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. viable alternative, particularly for large objects like entire steel hulls. The alloy often consists of an aluminum/ zinc mixture applied at approximately Above left—Although this hose 5 to 7 mils. Galvanized hardware has clamp is labeled “ALL its place aboard, particularly where STAINLESS,” the screw is mild it’s historically or aesthetically appro- steel, as the rust reveals. priate in wooden vessels. Anchors Left—One way you can identify and anchor chain are galvanized, an less-corrosion-resistant stainless application that makes sense for any alloys, like this 18-8 fastener: vessel if the hot-dipped method is they are mildly magnetic. employed. Keelbolts were frequently,

50 Professional BoatBuilder Occasionally seen aboard steel corrosion resistance of stainless steel, vessels, galvanized steel plumb- vary considerably within the grades of ing will have a limited life span. stainless. In addition to chrome and Because the inside of the pipe is nickel, these alloys include sulfur, inaccessible and unable to be phosphorus, titanium, molybdenum, primed and painted, stainless nitrogen, vanadium, and others. steel or reinforced plastic is There are three basic subcategories more appropriate here. of stainless steel. Martensitic is applied widely for cooking utensils and sharp cutlery—essentially any- thing that requires hardening. Ferritic is employed for automotive trim and salinity, even a hot-dipped- catalytic converters. Austenitic is best galvanized surface will for marine applications as well as “waste” at the rate of about 1 buildings and structures such as the mil per year when submerged Gateway Arch in St. Louis, Missouri, in still water, and faster in and New York’s Chrysler Building. moving water. Nickel is exclusive to the marine grades, which always fall in the 300 Stainless Steel series of stainless (see the sidebar Since its accidental discov- below). ery in 1913, stainless steel has Austenitic, or marine, grades of become extremely popular in stainless steel are further distilled into applications ranging from the American Iron and Steel Institute automobiles and jet turbines (AISI) heading of 300 series. For rec- to skyscrapers and flatware. reational and commercial vessels, this The term stainless steel primarily means two types: encompasses many alloys. • 304 is sometimes referred to as and sometimes still are, made of gal- Fortunately, for the sake of this dis- 18-8, reflecting the percentage of vanized steel. Because the bolt is cussion, the “marine grades” are a alloying elements chrome and nickel, high-carbon steel, it yields more fairly short list. respectively: 18–20% chromium, strength than one of stainless steel, Much like ordinary mild steel, stain- 8%–12% nickel; and bronze, or Monel (a trademarked less steel is made up primarily of iron. • 316 consists of 16–18% chromium, alloy of Specials Metal Corp., primar- To this base, chromium and nickel are ily composed of nickel and copper) added in varying amounts, depending for a given size and appreciable lon- on the grade being manufactured. Nickel in the Mix gevity, provided the galvanizing is These additions afford stainless steel Nickel plays a pivotal role in properly applied and not damaged its unique corrosion-resistant (not the durability and corrosion resis- during installation or subsequent use. corrosion-proof ) properties. Among tance of austenitic stainless steel. Complicating factors can sometimes other things, the nickel enhances the With the addition of this alloying make other materials preferable for alloy’s ability to resist acids. The chro- element, stainless steel becomes keelbolts. For example, if the bolts mium, when exposed to the elements, stronger and more stable than fer- are used as a connection point for a specifically oxygen, enables this alloy ritic stainless steel at higher tem- bonding or KISS-SSB counterpoise to form a tough oxide film almost peratures. Less nickel is needed to (the wire and terminal will be cop- instantly. The result is what was once retain an austenitic structure as per), the zinc coating on the portions overoptimistically referred to as rust- the nitrogen or carbon levels of the nuts and studs exposed to sea- less or rust-free steel. increase; when sufficient nickel is water or bilgewater may sacrificially, The alloying elements, and the added to a chromium stainless anodically corrode. Galvanizing effect they have on the strength and steel, the structure changes from doesn’t last forever, and there are bet- ferritic to austenitic. ter alternatives, particularly for keel Adding nickel also improves and other difficult-to-access fastenings stainless steel’s abrasion resis- or hardware. tance, toughness, ductility, and Galvanized steel’s applications weldability, as well as increasing aboard small vessels have other its resistance to oxidation, carburi- downsides. Depending upon environ- zation, nitriding, thermal fatigue, mental conditions, temperature, and and acids. It is an important alloying element in stainless steel and other alloys for corrosive and Stainless steel is not ideal in submerged high-temperature applications. fastener applications. One alternative is —Steve D’Antonio copper alloy such as this hex-head bronze bolt.

