Corrosion Resistant Fasteners
Corrosion Resistant Fasteners for Marine Environments
Marine environments are among the most demanding conditions a fastener can face. Saltwater, spray, humidity, and galvanic action combine to corrode standard fasteners within months. Choosing the wrong material or finish is not just a maintenance issue - in structural, offshore, or safety-critical applications it is a failure risk. Trojan Special Fasteners manufactures corrosion-resistant nuts and fasteners specifically for marine use, from M3 to M52 or 6BA to 2.1/4" in Imperial or Unified, to your drawing or specification.
| 316 Stainless as Standard |
| Phosphor Bronze PB102 |
| EN 10204 3.1 Material Certificates |
| Naval Brass CZ112 |
| ISO 9001:2015 Certified |
| No Minimum Order |
| Same-Day Quotations |
Why Standard Fasteners Fail in Marine Environments
Marine environments impose a combination of electrochemical, biological, and mechanical stresses that standard fasteners are not designed to withstand. Seawater is a near-perfect electrolyte: it contains approximately 35 grams of dissolved salts per litre, of which chloride ions are the most corrosively aggressive. Standard hardware begins to show visible pitting within 6 to 12 months in high-chloride coastal locations. There are six distinct failure mechanisms, and understanding all of them is essential for correct material selection.
| Chloride-Induced Pitting Corrosion | Stainless steels are protected by a thin chromium oxide passive film. Chloride ions attack this film at microscopic weak points, displacing oxygen and initiating pits that propagate rapidly inwards whilst the pit mouth remains small and difficult to detect visually. Grade 304 (A2) is highly susceptible because it lacks molybdenum: chloride ions break down its passive layer at concentrations commonly found in coastal air and salt spray. Grade 316 (A4) contains 2 to 3% molybdenum, which significantly stabilises the passive film in chloride environments. The resistance of a stainless steel to pitting is quantified by its Pitting Resistance Equivalent Number (PREN), calculated as: PREN = %Cr + (3.3 × %Mo) + (16 × %N). For reference: 304 has a PREN of approximately 19, 316 approximately 25, standard duplex 2205 approximately 35, and super duplex 2507 approximately 43. A PREN above 40 is generally specified for direct seawater immersion service. See the British Stainless Steel Association guidance on PREN for further detail. |
|---|---|
| Crevice Corrosion | Crevice corrosion occurs in geometrically confined spaces -- under washers, within threaded engagements, beneath nut faces, and inside connector nuts -- where seawater becomes stagnant and oxygen-depleted. The mechanism is electrochemical: the oxygen-deficient crevice becomes anodic relative to the surrounding aerated metal surface, and localised dissolution accelerates rapidly. Even Grade 316 stainless is susceptible to crevice corrosion in submerged and tidal applications, because its passive film requires oxygen to self-repair. This is why 316 performs well on aerated deck surfaces but can fail at fastener-to-fitting interfaces in continuously submerged conditions. Duplex stainless steel and copper alloys are significantly more resistant. As Fasten.one's technical analysis notes: 316 stainless "can suffer crevice corrosion or pitting if oxygen is excluded, e.g. under nuts, in wood joints, or in stagnant bilge water." |
| Galvanic Corrosion | When two dissimilar metals are in electrical contact in the presence of an electrolyte such as seawater, a galvanic cell is formed. The less noble (more anodic) metal corrodes preferentially whilst the more noble (cathodic) metal is protected. The rate of attack depends on the potential difference between the two metals and on the cathode-to-anode surface area ratio: a large cathodic surface paired with a small anodic fastener is the most aggressive configuration. The galvanic series in seawater places magnesium and zinc at the anodic (active) end and platinum and graphite at the cathodic (noble) end. Common problematic pairings include mild steel bolts in aluminium structures (large potential difference), copper alloy fittings with stainless fasteners (moderate, manageable), and stainless fasteners through aluminium plate (acceptable area ratio, lower risk than the inverse). A potential difference of 0.2 volts or more in the galvanic series is generally considered the threshold above which galvanic corrosion becomes a design concern. See the galvanic series table in seawater for corrosion potentials by material. |
