1. Quick lookup by process
Enter a process name or an alias to get the substrate, typical thickness, salt-spray reference and inspection method. Try zinc plating, yellow zinc, anodising, AA15, powder coating, Dacromet, electroless nickel, black oxide, QPQ.
2. Common metal finishing processes
3. Pre- and post-treatment (pickling / passivation / phosphating / sealing / baking / hydrogen removal)
Coating defects (blistering, flaking, mottling, skip plating, early white rust) more than 70% come from pre-treatment and rinsing, not from the plating bath itself. Below, the pre-plating and post-plating steps are listed with parameters, control points and inspection methods; your own process card or the supplier's process sheet governs the actual figures.
Typical sequence:degrease → rinse → pickle → rinse → activate → plate (electroplate / anodise / coat) → rinse → passivate or seal → bake or cure → hydrogen removal (high-strength parts)
1) Pre-treatment (before plating)
| Step | Purpose | Typical parameters (reference) | Control points | Inspection |
|---|---|---|---|---|
| Degreasing | Removes grease, fingerprints and polishing compound so the coating can bond | Alkaline cleaning pH 9–12 at 50–70 °C for 5–15 min; electrochemical or ultrasonic cleaning is more thorough | Pre-wipe heavy soil; wash blind holes and welds thoroughly; always rinse twice after alkaline cleaning | Water-break test: after rinsing the water film must stay unbroken for 30 s |
| Pickling | Removes scale, rust and hot-rolling oxide | Hydrochloric acid 10–15% at room temperature; sulphuric 10–15% at 60–70 °C; stainless in a nitric–hydrofluoric mixture | Use an inhibitor against over-pickling, control concentration, temperature and time, and watch high-strength steels and springs for hydrogen embrittlement | No rust or scale visually; use weight-loss or coupon tests on critical parts; hydrogen embrittlement per GB/T 3098.17 or a delayed-fracture test |
| Activation (light pickling) | Removes the thin oxide left by pickling and exposes active metal | 5–10% hydrochloric or sulphuric acid, 10–30 s at room temperature | Get into the plating bath quickly (≤ 2 min) to avoid re-oxidation; do not over-etch | Adhesion test (see the table below) |
| Blasting / shot peening | Mechanical descaling that also roughens the surface for adhesion | Abrasive 46–120 mesh; pressure 0.4–0.7 MPa; to Sa2.5–Sa3 | Nozzle angle 45°–75°; the compressed air must be oil- and water-free; coat or plate within 4 h | Compare with the ISO 8501 reference pictures (Sa2.5/Sa3) or use a roughness gauge (Rz 40–70 μm) |
| Aluminium alkaline etch + desmutting | Removes the natural oxide and extrusion lines for an even matt surface | Etch in NaOH 40–60 g/L at 50–60 °C for 1–3 min; desmut in HNO₃ 300–500 g/L at room temperature for 10–30 s | Rinse thoroughly after etching (keep alkali out of the anodising bath); change the desmut bath often to stop copper plating out | Even colour, no smut, no dark spots |
| Polishing / barrel finishing / vibratory finishing | Lowers roughness for decorative or mirror requirements | Mechanical polishing; barrel with media and soap; vibratory finishing 20–40 min | Avoid overheating and burning; do not round the edges too far | Visual or comparator plates for Ra; gloss meter |
2) Post-treatment (after plating)
| Step | Purpose | Typical parameters (reference) | Control points | Inspection |
|---|---|---|---|---|
