The Power and Peril of Stainless Despite stainless steel’s deserved reputation for corrosion resistance, longevity, and durability, this alloy is prone to a variety of failures depending on its actual composition and application in the marine environment. Text and photographs f you’ve worked in a service yard, through a cored deck; a pitted stain- Ithis list of problems will be familiar: less steel prop shaft breaks under by Steve D’Antonio rust from a rub strake leaves streaks load; or stainless steel keelbolts on the topsides, or deck hardware require replacement or fail outright stains the gelcoat; a stainless steel from corrosion. And as you explain chainplate corrodes where it passes the failure and propose an appropri- ate repair, a disappointed owner pre- dictably asks, “How could it corrode? Above—Even good-quality 300-series It’s stainless.” The short and alarming stainless steel, such as this steering answer is “Very easily.” But too often chain, can suffer crevice corrosion the builders or repairers doing the and microcracking, and then ultimate job have little more understanding structural failure. of the complexity of stainless steel Right—Rust stains running out from corrosion than the owner. With under or around stainless hardware growing concerns about our ability to nearly always portend crevice corrosion. verify the metallurgical quality of 48 PROFESSIONAL BOATBUILDER Left—This stainless steel water tank, built of 304 alloy, corroded and began leaking after just four years of service because it lacked the necessary resistance to corrosion. Uniform discoloration adjacent to the welds also points to weld migration, a result of using inappropriate higher-carbon stainless steel. Water tanks fabricated from 316L (low-carbon) stainless steel alloy are unlikely to fail. Right—Raw iron has little resistance to corrosion and thus few appropriate applications in a marine environment. Predictably, this cast iron centerboard rusted after its coating deteriorated. stainless steel hardware as yards are This can happen anywhere. I’ve drawback is vulnerability to corrosion. forced to source it in an increasingly encountered variations of this sce- If applied externally as a sailboat keel decentralized global marketplace, it’s nario repeatedly in Asian-, North or centerboard, for instance, iron will more important than ever to under- American–, and European-built craft. rust severely when its coating is stand just what we mean when we But common stainless steel failure is breached. Regular maintenance and specify, order, and install stainless avoidable when users fully under- preventive action can minimize but steel hardware and components. stand the metal’s weaknesses in the not entirely eliminate rust. Wholesale In my experience, boat builders and marine environment. corrective action—stripping, sand- repairers are equally responsible for blasting, and epoxy priming—can be misapplying various alloys of stainless Iron a significant expense. steel. I’ve made many such mistakes, Let’s start with stainless steel’s par- For sailing vessels internally bal- and I’ve also learned from seeing the ent alloying material, iron, which lasted with iron, there’s less concern failures of stainless steel alloys in appears in several forms aboard about rust, and no maintenance is boats I was servicing. nearly every boat. This metal is a key required as long as the iron is fully Recently, an exchange with a colleague component in ordinary steel (some- encapsulated within the keel. If water reminded me how misunderstood times referred to as mild steel or low- reaches this iron, however, even the stainless steel is, even within the or high-carbon steel) as well as in smallest intrusion can wreak havoc, as industry. The broker representing stainless steel. It may surprise many the oxidation and consequent expan- the vessel I had inspected was dumb- that stainless steel is made up of just sion may cause significant and costly founded when I told him its stainless over 50% iron, compared to conven- damage to the surrounding fiberglass steel water tanks were corroding, tional carbon steel, which typically structure. leaking, and had to be replaced—a contains more than 98% iron. costly proposition, particularly for Cast iron is most commonly Mild Steel a four-year-old vessel. employed for manufacturing engine Mild, or high-carbon, steel is most “How could that be?” he said. “They blocks, cylinder heads, and exhaust beloved by a select few recreational are stainless steel, and stainless steel manifolds and risers. It also serves in boat builders, although it is by far the doesn’t rust.” some boats as cast or shot ballast. most common boatbuilding material The error is common: the tanks had More commonly, lead is chosen for for commercial and military vessels. been built from 304 alloy, which is not ballast because of its extreme density It’s called mild because it is ductile, or the most corrosion-resistant stainless and resistance to corrosion, but