| Stress Corrosion Cracking (SCC) | Stress corrosion cracking is a particularly dangerous failure mode because it can result in sudden brittle fracture with little or no visible prior warning. It occurs when a susceptible metal is under sustained tensile stress in a corrosive environment containing specific aggressive ions. For stainless steels, chloride-induced SCC (CISCC) is the primary concern: austenitic grades such as 304 and 316 are susceptible at elevated temperatures or under high stress in chloride-rich environments. Cold-worked or heavily prestressed stainless fasteners are at greater risk. Duplex stainless steels are substantially more resistant to SCC than austenitic grades because the ferritic phase disrupts crack propagation. Bronze alloys are effectively immune to chloride SCC, which is one reason they are preferred in high-stress or safety-critical marine fastener applications. The Fastener and Fixing Magazine technical article on marine stainless also notes that cathodic protection systems can cause hydrogen-induced stress corrosion cracking (HISC) in ferritic or martensitic stainless grades - a relevant consideration for fasteners on cathodically protected offshore structures. |
| Dezincification of Brass Alloys | Dezincification is a dealloying process specific to copper-zinc alloys in which zinc is selectively leached from the alloy matrix, leaving behind a porous, spongy copper-rich residue that has lost most of its mechanical strength. The mechanism is electrochemical: in the presence of seawater or brackish water, the zinc-rich beta phase is anodic relative to the copper-rich alpha phase and dissolves preferentially. Standard brass alloys (CZ121, CZ108) with zinc content above approximately 15% are highly susceptible. The addition of 1.0 to 1.5% tin, as in naval brass CZ112, markedly reduces the rate of dezincification. The Copper Development Association's Guidelines for the Use of Copper Alloys in Seawater confirm that "the addition of 1% tin to admiralty, naval brass, and manganese bronze reduces the tendency toward dezincification." A 2002 peer-reviewed study in the Journal of Alloys and Compounds (Becceria et al., ScienceDirect) further characterises the passivating mechanism of tin additions in alpha-beta brasses. This is precisely why CZ121 free-cutting brass should never be substituted for CZ112 naval brass in marine applications. |
| Microbiologically Influenced Corrosion (MIC) | MIC is an underappreciated failure mechanism that accounts for an estimated 20% of all corrosion failures globally, according to research published in International Biodeterioration and Biodegradation (2021). In marine environments, microbial biofilms form on metal surfaces within hours of immersion. Sulphate-reducing bacteria (SRB) in anaerobic conditions, and iron-oxidising bacteria (IOB) in aerobic conditions, create localised electrochemical cells that accelerate pitting and crevice corrosion significantly beyond what chemistry alone would predict. Biofilms also change the local chemistry beneath them: oxygen is consumed, pH drops, and aggressive metabolic by-products including hydrogen sulphide and organic acids concentrate at the metal surface. Even high-performance alloys are not fully immune: research published in PLOS ONE demonstrated MIC attack on 2707 hyper-duplex stainless steel by marine Pseudomonas aeruginosa biofilm. Selecting inherently corrosion-resistant base materials - 316 stainless, phosphor bronze, naval brass - reduces but does not entirely eliminate MIC risk in long-term submerged service. |
Recommended Materials for Marine Fasteners
Material selection is the single most important decision in specifying marine fasteners. The table below summarises the options, followed by full detail on each.
| MATERIAL | CORROSION RESISTANCE | BEST FOR | AVAILABLE FROM TROJAN |
|---|---|---|---|
| 316 Stainless (A4) | Excellent | Above-waterline, splash zone, coastal | Yes - Standard Range |
| Duplex Stainless | Superior | Offshore, subsea, high-stress | Not currently available |
| Phosphor Bronze (PB102) | Excellent | Electrical, plumbing, heritage vessels | Yes - Standard Range |
| Naval Brass CZ112 (CW712R) | Very good | Marine plumbing, valves, fittings, fasteners | Yes - Standard Range |
| Aluminium Bronze (CA104) | Superior | Propeller shafts, pumps, offshore | Yes - Standard Range |
| 304 Stainless (A2) | Moderate | Inland or freshwater only | Not recommended for marine |
| Mild Steel/Zinc Plate | Poor | Not suitable for marine use | Not suitable for marine use. |
| 303 Stainless (free-machining) | Poor to moderate | Interior, non-corrosive environments | Not suitable for marine use |
Standard Marine Grade
316 Stainless Steel (A4 / UNS S31600)
The standard and most widely specified material for marine fasteners above the waterline and in the splash zone. 316 is an austenitic chromium-nickel-molybdenum stainless steel. Its nominal composition is approximately 16 to 18% chromium, 10 to 14% nickel, and 2 to 3% molybdenum, with the balance iron. The molybdenum addition is the critical differentiator from 304: it stabilises the passive chromium oxide film in chloride environments, improving resistance to both pitting and crevice corrosion. The PREN of 316 is approximately 24 to 26, compared to approximately 18 to 20 for 304.
