Marine fasteners – bolts, screws, studs and nuts used on boats, ships, docks and offshore installations – must withstand very aggressive environments. Seawater and salt spray contain chlorides that attack most metals, so material choice is critical. In this whitepaper we compare common marine fastener materials (316 and 304 stainless steel, silicon bronze, Monel nickel–copper alloy, titanium, and coated carbon steel) in terms of corrosion resistance (including galvanic compatibility), mechanical strength, availability and cost. We focus on corrosion-resistant fasteners for saltwater service and highlight when each material is preferred. Examples of use – hull fittings, engine components, underwater fasteners and deck hardware – are discussed along with performance in saltwater vs brackish water and under varied temperature or pressure. Readers will understand the trade-offs and maintenance needs of each option to select the right marine-grade bolts and screws for their application.

Stainless Steel Fasteners: 316 vs 304

316 Stainless Steel (AISI 316, A4)

Austenitic 316 stainless steel (also called A4 or UNS S31600) contains chromium, nickel and molybdenum (Mo) for added corrosion resistance. It is the most widely used marine-grade stainless steel for fasteners. 316 SS forms a thin oxide layer that protects it in oxidizing environments. It resists rust and pitting much better than ordinary steel or 304 SS in saltwater. Typical properties (cold-worked, annealed) include tensile strength around 75–100 ksi (520–690 MPa) and yield strength around 30–45 ksi (205–310 MPa). Mechanical strength: 316SS is strong enough for most fasteners, although specialty steels can exceed this.

Corrosion resistance: 316SS performs very well in aerated seawater and atmospheric salt spray. The molybdenum helps resist pitting from chlorides. In oxygenated conditions (decks, wells) 316 remains passive. However, 316 stainless can suffer crevice corrosion or pitting if oxygen is excluded (e.g. under nuts, in wood joints, or in stagnant bilge water). In low-oxygen or brackish conditions its passive film can break down, so it is not invulnerable. It also can undergo stress-corrosion cracking if heavily cold-worked in chloride environments.

Galvanic compatibility: In the galvanic series of seawater, 316SS is relatively noble (cathodic) compared to carbon steel and aluminum. This means that when a 316 stainless fastener contacts a steel or aluminum part, the steel or aluminum will tend to corrode preferentially. For a steel hull, 316 bolts actually stay intact while the steel hull is sacrificial (often aided by zinc anodes). But if a stainless fastener is paired with a copper alloy (bronze) or some carbon steels, the less noble metal will corrode. Use of isolating washers or compatible alloys is recommended to prevent dissimilar-metal attack.

Availability and cost: 316SS fasteners are widely stocked in every size and form (bolts, screws, nuts) by marine and industrial suppliers. They cost more than common steel or 304SS (often 2–3× the price of 304), but far less than exotic alloys. 316 fasteners are considered a standard “low cost” marine solution, especially for deck hardware and engine-room fittings.

Applications: 316SS is the go-to material for most boat and ship hardware above the waterline. Typical uses include deck cleats, handrails, stanchions, and through-deck fittings. In steel hulls or frames, 316 bolts are used because the steel body provides cathodic protection. It is also used for marine-engine components in contact with cooling water (pumps, manifolds) and for small underwater parts where oxygen is replenished (e.g. mooring cleats on a pier).

Pros:
– Excellent general corrosion resistance in saltwater (better than 304 or plain steel).
– Good mechanical strength and toughness.
– Extremely common and available in many shapes/sizes.
– Lower cost than Monel or titanium.

Cons:
– Susceptible to crevice and pitting corrosion where oxygen is limited (e.g. under head of bolt in wood or concrete, buried bolts).
– Galvanic issues: dissimilar contacts can corrode the mating part.
– More expensive than ordinary steel; not as corrosion-proof as bronze or titanium.
– Threads can gall when dry – lubrication often required for fasteners.

304 Stainless Steel (AISI 304, A2)

Austenitic 304 stainless steel is an 18-8 alloy (18% Cr, 8% Ni) with no molybdenum. It is the most common grade of stainless (often called A2 or UNS S30400) and is very cheap and readily available. However, 304SS has only moderate corrosion resistance in marine conditions. In fresh or mildly chlorinated water it resists rust, but in saltwater or constant splash it will quickly show corrosion (rust stains and pitting).

