Why do they call it stainless if it still rusts?
A couple of boat show attendees stopped at my booth and looked at the section of a sailboat bow and foredeck we had on display. My astute associate had rescued it from a boat that was being scrapped due to years of neglect. A number of the fasteners securing cleats, anchor-roller bracket, pulpit mounts, stem fitting, and so on, were heavily corroded — especially on the underside of the foredeck. One turned to the other and said with great certainty, “Those bolts aren’t stainless steel,” and off they walked. Unfortunately, that encounter is emblematic of the level of misunderstanding about stainless steel and corrosion.
In fact, the fasteners used by that sailboat manufacturer were likely a good quality 300 series stainless steel. So, why were the fasteners on this bow section, and so many other pleasure boats, so heavily corroded? The answer is as hidden as the source of the corrosion itself. First, we must learn a little about what makes stainless steel stainless.
The science of stainless steel is incredibly complex, and a deep understanding is a formidable task. Fortunately, the average boat owner doesn’t need to know much beyond the basics to be able to prevent or to recognize and correct most stainless-steel corrosion. Most marine-grade stainless used on production boats is from the 300 series. These stainless steels are suitable for a wide range of marine applications. Type 304 is a good, multipurpose steel. The Gateway Arch in St. Louis is clad with 304. Type 316 and 316L have a slightly higher nickel content and added molybdenum to improve their corrosion resistance over 304 — especially with regard to pitting and corrosion in chloride environments. Type 316L has a lower carbon content than 304 and 316 to avoid carbide precipitation in welds.
The single most important fact about stainless steel that all boat owners should know is that the chromium in the steel combines with oxygen to form an invisible surface layer of chromium oxide that prevents further corrosion from spreading into the metal’s internal structure. According to Anne Marie Helmenstine, Ph.D., “This protective film will self-repair if damaged, if sufficient oxygen is present. Stainless steels have poor corrosion resistance in low-oxygen and poor circulation environments. In seawater, chlorides from the salt can attack the passive film more quickly than it can be repaired in a low oxygen environment.”
With this basic knowledge, what makes decent quality stainless steel corrode aboard your boat? For simplicity, let’s assume that whenever the word stainless is used, we are referring to a grade-300 series such as 304, 316, or 316L, as these are typically very suitable for a majority of marine exterior applications. Also, in the spirit of simplicity, we will use the general term “corrosion” to represent the many different types of corrosion that can attack stainless steel. Differentiating between them can require a metallurgist and usually isn’t necessary for our purposes.
Stainless steel is “normally” corrosion resistant if the correct grade is selected, properly fabricated and finished, and correctly installed in the appropriate application. Most U.S. production boats were built with appropriate grade stainless steel and are reasonably well assembled. So, what caused the fasteners on our section of bow to corrode so heavily? The short answer is the same as the reason this vessel was cut up and sent to a landfill: lack of maintenance.
The knowledge of what causes stainless steel to corrode is one part of the equation. Knowledge of basic vessel construction of decks, cabin tops, and cockpits (horizontal surfaces) is the other crucial part. The horizontal surfaces on most production boats are a composite or sandwich consisting of an inner and outer layer of fiberglass with an inner core of plywood, end-grain balsa, or foam. The upside of this type of construction is that it adds rigidity with relatively lightweight; plus it provides excellent thermal and acoustic insulation. The downside is that the core can absorb water if the caulk used to seal the deck fittings breaks down. Since very few boatbuilders isolate this core from the fasteners, it will absorb moisture if there are any leaks. And, it holds the moisture in contact with the fasteners just like a sponge.
Deck hardware requires periodic recaulking to prevent leaks. This is especially true of hardware subject to high or sudden loads like stanchion and pulpit bases, anchor rollers, windlasses, cleats, and chainplates. Time and flex breaks the bond between the caulk and the hardware. This allows water to leak alongside the fastener where it passes through the structure. This moisture is held alongside the stainless steel where it penetrates the deck or cabin top. Over time, the oxygen naturally contained in the water is consumed, and the chromium oxide film on the fastener is eroded. The result is the stainless switches gears from passive to active. As the iron in the fastener corrodes, it expands and begins to push in all directions resulting in the rust blobs on the underside of our chunk of foredeck rather than as the result of using non-stainless steel fasteners. Boat owners often describe the staining around the base of deck hardware as a “blush” caused by saltwater. While a very light surface staining may fit this definition, in most cases it is the result of water getting in under hardware. Visible corrosion requires immediate action.
The lack of oxygen in the presence of an electrolyte (water) is only one reason stainless steel goes bad. A number of other factors can cause stainless steel to corrode or fracture. Some examples can be found on imported vessels of the 1970s and 1980s. Much of the stainless was of questionable heritage; welding was often improperly performed; installation techniques were often poor; and water leaks seem to have been engineered in. Corroded hose clamps, pulpits, hand rails, mufflers, and tanks were commonplace. A majority of them corroded in or adjacent to the welds.
Boat owners must be vigilant when purchasing stainless steel. Fasteners, clamps, valves, and other fittings are sold by a wide variety of vendors. Don’t be lured in by low price or the convenience of a neighborhood store. Specify the grade stainless you want. Ask the clerk to verify the grade he is selling you. Take a small magnet with you when you shop. 300-series stainless steel will be nonmagnetic or very close to it. When checking hose clamps, test the clamp screws as well the bands. Many “stainless steel” hose clamps have plated-steel screws that will turn into rust blobs allowing the clamps to take a swan dive into the bilge. Quality costs. But, it only costs once.
Vigilant Boat Owners Can Help Prevent Underwater Gear Corrosion
- Remove barnacles to prevent pitting that will develop under their crusty little bodies (even a small surface pit on a prop shaft can cause cavernous areas of crevice corrosion below the surface).
- Apply a proven anti-growth coating such as Prop Speed or Velox Plus at the next haul-out to prevent marine growth.
- Periodically run the engine(s) in gear to flush stagnant, oxygen deprived water out of shaft logs and cutlass bearings.
- And, don’t forget to keep anodes fresh and active to protect bronze and aluminum underwater gear from galvanic corrosion.