316 performs well in aerated seawater and coastal atmospheric conditions. It is suitable for deck hardware, rigging and stanchion fittings, keel bolts, engine room fixings, through-hull installations above the waterline, and structural fasteners in coastal construction. It is not, however, invulnerable: in low-oxygen submerged conditions, under washers, or in stagnant bilge water, the passive film can break down and crevice corrosion can initiate. For continuously submerged or offshore applications, duplex stainless or bronze should be evaluated.
316L (low carbon) is a variant with reduced carbon content (max 0.03%) that improves weld corrosion resistance by reducing sensitisation. For nut manufacture from bar stock, standard 316 is the correct and typical specification.
Trojan supplies 316 stainless steel nuts from M3 to M52 in metric and up to 2¼" in imperial and unified thread forms. Material certificates to EN 10204 3.1 are available with every order. 316 is Trojan's default stainless recommendation for all marine enquiries.
Offshore, Subsea and High-Stress
Duplex and Super Duplex Stainless Steel
Trojan does not currently machine duplex but provides this information for your reference.
Duplex stainless steels have a mixed austenitic-ferritic microstructure, typically approximately 50% of each phase. This dual microstructure delivers higher strength than standard austenitic grades (typically 450 to 550 MPa proof stress versus 200 MPa for 316) combined with superior resistance to both chloride pitting and stress corrosion cracking. The ferritic phase physically interrupts crack propagation, making duplex grades far more resistant to CISCC than austenitic 316.
Duplex stainless steels are divided into three performance bands by PREN:
| GRADE | COMMON DESIGNATION | PREN (APPROX) | TYPICAL APPLICATION |
|---|---|---|---|
| Standard Duplex | EN 1.4462/2205/UNS S31803 | ~35 | Offshore process piping, non-stagnant seawater cooling |
| Super Duplex | EN 1.4410/2507/UNS S32750 and Zeron 100 | ~43 | Direct seawater immersion, offshore structural |
| Hyper Duplex | EN 1.4658/2707/UNS S32707 | >45 | Subsea, aggressive offshore chemical environments |
Two points worth noting:
S31803 vs S32205 for duplex 2205. Both UNS numbers refer to the same grade. S31803 is the original designation with slightly broader composition limits. S32205 is a revised designation with tighter limits that better reflect current production practice. Both are in common use and either may appear on mill certificates. Listing both avoids ambiguity when buyers are cross-referencing documentation.
317L. This grade does not have a widely used BS product standard designation in the UK market and is more commonly encountered in North American specifications. It is included here for completeness but is rarely specified for UK marine fastener applications. You may wish to drop it from the table if you want to keep the focus on grades you are likely to be asked about.
A PREN above 40 is generally specified for direct seawater service in the oil and gas industry. Standard duplex 2205 (PREN ~35) is suitable for less aggressive marine duties. Super duplex 2507 meets or exceeds the PREN 40 threshold and is the standard specification for continuously submerged offshore fasteners. Reference: PREN calculator and grade comparison - WeldFabWorld; Duplex stainless steel - Wikipedia technical overview.
Trojan does not, at present, work with Duplex Stainless Steels
Electrical, Plumbing and Heritage Vessels
Phosphor Bronze (C51000 / CuSn5 / CW451K / PB102)
Phosphor bronze is a copper-tin-phosphorus alloy. The standard marine grade, C51000 (CuSn5), contains approximately 94 to 96% copper, 4 to 6% tin, and 0.01 to 0.35% phosphorus. The tin addition increases corrosion resistance and strength; the phosphorus deoxidises the melt and improves wear resistance by forming hard copper phosphide particles within the matrix.