Strength: 304SS has similar mechanical properties to 316SS (UTS ~65–85 ksi, yield ~30–40 ksi), so its strength is comparable.

Corrosion resistance: Poor for true marine use. 304’s lack of molybdenum makes it vulnerable to chloride attack. In full-strength seawater or salt spray, 304 stainless will pit, stain and eventually fail. It is sometimes used above the waterline in brackish or fresh environments (for example on a freshwater lakeboat or for electrical conduit), but it cannot be relied on in ocean or polluted harbor settings. Basically, treat 304SS as not corrosion-resistant in saltwater – it will eventually rust.

Galvanic compatibility: 304SS is slightly less noble than 316 but still more noble than carbon steel or copper alloys. When paired with steel, the steel tends to corrode. If 304 fasteners are used with bronze or copper, the bronze can corrode. In any case, 304 should not be mixed with more anodic metals without isolation.

Availability and cost: 304SS fasteners are ubiquitous and cheap (cheaper than 316 by a fair margin). Many hardware stores stock 304 or “A2” hardware. Marine supply houses often warn that their cheapest screws may be 304, not 316, so care is needed if true marine grade is required. Because of its low cost, 304SS is sometimes used in non-critical or short-term marine applications.

Applications: Use 304SS only in fresh or mildly corrosive conditions. It may be found on interior boat fittings, freshwater locks, or coastal structures where it is painted or not exposed to sea. It should never be used for underwater or open deck marine hardware in saltwater. Typical use cases are limited to interior fixtures, handrails in protected areas, or any application where exposure to seawater is minimal.

Pros:
– Very affordable and easy to obtain.
– Good mechanical properties for non-critical applications.
– Easy to form and weld.

Cons:
– Poor corrosion resistance in saltwater (will rust).
– Not truly “marine grade.”
– Stainless 304 can fail in about the same time an ordinary steel bolt would in salt spray.
– Galvanic issues similar to 316.

Copper Alloys: Silicon Bronze

Silicon bronze (also called marine bronze or C65500/C95400 alloy) is a copper–silicon–manganese alloy traditionally used in boatbuilding. Its composition (roughly 96–98% copper, 3% silicon, 0.5–1.3% manganese, small iron) gives it outstanding resistance to seawater corrosion. Silicon bronze fasteners are almost impervious to the pitting and crevice attack that trouble stainless steels. When exposed to seawater they develop a protective patina (copper oxide) that actually improves corrosion protection over time.

Strength: Silicon bronze has high strength for a copper alloy. Typical drawn or forged silicon bronze has tensile strength around 70–95 ksi (480–655 MPa) and yield strength around 30–70 ksi (205–483 MPa), depending on temper. This is comparable to common steel fasteners and higher than many brasses. The alloy is also tough and fatigue-resistant. Its hardness (generally 65–115 Brinell) allows it to perform under heavy load without deforming.

Corrosion resistance: Silicon bronze is one of the most corrosion-resistant fastener metals in seawater. It is not easily attacked by chlorides or marine biofouling. Because it does not rely on an oxygen-passive film, it resists localized corrosion in crevices and in stagnant conditions (e.g. wood-to-metal contact or mud lines) far better than stainless steel. The protective patina that forms is conductive and stable. Silicon bronze will not gall (freeze) against mating surfaces. It also resists corrosion in fresh and brackish water, and even in polluted or briny environments where stainless steel would pit.

Galvanic compatibility: As a copper-based alloy, silicon bronze is cathodic relative to steel and aluminum. In a galvanic couple with carbon steel, the steel will corrode, leaving the bronze intact. This makes bronze a good choice for fastening steel hull plates or wood to steel. However, if bronze fasteners contact stainless steel, the bronze is less noble and may corrode (though less likely than stainless attacking aluminum). In general, avoid mixing bronze fasteners with aluminum parts. Use insulating washers or compatible alloys if dissimilar metals must touch.

Availability and cost: Silicon bronze fasteners are available through marine suppliers and some industrial distributors, but they are less common than stainless hardware. Standard hex bolts, machine screws and wood screws come in Si-bronze grades (often listed as UNS C65500 or C95400). They cost significantly more than stainless – typically 2–4× the price of 316SS, because copper is expensive. However, for critical marine use (wooden boats, heritage vessels, or long-term underwater service) the investment is justified by vastly lower maintenance.

