The main finishes compared
| Finish | Standard | Typical thickness | Neutral salt spray (ISO 9227) | Hydrogen risk |
|---|---|---|---|---|
| Electroplated zinc (clear, blue, yellow, black) | ISO 4042 | 5–12 µm | about 96–200+ h, depends on passivate and sealer | Yes, for 10.9 and harder |
| Hot-dip galvanised | ISO 10684 | 40 µm minimum, often 50+ µm | 1,000 h and more | Very low |
| Zinc flake (Geomet, Delta-Protekt type) | ISO 10683 | about 5–20 µm | about 600–1,000+ h, with topcoat | None from the process |
| Phosphate and oil | ISO 9717 | thin | low, mainly for storage | Low, if not acid pickled |
| Black oxide | – | about 1 µm | very low, indoor only | None |
Salt spray hours are typical values to first red rust. Real life outdoors depends on the environment, and each specification sets its own requirement.
Electroplated zinc
Parts are cleaned, acid pickled and plated with zinc in an electrolytic bath, then given a passivate that sets the colour: clear or blue, yellow (iridescent) or black. Modern passivates are trivalent chromium (Cr III) and RoHS compliant; the old hexavalent yellow is being phased out. Plating is thin, so it keeps tight thread tolerances, and it is economical for indoor and mildly corrosive use.
Hot-dip galvanising
Parts are dipped in molten zinc at about 455–480 °C, then centrifuged (spun) to throw off excess zinc from the threads. The coating is thick and metallurgically bonded, so it is the standard choice for steel structures, towers and outdoor equipment. Two points matter:
- Thread fit. The zinc adds thickness to the bolt thread, so galvanised nuts are tapped oversize after galvanising to fit. A galvanised bolt will not take a plain nut, and a galvanised nut will be loose on a plain bolt. Always buy galvanised bolts and nuts as a set. ISO 10684 covers sizes M8 to M64.
- Bolt class. The zinc bath is hotter than the 425 °C minimum tempering temperature, so bolts made for galvanising should be tempered above the bath temperature, or the zinc can soften them. Hot-dip galvanising is generally used up to 10.9 and is not recommended for 12.9.
Zinc flake
A paint-like dispersion of zinc and aluminium flakes is applied by dip-spin (small parts in baskets) or spray, then cured in an oven at about 200–320 °C, depending on the product. Cleaning is by alkaline degreasing and shot blasting, not acid, so no hydrogen enters the steel. It gives high corrosion resistance in a thin layer that fits standard threads, which makes it the preferred finish for 10.9 and 12.9 bolts, especially in automotive and wind energy work.
Phosphate, black oxide and stainless passivation
- Zinc or manganese phosphate with oil gives a dark grey finish, good lubrication for consistent tightening and light rust protection. It is common on 12.9 socket screws and machine bolts.
- Black oxide is a conversion finish for appearance and minimal protection indoors, usually with oil.
- Passivation of stainless steel removes free iron from the surface with a nitric or citric acid treatment, so the natural chromium oxide layer can form evenly. It is not a coating and does not change size.
Hydrogen embrittlement: why it matters
Acid pickling and electroplating can put atomic hydrogen into the steel. In a hard, high-strength bolt under tension, that hydrogen can cause a delayed brittle fracture: the bolt is tightened correctly, then the head snaps off hours or days later. The risk rises steeply with hardness, which is why it is a concern for 10.9 and, above all, 12.9 bolts and case-hardened screws.
Two ways to manage it:
- Baking soon after plating, typically at around 200 °C for several hours, to drive the hydrogen out. ISO 4042 makes baking optional only where the process has been verified as safe; some specialists still prefer to bake.
- Choosing a finish without hydrogen, such as zinc flake or mechanical plating, for high-strength parts. This is the safer route.
Hydrogen can also enter in service from corrosion, which is another reason to avoid very high strength in wet or aggressive environments unless it is needed.
Finishes in Indian use
- Hot-dip galvanising to IS 1367 (Part 13) is standard for power transmission tower bolts (IS 12427), railway and bridge work, and outdoor structures. In coastal areas of Gujarat, Maharashtra and Tamil Nadu, galvanised or stainless A4 hardware lasts far longer than plated parts.
- Electroplating to IS 1367 (Part 11), which is identical to ISO 4042:2022, is mostly done by plating job-workers. Export and automotive buyers now ask for trivalent (Cr III) passivates; older yellow hexavalent finishes are still seen in the general market.
- Zinc flake lines are mainly found with automotive and wind-energy suppliers. It is the right finish for 10.9 and 12.9 parts needing corrosion protection.
- Phosphate finishes are covered by IS 1367 (Part 12). Black phosphate and oil is the usual finish on Allen bolts (12.9 socket cap screws).
If a high-strength bolt is to be electroplated by a job-worker, insist on baking after plating and ask for the plating lot record.
Figures are typical values for guidance. For design, inspection and purchase, always follow the current edition of the standard on your drawing.