Phosphor bronze develops a protective patina (copper oxide and tin oxide) on exposure to seawater that improves corrosion protection over time rather than degrading it. It is effectively immune to chloride-induced stress corrosion cracking, which is a significant advantage over stainless steel in high-stress or vibration-loaded fastener applications. It carries no risk of hydrogen embrittlement, making it suitable for electrochemically active environments. Its galvanic potential in seawater is close to that of other copper alloys, making it compatible with bronze, gunmetal, and naval brass fittings without significant galvanic risk.
| PROPERTY | C51000 VALUE |
|---|---|
| Copper (Cu) | 94% to 96% |
| Tin (Sn) | 4% to 6% |
| Phosphorus (P) | 0.01% to 0.35% |
| Tensile Strength | Up to 540 MPa (cold worked) |
| Machinability | Moderate (medium cutting performance) |
| SCC (Stress Corrosion Cracking) Resistance | Effectively immune to chloride SCC |
Common marine applications include electrical connectors and earthing straps (good electrical conductivity at approximately 15% IACS), marine plumbing fittings and valve components, hull penetration hardware on heritage and classic vessels, pump components, and fasteners in copper alloy substrate structures where galvanic compatibility is required. Reference: Wieland Concast C51000 technical data; Alloy 510 properties overview - MFG Shop.
Trojan machines phosphor bronze nuts to customer drawings as a standard material. EN 10204 3.1 material certificates available on request.
Plumbing, Valves and Marine Fasteners
Naval Brass CZ112 (CW712R)
Naval brass is an alpha-beta (duplex) copper-zinc alloy to which 1.0 to 1.5% tin has been added. The tin content is critical: it significantly improves resistance to dezincification and stress corrosion cracking in seawater and brackish water environments, which standard brass grades cannot withstand reliably. Trojan specifies CZ112 as its naval brass of choice.
Under the British Standard designation system, naval brass carries the CZ prefix (Copper-Zinc). CZ112 is the primary, nominally lead-free grade, with the European harmonised equivalent CW712R. The closely related CZ133 (CW719R) incorporates a controlled lead addition for improved machinability whilst retaining the same marine corrosion resistance.
Typical Nominal Composition - CZ112 / CW712R
| ELEMENT | CONTENT |
|---|---|
| Copper (Cu) | 61.0% to 63.0% |
| Tin (Sn) | 1.0% to 1.5% |
| Lead (Pb) | 0.2% to 0.6% (trace, for machinability) |
| Zinc (Zn) | Balance (approx. 36% to 38%) |
| PROPERTY | VALUE |
|---|---|
| Tensile Strength | 340 to 400 MPa |
| Proof Stress | 160 to 200 MPa |
| Machinability Rating | 60% to 70% relative to CZ121 (100% benchmark) |
Microstructure
CZ112 has a duplex alpha-beta microstructure. The alpha phase provides ductility and cold-working capacity. The beta phase enhances hardness, tensile strength, and hot-working properties, making it well suited to bar turning, hot forging, and pressing operations.
Relevant British and European Standards
| HISTORIC BS STANDARD | PRODUCT FROM | MODERN EN EQUIVALENT |
|---|---|---|
| BS 2874 | Bar, rod, and section | EN 12163 / EN 12167 |
| BS 2872 | Forgings and forging stock | EN 12165 |
| BS 2871 | Tubes and heat exchanger tubes | EN 12451 |
Trojan sources CZ112 naval brass bar as a standard stocked material for nut manufacture. Always confirm the specific alloy designation when ordering: standard free-cutting brass (CZ121) lacks the tin content required for marine use and performs significantly worse in saltwater and dezincification resistance. Material certificates to EN 10204 3.1 are available with every order.
High Performance Offshore and Propulsion
Aluminium Bronze (C95400 / CA104)
Aluminium bronzes are copper alloys containing 8 to 14% aluminium, often with additions of iron, nickel, and manganese. They combine superior corrosion resistance with high strength (tensile strength typically 600 to 700 MPa), excellent resistance to erosion and cavitation, and outstanding resistance to biofouling. The protective aluminium oxide film that forms on the surface is highly stable in seawater, making aluminium bronze one of the most corrosion-resistant copper alloys available.
