmetic one.
2. Functional surface properties. Wear resistance, low friction, lubricity for threaded assembly, electrical conductivity or insulation, and the ability to be bonded, printed or welded all depend on the surface layer.
3. Appearance and identification. Colour, gloss, texture and marking for traceability.

Two consequences follow. First, a finish belongs on the drawing with a process name, a standard designation and an acceptance method. Second, every finish changes the surface — by adding thickness, by growing into the substrate, by removing material during pickling or by changing the friction of a thread — so the features that must not change have to be identified before production starts.

2. Finishing processes at a glance

The table below maps the processes most often requested on custom metal parts to the substrates they apply to, their primary purpose, the governing specification most often cited, and the design caution that causes the most rework.

Process	Typical substrate	Primary purpose	Specification commonly cited	Main design caution
Decorative and protective anodising	Aluminium and aluminium alloys	Corrosion resistance plus colour and texture	ISO 7599	The oxide grows into and out of the surface; sealing and dyeing are separate process steps
Hard anodising	Aluminium and aluminium alloys	Wear and abrasion resistance on working surfaces	ISO 10074	Hard layers need masking and pre-machining allowance; not the same process as decorative anodising
Powder coating	Steel and aluminium fabricated parts	Durable, opaque barrier with colour	Powder properties and tests are covered by the ISO 8130 series	Film is relatively thick; threads, bores and fine detail usually need masking
Paint and coating systems	Steel structures and enclosures	Corrosion protection with defined durability	ISO 12944-5	Performance is dominated by surface preparation and by the full system, not by the topcoat alone
Electroplated zinc	Iron and steel parts	Sacrificial corrosion protection	ISO 2081, ASTM B633	Adds thickness to threads; high-strength parts need embrittlement relief
Electroplated coatings on fasteners	Steel fasteners	Corrosion protection plus controlled friction on threads	ISO 4042, ASTM F1941	Thread-fit limits cap usable coating thickness; barrel and rack plating leave different contact marks
Passivation	Stainless steel	Removes free iron and restores the passive film	ASTM A967, ASTM A380/A380M	Adds no measurable thickness; it is not a substitute for cleaning or descaling
Hot-dip galvanising	Iron and steel hardware	Heavy-duty, long-life corrosion protection	ASTM A153/A153M	High process temperature and thick coating; check distortion and thread fit
Chromate or non-hexavalent conversion coatings	Zinc-plated steel	Supplementary corrosion protection on plated zinc	Specified within ISO 2081, ISO 4042 and ASTM B633	Hexavalent chromium is restricted in electrical and electronic products placed on the EU market

Two rules apply to every row. Specify the process and the standard, not a colour word: "black" describes at least three different finishes. And state which inspection method will be used to accept the finish, because a requirement that cannot be measured will not be enforced.

3. Anodising: aluminium only, and two different processes

Anodising converts the aluminium surface itself into an oxide layer, which is why it applies to aluminium and aluminium alloys and never to steel, zinc or copper parts.

Decorative and protective anodising. ISO 7599:2018 is the method for specifying these coatings. The standard defines the characteristic properties of anodic oxidation coatings, lists the methods of test used to check those properties, provides minimum performance requirements, and gives information on the grades of aluminium suitable for anodising and on the importance of pretreatment for the required appearance. It deliberately excludes several families: barrier-layer coatings, coatings produced by chromic or phosphoric acid anodising, coatings intended only as a preparation for organic coating or electrodeposition, and hard anodic coatings used mainly for engineering purposes. If your requirement is wear resistance, you are outside ISO 7599.

Hard anodising. ISO 10074:2021 specifies requirements for hard anodic oxidation coatings on aluminium and its alloys, including test methods, and the information to be supplied by the customer to the anodizer. It excludes coatings made by plasma electrolytic oxidation, micro-arc oxidation, anodic spark deposition and similar processes — a useful boundary when a supplier offers "hard anodising" that is really a different spark process with different dimensional and surface consequences.

Practical points for the drawing. State which of the two standards applies and give the alloy and temper, since the same anodising cycle produces a different colour and hardness on different alloys. Decide the treatment of colour and sealing: colour from dyes sits inside the porous layer, and sealing affects both appearance and corrosion performance, so anodising for a decorative part is not interchangeable with anodising for a working surface. Finally, treat the anodic layer as a dimensional feature: because the layer grows into and out of the surface, features that must hold a tolerance after finishing — threads, bores, bearing seats — should either be masked or pre-machined with an agreed allowance.

