Corrosion and choice of material
316L stainless steel, 5000-series marine aluminium alloys (5083, 5086), bronzes and titanium are common. Each has its limits, notably in permanent immersion; coupling between dissimilar metals must be controlled.
Sector
Subcontracting for shipbuilding and boating
Stainless steel and aluminium parts, bespoke deck hardware, pipework, welding and fit-out: compare workshops for your ships, pleasure boats and marine equipment.
The marine environment is unforgiving: salt water, spray, vibration and humidity attack anything that has not been chosen with care. A boat part is therefore judged as much by its material and finish as by its shape. Between shipbuilding (ships, structures, defence) and boating (sailing yachts, motor launches, racing boats), needs range from a spare part for an older sailing boat to a large welded assembly for a shipyard.
Subcontracting falls into three families: deck hardware and fittings (stainless steel, bronze, aluminium), structures and fabrication (aluminium, marine steel, stainless steel) and fit-out (wood, textiles, light sheet metalwork). Many parts are one-offs, made from the shipyard’s drawing or a survey on site.
This page details the sector’s constraints, typical parts, materials, treatments and how to word a quote request. For materials, rely on the guides 304 or 316L stainless steel and aluminium 5083, 6061 and 7075.
Sector requirements
Standards, regulations and practices that buyers expect from their subcontractors.
Pleasure craft
Directive 2013/53/EU governs pleasure craft from 2.5 to 24 metres hull length and certain components: CE marking, design and construction requirements. The ISO 12215 standard covers hull scantlings.
Ships and classification
For ships, the rules of classification societies (for example Bureau Veritas, DNV or Lloyd’s Register) and conventions such as SOLAS set materials, construction and inspections. Marine equipment fitted on EU ships falls under Directive 2014/90/EU.
Welder and procedure qualification
On classed structures, welders and welding procedures are qualified to recognised standards (ISO 9606 and ISO 15614 series). State the requirement: not every workshop holds these qualifications.
Traceability and certificates
A 3.1 or classification society material certificate, plate marking, a weld inspection file: ask for the list of documents at the quote stage, because they determine acceptance.
Typical parts and sub-assemblies
- Propeller shaft lines, bushes and bearings
- Deck hardware: cleats, stanchions, pulpits, fittings
- Mast fittings, chainplates and bespoke rigging parts
- Through-hull fittings, valves and stainless steel or aluminium pipework
- Welded aluminium tanks and boxes
- Bathing ladders, gangways, platforms and handrails
- Like-for-like spare parts for older boats
- Interior fit-out in wood or light sheet metal
- Covers, tarpaulins and sprayhoods
Skills involved
Each skill has its own detailed page: services, materials, price factors.
Welding & boilermaking
Structures, tanks, pipework and superstructures in aluminium, stainless steel or steel. Welding aluminium calls for specific skills: check the qualifications.
CNC machining
One-off propeller shaft lines, bushes, fittings and deck hardware in stainless steel, bronze or aluminium, often from a survey or a broken part.
Sheet metalwork & cutting
Laser or waterjet cutting of aluminium and stainless steel plates, folding of covers, hatches and guardrails.
Surface treatment
Anodising, stainless steel passivation, painting and powder coating suited to a saline environment: the treatment determines service life.
Wood & joinery
Interior fit-out, furniture and worktops in wood or plywood suited to damp environments.
Textiles & garment making
Covers, tarpaulins, sprayhoods and marine upholstery in technical fabrics resistant to UV and sea water.
Materials
Stainless steel 316L (A4 for fasteners), duplex for heavily stressed partsMarine aluminium alloys 5083 and 5086 for hulls, structures and tanksAluminium 6061 and 6082 for profiles and machined partsBronzes, including aluminium bronze, for propellers and immersed partsTitanium for very light racing partsShipbuilding steels, to be protected by paint or galvanisingUHMW-PE, POM and PA for pulleys, bushings and slidesTeak, okoume and marine plywood for fit-out
Tips for your quote request
Post a request- State the exposure zone (below the waterline, exposed deck, interior): it dictates the material, treatment and joints.
- Specify the exact grade (316L, 5083, aluminium bronze) and the certificate expected: 3.1 material certificate or classification society certificate.
- For a spare part, attach photos, dimensions, the boat or engine reference, even the broken part: the workshop can measure or scan it.