In the galvanic series, aluminium bronze sits between the stainless steels and the lower copper alloys, making it compatible with bronze and brass fittings without aggressive galvanic action. It is immune to stress corrosion cracking in marine environments and has negligible susceptibility to dezincification (it contains no zinc).
Common marine applications include propeller shaft nuts and lock nuts, seawater pump impellers and housings, subsea valve components, offshore structural fasteners, and marine defence applications where high strength combined with corrosion resistance is required. This is a standard material Trojan manufactures from.
PREN Quick Reference and Material Selection Guide
The Pitting Resistance Equivalent Number (PREN) is the primary engineering metric for comparing the pitting corrosion resistance of stainless steels in chloride environments. It is calculated as: PREN = %Cr + (3.3 × %Mo) + (16 × %N). The higher the number, the more resistant the alloy. The table below provides typical PREN values and recommended service environments for the grades most relevant to marine fastener selection. For authoritative detail, see the British Stainless Steel Association PREN guidance.
| GRADE | BS DESIGNATION | EN DESIGNATION | UNS | PREN (APPROX) | MARINE SERVICE SUITABILITY |
|---|---|---|---|---|---|
| 303 | BS EN 10088-3: 1.4305 | EN 1.4305 | S30300 | 17 to 18 | Not suitable for any marine use. Sulphur additions for machinability create pitting initiation sites. For manufactured components, specify 316 instead. |
| 304 / A2 | BS EN 10088-3: 1.4301 | EN 1.4301 | S30400 | 18 to 20 | Coastal atmosphere only. Not suitable for salt spray or splash. |
| 316 / A4 | BS EN 10088-3: 1.4401 | EN 1.4401 | S31600 | 24 to 26 | Above-waterline, splash zone, deck hardware, coastal structural. |
| 317L | BS EN 10088-3: 1.4438 | EN 1.4438 | S31703 | ~30 | Improved pitting resistance. Considered where 316 has borderline performance. |
| Duplex 2205 | BS EN 10088-3: 1.4462 | EN 1.4462 | S31803 | ~35 | Offshore process piping, non-stagnant seawater cooling, structural. |
| Super Duplex 2507 | BS EN 10088-3: 1.4410 | EN 1.4410 | S32750 | ~43 | Direct seawater immersion, offshore structural, subsea. PREN >40 threshold met. |
| Super Duplex Zeron 100 | BS EN 10088-3: 1.4501 | EN 1.4501 | S32760 | 40 to 45 | Offshore, subsea, chemical process. Tungsten-adjusted PREN formula applies. |
| Hyper Duplex 2707 | No BS designation | EN 1.4658 | S32707 | above 48 | Subsea and aggressive offshore chemical environments. Outside EN ISO 3506. Supplied to project specification only. Not currently supplied by Trojan. |
Galvanic Compatibility in Marine Fastener Selection
Galvanic corrosion is one of the most common and most preventable causes of marine fastener failure. The table below shows approximate corrosion potentials in seawater for the materials most commonly encountered in marine fastener applications. The further apart two materials are in the series, the greater the galvanic risk when they are in contact. A potential difference of 0.2 volts or more is generally considered the practical threshold for design concern. Full galvanic series data and practical guidance on prevention is provided in Atlas Steels Technical Note No. 7: Galvanic Corrosion (PDF) - a widely cited engineering reference document - and discussed further on Corrosionpedia.
