A drawing that says "chrome plated" gives the supplier every freedom and the buyer no remedy. Write this instead: "Decorative Cu–Ni–Cr per ASTM B456, class Zn/Cu5 Ni20d Cr mc, minimum local thickness as class, CASS 16 h per ASTM B368, no basis-metal corrosion or blistering on significant surfaces." One line, and the argument about what "good plating" means is over before it starts.
This guide explains what each part of that line means, how to pick the numbers for your application, and which acceptance items buyers most often forget.
Why the trade words fail
"Chrome", "gold", "nickel", "antique brass" describe an appearance, not a coating system. Two parts can both be "gold plated" with one carrying 0.1 µm of gold over a single nickel layer and the other 0.3 µm over a three-layer nickel system with microporous chrome. They look identical on the sample table and behave completely differently after eight months outdoors, or after two weeks in a humid warehouse.
ASTM B456 exists precisely because that ambiguity is expensive. It defines a class code that fixes substrate, copper underlayer, nickel thickness and structure, and chromium type — in that order.
Reading the class code
Zn/Cu5 Ni20d Cr mc reads as: zinc alloy substrate, 5 µm copper, 20 µm double-layer nickel, microcracked chromium.
| Position | Meaning | Values you will see |
|---|---|---|
| Substrate | Base metal | Fe steel, Zn zinc alloy, Cu copper alloy, Al aluminium (needs zincate first), stainless steel |
| Cu | Copper underlayer | Cu5 (5 µm; mandatory on zinc alloy), Cu15a (15 µm ductile acid copper, steel) |
| Ni | Nickel thickness + structure | b fully bright single layer, d double/triple layer, p dull/semi-bright nickel that must be polished to full brightness |
| Cr | Chromium type | r regular, mc microcracked (≥ 30 cracks/mm), mp microporous (≥ 10,000 pores/cm²) |
Two clarifications that Chinese-language summaries frequently get wrong:
pis not "three-layer nickel". It is a dull or semi-bright nickel layer that only reaches full brightness after polishing, with sulphur below 0.005 % and elongation of at least 8 %. The "p = three-layer" translation circulating in domestic reposts is an error.Cu5on zinc is not a recommendation, it is the standard's notation. Every zinc-alloy class in B456 starts with it, because the copper layer protects the zinc from the acidic nickel and chromium baths that follow. A plater who skips it to save a step is producing parts that blister in the field.
Choosing the service condition
B456 ties thickness and corrosion testing to a service condition (SC1–SC5) that expresses the environment, not the price point.
| Service condition | Environment | Typical use |
|---|---|---|
| SC1 Mild | Indoors, dry, little condensation | Gift-box plates, indoor trim |
| SC2 Moderate | Indoors with occasional condensation | Handles, indoor hardware, cosmetic caps |
| SC3 Severe | Outdoors, rain, detergents | Bag hardware, buckles, outdoor fittings |
| SC4 Very severe | Salt, road salt, frequent wetting | Marine-adjacent, winter outerwear hardware |
| SC5 Extended very severe | Aggressive and prolonged | Specified by the buyer, rare in fashion hardware |
Nickel thickness on zinc alloy, by class (minimum, µm):
| Service condition | Typical class | Nickel minimum | Copper underlayer | CASS (ASTM B368) |
|---|---|---|---|---|
| SC1 | Zn/Cu5 Ni10b Cr r | 10 | Cu5 mandatory | Not rated |
| SC2 | Zn/Cu5 Ni15b Cr mc / mp; Zn/Cu5 Ni20b Cr r | 15 or 20 | Cu5 | 4 h |
| SC3 | Zn/Cu5 Ni20d Cr mc / mp; Zn/Cu5 Ni25d Cr r; Zn/Cu5 Ni25p Cr mc / mp; Zn/Cu5 Ni35p Cr r | 20 / 25 / 35 | Cu5 | 16 h |
| SC4 | Zn/Cu5 Ni30d Cr mc / mp; Zn/Cu5 Ni35d Cr r | 30 or 35 | Cu5 | 22 h |
| SC5 | Zn/Cu5 Ni35d Cr mc / mp | 35 | Cu5 | 44 h |
The Ni35p trap. At SC3, a p-class part needs 35 µm of nickel where a d-class part needs 25 µm — ten extra microns of real plating time and nickel consumption. A buyer who writes Ni35p on the drawing and then compares quotes against a supplier who quoted Ni25d is not comparing the same product. Either specify d and get 25 µm, or specify p and accept that the price is higher. Do not let the class letter drift between drawing and purchase order.
CASS hours differ by substrate. The test hours in B456 are tied to substrate as well as service condition. Public summaries for steel substrates quote SC2 at 8 h and SC5 at 66 h, against 4 h and 44 h on zinc. SC3 and SC4 agree at 16 h and 22 h on both. Since no single public source gives both columns side by side, treat the SC2/SC5 difference as needs verification against the standard text, and write the hours explicitly on the drawing rather than relying on the class alone.