| Passivation (after zinc or zinc-nickel) | Forms a conversion film that multiplies the salt-spray life of the zinc by 3–10 and prevents white rust and fingerprints | Trivalent chromium (the mainstream) at pH 1.8–2.5, 25–35 °C for 20–60 s; hexavalent chromium (blue-white or yellow, being phased out); chromium-free passivation | Rinse thoroughly before passivation (carry-over kills the bath), and hold pH and time tightly; test trivalent parts for hexavalent chromium (RoHS) | Even colour; nitric acid spot test (passive film on zinc); salt spray per GB/T 10125; hexavalent chromium by the diphenylcarbazide method of GB/T 26125/IEC 62321 |
| Phosphating | A base for painting (adhesion, rust prevention) or for friction reduction (cold heading, drawing) | Zinc type 40–55 °C for 5–15 min; iron type by spray at room temperature; manganese type (friction) 80–95 °C | Control the total-to-free acidity ratio (TA/FA) and accelerator concentration; rinse and seal after phosphating | Coating weight (gravimetric, GB/T 9792): 1–5 g/m² as a paint base, 5–15 g/m² for friction; copper sulphate spot test ≥ 3 min without discolouration; crystal density under the microscope |
| Sealing (mandatory after anodising) | Blocks the micropores of the oxide film, stops dye bleeding and improves corrosion and stain resistance | Boiling or hot-water sealing at 95–100 °C, about 1 min per μm; nickel-fluoride cold sealing at 25–35 °C, pH 5.5–6.5; nickel-free sealing | The water must be hot enough (<95 °C 封孔不良), 水质要纯(防白霜); 封孔后不可再机加工 | dye spot test per GB/T 8753.2: spots per cm² within limits; admittance test ISO 2931; acid-dissolution weight loss GB/T 8753.1) |
| Colouring / dyeing (after anodising) | Decorative and identification colours | Organic dyes at 50–60 °C for 5–15 min; electrolytic colouring (tin or nickel salts) | The film must be even before dyeing; control colour against a master sample | Colorimeter ΔE; visual comparison with the master; light and wash fastness |
| Baking / curing | Drives off moisture, cures the coating and sinters Dacromet | Powder coating 180–200 °C for 15–20 min; e-coat 170–180 °C for 20–30 min; Dacromet 300 °C for 30 min; drying after passivation 60–80 °C | Measure the part temperature, not the oven dial; watch load size and ramp time — over-baking embrittles, under-baking ruins adhesion | Cross-cut adhesion GB/T 9286 (class 1 or better); pencil hardness GB/T 6739; solvent rub (50 double rubs with MEK); colour and gloss |
| Hydrogen removal (de-embrittlement) | Drives hydrogen out of high-strength steel and springs after plating to prevent delayed fracture | 190–230 °C for 2–4 h (ordinary parts), 4–24 h for high-strength parts, starting within 1–4 h of plating | 温度不可过高(>stay below 250 °C to protect coating and substrate); start the clock when the parts reach temperature | Delayed-fracture test (GB/T 3098.17); hydrogen content; re-check for cracks after 48 h |
| Rinsing (multi-stage counterflow) | Prevents drag-in contamination and keeps the final rinse clean | Three-stage counterflow; pure or deionised water for the last stage | Final rinse conductivity <10μS/cm; 喷嘴通畅, 无死角 | Test the final rinse with a conductivity pen; no water marks or smut visually |
3) Should plating be sealed? Judge by the coating type
One distinction first:sealing after anodising is mandatory (the oxide is porous; unsealed dye bleeds and salt spray fails); sealing after electroplating is an extra post-treatment (the deposit is already dense metal — sealing fills micropores, prevents tarnish and adds a barrier), so whether to do it depends on the coating and the application, it is not automatic.