| MATERIAL | APPROX POTENTIAL Vs SCE (VOLTS) | POSITION | NOTES FOR FASTENER SELECTION |
|---|---|---|---|
| Magnesium alloys | -1.60 to -1.63 | Most anodic (active) | Corrodes rapidly when coupled with almost any other metal |
| Zinc | -0.98 to -1.03 | Anodic | Used as sacrificial anode. Standard zinc plate unsuitable as a fastener finish. |
| Mild steel (clean) | -0.60 to -0.71 | Anodic | Corrodes rapidly in seawater. Never specify for marine use. |
| Aluminium alloys | -0.76 to -1.00 | Anodic | Corrodes when coupled with stainless or copper alloys. Use compatible fasteners. |
| Naval Brass CZ112 | -0.30 to -0.40 | Intermediate | Compatible with phosphor bronze and other copper alloys. Good for copper alloy substrates. |
| Phosphor Bronze PB102 | -0.28 to -0.36 | Intermediate | Close to naval brass. Good galvanic compatibility with copper alloy fittings. |
| Aluminium Bronze CA104 | -0.16 to -0.27 | Intermediate-noble | Compatible with stainless and copper alloys. Low galvanic risk in most marine pairings. |
| 316 Stainless (passive) | -0.05 to +0.20 | Noble (passive state) | Noble relative to steel and aluminium. Stainless fasteners in aluminium: acceptable if area ratio favourable. |
| Titanium | -0.05 to +0.10 | Noble | Most noble common engineering metal. Very low galvanic risk with stainless. |
Key design rules:Always use a fastener material at least as noble as the substrate. Avoid large cathodic surfaces (stainless plate) with small anodic fasteners (aluminium or mild steel bolts). Where galvanic compatibility is critical and isolation is not possible, specify fasteners of the same alloy as the surrounding structure. Isolating washers and sealants can reduce galvanic risk by eliminating the direct electrical path, but should not be relied upon as the sole protective measure.
What to avoid: Mild steel and standard zinc-plated fasteners corrode rapidly in any marine environment and should not be specified. Clear or yellow zinc passivate offers no meaningful protection against saltwater. Grade 304 stainless is not suitable for coastal or marine use - it lacks the molybdenum content needed to resist chloride attack and will develop pitting corrosion.Grade 303 stainless must not be specified for marine use under any circumstances. Although widely used in CNC bar turning for its excellent machinability, 303 contains sulphur additions (typically 0.15% minimum) that create microscopic sulphide inclusions in the metal matrix. These inclusions are preferential initiation sites for pitting corrosion and disrupt the passive chromium oxide film. Its PREN of approximately 17 to 18 is lower than even 304, and it has no molybdenum. If you receive a quote for marine fasteners in 303, insist on 316 as the minimum substitute. Black phosphate is also unsuitable. Standard free-cutting brass (CZ121) must not be substituted for CZ112 naval brass in marine applications: it lacks the tin content required to resist dezincification.
Finishes for Marine Fasteners
For most marine applications, material selection does more work than the finish. However, where steel fasteners are specified or where additional protection is required, the choice of finish matters significantly. See our full finishes guide for comprehensive detail.
| PASSIVATION | The standard finish for stainless steel marine fasteners. Removes free iron from the surface using an acid treatment and strengthens the passive oxide layer. Recommended for all 316 stainless marine applications. Does not add dimensional thickness, so it has no effect on thread fit. |
|---|---|
| ZINC-FLAKE COATINGS (DACROMET / GEOMET) | The best corrosion protection available for steel fasteners used in marine environments. Provides typically 1,000 or more hours of salt spray resistance to BS EN ISO 9227. Trojan supplies Dacromet and Geomet coated fasteners via approved sub-contractors. Suitable for structural marine applications where stainless steel is not specified. HDG Hot-Dip Galvanising |
| HOT-DIP GALVANISING | Suitable for structural marine applications above the waterline, such as jetty hardware and sea defence fixings. Not suitable for submerged use or applications requiring close thread tolerances, as the coating thickness (typically 45 to 85 microns) affects thread fit and may require thread chasing after galvanising. |
| NICKEL PLATING | Good corrosion resistance and used in marine electrical applications where appearance and conductivity are important. Available from Trojan via approved sub-contractors. Electroless nickel provides more uniform coverage than electrolytic nickel and is preferred for threaded components. |
Finishes to avoid in marine environments: Standard zinc plating (clear or yellow passivate) offers very limited salt spray resistance and corrodes rapidly in marine conditions. Black phosphate provides minimal corrosion protection and is entirely unsuitable for any wet or coastal environment. Neither should be specified for marine fasteners.
Non-Standard and Bespoke Marine Fasteners
Marine applications frequently require fasteners that are not available from standard catalogues. Unusual sizes, mixed thread forms, legacy imperial threads for heritage vessel restoration, or specific dimensional requirements for OEM components are all common requests. Trojan manufactures marine fasteners to customer drawings, samples, or verbal specifications.
