Have you ever wondered why some stainless-steel fittings rust while others remain shiny? Would aluminum be suitable for a freshwater tank? Naval brass sounds admirable, but does it hold up well underwater?
In this column, we will look into the pros and cons of various metals for the marine environment. This article will not delve into hull materials and boat construction, but rather the metal components that all boats rely upon.
All of these metals consist of alloys, which are mixtures of metals that produce a stronger and more durable result than the pure element. The mixing of alloys can be traced all the way back to 4500 B.C., beginning an era that evolved into the Bronze Age. During this time, humans learned how to alloy copper and tin into bronze, producing major advances in tools and weapons.
It all started with bronze, so let’s begin there. Pure copper is too soft and too flexible to be useful for tools and boat parts (think copper wire). Adding tin to molten copper produces a much more useful result. Basic bronze consists of copper and tin, but similar results can be achieved by alloying copper and zinc. When zinc is the primary additive, we call the result brass.
Sounds easy enough, but it gets more complicated. It turns out that adding tin and zinc to copper can produce even better results, but then our bronze/brass distinction gets blurry.
Zinc placed on your propeller shaft will deteriorate as a result of its connection to the shaft. The same applies to the zinc in an alloy; under certain conditions, it will deteriorate, a process known as dezincification. Once that happens, the part becomes porous and weak. It stands to reason, then, that the less zinc in the alloy, the better.
Naval brass and admiralty brass, despite their impressive names, contain 30 percent or more zinc. By contrast, silicon bronze contains less than 2 percent. That is why wooden boats were built with silicon bronze screws. Brass works well for ornamental objects, but not for any parts you are counting on to keep you afloat and going. It has been said that brass works well on ships, as long as it is used for bells, buttons and bugles. (Pictured at top: Once this boat's sacrificial anode disappeared, the zinc in its manganese bronze prop became sacrificial, resulting in a dangerously weak alloy.)
We take aluminum for granted, but it is a wonder metal. Unlike steel, aluminum can be cut on a table saw or shaped with a router, and can be drilled and tapped easily. Aluminum requires no paint and will not rust. On the negative side of the ledger, it is soft and does not handle repeated flexing well. Aluminum would be a poor choice for an alternator bracket, but a good choice for a brace under a floorboard or hatch. Aluminum can be purchased in a variety of alloys, most of which are not suitable for the marine environment. While bronze alloys have promising (and misleading) names, aluminum alloys come with a number. Here is a partial list of the most common alloys suitable for marine exposure: 5052, 5083, 5086, 5154, 6061, 6082.
Generally, alloys in the 5000 and 6000 series can be used. For critical parts where failure would lead to physical harm or disabling the vessel, you will need to do some homework to find the best choice. Aluminum extrusion purchased at a local hardware store might not be suitable; you have to identify the alloy.
Aluminum has a bit of a split personality. It forms a thin oxide layer that protects it from corrosion, but it is way down on the galvanic scale, making it sacrificial to most other metals (that’s why aluminum can be used instead of zinc as a sacrificial anode). In other words, it does not play well with others. Never use bronze fasteners with aluminum; you will set up galvanic corrosion, and the aluminum will be the loser. Stainless steel fares much better, but even stainless in contact with aluminum will set up some reactivity in a wet environment.
Aluminum fares well if left bare. Where aesthetics matter, you have three options: paint, powder coat or anodizing. For a long-lasting paint finish, you will need a two-part catalyzed paint with a primer system designed for aluminum. The paint will likely fail wherever there are stainless fasteners. As much as possible, insulate any fasteners from the aluminum, ideally with nylon sleeves, or at least with a healthy dose of marine sealant.
Powder coating requires sending the part out. I am not a fan of powder coating for aluminum in a marine environment. As the aluminum oxidizes because of contact with fasteners or ongoing moisture, the coating fails from the underside and bubbles up. Once it starts to fail, refinishing often costs more than a new part.