What "thickness" should mean on a drawing
Write minimum local thickness on significant surfaces, not average thickness over the part. Average thickness lets a plater satisfy the number while leaving the recessed areas — the ones that corrode first — thin.
Verification methods, per B456:
| Method | Standard | Note |
|---|---|---|
| Coulometric | ASTM B504 | Fast, point measurement, destructive to a small spot |
| Microscopy | ASTM B487 | Cross-section; the referee method when there is a dispute |
| XRF | ASTM B568 | Non-destructive, good for screening and for chromium |
| Magnetic | ASTM B499 / B530 | Nickel on non-magnetic substrate; check the instrument is calibrated for zinc |
For a 20 µm nickel requirement, expect the plater to hold 20 µm minimum everywhere on the significant surface, which in practice means aiming higher on the rack average. Ask what they target, not what they guarantee.
The acceptance items buyers forget
Thickness is the easiest thing to check and the least informative on its own. B456 lists seven acceptance items; most purchase orders mention one.
| Item | Method | Criterion |
|---|---|---|
| Thickness | B504 / B487 / B568 / B499 | Minimum local thickness on significant surfaces |
| Appearance | Visual | No blistering, pitting, roughness, cracks, skip plating, stains, discolouration on significant surfaces |
| Adhesion | B571 bend or thermal shock | No peeling, no flaking |
| Nickel ductility | B490 | Semi-bright nickel ≥ 67 %, bright nickel ≥ 11 % |
| Multilayer nickel potential | B764 STEP | 100–200 mV between semi-bright and bright (120–160 mV recommended) |
| Chromium cracks / pores | Metallographic or Dubermell | Microcracked ≥ 30 cracks/mm; microporous ≥ 10,000 pores per 10 × 10 mm |
| Corrosion resistance | B368 CASS, B380 Corrodkote, B287 | Hours per the class table; basis-metal corrosion or blistering = fail |
The STEP test is the one that separates a real three-layer nickel system from a cosmetic one. Without it, a plater can claim d and deliver two layers whose electrochemical difference is too small to spread corrosion sideways — which is the entire reason the double layer exists.
A clause you can paste onto a drawing
Finish: Decorative copper–nickel–chromium per ASTM B456, class
Zn/Cu5 Ni20d Cr mc. Minimum local thickness on significant surfaces: Cu 5 µm, Ni 20 µm. Corrosion: CASS per ASTM B368, 16 h; no basis-metal corrosion, blistering or coating breakdown on significant surfaces. Adhesion: ASTM B571 (thermal shock or bend), no peeling. Nickel ductility per ASTM B490. STEP per ASTM B764, 100–200 mV. Substrate: castings free of cold shuts, porosity, cracks and polishing-compound residue on significant surfaces; parts showing these shall be rejected before plating. Chromium: trivalent, hexavalent chromium not permitted (RoHS).
That last line matters commercially. Hexavalent chromium passivations and plating baths are restricted under RoHS, and a supplier who "still has some yellow" is selling you a compliance problem. Specify trivalent and put it in writing.
Three failure modes and what to do about them
Blistering, two to four weeks after shipment. Zinc die castings blister when pre-treatment is wrong: wax and polishing compound not removed, or a caustic degreaser strong enough to attack the zinc surface. It is the single most common failure of plated zinc, and it does not show at final inspection. Countermeasure: require a documented pre-treatment sequence, and run a thermal shock or 24 h humidity check on the first batch rather than only a visual pass.
Pitting that follows the casting, not the plating. Gas porosity just below the surface opens up during polishing and plating. No plating specification fixes a porous casting; the fix is casting quality, agreed as a cosmetic grade before the tool is cut.
Thickness that passes the certificate and fails the corner. Recessed areas, holes and edges plate thin. If the part has a deep recess that will be touched or exposed, say so on the drawing and require a cross-section at that location.
On salt spray. Neutral salt spray (ASTM B117 / ISO 9227) is widely used on decorative coatings but is best treated as a porosity check; industry practice is that its hours do not translate into outdoor service life, which is why B456 uses CASS and Corrodkote instead. If a buyer insists on B117 hours, agree a number, and treat it as a screening test rather than a lifetime prediction.
On plastic parts. If the component is injection moulded rather than cast, the equivalent standard for decorative Cu–Ni–Cr on plastics is ASTM B604; do not put a B456 class on a plastic part.
Next step
Send the drawing — or the part with the finish you want to match — to 13928445679@163.com. Tell us the environment it will live in (indoor, outdoor, coastal, cosmetic contact) and we will propose a class and test hours, then quote plating as a separate line so you can see what the class difference actually costs. Plating is done by our partner lines; the class, the thickness and the acceptance criteria are agreed with you before the first batch, and verified per lot.