Common bespoke marine requests include:
| Oversized or heavy-series nuts in 316 stainless for structural marine applications |
| Castle and slotted nuts in 316 stainless for split-pin retention in steering and rigging components |
| Connector nuts with mixed thread forms for rod-to-fitting connections where metric and imperial threads meet |
| Round and boss nuts in bronze or stainless for marine machinery and pump housings |
| Stainless wheel nuts for trailer and boat-launch applications |
| Non-standard thread pitches in 316 stainless or bronze for legacy or imported marine equipment |
| BSW, BSF, and BA thread forms in phosphor bronze for heritage and classic vessel restoration |
No minimum order quantity. Single prototypes through to production volumes, with the same level of material traceability and documentation on every order.
Industries and Applications
| Boatbuilding and Yacht Construction | Deck hardware, rigging fittings, keel bolts, through-hull connections, and structural fixings in 316 stainless, bronze, or naval brass. |
|---|---|
| Commercial Shipping and Ports | Structural fixings, deck equipment, hatch hardware, and mooring fittings where corrosion resistance and documentation are required. |
| Offshore Oil and Gas | Subsea fasteners, topside structural fixings, and flange connections in duplex stainless or high-performance alloys with full material traceability. |
| Marine Engineering and Repair | Engine room, shaft, propulsion, and pump components in 316 stainless, phosphor bronze, and aluminium bronze. |
| Coastal and Waterfront Construction | Jetties, pontoons, sea defence structures, and lock gate hardware where long-term corrosion resistance is a structural requirement. |
| Heritage and Classic Vessel Restoration | BSW, BSF, and BA thread forms in phosphor bronze and stainless to match original specifications on pre-war and classic vessels. |
| Trailer and Boat-Launch Equipment | Wheel nuts, axle nuts, and hub nuts in 316 stainless for boat trailers and launch equipment operating in tidal and saltwater conditions. |
Why Choose Trojan for Marine Fasteners
| ✓ UK manufacturer, not a distributor | All machining is carried out in-house at our Birmingham facility. We do not buy in and resell - every fastener we supply is machined on our own equipment by our own team. |
|---|---|
| ✓ 316 stainless as standard | We machine in 304, and 316 stainless steel. For marine enquiries, 316 is our default recommendation unless the application specifies otherwise. |
| ✓ Bronze and brass capability | We machine phosphor bronze (PB102), aluminium bronze (CA104), and Naval Brass CZ112 (CW712R) as standard materials - not just stainless steel. |
| ✓ Full material traceability | Material certificates to EN 10204 3.1 are available with every order, maintaining full traceability from bar stock to finished component. |
| ✓ ISO 9001:2015 certified | Full quality management system with documented inspection, first-off verification, and traceability throughout production. |
| ✓ Bespoke capability | Non-standard sizes, unusual thread forms, mixed-thread connectors, and custom designs manufactured to your drawing or sample. |
| ✓ No minimum order quantity | Single prototypes and small batches are as straightforward as production runs. Pricing scales with volume, but access does not. |
| ✓ Fast turnaround | Same-day quotations, 3 to 5 working days for standard items, 7 to 10 working days for bespoke marine fasteners. |
Technical References and Further Reading
The following sources were used in compiling this guide and are recommended for engineers and specifiers who require primary technical data on marine corrosion and material selection.
