A Guide To Marine-Grade Metals

Which Metal To Use On A Boat 

Saltwater is a harsh environment for most construction materials. If a metal isn’t suitable for being submerged in water or exposed to salty air, it will corrode. For this reason, only marine-grade alloy is used on boats In this article, we’ll look at the types of metal that can withstand the marine environment and what they are used for. 

Most of the metals we will mention are alloys, which are mixtures of elements. Alloys are stronger than pure metals. 

Using Bronze On Boats: 

You’ll often see bronze fittings on traditional wooden sailboats. It can be used for chainplates, propellers, turnbuckles, and more. 

Bronze is an alloy of copper with other metals, such as tin and aluminium. When combined with silicon and tin, it forms silicon bronze, which is especially durable. Gunmetal, also called G metal, is a type of bronze composed of 88% copper, 10% tin, and 2% zinc. It’s used to make gears and bearings that need to withstand heavy loads. 

Bronze is corrosion-resistant and produces little friction against other metals. It’s the most robust marine-grade alloy because it has low oxidation rates at high temperatures, and it fights off marine organisms, such as algae and mussels. For this reason, it’s the only metal that’s recommended for structural parts located under the waterline. Bronze alloys suitable for marine use include C65500, C95400, and C51000. You won’t see any bronze on modern recreational sailboats, as it’s an expensive material compared to steel. 

Using Copper On Boats:

Copper is a soft metal that resists corrosion and ductility well. It can’t be used for any structural boat parts, but it’s useful for flexible pieces, such as copper wire or tubes. Copper nails and roves (washers with a slight conical shape) are used as fasteners on traditional wooden clinker or carvel-built boats. 

Using Brass On Boats:

Brass is an alloy made of copper and zinc, which can withstand heat and corrosion. It’s used on classic sailboats for below-decks hardware and ornamental objects, such as hinges, cabinet fittings, bells, and oil lamps. On big ships, it’s also used for piping, pumps, shafting, and more. Marine-grade brass alloys include C46200, C46400, and C48500. 

Using Stainless Steel On Boats:

Stainless steel is made of steel and chromium (minimum 11%), which prevents rust. It’s a very strong alloy that can be used for structural parts, such as chainplates, stanchions, and more. 

The chromium reacts with oxygen, creating a thin film on the steel’s surface. This protects it from rust. Removing the film will result in corrosion. If it’s scratched, it can reform, but it needs oxygen to do so. For the same reason, stainless steel resists corrosion much better in flowing water than in stagnant water. 

Different grades of stainless steel are suitable for different uses. For example, grade 316 is the most common kind, and it resists pitting and other saltwater corrosion very well. It’s the only stainless steel grade typically recommended for parts that come into contact with water. 

Grade 304 withstands high temperatures and is cheaper. It’s used for any internal parts of a boat that aren’t likely to get wet or exposed to salty air a lot. 

To make sure you’re buying grade 316, check that the piece is stamped 316 or A4. For an emergency fix, take a magnet with you and buy the least magnetic stainless you can find - 304 and 316 don’t attract magnets. For attaching parts that are welded, look for the L designation on the stainless. If you use regular stainless, the welds can fail. 

Using Galvanised Steel On Boats:

Galvanised steel is steel with a zinc coating. Only hot-dip galvanised steel is suitable for use in the saltwater environment, as the chlorides in the water will quickly corrode the zinc from electroplated (bzp) steel. Higher temperatures speed up the process. 

Galvanised steel is typically used for anchors, chains, and deck fittings, or for building marine structures. 

Using Alloy Steel On Boats:

Alloy steel is steel alloyed with a variety of elements, in varying proportions. It’s mainly used in shipbuilding to build structures and offshore buildings. Marine alloy steel comes in grades MD, ME, MF, and MG. 

Using Aluminium on Boats:

Aluminium is durable and lightweight. It can corrode and is usually anodised to prevent this, but see notes below regarding corrosion between the aluminium and any stainless steel fittings or fixings. The most common alloys suitable for the marine environment are grades 5052, 5083, 5086, 5154, 6061, 6082. 

Aluminium is suitable for masts and spars, deck fittings and fuel tanks, but not for freshwater or holding tanks. The chlorine in the water and the acidity in the waste would attack the metal. 

Types and dangers of corrosion on a boat 

Exposure to the marine environment causes boat parts to corrode over time. You can, however, prevent it and minimise it through regular inspection and maintenance. Let’s look at the main types of corrosion. 

Galvanic corrosion in the marine environment 

Galvanic corrosion, also called bimetallic corrosion, is an electrochemical reaction that happens when two or more dissimilar metals are immersed in saltwater, creating an electrical exchange. As a result, the more chemically active (or less stable) metal will corrode, sometimes quite rapidly. Conductivity increases with water temperature, which is why boats experience more galvanic corrosion in warmer climates. 

This is why sacrificial anodes are regularly attached to the hull, rudder, propellers, propeller shafts, and trim tabs of boats. A sacrificial anode is made of a metal lower on the galvanic table than the metals around it. It slowly corrodes until it gets replaced. 

Avoid galvanic corrosion. Don’t use brass fasteners in stainless steel fittings, or use aluminium rivets to attach steel parts. If unavoidable, place plastic or rubber washers or gaskets between the components and coat the metals with an anti-corrosive compound, such as Tef-Gel or Duralac, to isolate them electrically. 

Crevice corrosion 

Crevice corrosion threatens stainless steel, which is deprived of oxygen. For example, a leak that floods steel bolts and fittings will lead to crevice corrosion. 

Electrolytic corrosion 

12-volt battery systems are subject to shorts. When this happens, an electrical current may reach an underwater fitting, setting off electrolytic corrosion. This can cause significant damage.