December/January 2014 51 8%–12% nickel, with an addition of Welded components such as 2–3% molybdenum. That final ingredi- this rudder armature should ent enables 316 stainless steel to be fabricated of 316L series better resist a phenomenon referred stainless, a corrosion-resistant to as crevice corrosion, arguably the alloy low in carbon and thus less metal’s greatest vulnerability and a prone to weld decay. The stock menace to those who specify the should be made of an even more material for unsuitable applications. corrosion-resistant shaft stock alloy. Most stainless steel available to the marine industry and retail purchasers is 304. Stronger than 316, technically it is resistance is considerably the least-corrosion-resistant commonly higher than that of ordinary available grade of marine-approved steel, it’s far from corrosion- stainless steel. While its corrosion proof. The popularity of 304

Duplex Stainless Steel: Strength and Corrosion Resistance

any of the stainless steel alloys industry, duplex stainless steel is cracking (SCC), pitting, and crevice M familiar to boat builders and now widely used and readily avail- corrosion. repairers include as much tensile able from several manufacturers and As a result of these attributes, the strength and corrosion resistance as distributors. And because this alloy popularity of these so-called duplex possible, yet they remain far from contains less nickel—whose cost fluc- stainless steel alloys has grown. perfect. As illustrated in the main tuates considerably—than other Because the alloys are still relatively text, propeller shafts in particular are equivalent conventional stainless new, it seems each mill or producer run hard in a corrosive environment, steel alloys, its price is more stable offers its own grade. As volume making almost any improvement to and at times lower. increases, those that offer the best their alloy worthwhile, especially at Common austenitic and ferritic stain- properties will likely rise to the top. the right price. less steel alloys are referred to as A key factor in determining the Although Sweden’s Avesta single-phase austenite or ferrite, corrosion resistance of stainless steel Steelworks—now owned by Finnish respectively, and each has strengths is its pitting resistance equivalent metallurgical giant Outokumpu, one and weaknesses in corrosion resis- number, or PREN (see the table). of the world’s largest and oldest tance, machinability, cost, and tensile While it’s not the whole story, a stainless steel manufacturers—cast strength. In the 1920s it was deter- metallurgical law dictates that greater the first duplex stainless steel close to mined that a hybrid alloy of the two corrosion resistance results in a century ago, it wasn’t really could offer distinct advantages, pri- reduced strength. Duplex stainless adopted until about 30 years ago. marily higher strength pound for alloys come close to disproving that Due in part to the need for stronger, pound, good weldability, greater law, and are particularly useful for lighter, more corrosion-resistant toughness than that of ferritic alone, certain applications such as propeller alloys for the offshore oil and gas and better resistance to stress corrosion shafts, where SCC is especially common. With some exceptions, formability Comparison of Stainless Steel Alloys and machinability of duplex stainless Grade EN No/UNS Type Typical PREN steel are not as good as those of aus- 430 1.4016/S43000 Ferritic 18 tenitic and ferritic alloys. Its ductil- 304 1.4301/S30400 Austenitic 19 ity is also compromised, and its 441 1.4509/S43932 Ferritic 19 complex metallurgy makes it more RDN 903 1.4482/S32001 Duplex 22 challenging to produce. Still, its 316 1.4401/S31600 Austenitic 24 enhanced corrosion resistance is very 444 1.4521/S44400 Ferritic 24 attractive. There are three grades of 316L 2.5 Mo 1.4435 Austenitic 26 duplex stainless steel: standard, 2101 LDX 1.4162/S32101 Duplex 26 which includes 22% chrome; super, 2304 1.4362/S32304 Duplex 26 having 25% chrome and a PREN of DX2202 1.4062/S32202 Duplex 27 40; and hyper, with a PREN of 48. 904L 1.4539/N08904 Austenitic 34 Again, each of these alloys, like most 2205 1.4462/S31803/S32205 Duplex 35 Zeron 100 1.4501/S32760 Duplex 41 metals, has trade-offs that we must recognize and work with or around. Ferrinox 255/ 1.4507/S32520/S32550 Duplex 41 Uranus 2507Cu If your application calls for a 2507 1.4410/S32750 Duplex 43 highly corrosion-resistant stainless 6% Mo 1.4547/S31254 Austenitic 44 alloy, duplex stainless steel may be This table lists the pitting resistance equivalent number, or PREN, of some worth a closer look. duplex stainless steels and a selection of austenitic and ferritic grades. —S.D.