| Coating | Sealing? | Usual practice | Purpose / cautions |
|---|---|---|---|
| Zinc plating (blue-white / yellow / black) | Yes (passivation is essential, sealing is extra protection) | Passivation (trivalent or chromium-free) is the basic step; add sealing when salt spray must reach 96 h or more Organic or silicate sealer | Fills the pores of the passive film and lifts salt-spray life and white-rust resistance; the colour darkens and contact resistance rises, so be careful with conductive parts |
| Zinc-nickel plating | Yes | Passivation plus sealing (standard for automotive parts) | Salt spray up to 720–1 000 h; rinse thoroughly before sealing or the bath is ruined |
| Electroless nickel (Ni-P, including on aluminium) | Recommended | Passivate, use an anti-tarnish or a dedicated sealer after plating (especially when the deposit <10μm, 铝基体) | Electroless nickel is microporous, so unsealed it pits and tarnishes; note the effect on solderability and contact resistance |
| Nickel plating (decorative or engineering) | Normally not sealed on its own | Use an anti-tarnish where required; in a nickel-plus-chromium system the chromium does the protecting | Plain nickel yellows in hot humid conditions — add anti-tarnish if it matters |
| Decorative / hard chromium | No | Neutralise (alkali) and hot rinse only | The microcracks in chromium self-passivate; the point is to wash out hexavalent chromium residues that later cause rust spots |
| Silver plating | Yes (anti-tarnish) | Passivation, silver anti-tarnish, or thicker protection in sulphur-bearing environments | Silver blackens with sulphur; check contact resistance on contacts and connectors |
| Tin plating | Usually no | Neutralise and anti-tarnish only (dichromate or benzotriazole, with customer approval) | Sealing clearly damages solderability, so electronic parts are not sealed by default |
| Gold / palladium plating | No | — | The noble metals are stable; sealing contaminates the surface and hurts wire bonding and contact resistance |
| Copper plating | Yes (anti-tarnish) | Benzotriazole (BTA) anti-tarnish | Copper oxidises and blackens easily — protect work in process especially |
Three questions: ① Is the deposit porous or microcracked (electroless nickel, zinc, decorative chromium)? ② Is there a salt-spray, damp-heat, sea-freight or long-storage requirement? ③ Will it be welded, conducting, painted or bonded?？
→ ① yes, or ② yes with ③ no = seal it; ③ yes (especially tin solderability, gold wire bonding, contact resistance) = do not seal— just neutralise and anti-tarnish.
Order matters: high-strength parts (bolts of grade 10.9 and above, springs) must be de-embrittled within 1–4 h of plating before passivation and sealing — passivating first and baking afterwards bakes the passive film away; if you did passivate first, re-passivate after de-embrittlement. A sealed part is normally not machined or bent again.
→ ① yes, or ② yes with ③ no = seal it; ③ yes (especially tin solderability, gold wire bonding, contact resistance) = do not seal— just neutralise and anti-tarnish.
Order matters: high-strength parts (bolts of grade 10.9 and above, springs) must be de-embrittled within 1–4 h of plating before passivation and sealing — passivating first and baking afterwards bakes the passive film away; if you did passivate first, re-passivate after de-embrittlement. A sealed part is normally not machined or bent again.
Judging adhesion (and whether pre-treatment was done properly): ① Cross-cut per GB/T 9286 (coatings, class 0–1 passes); ② bend test: fold 180° with the coating outside and inspect under a magnifier for lifting or flaking; ③ thermal shock: hold 1 h at 200 °C then quench in water — the coating must not blister or peel; ④ file or scratch test for flaking. If any fails, check degreasing, over-pickling and how long parts sat after activation before blaming the bath.
Points that are easily confused:· Pickling is not passivation — pickling removes scale (and activates the surface, so it rusts again quickly) while passivation forms a protective film; stainless must be passivated after pickling or its corrosion resistance actually drops; · phosphating can be a pre-treatment (paint base) or a final finish (friction reduction, rust prevention), depending on coating weight and whether it is painted; · sealing after anodising is not optional — unsealed dye bleeds and salt spray fails; · do not dry zinc-plated parts above 60–80 °C after passivation, or the film cracks and corrosion resistance falls.
Points that are easily confused:· Pickling is not passivation — pickling removes scale (and activates the surface, so it rusts again quickly) while passivation forms a protective film; stainless must be passivated after pickling or its corrosion resistance actually drops; · phosphating can be a pre-treatment (paint base) or a final finish (friction reduction, rust prevention), depending on coating weight and whether it is painted; · sealing after anodising is not optional — unsealed dye bleeds and salt spray fails; · do not dry zinc-plated parts above 60–80 °C after passivation, or the film cracks and corrosion resistance falls.