| British Stainless Steel Association: | Calculation of Pitting Resistance Equivalent Numbers (PREN) | the authoritative UK reference for comparing stainless steel grades in chloride environments. |
|---|---|---|
| Copper Development Association: | Guidelines for the Use of Copper Alloys in Seawater | covers dezincification, galvanic compatibility, and alloy selection for all copper-based marine materials including naval brass and phosphor bronze. |
| Fasten.one: | Best Materials for Marine Fasteners -- Corrosion Resistance, Strength and Cost Analysis | a detailed technical whitepaper covering 316, silicon bronze, Monel, titanium, and coated carbon steel in saltwater service. |
| Fastener and Fixing Magazine: | Stainless Steel Fasteners for the Most Demanding Marine Applications | covers PREN, hydrogen-induced stress corrosion cracking (HISC), and grade selection for offshore wind and oil and gas fastener applications. |
| Wikipedia - Pitting Resistance Equivalent Number: | PREN - formula variants, tungsten-adjusted versions, and grade comparison | |
| Wikipedia - Duplex Stainless Steel: | Duplex stainless steel - microstructure, PREN banding, and offshore applications | |
| Peer-reviewed research - dezincification: | Becceria et al., "The effects of tin and nickel on the corrosion behaviour of 60Cu-40Zn alloys" | Journal of Alloys and Compounds, 2002. Characterises the passivating mechanism of tin in alpha-beta brasses in seawater. |
| Peer-reviewed research - MIC: | Xu et al., "Microbiologically Influenced Corrosion of 2707 Hyper-Duplex Stainless Steel by Marine Pseudomonas aeruginosa Biofilm" | PLOS ONE, 2016. Demonstrates MIC susceptibility even in high-PREN alloys. |
| Peer-reviewed research - MIC prevalence: | International Biodeterioration and Biodegradation, 2021 | estimates MIC accounts for 20% of all corrosion failures and $30 to $50 billion in annual economic losses. |
| Atlas Steels Technical Note No. 7 - Galvanic Corrosion (PDF): | Atlas Steels Tech Note No. 7: Galvanic Corrosion | a properly authored engineering reference document covering the galvanic series in seawater, the three conditions required for galvanic corrosion, area effects, and practical prevention methods. Widely cited in engineering literature and referenced by Wikipedia's galvanic series article. |
| Corrosionpedia: | Introduction to the Galvanic Series - galvanic compatibility and corrosion | explains how to read and apply the galvanic series in engineering design. |
Frequently Asked Questions
1. What is the best material for marine fasteners?
For most marine applications, Grade 316 stainless steel (A4) is the standard recommendation. It contains molybdenum which gives significantly better resistance to chloride corrosion than Grade 304. For submerged or offshore applications, duplex stainless steel or phosphor bronze offer superior performance. The right choice depends on the specific environment, the substrate material, and whether galvanic corrosion is a risk.
2. What is the difference between 304 and 316 stainless steel for marine use?
Both grades form a passive oxide layer that protects against corrosion, but 316 contains 2 to 3% molybdenum which dramatically improves resistance to chloride attack - the primary corrosion mechanism in seawater and salt spray. In coastal and marine environments, 304 is prone to pitting and crevice corrosion. 316 should be specified as the minimum for any marine application.
3. Can you supply stainless steel marine fasteners with marine certificates?
Yes. Material certificates to EN 10204 3.1 are available for all orders. We maintain full traceability from bar stock to finished component and can provide certificates of conformity, material test reports, and inspection documentation as required.
4. Do you manufacture non-standard marine fasteners?
Yes. We regularly manufacture marine fasteners to customer drawings, samples, or specifications where standard catalogue sizes are not suitable. Common requests include oversized nuts for structural applications, castle nuts in 316 stainless, connector nuts with mixed thread forms, and legacy imperial thread forms for heritage vessel restoration.
5. What finishes are suitable for marine fasteners?
For stainless steel, passivation is the standard finish. For steel fasteners in marine environments, zinc-flake coatings such as Dacromet or Geomet provide the best corrosion resistance. Standard zinc plating is not suitable for marine use. Hot-dip galvanising is acceptable for above-waterline structural applications only.
6. Do you have a minimum order quantity for marine fasteners?
No. We manufacture from single prototypes through to full production runs. There is no minimum order quantity, but having a bar length's worth of nuts is usually more financially appropriate for all concerned.
*7. What is CZ112 Naval Brass and why is it specified for marine fasteners?"
CZ112 (European equivalent CW712R) is a duplex alpha-beta copper-zinc alloy containing 61 to 63% copper, 1.0 to 1.5% tin, and a balance of zinc. The tin addition is critical: it provides significantly improved resistance to dezincification and stress corrosion cracking in seawater and brackish water compared to standard brass grades such as CZ121. It should not be confused with standard free-cutting brass, which lacks the tin content needed for marine use. CZ112 is covered under BS 2874 (bar and rod) with the modern European equivalent EN 12163 / EN 12167. Trojan supplies CZ112 as a standard bar stock material for nut manufacture, with EN 10204 3.1 material certificates available on every order.
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