52 Professional BoatBuilder in the marine trades is a function of (formerly columbium), and manga- steel alloys. Ideally, these mixtures cost and availability. It’s less expen- nese. These formulations are almost yield the preferred combination for sive than 316 and does a reasonably exclusively for solid propeller and the environment in which shafts good job when not called upon to do rudder shafts, although keelbolts operate. something it was not designed for. As and rigging are also available in these As a boatyard manager and now as for availability, a 316 hex-head cap or similar alloys. Such shafting comes a marine consultant, I have seen screw, say, is rarely stocked by com- in several grades, typically 17, 19, and countless severely corroded propeller mon chandleries and must be special- 22, with the last being the most resis- shafts. I suspect, with some excep- ordered from hardware suppliers. tant to corrosion, and the first more tions, that they are made of lower Most builders just go with the 304 resistant to yielding, meaning it’s grades of shaft alloys, specifically 17, screw on the shelf. stronger. or less-exotic and thus less-expensive Note that 304 or 316 stainless to be Because propeller shafts require and less-corrosion-resistant non- welded must include the suffix “L,” as exceptional strength and corrosion proprietary shaft alloys such as 304 in 304L or 316L, denoting a lower car- resistance, they are usually fabricated or even 316. As one might expect, bon content than in ordinary stainless from one of these proprietary stainless super-corrosion-resistant alloys steel; lower carbon helps prevent weld migration. I’ll discuss this later. Another derivative of stainless steel Pitting corrosion on could be considered a super-corrosion- a propeller shaft resistant metal of sorts. Commonly, almost certainly fab- and often mistakenly, referred to by ricated from conven- its well-known trade name, AquaMet, tional shaft stock. it is simply the best known of a num- The need for high ber of proprietary stainless-steel-based strength and corro- shafting alloys. sion resistance has With slight variations, these alloys driven the develop- possess the basic ingredients of 316 ment of proprietary stainless steel plus several other stainless-steel-based elements such as tantalum, niobium shafting alloys.

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54 Professional BoatBuilder come with a justifiably higher price than 22 series shaft alloys unless like holes. The chlorides in seawater tag, and they still require cathodic specifically required by a vessel’s accelerate the destruction of the oxide protection in the form of zinc or alu- naval architect or builder. layer, which is readily replaced as minum anodes. Despite that, when long as oxygen is present. But with- comparing the potential cost of shaft Stainless Problems out oxygen, the chlorides win, the corrosion and failure, it’s difficult to Although the advantages and oxide layer diminishes, and the stain- make an argument for the initially less popularity of stainless steel make it less steel starts to corrode more like costly alloys, particularly in recre- seem virtually indestructible, it mild steel. ational vessels where disuse exacer- suffers primarily from two types The most likely location for crev- bates crevice corrosion. of common corrosion: crevice and ice corrosion is in stainless steel Shaft failure includes shafts that pit carbide precipitation. alloys used for raw-water and under- severely in the region of cutless bear- Crevice, or pitting, corrosion water hardware, most commonly ings and stuffing boxes, which while occurs when stainless steel is exposed propeller shafts, struts, raw-water not necessarily in danger of breaking to an oxygen-depleted environment intakes and discharges, sanitation can still be condemned and require for an extended period. The tough, plumbing, exhaust systems (particularly replacement. Shafts manufactured nearly impenetrable oxide film that diesel exhaust, which when mixed from ordinary 304 or 316 stainless fre- forms as soon as stainless steel is with water becomes acidic), shaftlogs quently suffer from crevice corrosion exposed to air (the term inox on many and related fasteners, and bolts and attacks (discussed in the following European stainless components is sim- screws installed below the waterline section), occasionally within less ply an abbreviation for inoxodizable, to secure hardware such as struts, than a year of service, often beneath which refers to stainless steel’s resis- seacocks, strainers, swim platform cutless bearings and stuffing-box tance to staining) remains intact only supports, etc. packing, where the damage cannot as long as the metal is exposed to Because nearly all commonly avail- be easily observed. oxygen. When stainless steel is placed able stainless steel fasteners are made When sourcing shafts for your in an environment starved of oxygen of 304 alloy, they are exceptionally customers, ask for verification, in and is exposed to water, fresh or salt, prone to crevice corrosion when writing, of the specific alloy and it becomes susceptible to crevice cor- installed below the waterline or in a brand your supplier delivers. Again, rosion, which typically manifests as splash zone. They are particularly vul- it’s difficult to justify anything other roughness, valleys, pitting, or worm- nerable where they cannot be easily IYRS.EDU IYRS School of Technology & Trades - Immerse yourself for six months with skilled instructors, industry experts and craftspeople. Learn career skills in composites technology and construction. Be in demand, and ready to work in almost any industry. Register for an open house at IYRS.edu. Make anything possible.