4. Multilayer plating: copper strike, nickel, chromium
Yes — it is the mainstream for decorative and protective plating. Chromium is hardly ever plated straight onto steel: the deposit is highly stressed with a natural microcrack network that runs down to the substrate, so moisture pits it and rust bleeds out. A drawing that says only "chrome plated" is in practice a copper undercoat → nickel → chromium (Cu-Ni-Cr) multilayer system. A decorative chromium callout should spell out the layer structure, e.g. Fe/Cu15/Ni15b/Cr0.3.
1) Why the nickel layer must sit in between
| Layer | Role | Typical thickness | Control points / common problems |
|---|---|---|---|
| Copper | Levels and fills (polishing lines, pits, scratches), cuts cost (copper is cheaper than nickel) and relieves stress, giving the nickel a bright base | Copper strike 3–5 μm; acid copper 10–25 μm | Copper oxidises fast, so do not leave it in air (into the nickel bath within about 10 min) or interlayer adhesion drops; too thin a copper layer will not level, and substrate marks show through the chromium |
| Semi-bright nickel (sulphur < 0.005%) | The corrosion-resistance workhorse: columnar, ductile, and at a different potential from bright nickel so corrosion spreads sideways instead of through | 10–15 μm, 60–70% of the total nickel | Keep 100–150 mV of potential difference from the bright nickel (verify by STEP test); too small a difference is no better than single-layer nickel |
| High-sulphur nickel (triple nickel, 0.1–0.2% sulphur) | A sacrificial layer that keeps corrosion spreading sideways within that thin layer, lifting corrosion resistance another notch | 1–2μm | Too thick it darkens and adhesion to the bright nickel falls; triple nickel is expensive and usually reserved for outdoor high-corrosion parts |
| Bright nickel (0.03–0.08% sulphur) | Brightness, levelling and a carrier for chromium (chromium straight on iron adheres poorly and yellows) | 5–10μm | Additive breakdown causes pitting and cloudiness, so filter continuously through carbon; sitting too long before chromium passivates the nickel and clouds the chromium |
| Chromium | Wear resistance, tarnish resistance, decoration; the microcrack or micropore structure spreads the corrosion current over countless tiny cells | Decorative 0.1–0.5 μm (0.25 usual); hard chromium 10–100 μm | Too thin it clouds and yellows and wears quickly; too thick the cracks deepen and corrosion resistance drops; hexavalent chromium is restricted under RoHS/REACH |
| Electroless nickel (EN) | Throws into recesses better than anything else — a conductive layer on plastics, a base on aluminium and in deep holes, or a wear-resistant functional layer | Conductive layer 0.2–0.5 μm; functional layer 5–25 μm | Plastics must be fully covered; a skip burns and turns into a black spot in the following plating steps |
2) Common multilayer systems, substrate to surface
| System | Layer structure / typical thickness | Typical use | Acceptance focus / standard |
|---|---|---|---|
| Decorative chromium on steel Cu-Ni-Cr | Cu 10–25 + bright Ni 10–20 + Cr 0.1–0.5 | Sanitary ware, furniture, automotive and motorcycle trim, decorative fasteners | GB/T 9797 (Ni+Cr, Cu+Ni+Cr); write the full system on the drawing |
| Double or triple nickel + microporous chromium | Semi-bright Ni 10–15 + (high-sulphur Ni 1–2) + bright Ni 5–10 + microporous Cr 0.1–0.3 | Automotive exterior, outdoor high-corrosion parts | STEP test for interlayer potential; CASS 24–48 h; micropore density ≥ 10⁶ per cm² |
| Copper undercoat + hard chromium (hydraulic rods, rollers) | Cu 5–15 + hard chromium 20–50 | Cylinder rods, piston rods, guide rollers, moulds | GB/T 11379; ground after plating, so the drawing must give the "after plating" size; check adhesion by thermal shock |
| Aluminium parts Double zincate + electroless nickel + Cu + Ni + Cr | Zincate twice + EN 3–5 + Cu 10–20 + Ni 8–12 + Cr 0.2–0.5 | Aluminium handles, trim and bodies | Aluminium cannot be plated directly — zincate or electroless nickel first; measure thickness by XRF |
| Zinc die castings Cu-Ni-Cr | Pyro or cyanide copper 8–15 + acid copper 10–20 + Ni 8–12 + Cr 0.2–0.3 | Handles, locks, lighting, hardware | Die castings are porous and blister readily; check the copper covers completely (no exposure of the zinc substrate) |
