Engineer’s Angle: by JD Anson
Time and again, cursing and screams can be heard up and down the dock from crew attempting to remove corroded fasteners. Voodoo dolls of previous crew hang in bosun lockers, and bad tidings are wished upon them regularly.
They usually did not have bad intentions. A lack of understanding of how to choose the correct materials for the job and how to install them properly caused headaches down the road. Many times, stop-gap fasteners are used with the good intention of changing them later, but are forgotten.
Many people assume that “stainless” means “rustproof.” More accurately, stainless is rust-resistant. The metal is formed of iron mixed with other materials to make it more noble – that is, less prone to corrosion.
Chromium, along with lesser amounts of other metals, is mixed in with the iron to create stainless steel. These metals react with oxygen from water or air to produce a barrier film of metal oxidation (corrosion) on the metal’s surface just a few atomic layers thick. This layer prevents oxygen from interacting with the iron below. Further, highly polished metal removes tiny crannies that corrosive salt molecules can hide in. This enhances the corrosion resistance of the stainless steel. It is important to fresh-water rinse the metals frequently, to prevent salt concentrations from build up by evaporation.
All metals are not created equal, and certain materials are better suited for use on yachts because of the corrosive environment. Being able to tell them apart is especially important in the marine environment. All metals should have a numerical designation. This is a code reflecting the amount of each material that is present in the alloy. Multiple types of designations exist for metals. Most common are the American ANSI/SAE and the European BS and EN nomenclatures.
While a fastener may be labeled as stainless steel, it is best to determine the exact alloy to decide if it is usable on board. For marine environments, 18/8 (alloy that is iron mixed with 18 percent chromium and 8 percent nickel) with added molybdenum is the preferred material. This is also known as AISI/SAE 316 and BS EN 1.44xx. Another common fastener material is 304 or 1.43xx stainless steel. This is the same 18/8 material without the added molybdenum. This metal is suitable for interiors, kitchens and regions away from coastal salt air influence. It is not recommended for use on board, especially on the exterior.
Now that the proper fasteners have been obtained, the next crucial step is installing them properly. Just as underwater metal components are protected from galvanic corrosion, so must be topside fasteners. Two factors come into play: salt corrosion and galvanic corrosion created by dissimilar metals.
Exposed stainless surfaces will create their own protective layer. But any portions not exposed to air will not receive the oxygen necessary to self-protect, and salt water can creep in via capillary action and corrode the fastener inside the screw hole, even in fiberglass and wood. When it comes to fasteners into metals such as aluminum yachts, the similarity or lack thereof is an even greater problem.
Whereas stainless steel is a noble metal, aluminum is less stable and becomes the anode to the stainless’ cathode. There is a potential difference of 840 millivolts between the two. Coupled with salt air, galvanic corrosion is virtually guaranteed to happen. This first shows as paint blisters around screw holes as the aluminum underneath corrodes and lifts the paint. While this appears to be cosmetic, real damage from pitting is taking place to the aluminum structure. The oxidation of the aluminum is creating a tighter hole, thus seizing the fastener into it. Here is where the cursing begins.
To alleviate the problem, the two metals must be either isolated from one another or kept from salt water. Nonmetallic washers can keep the heads from contacting the surface, physically separating the two metals. A product such as Tef-Gel or Duralac may be used on the threads. Rather than electrically isolating the two metals, when applied to both the screw and hole threads they create a seal that prevents saltwater from entering between. Normally, saltwater would be an electrolyte in the galvanic cell. But if it cannot get inside, the anode and cathode will not interact. This is even good practice in wood or fiberglass. A bonus is that they act as a lubricant to allow fasteners to be removed easier later.
As with the rest of the boat, using the right materials in the right way will make life easier for everyone, now and in the future. And prevent the strange stabbing pains that come from a voodoo doll.
JD Anson has more than 20 years of experience as a chief engineer on megayachts. He is currently project manager at Fine Line Marine Electric (finelinemarineelectric.com) in Fort Lauderdale. Comments are welcome below.