Applications: Silicon bronze is favored in wooden boat construction and classic yacht rigging. Common uses include planking screws below the waterline, chainplates and chainplates bolts, mast base bolts, and bronze thru-hull fittings. Its gold color is also aesthetically prized in luxury fittings. Because of its fatigue and corrosion resistance, silicon bronze is chosen for underwater shackle pins, electrical connectors (marine wiring), and in any design where ultimate seawater durability is needed without sacrificial anodes. It is excellent for freshwater applications too, and is often used in cabinetry and ornamental work for its warm hue.

Pros:
– Exceptional resistance to seawater corrosion (top of the list for marine alloys).
– Performs well even in crevices and wood joints (no crevice corrosion issues).
– Good strength and ductility; slow, visible deformation before failure (warning before breakage).
– Attractive appearance and patina, popular in historic restorations.
– Cathodic to steel, providing galvanic protection to iron parts.

Cons:
– Higher cost and limited availability; not stocked by general suppliers.
– Heavier density (8.5 g/cm³) than steel (7.85 g/cm³).
– Lower strength than high-grade steels – bolts may need larger size for same load.
– Hard on drilling equipment (more abrasive than brass).
– Can still suffer dezincification if used near aggressive acids.

Nickel–Copper Alloys: Monel (Alloy 400)

Monel Alloy 400 is a nickel–copper alloy (UNS N04400) containing about 63–70% nickel and 28–34% copper (plus small iron, Mn, Si). It is renowned for excellent corrosion resistance in seawater and many harsh chemicals. Monel is more noble than bronze and stainless steel, giving it outstanding performance in marine environments.

Strength: Monel 400 in the annealed condition has a tensile strength of about 75–85 ksi (520–585 MPa) and yield strength around 25–45 ksi (170–310 MPa). It can be cold-worked (hard-rolled or drawn) to much higher strengths (up to 100–110 ksi) when needed. Its fracture toughness and fatigue strength are very high, and it remains ductile at low temperatures. Overall, Monel’s strength is comparable to 316SS and can exceed it when work-hardened.

Corrosion resistance: Monel is extremely resistant to chloride stress corrosion cracking and general corrosion in seawater. It will not pit in ordinary sea service and resists corrosion in a wide pH range. It is also highly resistant to acids (such as HCl, H2SO4) and alkalis, making it suitable for chemical plants and marine engines. In saltwater contact, Monel may actually form a thin oxide with a slight brown tint, but it protects the metal. For temperatures up to a few hundred °C, its corrosion resistance remains excellent.

Galvanic compatibility: Monel is very noble; it sits above bronze in the galvanic series. When Monel fasteners contact steel, the steel becomes the anode and will corrode before the Monel. This is acceptable in steel structures. Monel is compatible with copper-nickel piping and is often used as fasteners in copper-nickel heat exchangers. It should still be isolated from aluminum, which is far less noble. In general, Monel fasteners have low risk of self-corrosion, but they could accelerate corrosion of dissimilar less-noble metals if no isolation is used.

Availability and cost: Monel fasteners are specialty items. They can be ordered from industrial distributors and specialty suppliers, though not as widely stocked as stainless or bronze. The material cost is high – often several times that of stainless steel – and machining is slower. Monel is non-magnetic and can be difficult to find in all hardware grades (typically available in hex bolts, nuts, and some screws). Expect premium pricing and possible long lead times.

Applications: Monel is used where 316SS might corrode or where chemical exposure is severe. Examples include saltwater pump and valve components (impellers, shafts), marine heat exchangers, and high-pressure sea valves. It is also used for propeller shafts and rudders in some applications. In oil and gas, Monel fasteners hold seawater coolers, piping and pressure vessel parts. Because Monel is non-sparking, it is used in flammable or explosive marine environments as well.

Pros:
– Superior corrosion resistance to seawater (better than 316SS, bronze, etc.).
– Resistant to acids and alkalis; very versatile.
– Good mechanical properties and toughness; can be work-hardened for extra strength.
– Non-sparking (good for hazardous environments).

Cons:
– Very high cost and limited supply.
– Difficult to machine and fabricate (Ni alloys are tough).
– Can gall or seize when threaded; requires lubrication or special anti-seize coating.
– Unnecessary for many applications where 316 is sufficient (overkill).
– Denser than steel (8.8 g/cm³), adding weight.