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December/January 2014 55 Left—This bolt’s head and shank parted ways, likely from a combination of a low-quality manufacturing process and subsequent stress corrosion. Right—Crevice corrosion, occurring where the metal is simultaneously starved of oxygen and exposed to stagnant water in a bilge or stuffing box, is a common failure mode on T-bolt clamps like this one, manufactured with a metal fold that traps water. inspected: as they pass through the steel fasteners pass. Exacerbating this lets the boat owner know it’s time to hull, for instance. The exposure to problem is the common practice of bed- re-bed deck hardware. water does not require submergence. ding the inside portion of hardware— To avoid installing stainless steel in Stainless steel imbedded in hydro- washers and nuts—virtually guaran- applications where it is regularly scopic materials such as wood, foam teeing that when deck bedding fails, exposed to stagnant or still water for or balsa core, or even fiberglass may water will intrude and then stagnate extended periods, builders should suffer from crevice corrosion. in the void. Without inside bedding, choose alternatives such as high-quality This type of corrosion also occurs water is more likely to drain, reducing the silicon-bronze, cupronickel, Inconel on deck where water migrates into likelihood of crevice corrosion and 625, Monel, or nonmetallic alternatives. penetrations through which stainless core saturation; and the annoying drip This is particularly true for fasteners

56 Professional BoatBuilder As shown here, crevice carbide, at which point the chrome is corrosion also manifests no longer available to help the alloy where fasteners or deck resist corrosion. Essentially, this leaves hardware penetrate a cored or a strip of what amounts to ordinary solid deck, allowing water to mild steel on either side of the weld. intrude and stagnate around This region, known as the heat- the metal component. Don’t affected zone, becomes susceptible to rebed these components on corrosion. This is usually noticeable the interior, as the bedding to even the inexperienced observer will trap water in the bore as a discolored brownish area extend- hole. ing outward from the center of the weld as much as an inch (25mm). Other than reduced corrosion resis- Carbide precipitation, tance, the material in the weld area sometimes referred to as is unaffected. weld decay, can occur when Related to carbide precipitation is a the incorrect alloy of stainless form of corrosion caused by galvanic and raw-water plumbing. Avoid stain- steel is used in welded applications. or electrical interaction across the less steel fasteners for struts, strainers, Low-carbon-content stainless steel, metallic grain structure itself, and and other underwater hardware instal- noted as 304L, 316L, or 317L, must be between the boundaries of grains, lations. Bronze (true zinc-free bronze, used in fabrication. When carbon is impurities, and the alloying elements. not brass) or Monel is a better choice. not present, chrome carbide will not Known as intergranular corrosion, If stainless steel must be used for a form when the metal is heated. In this can be caused by welding, over- custom-made strut or shaftlog, for common higher-carbon alloys the heating, or stress annealing. It can instance, it should be fabricated from heat of welding (or any heat between occur also as a result of overly aggres- the most corrosion-resistant, readily 800°F and 1,600°F/427°C and 871°C) sive grinding (which causes local weldable marine alloy available, 316L drives chrome ions out of the weld heating), or when the metal is not or 317L, primed and antifouled and area; the chrome becomes tied up adequately gas-shielded during weld- provided with cathodic protection. with the carbon, forming chrome ing (in the latter case particularly for

December/January 2014 57 Left—Corrosion emanating from weld decay, also known as weld migration, on this stainless steel exhaust system led to complete failure. Right—Susceptibility to crevice corrosion makes stainless plumbing less than ideal for raw-water applications. In this case, the problem is amplified by the use of the incorrect high-carbon stainless alloy.