4. Powder coating and paint systems

Powder coating applies a dry polymer powder electrostatically and cures it into a continuous film. It is normally chosen for steel and aluminium fabricated parts, enclosures and frames where a durable, opaque, decorative finish is wanted, and it is the finish most often used on hollow sections and welded assemblies.

The coating powders themselves are characterised under the ISO 8130 series, which covers test methods such as the determination of particle size distribution of coating powders. For protective paint systems on steel structures, ISO 12944-5 describes the types of paint and paint system commonly used for corrosion protection and gives guidelines for selecting systems for different environments and expected durability, referring out to the other parts of the ISO 12944 series for corrosivity categories and surface-preparation grades. That structure is the point for a buyer: durability comes from the system and the preparation, not from the name of the topcoat.

Design cautions. A powder film is thicker than most plating or anodising layers, so it fills and bridges fine detail; threads, close-tolerance bores and mating faces are normally masked, and threaded joints often use a separately plated or anti-seize approach. Damaged powder cannot usually be touched up invisibly, so specify how cosmetic acceptance will be judged: a reference panel, an agreed gloss and colour reference, and the surfaces that will be visible in service. Adhesion is verified by the tape test methods in ASTM D3359, which is intended for ductile organic films on metallic substrates and is not sensitive to small differences in adhesion; the standard also makes clear that the method chosen depends on coating thickness and that operator technique matters, so the acceptance criterion and the method variant must be agreed rather than assumed.

5. Electroplating and conversion coatings

Electroplating deposits a metal layer for corrosion protection, conductivity, wear resistance or appearance. In general engineering parts the dominant specification is electroplated zinc on steel.

Zinc plating. ISO 2081:2025 is the current edition of the zinc electroplating standard for iron and steel. Its current title is specific about the chemistry: electroplated coatings on iron and steel using zinc treated with solutions containing chromium (VI). The standard specifies requirements for the coating with supplementary treatments, the information the purchaser must supply to the electroplater, and the requirements for heat treatment before and after electroplating, and it notes that the coating thickness that can be applied to threaded components is limited by dimensional requirements such as class or fit. That is a direct warning to state the thread class when you specify a plating thickness.

Alternative general specification. ASTM B633 covers electrodeposited zinc coatings on iron and steel articles for corrosion protection, provided in four standard thickness classes in the as-plated condition or with one of five types of supplementary finish. It expects the purchaser to read the full specification including appendices rather than treating it as a pick-one-line document. Two limitations are worth carrying into the drawing: the specification states that high-strength metals, including steels above the strength it defines, should not be zinc electroplated to it; and it has been revised to address RoHS requirements, with additional finish types added so that non-chromate passivates can replace hexavalent chromium.

Fasteners. For threaded fasteners the more specific documents take precedence. ISO 4042:2022 specifies requirements for steel fasteners with electroplated coatings and coating systems, mainly zinc, zinc-nickel and zinc-iron, with or without conversion coating, sealant, top coat or lubricant; it also covers dimensional requirements for fasteners made of copper or copper alloys, applies to bolts, screws, studs, nuts, washers, pins, clips and rivets, and states that its requirements for electroplated fasteners take precedence over other documents dealing with electroplating. It also includes requirements and recommendations to minimise the risk of hydrogen embrittlement, which matters on high-strength and surface-hardened parts. ASTM F1941/F1941M covers the same ground for inch and metric mechanical fasteners, specifying coating thickness, hexavalent chromate or non-hexavalent conversion coatings, corrosion resistance, precautions against hydrogen embrittlement and relief treatment after plating, and the applicability of barrel versus rack plating.

Process consequences. Parts are plated on racks or in barrels, so expect contact marks and plan the contact points. Plating increases dimensions on every surface it reaches, including threads. And because ISO 2081 and ISO 4042 both separate the coating from the conversion coating on top of it, the purchase specification should name all three: coating type, thickness class and the supplementary treatment.

Adhesion and appearance. ASTM B571 covers qualitative adhesion testing of metallic coatings, and warns that the interpretation of qualitative results is often contested and that the method must be specified for acceptance work; several of its tests apply only within certain coating types and thickness ranges. Choose the test that matches the part's duty — for example a bend or draw test for parts that will be formed afterwards.