Titanium Alloys

Titanium alloys (commercially pure Grade 2 or alloyed Grade 5, Ti-6Al-4V) are among the most corrosion-resistant materials available. Titanium forms an extremely stable, adherent oxide film (TiO₂) that protects it in virtually all environments, including seawater, brackish water, and chemical exposure. Titanium is essentially immune to corrosion in marine environments – it does not pit or crack in chloride conditions.

Strength: Titanium has an exceptional strength-to-weight ratio. Grade 2 (unalloyed CP-Ti) has a tensile strength around 50–60 ksi (345–415 MPa) and yield ~35–45 ksi (240–310 MPa), with very low density (~4.5 g/cm³). Ti-6Al-4V (Grade 5) is much stronger: annealed it has UTS ~150–170 ksi (1000–1170 MPa) and yield ~135 ksi (930 MPa). Even at elevated temperatures (200–400 °C), titanium retains much of its strength. Thus, a Grade-5 titanium bolt can match or exceed high-strength steel in load capacity while weighing about half as much.

Corrosion resistance: Titanium’s corrosion immunity in seawater is unmatched. It resists chloride pitting and crevice corrosion even under very aggressive conditions (high-pressure chlorides, hot seawater). Unlike stainless steel, titanium does not rely on oxygen to re-passivate and is unaffected by water chemistry or temperature (below red heat). In low-oxygen and high-pressure deepwater, titanium remains passive. Only certain very strong oxidizers (fluoride solutions, concentrated nitric acid with fluorides) can attack titanium. In any practical marine setting, titanium fasteners will outlast the vessel.

Galvanic compatibility: Titanium is very noble in seawater, more so than steel, bronze, and stainless. If titanium fasteners contact steel or aluminum, those metals will corrode first. In steel structures, this means the steel hull could sacrifice itself to protect titanium bolts (acceptable if planned with anodes). Because titanium is so noble, it should be isolated from other high-nobility metals (e.g. copper alloys) as well. In practice, insulating washers or PTFE sleeves are often used under titanium bolt heads to prevent contact with less-noble parts.

Availability and cost: Titanium fasteners are specialty and expensive. Grade 2 titanium bolts and screws can be sourced, and Grade 5 is used for critical fasteners. However, cost is 5–10× that of stainless steel equivalents. Only select suppliers stock titanium hardware, and minimum order quantities may apply. Titanium’s cost and difficulty of manufacture (including galling propensity) keep its use limited to high-performance or high-value applications.

Applications: When ultimate corrosion resistance and light weight are needed, titanium fasteners are chosen. Examples include oceanographic instrumentation, submersible vehicles, underwater robotics, and naval ship fittings. Titanium is used in seawater-cooled heat exchangers, deep-sea sensor mounts, and anywhere replacement is extremely difficult. It is also valued in high-end yachts for rigging and deck gear. In industry, titanium fasteners hold together seawater piping, valves, and marine heat exchanger packs that face severe service.

Pros:
– Outstanding corrosion resistance – essentially immune in saltwater.
– Extremely high strength-to-weight ratio (light and strong).
– Non-magnetic and biocompatible (safe for sensitive equipment and divers).
– Very high fatigue strength and excellent performance over temperature (–250 to 400 °C).

Cons:
– Very high material and fabrication cost.
– Susceptible to galling in threads – requires lubrication or specialized coating.
– Can suffer hydrogen embrittlement under certain cathodic potentials or hot hydrogen gas.
– Requires careful galvanic isolation (will corrode adjacent steel if not isolated).
– Less common; metric/imperial sizes limited.

Coated and Galvanized Carbon Steel

Galvanized Steel Fasteners

Galvanized steel fasteners are carbon steel bolts or screws with a protective zinc coating (usually hot-dip galvanized). The zinc acts as a sacrificial anode, corroding in place of the steel. Galvanizing is cheap and provides a measure of rust resistance, but its performance in marine environments is modest.

Corrosion resistance: A galvanized fastener resists rust longer than bare steel, especially in mild outdoor conditions. However, in saltwater exposure the zinc coating erodes relatively quickly. A hot-dip galvanizing layer can last months to a couple of years in a marine atmosphere or occasional immersion, but underwater in seawater the zinc dissolves in days to weeks. Once the zinc layer is consumed or damaged (scratches), the exposed steel rapidly rusts. Galvanizing is best for atmospheric or brackish exposure where direct seawater immersion is minimal.