Anodizing provides the best results. The finished part must be sent out for this electrochemical process, but it is well worth the effort. Anodizing produces a durable, aesthetically pleasing appearance. You won’t have the bold color of paint or powder coat, but you will have a maintenance-free finish that will last for years.
Aluminum works well for fuel tanks but makes a poor choice for a freshwater tank. Chlorine in the water attacks the aluminum, releasing undesirable by-products into the drinking water. Aluminum would not be my choice for a holding tank either; acidity in the waste will attack the aluminum.
While bronze alloys appeared more than 4,000 years ago, we didn’t come up with stainless steel until about 100 years ago. A foundry discovered, accidentally, that adding chromium to steel produced a remarkable outcome: no rust.
The chromium reacts with oxygen in the air, creating a thin film of oxygen on the metal’s surface. The film seals the metal, preventing it from rusting. Removing or violating the film is kryptonite to stainless steel’s superpowers. If scratched or scraped, the film will rejuvenate, but if deprived of oxygen, the film cannot form, and the alloy becomes susceptible to corrosion.
Tests have shown that stainless steel placed in flowing water resists corrosion far better than the same alloy placed in stagnant water. Stainless in moving water survives for the same reason that fish in a baitwell survive: moving water contains more oxygen.
For this reason, propeller shafts in dripless shaft seals are more prone to corrosion than shafts in conventional stuffing boxes. The continuous drip helps to oxygenate the water.
Stainless comes in an array of alloys, and the application determines which one you should use.
For noncritical applications, such as inside the boat where it will not get wet, use 304 (also referred to as 18-8). This alloy would be suitable for mounting a mirror or fastening cabinetry.
For attaching hardware to the deck or cabin (exterior above the waterline), use 316. You will pay more for 316, but it will not bleed rust stains, and it has far greater resistance to corrosion than 304.
For parts that will be welded, use the same categories as above, but designated L (304L, 316L). Without the L designation, the welds are susceptible to failure.
For fasteners below the waterline, know that even with 316, the application can be questionable. Silicon bronze resists corrosion in a low-oxygen environment more reliably than stainless.
Identifying the alloy can be difficult. Any stainless that strongly attracts to a magnet should be avoided. In sheet form, 304 and 316 are nonmagnetic. When machined or cold formed, however, they can become slightly magnetic. Fastener manufacturing uses one or both of those processes, and for that reason a magnet is not a reliable test.
All 316 fasteners will be stamped with a 316 (or A4) designation (see photo). If you are purchasing from a marine- or home-supply store, you can usually look up the alloy.
For almost all fabricated parts on a cruising boat, aluminum, bronze or stainless will serve you well. Titanium offers many of the same properties as stainless steel, but with far less weight and much higher cost. While a racing boat might be willing to pay the premium, using titanium makes no sense on a cruising boat.
Inconel and Monel also have a place in the marine environment. Inconel refers to an alloy of nickel and chromium, while Monel uses nickel and copper. Both offer excellent resistance to corrosion in salt water.
For stainless alloys, there is a formula that predicts corrosion resistance. The PREN (pitting resistance equivalent number) provides a valuable comparative tool. For seawater exposure, a minimum PREN above 32 is recommended. The PREN for 316 stainless is 22-28; for Inconel 625, the PREN is 46-56.
The PREN only applies to stainless alloys, and Monel falls outside that category, but Monel is high strength and does offer outstanding resistance to corrosion from seawater. Inconel would be an excellent choice for an exhaust riser, where repeated exposure to high temperatures and salt water combine to create a high-risk environment. Monel excels as a fastener for below-waterline applications, such as bolts to hold a strut in place.
We have a 60-year-old sailboat that we have maintained for the past 40 years, and it still has its original Monel fuel tank—a testament to the wisdom of choosing the best alloy for the job at hand.
Steve Zimmerman is the president of Zimmerman Marine, which operates five boatyards in Maryland, Virginia, North Carolina and South Carolina. Zimmerman has been building and repairing boats for more than four decades.