| Plating on ABS | EN 0.2–0.5 + pyro copper 20–30 + semi-bright or bright Ni 10–15 + Cr 0.1–0.3 | Automotive grilles, appliance panels, sanitary plastic parts | GB/T 12600; no blistering after thermal cycling (−40 to 80 °C); too thin a copper layer always blisters |
| Electroless nickel base + electroplated nickel build-up | EN 3–5 + electroplated Ni 10–30 | Deep holes, blind holes, complex cavities | Electroless nickel covers the inside, electroplating builds thickness; measure the cavity thickness at its own points |
| Cu / Ni / Au (connectors, PCB) | Cu 15–25 + Ni 1.3–5 + Au 0.05–1.27 | Contacts, gold fingers, lead frames | Nickel is the diffusion barrier that stops copper diffusing into gold and raising contact resistance; measure the layers by XRF |
| Cu / Ni / Sn | Cu base + Ni 1–3 + Sn 3–10 | Terminals, busbars, solderable parts | Solderability (GB/T 2423.32) plus whisker assessment; the nickel blocks copper-tin interdiffusion |
| Cu / Ni / Ag | Cu + Ni 2–5 + Ag 5–20 | Cutlery, conductive parts, high-frequency parts | Silver anti-tarnish; contact resistance; adhesion (silver is soft and scratches easily) |
3) Measuring multilayer thickness (where inspection trips up most)
| Method | Resolves layers? | Notes / standard |
|---|---|---|
| Metallographic cross-section | Yes, the most accurate | Section, mount, polish and etch to reveal the interfaces, then measure each layer under the microscope; always the referee in multilayer disputes (GB/T 6462 / ISO 1463) |
| Coulometric (electrolytic) method | Yes, dissolving layer by layer | The potential jumps as each layer dissolves through, so Cu/Ni/Cr and double nickel can be resolved; destructive, small area (GB/T 4955) |
| XRF (X-ray fluorescence) | Yes, when the layers are thick enough | Reads several layers at once and works well for Cu/Ni/Au; error grows when a layer is under 1 μm or neighbouring lines interfere, so calibrate with standards (GB/T 16921) |
| Magnetic / eddy current | No | They measure total thickness only, and since nickel is both ferromagnetic and conductive neither method is reliable on Cu-Ni-Cr — do not use a gauge reading on a multilayer chromium part to judge a single layer |
Acceptance reminder:"Chrome 0.3 μm" alone cannot be accepted — without the layer structure there is no basis for arbitration. Correct callouts look like:
Fe/Cu15/Ni15b/Cr0.3 (b = double nickel),Fe/Cu10/Ni20/Cr0.5; for plastics write ABS/EN0.3/Cu25/Ni12/Cr0.25. Submit samples with all three: each layer's thickness, adhesion and salt spray (or CASS) — miss one and you cannot tell a coating fault from a pre-treatment fault.4) Which layer to blame for a defect
| Symptom | Usually traced to |
|---|---|
| Large blisters, sheets of peeling | Incomplete degreasing, or poor copper-to-substrate adhesion (parts left too long after activation) |
| Local pinholes, pits, roughness | Organic contamination or additive breakdown in the nickel bath with poor filtration; insufficient copper levelling |
| Cloudy, hazy or bluish chromium | The nickel passivated before chromium (held too long, poor rinsing), or the chromium bath is too cold or the current density too low |
| Burning and blackening at edges and sharp corners | Current density too high, poor jig contact, no shielding at the edges |
| Skip plating, white spots or round marks in high-current areas | Jig contact in the wrong place (nothing plates under it); put contacts on non-cosmetic faces and add a plating boss |
| Pitting red rust in salt spray (not sheets of it) | Chromium cracks reaching the nickel, nickel too thin, or too little potential difference in double nickel — "chromium too thin" is not the only cause |
| Blistering after thermal cycling (plastics) | Copper too thin to absorb the expansion mismatch, coating stress not relieved, or moulded-in stress not annealed out |
| Flaking or chrome loss on edges after assembly | Poor interlayer adhesion plus brittle chromium; edges carry concentrated current and high stress, so chamfer them or mask them from plating |
5) Three hard rules on sequence
- No pause after copper, no passivation before chromium: oxidised copper and passivated nickel both kill adhesion, so Cu → Ni → Cr must run on a continuous line, into the next tank straight after rinsing.