Strength: The steel core provides the strength. Galvanized fasteners made from Grade 2–8 carbon steel have tensile strengths similar to their uncoated grades (up to ~150 ksi for high-grade bolts). The zinc layer does not significantly affect mechanical properties, aside from making threads slightly thicker.

Galvanic compatibility: Zinc is very anodic relative to most metals (including steel, stainless, copper and aluminum). This means a galvanized bolt will sacrifice itself to protect steel. If a galvanized fastener is used on a steel structure, the zinc will corrode first, protecting the steel. However, in contact with stainless steel or copper alloys, the zinc quickly dissolves and can cause rapid corrosion. Galvanized coatings are usually not recommended in assemblies with non-ferrous metals unless separated. In general, avoid coupling galvanized steel with aluminum in seawater (both metals corrode – steel/bz).

Availability and cost: Galvanized bolts and screws are among the cheapest corrosion-resisting fasteners. They are widely available for structural and general-purpose use (carriage bolts, eye bolts, lag screws, washers). Hot-dip galvanizing is common and there are ASTM specs (F2329) for galvanizing fasteners. Galvanized hardware is ubiquitous in construction stores and industrial suppliers. The low cost makes it attractive, but it is a short-term fix in marine service.

Applications: Galvanized fasteners are used where cost is critical and long-term exposure is limited. Examples: temporary dock structures, fender systems, tie-downs and lumber connections on piers. They are also used for chain links and anchor chain (e.g. galvanized rigging cable) even though lifetime is limited, because replacement is expected. Note that most serious marine engineers would not specify galvanized fasteners for critical components on a vessel due to the rapid degradation.

Pros:
– Very low cost and readily available.
– Sacrificial protection for steel in mild exposures.
– Uses standard steel strengths and hardware forms.

Cons:
– Poor long-term performance in saltwater (zinc dissolves).
– Requires maintenance (re-galvanizing or repainting).
– Heavy and magnetic.
– Coating thickness is limited – even “hot-dip” may only provide 10–50 µm.
– Not truly “corrosion-resistant” under continuous marine service.

Other Coated Carbon Steel

Beyond galvanizing, carbon steel fasteners can be given various protective coatings or platings:

• Zinc/Nickel or Zinc/Aluminum Plating: Thicker electroplated coatings (e.g. Zn-Ni) last longer than ordinary zinc plating. Some coatings meet several-hundred-hour salt-spray tests. However, these are still generally insufficient for seawater immersion.
• Cadmium Plating: Cadmium used to be popular for naval fasteners because its thin plating resists seawater (it is very anodic and low solubility). Today cadmium is banned for most uses (toxic), so plating is rare.
• Polymer/Epoxy Coatings: Fasteners can be dip-coated in resin or paint (yellow chromate, epoxy, PTFE coatings). These provide some barrier to corrosion, but any scratch or chip leads to catastrophic rust at the defect. Under constant splash or immersion, coated steel still corrodes once penetrated.

Applications: Coated or plated steel is used where a small improvement over bare steel is needed on a budget. Example: brackish-water docks where full stainless isn’t justified, or interior marine applications (electrical rooms) where aesthetics benefit from a coating. Fasteners in sprayed cooling water (non-potable) systems often use zinc-nickel plating. In summary, coated steel is a stop-gap measure – not truly seawater-grade.

Pros:
– Cheaper than stainless or exotic alloys.
– Can delay rust on mild steel for some time.
– Can still use high-strength carbon steel base material.

Cons:
– Not maintenance-free – coatings degrade.
– Potential galvanic issues similar to galvanizing.
– Under constant wet conditions, eventually fails.

Environmental Conditions and Galvanic Considerations

Saltwater vs Brackish vs Fresh Water

• Saltwater: The most aggressive environment. High chloride content attacks alloys relentlessly. Top performers: Titanium and Monel (virtually immune), silicon bronze (excellent), 316SS (good in oxygenated zones). Marginal: 304SS and coated steel (rust quickly), galvanized (zinc dissolves in weeks).
• Brackish water (estuarine): Chloride levels are lower, so 316SS and even 304SS can last far longer. Galvanized or zinc-plated steel may hold up for months. Bronze and titanium remain safe choices.
• Freshwater: Least aggressive. Even 304SS can be serviceable (though for safety most still use 316 or bronze). Galvanized and coated steel will last a long time in freshwater (often decades if painted), making them acceptable in non-critical parts on lakes and rivers.