iron is flexible (rather than brittle), which choice and is therefore inappropriate desirable because of its low cost and generally makes it good boat- and for a water tank. (See page 51 for an environmental friendliness. Its primary shipbuilding material. The carbon explanation of 300-series stainless steel.) Yet the tank was clearly labeled as 304 series stainless steel When covered by protective alloy—proof, I believe, that no subter- coatings, mild steel is a fuge was intended. It was simply an stout and forgiving building error in design or selection that no material, especially for one on the boat’s build or mainte- commercial vessels. This nance crews caught. It seemed ade- hull, deformed under lifting quate that the tank was stainless, stresses during salvage, thought to be impervious to decay or shows just how ductile corrosion. the metal is. DECEMBER/JANUARY 2014 49 Combining mild steel, Experienced pros who have handled galvanized steel, and thousands of hose clamps and fasten- bronze in this raw- ers of stainless and anodized mild- water application can steel can discern the difference lead to a variety of between the two, as plated-steel corrosion-induced screws often have a slightly different failures. hue than stainless steel ones. The rest of us have to rely on a magnet. Hose clamps with plated mild-steel composite or non- screws will come to grief rather metallic options now quickly in the marine environment, exist for many of even when not directly exposed to these applications, seawater; the same can be said of habit and cost will some varieties of genuine all-stainless likely dictate mild steel clamps (see the photograph of a content of mild steel is approximately steel’s continued use in select loca- deteriorated all-stainless clamp 0.08%. (High-carbon steel, on the tions aboard small and large vessels. below). Although many marine other hand, with carbon content up to It should be applied sparingly and, engines have “automotive” mild-steel- 0.77%, is strong, a relative term to be with some notable exceptions, never screw-equipped clamps, some now sure, yet brittle, and is applied where above deck or below the waterline. In have all-stainless constant-tension rigidity is favored over ductility.) Mild most cases, better alternatives include clamps, which are better mechanically steel is inexpensive and strong, and stainless steel alloys. as well as more corrosion resistant. will flex and bend dramatically before An unfortunate characteristic of Elsewhere aboard, test with a magnet fracturing. Consider how much the steel, and most alloys for that matter, to determine marine suitability; if it is thin sheet metal in an automobile is that it’s impossible to determine the attracted to any part of the clamp, body distorts before failing. However, alloy or grade by simple visual replace the clamp with an all-stainless mild steel shares an unfortunate inspection. Hose clamps provide an steel model designed for marine weakness with iron: it rusts readily excellent and common example. applications. when unprotected, especially in a Clamps designed for marine applica- marine environment. tions are constructed of all 300-series Galvanized Steel Despite this, mild steel is used for stainless steel—either 304 or 316 The other mild-steel category wor- myriad gear aboard recreational and alloy—while industrial or automotive- thy of note is galvanized hardware. commercial boats, most commonly style clamps are usually made with a Galvanizing is nothing more than zinc in engine beds, fuel tanks, various stainless steel band and a mild-steel plating. Hot dipping, or literally engine brackets and components, screw. New mild-steel screws are vir- immersing the steel component in radio and other equipment chassis, tually indistinguishable from stainless molten zinc, typically leaves a slightly chain rode, rudder webbing, and hose steel screws without the aid of a mag- mottled or rough surface and is by far clamp screws. Although more desir- net, which is attracted to the iron in the preferred galvanizing method, as the able, lighter-weight corrosion-proof non-stainless steel fasteners. Further coating it provides is comparatively complicating the issue, some thick—about 3 mils (a mil is one- stainless-band/mild-steel-screw thousandth of an inch). hose clamps are labeled “all Another popular though inferior stainless,” evidently referring to method is electroplating, where the the band alone. I’ve seen boats zinc is applied with electricity much that were built mistakenly, I the same way chrome coating is presume, with this inferior-style applied to steel. For most marine clamp throughout their plumb- applications, the 1.5-mil coating is not ing systems. Unfortunately, it’s thick enough for what’s referred to as up to the builder or service yard a “class A marine standard.” to determine the metallurgy So-called flame spraying, which is of the clamps they install, essentially spraying molten metal, is a regardless of the labeling.