Catalogue data and standard-part designations in this guide follow the cBallast C-Parts range, supplied through Bufab (Värnamo, Sweden). Values are nominal reference data; confirm against the controlling standard revision before procurement.Comparison of the finishes used on standard fasteners: zinc electroplating with Cr(III) passivation (blue/clear, yellow, black, with sealer), zinc-nickel, zinc flake (Geomet, Delta-Protekt, Dacromet-type), hot-dip galvanising, zinc and manganese phosphate, black oxide and stainless steel — with typical coating thickness, neutral-salt-spray life to white and red rust (ISO 9227), suitability per property class (hydrogen embrittlement), thread-tolerance effects (6g/6H vs 6az/6ax oversize nuts), galvanic compatibility, and which finish to specify for corrosivity categories C1–C5 of ISO 12944.
Catalogue data and standard-part designations in this guide follow the cBallast C-Parts range, supplied through Bufab (Värnamo, Sweden). Values are nominal reference data; confirm against the controlling standard revision before procurement.
A fastener finish does three jobs: it delays corrosion of the steel underneath, it sets the friction (and therefore the torque-to-preload relationship) of the joint, and it must not damage the fastener in the process — the hydrogen absorbed during acid pickling and electroplating can embrittle steel above about 320 HV (property class 10.9 and up). The right choice depends on the service environment (ISO 12944 corrosivity category), the property class, the mating parts and the tolerance budget of the thread. The table gives typical values; the standards referenced fix the requirements.
| Finish (common codes) | Typical thickness | NSS to white / red rust (ISO 9227, typical) | Property class limit | Thread effect | Notes |
|---|---|---|---|---|---|
| Plain (untreated, may be lightly oiled) | — | hours | any | none | Indoor dry service or where the joint is painted after assembly |
| Zinc electroplated, Cr(III) clear/blue passivated (ZPL, Fe/Zn 5–8 µm, ISO 4042) | 5 – 8 µm | 24–72 h / 96–150 h | ≤ 10.9 with baking; avoid 12.9 | 6g bolt before plating; nut 6H | Standard industrial finish; decorative; C2–C3 indoors |
| Zinc electroplated, Cr(III) thick-film / yellow passivated (YZPL) | 8 – 12 µm | 96–150 h / 200–300 h | ≤ 10.9 with baking | as above | Higher salt-spray life than blue; colour is not a guarantee of Cr(VI)-free unless stated |
| Zinc electroplated + sealer / top coat | 8 – 12 µm + seal | 150–250 h / 300–500 h | ≤ 10.9 with baking | as above | Sealer also fixes friction (µ 0.12–0.14 typical) |
| Zinc-nickel (Zn-Ni 12–15 % Ni, 8 µm, ISO 19598) | 8 – 12 µm | 150–300 h / 720–1000 h | ≤ 10.9 (low embrittlement risk with control) | as above | Automotive and off-highway standard; good at 150–200 °C; silver or black |
| Zinc flake, non-electrolytic (ISO 10683; Geomet 500, Delta-Protekt KL 100, Dacromet-type) | 8 – 12 µm | 240–480 h / 600–1000 h | all classes incl. 10.9 and 12.9 (no hydrogen) | thin — fits 6g/6H | Default for 10.9/12.9 outdoor; friction controlled by top coat (e.g. Geomet 500 A = µ 0.12–0.18 range with lubricant) |
| Hot-dip galvanised (HDG, ISO 10684; tZn) | ≥ 40 – 50 µm (≥ 45 µm local typ.) | not NSS-rated; 1–2 µm/yr loss in C3, 4–8 in C5 | ≤ 8.8 routinely; 10.9 only with qualified process | bolt normal; nut tapped oversize 6AZ/6AX | Structural steel, outdoor, marine deck; rough surface; ISO 1461 for the structure |
| Zinc phosphate + oil (PHM) | 2 – 8 µm | 24–72 h (oil-dependent) | all | none | Corrosion protection in transit and storage; paint base; good lubricity |
| Manganese phosphate | 5 – 15 µm | 48–96 h with oil | all | none | Wear-resistant, anti-galling for sliding parts and set screws |
| Black oxide (BLACK OXIDE) + oil | < 1 µm | < 24 h | all (no hydrogen) | none | Cosmetic / indoor; socket screws 12.9 are usually black oxide |
| Nickel plated | 5 – 10 µm | 48–150 h | ≤ 10.9 with baking | as plating | Decorative, brass and copper parts |
| Stainless A2 (EN 1.4301/1.4307) | n/a | no red rust; tea staining possible in C4+ | A2-70 ≤ M20 | none | Atmospheric, food, water; not for chloride-rich or splash zones |
| Stainless A4 (EN 1.4401/1.4404) | n/a | resists chlorides; pitting possible in stagnant seawater | A4-70 / A4-80 ≤ M20 | none | Marine, chemical, coastal; isolate from carbon steel and aluminium |
Electroplating and acid pickling charge steel with hydrogen. In fasteners harder than about 320 HV (class 10.9 and above, spring steel, case-hardened screws) this can cause delayed brittle fracture under static load, days after assembly. ISO 4042 requires the plater to assess the risk and, where needed, bake the parts (typically 190–230 °C for 4–24 h, started within 4 h of plating) and to run process verification per ISO 15330 (sustained-load test). Zinc flake and mechanical plating avoid the problem because no hydrogen is generated — that is why 10.9 and 12.9 outdoor fasteners are normally zinc flake. Hot-dip galvanising at ~450 °C can temper 10.9 steel; ISO 10684 allows it only with process controls and excludes 12.9.
A coating adds to the pitch diameter four times its thickness (it sits on both flanks). Electroplated bolts are therefore made to 6g and plated to a thickness that keeps them inside 6h; nuts stay 6H. Hot-dip zinc is too thick for that: ISO 10684 keeps the bolt thread normal (6g before dip) and requires the nut to be tapped oversize after galvanising — tolerance class 6AZ (nut over-tapped by 0.3–0.5 mm on the pitch diameter, standard) or 6AX (larger oversize for plating the nut too). An HDG bolt with an ordinary 6H nut will seize; an oversize nut on an ordinary bolt loses about 15 % load capacity and must be class-checked.
The finish sets the friction coefficient the torque table assumes. Plain and black-oxide steel with light oil: µ ≈ 0.14–0.20; zinc plated (dry): 0.12–0.18; zinc flake with integrated lubricant (Geomet 500 A/B, Delta-Protekt KL 100 with VH 301 top coat): 0.09–0.15 and tightly controlled; hot-dip galvanised: 0.16–0.25 and scattered — HDG joints are normally tightened by turn-of-nut rather than torque. Zn-Ni without sealer runs high (0.20+). Always take the torque value from a table for the actual finish and lubrication state; the same bolt with a different finish can need 40 % more or less torque for the same preload.
Use the standard's designation so the plater and the buyer mean the same thing. Examples: "ISO 4042 – Fe//Zn8//An//T0" = 8 µm zinc, trivalent clear passivation, no top coat; "Fe//Zn8//Cn//T2" = 8 µm zinc, thick-film/iridescent passivation, sealed; "ISO 10683 – flZn nc 480 h" = zinc flake, no Cr(VI), 480 h neutral salt spray to red rust; "ISO 10684 tZn" = hot-dip galvanised with oversize-tapped nuts. Add the required salt-spray hours, whether the value is to white or red rust, and the friction window (e.g. µ_tot 0.09–0.14) if the joint is torque-controlled.
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