Temperature and Pressure Effects

• Temperature: Elevated temperature accelerates corrosion. Alloys rate themselves by max safe use temperature: 316SS holds up to ~500 °C, Monel ~550 °C, bronze ~300 °C, titanium grades 2–5 up to 400–600 °C. For marine hardware (usually below 100 °C), all of the listed materials perform at maximum capability. However, in steam or engine exhaust, choose accordingly: stainless or Monel for hot valves, not bronze. In Arctic or sub-zero sea, alloys become more brittle. Titanium and 316 maintain toughness at low T (Monel too), whereas some steels lose some ductility in extreme cold.
• Pressure/Depth: Hydrostatic pressure itself doesn’t corrode, but deep submersion means low oxygen. Low-oxygen water can cause stainless steels (316) to break down in crevices. Bronze and Monel rely less on oxygen, so they do better in stagnant crevices or at depth. Titanium is unaffected by pressure or oxygen level. For deep-sea operations (submarines, ROVs) titanium or high-Ni alloys are often chosen.
• Cavitation and Flow: In fast-flowing seawater (propeller shafts, strainers), erosion-corrosion can occur. Alloys like 316SS and Monel resist this due to toughness, whereas soft metals (brass) can erode. Titanium is extremely good under cavitation because its oxide quickly reforms.

Galvanic Couples

When designing assemblies with dissimilar materials, consult the galvanic series for seawater. Rule of thumb: The more different the alloy potentials, the worse the galvanic attack on the less-noble (more anodic) metal. For example: titanium or Monel fasteners touching aluminum or zinc will rapidly corrode the aluminum/zinc. Stainless on bronze or aluminum will corrode the bronze/aluminum. Bronze on steel actually protects the steel.

Use insulating barriers (plastic or rubber washers), or sacrificial anodes, or choose metals close together on the galvanic chart to minimize current. The tables below summarize compatibility in marine conditions (where G = safe, Y = caution, R = avoid):

• Steel Hull: Steel boats can use stainless, Monel or bronze fasteners freely (the steel will protect them). Aluminum fasteners on steel hulls will corrode quickly (avoid).
• Wood/FRP Hull: In oxygen-poor wood or fiberglass, 316SS is only marginal. Bronze or Monel fasteners are generally satisfactory below water.

Material Comparisons and Trade-Offs

Each material brings its own balance of corrosion resistance, strength and cost:

• 316 Stainless Steel: Widely used for general marine hardware. Offers a good compromise of corrosion resistance and strength at moderate cost. Trade-off: Not indestructible in seawater – can corrode in crevices and must be used judiciously.
• 304 Stainless Steel: Affordable but not truly marine. Best only for light duty above water. Trade-off: Lower cost versus very limited corrosion performance.
• Silicon Bronze: Premium corrosion resistance in salt or fresh water, good strength. Trade-off: Higher weight and cost, fewer hardware options. Bronze fasteners excel in wood boats and historical vessels.
• Monel (Ni–Cu): Superior resistance in harsh seawater and acid, with reasonable strength. Trade-off: Very high cost and weight, limited availability. Use when nothing else will suffice (e.g. critical sea valves, acid tanks).
• Titanium: Ultimate corrosion resistance and strength-to-weight. Trade-off: Extremely high cost, prone to galling, specialized use only. Chosen when long service life and light weight justify expense (deep-sea, military, aerospace marine).
• Galvanized/Coated Steel: Lowest cost and good strength, but poorest corrosion performance. Trade-off: Cheap versus very limited life in saltwater. Suitable for non-critical or short-term uses where frequent replacement or maintenance is planned.

Cost vs Availability: In general, stainless steels and galvanized fasteners are inexpensive and easy to buy. Copper and nickel alloys are pricier and often made to order. Titanium is the most expensive and least common. Budget constraints often push designers toward 316SS or galvanized steel, but for long-term reliability in seawater, spending more on bronze/Monel can save on maintenance over the vessel’s life.

