More than 400 years ago, Sheffield, England, became renowned for its steel-working, fine cutlery, and swords. Over the decades and centuries, local artisans continued to refine the process of converting iron ore into workable alloys. In the early 1900s, a research lab in Sheffield worked on finding ways to eliminate rust in gun barrels. The head of the laboratory happened to notice that, unlike the other samples, a discarded sample from a previous test had not rusted yet. They quickly determined that by adding chromium to the steel alloy the metal could be made far more resistant to rusting than ever. Two months later, in the summer of 1912, they produced a stainless steel casting for the first time. Initially they called it “rustless steel” but for marketing reasons, soon changed it to stainless.
Many consider stainless steel to be a technological marvel. This material has had a major impact in many critical industries, including food handling, medical supplies, and, of course, boats. Stainless steel refers to a family of alloys, with critical differences within that group. The stainless steel bolt might not be the same alloy as the cleat it holds. The cleat will not be the same alloy as the stainless propeller shaft. Knowing the differences among the choices can make the difference between cruising and sinking. If you are buying a component made out of stainless, you will hear a variety of confusing terms, such as 18-8, 304 or 316, Aquamet 22, and more. Understanding these terms will help you make smart choices.
The Stainless Recipe Book
As accidentally discovered in Sheffield, adding chromium to the iron is a game changer. And here’s why: Chromium readily reacts with oxygen and forms a hard, tough surface film that protects the underlying metal from attack. The film’s thickness equates to about 1/10,000 of the thickness of a strand of human hair. When polished, this skin thickens and hardens, improving its protective properties. In the same way that the bark on a tree protects the wood from invasive pests and diseases, the oxide film protects the base iron from corrosion. If the film gets scratched or scraped it will reform, provided that oxygen is available. Table cutlery alloys contain 12-14% chromium. For stainless that will be more exposed to the atmosphere, such as automotive trim, the chromium content must be increased to about 18%.
If we want to increase resistance to acids and corrosion, then nickel must be added. Nickel also happens to make the alloy non-magnetic. The lowest grade of stainless that should be used on a boat contains 18% chromium and 8% nickel. We commonly refer to this alloy as 18-8. To be more precise, stainless steels can be identified via a numerical series, and for marine applications they must be within the 300-series—18-8 equates to grade 304. Three-zero-four stainless, though, is susceptible to corrosion from chloride solutions, and salt water contains chloride. The chloride ions create small holes in the protective film, inviting pitting corrosion. Three-zero-four stainless can be used for non-critical interior applications where the hardware will not be directly exposed to seawater. Screws holding wires or hoses in place would be a typical application.
To achieve a higher level of protection from the elements, foundries increase the nickel content and add one more element to the recipe, 2-3% molybdenum, which boosts the stainless grade to 316. This alloy consists of 16% chromium, 10% nickel, and 2% molybdenum. Molybdenum greatly increases the alloy’s resistance to corrosion from salt and acid. Deck hardware and fasteners should be 316.
You might be wondering at this point, “How can I tell what kind of stainless I have?” You usually can’t. The grade must be known at the time of ordering. Some 316 hardware comes with the designation stamped into the part. Neither 304 nor 316 stainless will attract a magnet. In critical applications, if the magnet pulls to the metal, replace it with higher-grade stainless, but this test does not work well on stainless bolts because the cold-forming manufacturing process imparts a small amount of magnetism to the fastener.
Hose clamps provide an excellent illustration of the differences. Cheap hose clamps use 18-8 and will be short-lived in a marine environment. The next grade up uses 316 for the band, but a lesser grade for the tightening screw. The best quality, and the ones you should be using, have 316 stainless bands and a 316 screw (to go one step further, the band should not have slots which will cut the rubber, but stamped grooves instead). Checking hose clamps with a magnet won’t help much, because the forming process imparts a surprising amount of magnetism and even an all-316 clamp will give the impression it’s a lesser grade. It’s all about cost/benefit: You’ll pay more for good, all-316 stainless clamps.
Below The Waterline
Just as deck hardware calls for a higher grade of stainless alloy than required in the accommodations, going below the waterline takes the bar even higher. Remember that the game-changer for creating stainless steel is chromium because of the way it reacts with oxygen. In the absence of oxygen, the protective film cannot be reliably formed. Strut bolts serve a critical purpose. The head of a stainless strut bolt usually seats in the strut, under water. Oxygen in the seawater enables the head to maintain its protective layer. The threaded end protrudes into the boat, where it has access to oxygen in the air. In between, where it passes through the metal casting and the hull, the bolt will be starved of oxygen and that is where corrosion usually occurs (which also happens to be in a place you can’t see). Lower grades of stainless will be eaten away in the middle, forming a “waist”. For this type of application, if you are going to use stainless steel, it must be 316. For a bronze strut, bronze bolts would be preferable, but they are becoming harder to obtain.
Propeller shafts face similar issues. Shafts rest for long periods of time in stagnant oxygen-deprived water in stern tubes. Barnacles form on shafts, also restricting access to oxygenated seawater. These demands call for higher levels of corrosion resistance. Various trade names exist, such as Aquamet® and Aqualloy®, but the different brands share the same numerical designations: 17, 19, and 22. Only 19 and 22 should be used on cruising boats—19 compares to 304 stainless, 22 compares to 316 and offers superior corrosion resistance due to its molybdenum content.
Stainless steel does not conduct electricity very well. On a relative scale, if copper rates at 100% conductivity, stainless steels fall in the 2-5% range. For this reason, stainless washers should not be used between the electrical terminal and the connecting point. It is acceptable to place the ring terminal over the mounting stud first and then use a stainless washer under the nut that will secure it. If, however, you place the stainless washer under the ring terminal, you will create resistance in the circuit.
Despite its excellent non-corrosive properties, stainless should not be used in critical high-strength applications. Grade 8 bolts have roughly 50% higher tensile strength than 316 stainless. Shaft coupling bolts provide a good example of an application that calls for grade 8 steel and not stainless. Grade 8 bolts can be identified by six radial lines stamped on the head (as shown in the photo).
Welding stainless steel for marine use requires special consideration. Failure to use the proper alloy combined with the appropriate filler rod and welding technique can result in welds that decay and fail suddenly. Executed properly, stainless steel welds beautifully and reliably.
Occasionally stainless steel will stain. Surface contaminants such as residual oils, dirt, iron particles (never use steel wool), and salt crystals can violate that critical microscopic film that protects the alloy. Rinsing the hardware after exposure to salt can be helpful and the marketplace offers an abundance of stainless steel cleaners and polishes. Cleaning and polishing helps restore the protective oxide layer and, unlike a teak deck, cleaning will not wear away the material. The higher corrosion resistance of 316 makes it less susceptible to staining than the lower grades.
Stainless steel plays a critical role aboard cruising boats. It can be found on windlasses, deck hardware fasteners, portholes, prop shafts, through-hulls, galley appliances, and much more. Given its widespread use, it pays to understand how this remarkable material maintains its condition and what the various designations signify. Choose poorly and you might experience a critical failure. Choose well and you have an amazing material that will maintain its strength and beauty for more than a lifetime.