6. Passivation and hot-dip galvanising

Passivation of stainless steel. Passivation is a chemical treatment that removes free iron and other exogenous contamination from a stainless surface and supports the natural re-formation of the passive oxide film. ASTM A967/A967M covers the chemical passivation treatments themselves — immersion in nitric acid solution, immersion in citric acid solution, and electrochemical treatment — together with rinsing and neutralisation, and it lists the alternative tests used to confirm effectiveness: water immersion, high humidity, salt spray, copper sulfate, potassium ferricyanide-nitric acid, damp cloth and boiling water immersion. It adds no measurable thickness, which is why it is the usual final step on machined stainless parts where the machined surface must resist corrosion.

ASTM A380/A380M covers cleaning, descaling and passivation of stainless steel parts, equipment and systems, and contains an important warning for buyers: the word passivation is applied to several different operations, and unless the requirement is defined by the purchaser the meaning taken is removal of exogenous iron by chemical dissolution. If your requirement is really descaling, or really an oxidising treatment, say so, because the supplier will otherwise deliver the standard interpretation.

Hot-dip galvanising. ASTM A153/A153M specifies zinc coatings applied by the hot-dip process on iron and steel hardware. It is the appropriate route where long-life corrosion protection is needed on fittings, brackets and similar thick-section hardware, but the part must survive the process: the coating is thick, it can affect thread fit, and thin or distortion-sensitive parts are usually better served by zinc plating or a coating system. Where a galvanised thread must assemble, agree whether the thread will be re-tapped after coating, and record that decision on the drawing.

7. How to specify a finish correctly

A finish requirement is complete only when all of the following are on the drawing or in the purchase specification. Missing items are the usual reason a shipment arrives with a finish that is technically acceptable and commercially useless.

1. Process and specification. Name the process, the standard designation and, where the standard has editions in circulation, the edition or at least a requirement to state the edition used.
2. Substrate. Alloy and temper for aluminium, grade family for steel, and grade and condition for stainless steel. Anodising and passivation both behave differently across grades.
3. Extent of finishing. A masking map: which surfaces are finished, which are masked, and which are contact or witness areas that will be left unfinished intentionally.
4. Coating build-up. Whether the coating thickness is included in the dimensional tolerances or must be removed afterwards, and which features are dimensionally critical — threads, bores, sealing faces, press fits.
5. Supplementary treatment. Sealing, chromate or non-hexavalent conversion coating, lubricant, top coat, oil or wax. These are separate decisions in the standards and separate line items in the process.
6. Heat treatment and embrittlement relief. For plated high-strength parts, state whether relief treatment is required before or after plating, and require the finisher to confirm what was applied.
7. Acceptance criteria and method. The test, the sampling and the pass/fail criterion: adhesion by tape test or by bend test, coating thickness by a named measurement practice, corrosion testing by a named method, or visual acceptance against an agreed sample.
8. Documentation. Certificate of conformity naming the standard and edition, the batch, the measured results, and any test reports from an external laboratory.
9. Handling and packaging. Which surfaces are cosmetic and how they must be protected during transport, since cosmetic rejection is far more common than functional rejection.

8. How to inspect a finish

Inspection should test the acceptance criteria you wrote, not a generic checklist. Four measurements cover most disputes.

Coating thickness. ASTM D7091 describes non-destructive measurement of dry film thickness using magnetic and eddy-current gauges, including the three operational steps of calibration, verification and adjustment of the instrument, and notes that a single reading may not represent the coating across a surface, so frequencies of measurement are set by the governing specification. The lesson for a buyer is that the specification must state the minimum and maximum thickness for each layer and for the total system; without those limits there is nothing to compare a reading against.

Adhesion. For organic films such as powder coatings, ASTM D3359 sets out tape test methods, with the appropriate method depending on the coating thickness and with acceptance rankings limited to a small scale because the method cannot make fine distinctions. For metallic coatings, ASTM B571 is the qualitative reference, and it requires the applicable method to be named before the test is used for acceptance.

Corrosion testing. ASTM B117 provides a controlled salt spray environment for specimens of metals and coated metals. Its stated significance is the caution: prediction of performance in natural environments has seldom been correlated with salt spray results when used as standalone data, reproducibility depends heavily on specimen type, evaluation criteria and control of operating variables, and variability has been observed between chambers operating nominally under the same conditions. ISO 9227 covers neutral salt spray, acetic acid salt spray and copper-accelerated acetic acid salt spray, and is explicit that it does not specify specimen type or dimensions, exposure period or the interpretation of results — those belong in the product specification. ISO 9227 also notes that these tests are useful for detecting discontinuities such as pores in metallic, organic, anodic oxide and conversion coatings, and that they are not intended for ranking materials or for predicting long-term corrosion resistance. Use salt spray as a comparative, process-control test with pre-agreed specimens and criteria — not as a warranty of service life.