- Hydrogen embrittlement accumulates: each plating step evolves hydrogen at the cathode, so bolts of grade 10.9 and above, springs and high-strength stampings cannot rely on "we de-embrittled after each step" — they need a single de-embrittlement within 1–4 h of finishing all plating (200–230 °C for 2–4 h, 4–24 h for high-strength parts), with passivation and sealing afterwards.
- Design jigs and masking up front: multilayer parts always skip at the jig contact, so fix the contact points at the design stage (non-cosmetic, non-mating faces) and plug or tape threaded and precision fits — do not wait until after plating to rework.
5. Choosing a thickness gauge: magnetic vs eddy current vs XRF
| Method | Principle | Applies to (substrate → coating) | Standard |
|---|---|---|---|
| Magnetic method (magnetic induction or pull-off) | The magnetic circuit changes with coating thickness | Ferromagnetic substrate (carbon steel, iron, alloy steel) with a non-magnetic coating: zinc, nickel, tin, powder coat, paint, e-coat, phosphate, black oxide | GB/T 4956 ISO 2178 |
| Eddy current | High-frequency eddy currents change with coating thickness | Non-magnetic metal substrate (aluminium and its alloys, copper, zinc die castings, 304/316 austenitic stainless) with a non-conductive coating: anodic film, powder coat, paint, e-coat, powder coating | GB/T 4957 ISO 2360 |
| X-ray fluorescence (XRF) | Characteristic fluorescence of the coating | Metal coatings on metal: zinc (blue-white or yellow passivate), nickel, electroless nickel, tin, zinc-nickel, gold and silver — measures several layers and alloy composition at once | GB/T 16921 ISO 3497 |
| Coulometric (electrolytic) method | Coulometric dissolution | Metal coatings (multilayer capable); destructive, used as the referee | GB/T 4955 |
| Metallography (cross-section) | Measured on a magnified section | Any coating or anodic film; destructive; the referee method | GB/T 6462 ISO 1463 |
| Split-beam microscope | Groove and microscope | Anodic film on aluminium (after sealing) | ISO 2128 |
In one line: zinc or powder coat on steel → magnetic; anodising or powder coat on aluminium → eddy current; nickel on steel, nickel on aluminium, precise zinc thickness and multilayer systems → XRF. The common "dual" gauges that switch between Fe and NFe carry both magnetic and eddy-current logic and pick the right mode automatically.
6. Where an eddy-current gauge applies
- Suitable for: non-conductive coatings on non-conductive coating——
- anodic films on aluminium and its alloys (the AA10/AA15/AA20 acceptance checks on this site's aluminium extrusions use an eddy-current gauge);
- powder coat, paint, e-coat and other organic coatings on aluminium, copper, zinc die castings and stainless.
- Not suitable for:
- any coating on a ferrous (magnetic) substrate — eddy-current readings on steel are unreliable, use Magnetic method (zinc or powder coat on steel is magnetic only);
- metallic conductive coatings on a non-magnetic substrate (electroless nickel on aluminium, nickel on copper, zinc itself) — coating and substrate both conduct and the eddy-current signal blurs, so use XRF or metallography.