high-carbon-stainless alloys). layer and the chrome-poor region likelihood of corrosion via anodic While stainless steel owes its corro- must be removed, either mechanically interaction. sion resistance to the oxide layer that or chemically. Ideally, mechanical A final note on stainless steel raw- forms on its surface when exposed to removal is by fine-grit sanding media water plumbing: I often see attractive, air, an overly thick oxide layer, often or less desirably by shot-blasting or intricately welded, and polished raw- identifiable by its blue hue, can be wire-brushing (the alloy of the wire water components that are assidu- detrimental and can form as a result brush can actually exacerbate the ously bonded. I presume builders of the above-described scenarios. The problem). Chemical treatment with a include these bonding connections area beneath the layer is depleted of mixture of nitric and hydrofluoric acid for the perceived cathodic protection chrome, impairing its corrosion resis- is another option. A final treatment of they offer, but in virtually every case, tance. To resolve this issue, the oxide chemical passivation will reduce the the anode is located too far from the

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58 Professional BoatBuilder The hard-to-identify those that remain idle for long peri- characteristics of this ods, when oxygen levels are often low failure, which is limited to in the water surrounding the shafts in the weld material rather the logs and under packing and cut- than the parent metal, less bearings—greater corrosion resis- are most likely caused by tance is usually of more value. I intergranular corrosion. would be hard pressed to select any- thing other than a 22 series propeller shaft unless the modest additional that includes the name strength was absolutely essential. of the manufacturer or Not long ago I was involved in a distributor, as well as the case wherein two propeller shafts alloy from which they aboard the same vessel were afflicted are made. with the same type of corrosion in the internal plumbing component to offer It’s also important to specify what’s same location after approximately six any protection. Bonding fittings like best suited for the application and to months and 400 hours of operation in this may afford some protection from know what you are getting. For tropical water. The corrosion was dis- stray-current corrosion; however, instance, the most corrosion-resistant, covered because of a seemingly unre- when compared to crevice corrosion readily available proprietary shaft alloys lated cutless bearing failure. The bear- it is far less common. While such supplied in North America carry a ing had been added when the shafts bonding connections can’t hurt, they 22 suffix (they’re nonmagnetic); less- were replaced, a detail that became offer little if any protection from crev- corrosion-resistant versions (which relevant later in the diagnosis. ice corrosion. are magnetic) carry a 17 and 19 suffix. During shaft removal to diagnose The less-corrosion-resistant shafts and replace the failed bearing, we Conclusions possess greater tensile strength for a found corrosion where stuffing box Obtain stainless steel stock and given diameter, so they may be speci- packing made contact with the shaft components from reputable suppliers, fied by naval architects for applica- and where the shaft was supported by especially proprietary shaft material. tions where this strength is required. the added bearing. We confirmed the Request documentation with shafts For most recreational vessels—and shaft alloy by sending a sample to a THANK YOU

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December/January 2014 59 Slight misalignment caused this with optical and laser alignment. shaft to be stressed at a support- Ultimately, to avoid corrosion and ing bearing, which led to stress failures, one should employ stainless corrosion cracking and the pitting steel only where it’s best suited for visible here, after just 400 hours the task, meaning, where it’s better of operation over six months in than other alloys, and specify the tropical water. High friction and poor right stainless alloy for the job. The water flow raised the temperature tanks mentioned at the beginning of of the alloy. Combined with exposure this article were fabricated from 304 to seawater, these conditions series stainless steel; 316L alloy created an ideal environment for almost certainly would not have suf- this type of corrosion. fered a similar fate. For submerged applications such as struts, rudders, and skegs, rely on 316L only as an the shaft to run hot in this alternative to other preferred alloys region. This condition, exacer- such as bronze, and be sure to coat lab for analysis. The percentage of bated by the misalignment, led to them with epoxy primer and antifoul­ each alloy was within the manufactur- localized pitting. ant. And always use and maintain er’s specifications, ruling out an infe- Sea trials with the original setup, cathodic protection. rior alloy as the culprit. taken before the corrosion was dis- The ultimate cause of the corrosion covered, confirmed the stuffing boxes About the Author: For many years a was stress corrosion cracking, i.e., were overheating. Subsequent sea trials full-service yard manager, Steve now being hot, salty, and stressed (it was after the corrosion was discovered works with boat builders and owners surmised that slight misalignment confirmed that the added shaftlog and others in the industry as “Steve was causing the shafts to “work” over bearing was also overheating. When D’Antonio Marine Consulting.” He is the bearing). The boat had long the shafts and bearings were the technical editor of Professional shaftlogs, and while under way, the replaced, an added water-injection BoatBuilder, and is writing a book on stuffing boxes and cutless bearings system reduced the temperature; and marine systems, to be published by were insufficiently lubricated, causing shaft-to-bearing interface was adjusted McGraw-Hill/International Marine.

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60 Professional BoatBuilder