A2 vs A4 stainless — choosing fasteners for a yacht
How A2 (304) steel differs from A4 (316), where to use each on a yacht, and how to avoid galvanic and crevice corrosion.
Choosing the right stainless steel for fasteners is not cosmetics but a matter of safety and durability. The most common bolt markings are A2 and A4 to ISO 3506. A2 corresponds roughly to grade AISI 304, and A4 to AISI 316 with an addition of molybdenum that radically improves resistance to chlorides. The difference in price is small, but in a marine environment it is enormous.
A2 (304) steel works inside the yacht and in places sheltered from constant contact with salt water and spray. It does well in fresh water and dry applications, but in a salt environment it gradually picks up rust staining and can corrode in crevices. It is a sensible, cheaper choice for furniture, linings and hardware below deck, where there is no constant damp. Outside, in the spray zone, it is better avoided.
A4 (316) steel is the standard for everything that works on deck and under water: guard rails, fittings, through-bolts, hardware mountings, hose clamps and parts in the bilge. The molybdenum addition improves resistance to pitting and crevice corrosion caused by chlorides, so A4 keeps its looks and strength far longer in salt water. In practice, if in doubt, choose A4 for a seagoing yacht — the small extra cost pays for itself in durability.
Remember that even stainless steel is not indestructible. Its resistance relies on a thin passive layer of chromium oxide, which needs access to oxygen to rebuild itself. In crevices, under deposits and in permanently flooded, oxygen-free places passivation fails and crevice corrosion appears — hence the rust streaks around bolts and under washers. That is why clean assembly, good sealing and avoiding standing water matter.
A separate hazard is galvanic corrosion, when different metals meet in the presence of an electrolyte (salt water). Stainless steel paired with aluminium makes the aluminium corrode as the less noble metal — a typical problem when mounting stainless hardware on a mast or aluminium parts. To prevent it, use insulating spacers, pastes (such as Tef-Gel or lanolin) and avoid direct contact between metals far apart in potential. It is worth checking material pairs against a galvanic series table.
When choosing fasteners, mechanics matter too, not only corrosion. Austenitic stainless steels A2 and A4 have moderate strength compared with carbon steel classes and come in grades such as 70 or 80 — for loaded structural joints choose the right strength class. Stainless steel is also prone to thread galling, especially a nut on a bolt of the same grade. Lubricate the threads with anti-seize paste and tighten slowly, so the thread does not weld itself dry.
Finally, a few practical rules. Do not mix bolts of unknown origin in one joint — buy sets marked A2 or A4. After fitting on deck it is worth wiping the fasteners and fittings, and periodically passivating or cleaning off staining to restore the protective layer. Keep a simple record of where you used A4 and where A2 — at the next repair it will stop you putting the poorer grade into a critical place.
Material prices — products and options
Indicative prices in EUR (incl. VAT), as of July 2026. Products widely available in Europe; prices depend on the shop, the pack size and consumption. The links lead to example shops that ship across the EU. Dollar amounts are converted at 1 EUR = 1.1622 USD (ECB, 7 September 2026) and rounded — treat them as a rough guide, not a price in a US shop. Imperial values in brackets are conversions of the metric figure.
A4 (AISI 316) fasteners — deck and underwater
- A4 DIN 933 hex bolts (per piece)
acid-resistant A4/316 for loaded joints, M6–M20€0.30–4 (≈$0.35–5) / piece - A4/316 nuts and washers (set)
nuts, flat and spring washers in 316€8–25 (≈$9–29) - Osculati — eye bolt DIN 580 A4 M6
marine deck fitting, premium 316€2–6 (≈$2–7) / piece - A4/316 workshop bolt set
assorted sizes in an organiser€25–60 (≈$29–70)
A2 (AISI 304) fasteners — interior and dry places
- A2 DIN 933 bolts (per piece)
A2/304 for interiors and sheltered spots, cheaper€0.10–2 (≈$0.12–2) / piece - A2 bolt/screw set (workshop)
M3–M10 set, A2/AISI 304€12–30 (≈$14–35)
Assembly pastes and greases
- Tef-Gel — paste for steel-aluminium pairs
protects against galvanic corrosion and seizing€25–60 (≈$29–70) - Anti-seize compound
prevents stainless threads from galling€8–20 (≈$9–23) - Lanolin spray or paste
budget protection for threads and contacts€8–18 (≈$9–21)
Isolating galvanic metals
- Insulating nylon spacers / bushes
separating steel from aluminium in a joint€6–18 (≈$7–21) - Duralac — jointing compound
chromate barrier paste against contact corrosion€12–25 (≈$14–29)
Cleaning and passivation
- Cleaner / passivator for stainless steel
removes surface rust staining, restores the passive layer€12–28 (≈$14–33) - Yachticon / Star brite — stainless steel paste
polishing and protecting stainless fittings€10–22 (≈$12–26)
Tools
- Torque wrench
tightening joints to the correct torque€30–90 (≈$35–105) - Combination spanner set
a basic set for fitting fasteners€20–60 (≈$23–70)