Passivation effectiveness. ASTM A967/A967M provides the alternative confirmation tests listed in Section 6, and because they are qualitative, the specific practice and its acceptance criterion should be named in the specification. Also confirm the test is applied to the parts and the lot you are buying, not to a generic sample.

Two supporting checks close the loop. Ask for the finisher's process records for the actual batch — pretreatment sequence, bath or powder identification, cure or sealing step — because a certificate without batch records only proves that a procedure exists. And when a test result is used as evidence in a commercial decision, confirm that the laboratory works to ISO/IEC 17025:2017, which sets out the requirements for the competence, impartiality and consistent operation of testing and calibration laboratories.

9. Common mistakes when buying finished parts

• Specifying a finish by colour or by a trade name only, with no process or standard.
• Forgetting that coatings change dimensions on threads, bores and press fits, and then rejecting parts that meet the coating specification but not the assembly.
• Using decorative and protective anodising on a wear surface, or hard anodising for a purely decorative part, instead of matching the process to the duty.
• Applying anodising requirements to steel parts, or expecting passivation to protect carbon steel.
• Ordering hexavalent-chromium conversion coatings for electrical and electronic products destined for the EU market without checking the restricted-substances position.
• Omitting embrittlement relief on high-strength plated parts, especially fasteners and spring steel components.
• Treating salt spray hours as a universal durability ranking and comparing suppliers on that basis.
• Leaving masking and cosmetic acceptance undefined, so the finisher makes the decision.
• Accepting a supplier certificate as proof of performance on your specific part, geometry and lot.
• Specifying an anodising or plating standard without stating the alloy, grade or thread class on which the finish is expected to perform.

Request a quote

Send us the part and the requirement, not a finish written from scratch. We work from the checklist in Section 7 — process and specification, substrate, masking map, coating build-up, supplementary treatment, acceptance method and documentation — and we will tell you what is missing before a finisher starts.

Request a quote → Upload the drawing, state the substrate and the finish requirement, and describe the service environment. Questions about masking, thread fit after coating and inspection criteria are welcome; the answer costs nothing and usually saves a rejected batch.

Related pages

• Manufacturing guides: custom processes and design rules — the hub for machining, forming and finishing guides.
• CNC machining in China: design and sourcing guide — tolerances, materials and finish options for machined parts.
• Sheet metal design guide: bend radius, tolerances and materials — for fabricated parts that are powder coated or painted after forming.
• Fasteners from China: materials, grades and inspection — coating and thread-fit decisions for threaded parts.
• Certification and compliance hub — how product compliance obligations sit with the importer.
• ASTM vs CE vs UL: standards you need before importing — mapping standards to destination markets.
• Quality control for imported parts — inspection planning, sampling and documentation.
• Request a quote — upload a drawing and get finishing feedback.

Last updated: 16 September 2026.

Frequently asked questions

Sources

All sources retrieved 16 September 2026. This page is an independent reading of the sources listed; the standards and official pages themselves are the specification.

Fact used on this page	Source
S1	ISO https://www.iso.org/standard/70156.html
S2	ISO https://www.iso.org/standard/80119.html
S3	ISO https://www.iso.org/standard/88302.html
S4	ASTM https://store.astm.org/b0633-19.html
S5	ISO https://www.iso.org/standard/77913.html
S6	ASTM https://store.astm.org/f1941_f1941m-16r25.html
S7	ASTM https://store.astm.org/a0967_a0967m-25.html
S8	ASTM https://store.astm.org/a0380_a0380m-17.html
S9	ASTM https://www.astm.org/a0153_a0153m-16a.html
S10	ASTM https://store.astm.org/d3359-23.html
S11	ASTM https://store.astm.org/d7091-22.html
S12	ASTM https://store.astm.org/b0117-19.html
S13	ISO https://www.iso.org/standard/81744.html
S14	ASTM https://store.astm.org/b0571-18.html
S15	ISO https://www.iso.org/standard/15200.html
S16	ISO https://www.iso.org/standard/77795.html
S18	ISO/IEC https://www.iso.org/standard/66912.html