An Explanation of Fastener Metals
Fair Wind Fasteners strives to be your source for non-ferrous marine grade fasteners. At the time of writing this guide we specialize in silicon bronze fasteners such as wood screws, nuts, bolts, etc. but will soon be branching out in to other alloys that are common in the marine industry. We often get the question of why to use silicon bronze fasteners versus stainless steel, galvanized, or even just cheapo screws from your local big box store (for shame!), and I'm hoping to clear some of that up with this guide.
Silicon Bronze
Silicon bronze fasteners are well known to traditional boat builders and restorers, but are often used by other industries and home DIY people as well. Their beautiful luster when polished and the patina that they develop over time makes them excellent for use when aesthetics are important, but the main reason they are so prized is their excellent corrosion resistance properties. I've written a whole other article on why bronze fasteners are used on boats, but it boils down to their corrosion resistance and their lack of susceptibility to a little known phenomenon called crevice corrosion. Bronze fasteners will never leave a rust streak down the side of your project, and when properly installed and isolated from electrical sources they will last for decades to come.Speaking of electrical sources, that brings up another use case for bronze fasteners. They are often used in electrical grid applications because of their high degree of conductivity. Other uses include use in water treatment and plants where concentrated chemicals would corrode other alloys quickly, or in heavy duty seawater pumps where cavitation and salinity would otherwise be a cause for corrosion.
Brass
Brass fasteners work well when aesthetics are the primary concern. Many yacht interiors have brass fittings that are held on by brass fasteners. They shine up to an almost gold color and are quite beautiful as well as traditional in the right setting. You'll often see brass fasteners holding on hinges, doorknobs, light fixtures, and other fittings. Where brass fasteners don't excel as much is their general corrosion resistance and softness. Brass, when exposed to seawater and even the slightest amount of electrical current will honeycomb and corrode in to a dust due to their high zinc content. The zinc content in brass will work as a sacrificial anode, corrode away, and leave the other metals that make up the brass alloy behind which leaves behind a highly compromised fastener. Brass is also a comparatively soft metal. Be sure to drill properly sized countersink holes and get a good fitting driver bit or it will be easily twisted and stripped.Due the lack of corrosion resistance, brass should never be used in a corrosive environment and belongs on the interior of a vessel only. The boatbuilder that mistakes brass for bronze and uses it in a planking application, for example, is going to have a bad time and likely and eventual call to the coast guard!
Stainless Steel
Stainless steel fasteners are used all throughout the marine industry and are the go-to for many applications in more modern boatbuilding. They won't often be found in more traditional vessels like the beautful Dorade, or used for teaching traditional boatbuilding like at IYRS. However, traditional craft are in the minority and stainless steel fasteners are ubiquitous in more modern craft.
It's important to note that "Stainless Steel" is a broad term that encompasses a variety of different alloys, and while they may share some similar properties the properties of the various stainless steels can vary greatly. Be wary when a company simply advertises and sells "Stainless Steel" fasteners without naming the grade such as 18-8 stainless steel, A2 stainless, or 316 stainless steel. The grade lets you know what metals make up the alloy, and what the properties of that stainless steel alloy will be.
For boatbuilding, we recommend none other than 316 stainless steel. Alloy 316 combines a good amount of strength, lack of brittleness, and an excellent amount of corrosion resistance. It's the only alloy of stainless that we would use on our boat, so it's the only one I can recommend that you should. Other alloys, specifically 18-8 stainless steel, are far less corrosion resistant and will eventually leave orange rust streaks down the side of your craft. As the old saying goes, stainless is not stain free, it just stains LESS!
Note that stainless steel fasteners should never be used in an anaerobic environment such as below the waterline on a vessel do to the fact that it is subject to crevice corrosion, which you can read about here.
Aluminum
As with stainless steel, "Aluminum" is an all encompassing term that also refers to a variety of alloys that fall under the umbrella of aluminum. As a general rule, aluminum fasteners are very lightweight and highly corrosion resistant, but suffer from a severe lack of strength due to how soft and brittle they can be. It's all to easy to over torque an aluminum bolt and have it crack in half.
Aluminum fasteners (or aluminium if you're from anywhere other than the United States) are often used when fastening aluminum parts due to the fact that aluminum doesn't play too nicely with other alloys. It's position far down on the galvanic series chart means that when almost any other alloy comes in to contact with it there will be corrosion issues over time. However, if strength isn't too much of an issue, aluminum fasteners can be safely used and are exactly what should be used when fastening aluminum parts or to an aluminum boat hull.
Copper
Copper fasteners are often used in lapstrake boatbuilding. Planks will be riveted or clenched on with copper nails. A copper nail is driven in to a pilot hole, and then the pointy end will be rounded over to create a rivet, or bent back in towards the frame to clench it.
Copper is very corrosion resistant when in a corrosive environment such as seawater, which is why it is suitable for building boats.
Solid copper fasteners are however quite soft. As a metal, copper is too soft for threaded fasteners such as hex bolts, which is why when a corrosion resistant and conductive alloy is needed for a bolt something like a silicon bronze hex bolt will be used. Copper is after all the main component of silicon bronze, with the silicon being added to harden it.
Copper nails aren't just for boatbuilding. You'll find them also used for copper roofing, copper flashing, copper gutters, and other applications where fastening a copper component is necessary. They won't rust and age to a beautiful patina. Like with aluminum, they are used to avoid having a galvanic corrosion issue between dissimilar metals.
Galvanized Steel
Older wood boats that were built on a budget can often be found to have galvanized steel fasteners, as well as newer boats with galvanized steel keel bolts since galvanized steel won't succumb to crevice corrosion like stainless steel will. Steel is also exceptionally strong while being relatively inexpensive. It's cost to strength ratio is easily the best on this list.Galvanized steel fasteners do unfortunately begin to rust over time as the galvanized coating tends to break off or crack, exposing the steel beneath. The galvanized coatings are resistant to corrosion in salt water, but over time will also break down and corrode even if not cracked or broken away. When rust does begin, it will leave ugly stains but more importantly, the iron oxide will cause iron rot in the wood around the rusty fastener. You can read all about iron rot here in this publication by the US Dept of Agriculture.
Titanium
Like stainless steel, "titanium" refers to a whole family of alloys, but in general titanium is an excellent alloy for fasteners and excels in almost every category apart from price. Titanium is stronger and lighter than steel, extremely corrosion resistant, and very hard without being brittle. It's most often used when high strength is necessary, weight is a major concern, and the budget is no concern at all!
In the maritime world you can find titanium on high end racing yachts or superyachts, but it is most often used in the aerospace and aviation industries. It's is non-magnetic, has low thermal conductivity, and a low thermal expansion coefficient which makes it the go-to material if what you're looking to do is re-enter the atmosphere from orbit and have deep pockets.
Monel
High up on the nobility chart, monel is extremely corrosion resistant, and comes in at a fraction of the price of titanium. Monel is primarily a nickel based alloy with a high copper content which makes it strong, corrosion resistant, and unfortunately also expensive. While not the price of titanium, monel fasteners do come in at a price that is multiples higher than their silicon bronze alternatives for a fastener that is only slightly more corrosion resistant.
You will find monel fasteners, keel bolts, and sometimes monel fuel and water tanks on wood boats built with high budgets. It has a nice bright look, is very corrosion resistant, strong, and is an excellent choice if budget is less of a concern. If you're looking for monel fasteners get in touch, and we can provide them for you.