- Flag the metals your part will be in contact with, so that the workshop can plan insulation or a compatible material.
- Mention welding requirements: welder qualification, non-destructive testing, tank leak testing, and the acceptance planned.
- Give your deadlines (launch, season, race): a workshop can commit to a firm date.
Marine corrosion: choosing the material and treatment
Sea water is an efficient electrolyte. As soon as two dissimilar metals are connected and exposed to spray or immersed, the less noble one corrodes in favour of the other: this is galvanic corrosion. A stainless steel part fixed to aluminium, poorly chosen fasteners or contact with a bronze are enough to trigger it. Four habits limit the risk.
- Isolate dissimilar metals with non-conductive washers, sleeves or gaskets.
- Choose metals close together in the galvanic series, or provide a sacrificial part, such as an anode.
- Treat: anodising protects aluminium, passivation protects stainless steel, suitable paint protects steel.
- Take care over welds: welded stainless steel that has not been pickled corrodes faster along the weld bead.
Thanks to its molybdenum, 316L resists chlorides better than 304, but it is not invulnerable in permanent immersion or in confined areas. 5000-series aluminium alloys are sought after for their resistance to sea water. To compare treatments by metal, see the guide to surface treatment of metals.
Corrosion is often decided in the details: a crevice where water stands, stainless steel fasteners fitted dry into aluminium, an unprotected drilled edge. Flag these areas to the workshop.
From spare part to welded assembly: how to order
Requests fall into three situations, which call for different workshops. The one-off or spare part mainly needs a good survey and a machinist used to marine materials: bronze, stainless steel, aluminium. The small batch, for example deck hardware repeated for a shipyard, benefits from a validated first article before launch. The welded assembly (tank, gangway, structure) calls for qualified welders, a precise drawing and suitable inspection: dye-penetrant testing or radiography of the welds, leak testing for a tank.
In every case, describe the part’s real environment and the schedule: a boat laid up at the yard and a sailing season do not leave the same margin. Also specify the logistics: heavy or bulky parts, delivery to the quayside or the yard, handling conditions. For a series requirement or a large assembly, the guide to choosing an industrial subcontractor helps you compare workshops on more than price.
On Meaflow, you post your requirement for free; interested workshops ask their questions, then send you their quotes. You can also explore workshops in Brittany or in Pays de la Loire. Post your request to receive quotes.
Frequently asked questions
A question not answered here? Visit the help centre or write to us.
Which stainless steel should I choose for a boat?
316L, or A4 for fasteners, is the usual choice in a marine environment, thanks to its better resistance to chlorides than 304. It is not immune: in permanent immersion or in a confined, oxygen-starved area, crevice corrosion is still possible. State the exposure zone: the workshop can then suggest a duplex or another alloy.
Can a workshop remake a boat part from a sample?
Yes, this is very common for older boats and engines whose part no longer exists. The workshop takes the dimensions, sometimes by scanning, and can suggest a more resistant material. Attach photos and known dimensions, and say whether the part is immersed.
Do I need qualified welders to weld marine aluminium?
For a part subject to classification or acceptance inspection, yes: welders and procedures must be qualified to the applicable standards. For an unclassed structure, qualification is not always required, but aluminium welds very differently from steel. Ask for the workshop’s references.
Do parts for a pleasure boat have to be certified?
Not individually in most cases. A new boat placed on the European market falls under Directive 2013/53/EU, under the responsibility of its builder, who may require materials and supporting documents from its suppliers. Certain components have their own requirements: check with the yard.
Guides to go further
Stainless steel 304 or 316L: which grade should you choose for your parts?
EN designations, the role of molybdenum, chloride resistance, machining, welding and relative price: how to choose between stainless steel 304 (1.4301) and 316L (1.4404) according to the use of the part.
Anodising, powder coating, zinc plating: choosing a surface treatment
Anodising, powder paint, zinc, passivation, nickel, chrome: the strengths and limits of each treatment, their effect on dimensions and how to specify it in a request.
Machining aluminium: choosing between 5083, 6061, 6082, 2017 and 7075
Alloy 6082, 6061, 5083, 2017A or 7075: how to read an EN AW designation, compare machinability, welding, anodising and strength, and choose without overpaying.
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