- Substrate check:304/316 austenitic stainless is weakly magnetic (more so after cold work) and should be measured in eddy-current mode as a non-magnetic substrate; if the gauge auto-detects Fe, switch to NFe and check again.
- Operating points: calibrate with the supplied standards each shift; keep test points 10 mm from edges and 10 mm apart and take 3–5 per part; a ferrous table or fixture behind the part biases magnetic readings, so isolate it.
- Acceptance criteria: anodic film on aluminium is graded by GB/T 8013.1 (AA5/AA10/AA15/AA20/AA25 correspond to mean thicknesses of 5/10/15/20/25 μm); powder coat and paint are usually 60–120 μm per the drawing; zinc plating per GB/T 5267.1 / ISO 4042 (e.g. Fe/Zn 5, Fe/Zn 8, Fe/Zn 12).
7. Inspection items and methods
| Item | Method / instrument | Standard | Applies to |
|---|---|---|---|
| Appearance (colour, bare spots, burning, burrs, runs) | Visual plus a 6–10× magnifier; colour by colorimeter (ΔE usually ≤ 1.0–1.5) | per drawing / GB/T 3181 (colour chips) | All |
| Thickness | Magnetic / eddy current / XRF (see section 5); metallography for arbitration | GB/T 4956, GB/T 4957, GB/T 16921, GB/T 6462 | All |
| Adhesion | Cross-cut with tape pull-off; coatings can use thermal shock, file or bend tests | GB/T 9286 (coatings), GB/T 5270 (platings) | Powder coat, paint, e-coat, platings |
| Corrosion resistance | Neutral salt spray (time to white rust or red rust); CASS as an accelerator for passivated zinc | GB/T 10125 / ISO 9227 | Zinc, zinc-nickel, Dacromet, electroless nickel, coatings |
| Coating hardness | Microhardness tester (HV0.1/HV0.3) | GB/T 4340.1 | Hard chromium, electroless nickel, QPQ, nitriding |
| Coating hardness | Pencil hardness (usually ≥ H/2H) | GB/T 6739 | Powder coat, paint |
| Impact resistance | Impact test (50 kg·cm without cracking is usual) | GB/T 1732 | Powder coat, paint |
| Seal quality of anodic film | Dye stain spot test; phosphoric-chromic acid weight loss | GB/T 8753 series | anodising |
| hydrogen embrittlement | Delayed-failure or dedicated hydrogen embrittlement testing; for high-strength parts prefer Dacromet or zinc-nickel, which are hydrogen-free processes | GB/T 3098 series, ISO 4042 | Electroplated bolts of grade 10.9 and above, spring parts |
| Coating composition (alloy ratio) | XRF (zinc-nickel at 12–15% Ni, phosphorus in electroless nickel) | GB/T 16921 | Zinc-nickel, electroless nickel |
Sampling and acceptance: sample thickness and salt spray per GB/T 2828.1; salt spray is destructive, so sample it by lot. Acceptance always rests on the drawing or purchase specification plus the supplier's finishing report (with measured thickness records and the salt spray report).
8. Salt-spray reference by coating (neutral salt spray)
Note: salt-spray hours depend strongly on thickness, passivation system, substrate and sealing; the table shows the order of magnitude usually asked for in purchase specs, andthe drawing governs actual acceptance (e.g. "yellow zinc, 72 h, no white rust"). White rust does not mean scrap: on zinc it is a change in the passive film, while red rust is substrate corrosion; white rust on a non-cosmetic mating face of a structural part is usually acceptable, on cosmetic parts it is another matter.