Use Case Examples

• Hull Fittings (chainplates, thru-hulls, cleats):
  • Fiberglass or wood hulls: Use silicon bronze or Monel bolts for below-water attachments (through-deck stanchions, chainplates). These resist crevice corrosion in wood or FRP. 316SS is acceptable above the waterline where oxygen is plentiful.
  • Steel hulls: 316SS fasteners are common (steel hull provides cathodic protection). 316 or Monel chainplates, bronze flange bolts and copper-nickel pipe connectors are also used. Avoid aluminum fasteners on steel – aluminum will oxidize rapidly (R rating).
  • Brass/bronze hulls: Use like-for-like bronze fasteners to avoid galvanic issues.
• Engine and Machinery:
  • Cooling Systems and Pumps: 316SS or bronze are often used in seawater pump housings and valves. 304SS is inadequate for sea strainer bolts. Monel is used in highly corrosive or high-temperature cooling circuits.
  • Exhaust Systems: Hot sections may use 316SS or higher alloys (some marine exhaust elbows use 16-8-2 SS or higher). Bronze will soften and deteriorate at exhaust heat. Titanium is rarely needed unless weight is critical.
  • Fasteners in Engine Compartment: 316SS bolts and studs for mounting (protected by engine room ventilation). For raw-water heat exchangers, Monel or copper-nickel tubing is common.
• Underwater and Diving Applications:
  • Submersibles/ROVs: Titanium or Monel fasteners are preferred for external hull closures and instrumentation. 316SS can be used on shallower vehicles or with sacrificial anodes.
  • Propulsion Shafts/Hardware: Prop shafts are often bronze or nickel-aluminum bronze, attached with bronze fasteners. 316SS shaft collars or keyways are common.
  • Diving Gear: Titanium is used for tank valves and regulators due to absolute corrosion immunity and low weight. Stainless fasteners may be used in enclosures but require regular inspection.
• Deck Hardware (cleats, rails, stanchions):
  • Typically made from 316 stainless for appearance and strength. Bronze is used on high-end or classic yachts. Galvanized steel hardware is rarely used on the open deck (will rust within years).
  • Rigging: Turnbuckles and chainplates often use 316SS rod fittings. Racing sailboats may specify rigging screws in titanium to save weight aloft.
  • Anchoring Gear: Anchor shackles and swivel hooks are often made of high-strength stainless (sometimes marked “U.S. Govt Grade A4”). Bronze or Monel hooks exist for specialized use.

Maintenance and Compatibility Notes

• Cleaning and Inspection: Stainless steel hardware should be kept clean and occasionally polished to remove salt deposits. Bronze naturally resists surface fouling but should be checked for electrochemical damage at dissimilar joints. Coated or painted steel bolts must be recoated if damaged.
• Thread Lubrication: Many marine fasteners (especially Monel, titanium, and stainless) tend to gall under high clamping loads. Use appropriate lubricants (bronze/silver anti-seize) on threads to ensure proper torque and allow disassembly. Lubricated galvanic coatings also improve torque consistency.
• Sacrificial Anodes: In steel and some aluminum assemblies, sacrificial zinc or aluminum anodes protect both the hull and steel fasteners. However, these can cause accelerated hydrogen embrittlement of high-strength steels if hydrogen evolves at protected surfaces. Careful selection (ferritic 410SS for high-strength) or using anodes only where needed is advised.
• Compatibility: Never mix aluminum fasteners with copper/nickel or stainless on boats. Use isolators (nylon washers, gaskets) when dissimilar metals contact. For example, use a plastic-lined bushing when a bronze bolt passes through an aluminum hull, to prevent bimetal corrosion.
• Replacements: In existing vessels, replace failed rusty hardware with higher-grade material if corrosion was an issue. Upgrading from 304SS to 316SS or bronze is a common refit.

Summary

No single material is best for every marine fastening situation. The choice hinges on balancing corrosion resistance, strength, availability and cost:

• For general marine use, 316 stainless steel is usually the default choice: widely available, moderate cost, and reasonably corrosion-resistant.
• For extreme environments or long life, copper or nickel alloys (silicon bronze, Monel) or even titanium are preferred despite higher cost.
• For budget-sensitive or temporary applications, galvanized or coated carbon steel may be acceptable in mild conditions, but they require frequent maintenance or replacement.
• Engineers must also consider galvanic effects: pairing metals close in nobility (e.g. 316SS with bronze or steel) reduces risk. In all cases, design details (sealants, insulation, anodes) complement the material choice.

By comparing the marine fastener materials outlined here, a marine engineer can specify the right marine-grade bolts and screws for each application, ensuring safety and durability in saltwater, brackish, or variable conditions.