9. Pre-plating allowance for precision holes and shafts
Basic formula (t = coating thickness per side):
· Shaft (outside diameter): grows after plating → pre-plating diameter = finished diameter − 2t (allow towards the low limit)
· Hole (bore): shrinks after plating → pre-plating diameter = finished diameter + 2t (allow towards the high limit)
Acceptance basis: thickness is accepted on the minimum (μmin) while real coatings run 20–50% heavy, so calculate the allowance on the upper thickness limit— otherwise being out of tolerance after plating is the likely outcome.
| Process | Typical t per side | Total diameter allowance 2t | Practical notes |
|---|---|---|---|
| Zinc plating (blue-white / yellow / black) | 5–12 μm | 10–24 μm | Normally plated to size and not machined afterwards. Fasteners follow GB/T 5267.1; for threads see the 4t rule below. |
| Nickel plating | 8–20 μm | 16–40 μm | Throw is only fair (current density varies, edges run thick), so avoid it on precision fits; electroless nickel is better. |
| Electroless nickel | 10–50 μm | 20–100 μm | Uniform to ±1–2 μm —first choice for precision holes and shafts; no machining afterwards, and it plates inside holes. |
| Decorative Cr (Cu-Ni-Cr) | 15–30 μm total | 30–60 μm | Mainly cosmetic; where a mating face needs precision, allow the full thickness or plug it and leave it unplated. |
| Hard chromium | 20–50 μm | 40–100 μm | Usual route: grind to the pre-plating size → hard chrome (about 0.05 oversize) → finish grind after plating. Pre-plating size = finished − 2t. |
| Anodising (aluminium) | Film 10–25 μm | ≈ 10–25 μm | Half the film grows into the metal: an outside diameter grows by about t and a bore shrinks by about t. Precision bores in aluminium must be opened up or honed after anodising. |
| Hard anodising | Film 25–80 μm | ≈ 25–80 μm | As above, and the hard film is brittle with chipping edges; for IT7 or better on mating faces, grind or hone after anodising. |
| Dacromet / zinc-aluminium coating | 6–12 μm | 12–24 μm | Thin and hydrogen-free, so threaded parts usually need no allowance; high-strength bolts are certified to 5267. |
| Black oxide / phosphating | 1–5 μm | 2–10 μm | Normally no allowance, but on precision sliding fits (a spool valve, for instance) the film is thick enough to change the clearance — verify it. |
| Nitriding (not a coating) | Case 0.3–0.6 mm | OD grows 0.01–0.05 | Distortion dominates: leave grinding or honing stock after nitriding, no more than 0.05 per side — grind more and you remove the strongest part of the case. |
| Carburize and harden | Case 0.5–1.5 mm | Grinding stock 0.4–0.6 (diameter) | Distortion plus surface oxidation and decarburisation: leave 0.4–0.6 on diameter for finish grinding; bores shrink during hardening, so bore them to the high limit before carburising. |
| Hot-dip galvanizing | 70–100 μm | 140–200 μm | Thick and uneven, not for precision parts; bolt threads are re-chased after coating (tapped oversize) and internal threads cleaned out. |
The 4t rule for threads: coating builds on both flanks of the 60° form, so the pitch diameter grows by about four times the thickness per side (4t). The M10 6g pitch diameter tolerance is about 0.15 mm; 8 μm of zinc adds 32 μm (about a fifth), which normally still assembles. At 10 μm or more, or where interchangeability is required, machine the pitch diameter undersize before plating or use a looser class (6e).
Three hard rules:
1. The drawing must state the measuring basis: "φ40h7 + 10 μm nickel" without saying before or after plating always ends in an argument. Write instead:"φ40 h7 (measured after plating)" or "pre-plating φ39.97 ± 0.01, 10 μm Ni, finished φ40 h7".
2. Do not plate interference or transition fits: thickness variation eats the interference. Plug or mask with peelable lacquer, or brush plate and finish grind.
3. Deep holes and internal corners do not plate: plating follows current distribution, so deep cavities and bores run thin or skip; where the inside must be uniform, switch to electroless nickel.
Pre-plating size calculator: enter the finished diameter and the thickness per side to get the pre-plating target.
mm
μm
%
Enter the values and click Calculate. Example: φ40 h7 shaft, 10 μm nickel per side, 50% over-thickness → pre-plating target φ39.97.