Other Forms of Corrosion
There are many other specific forms of corrosion in the marine environment common to the boating public. These include Stress Corrosion Cracking of stainless steel, Poultice Corrosion, Microbial Induced Corrosion (diesel fuel tanks a likely source), General Corrosion (corrosion which attacks the internals of exhaust manifolds and risers), Cavitation Corrosion (from collapsing air bubbles impinging metal surfaces), to mention a few. There’s a lot out there that can damage your investment! Stress cracking and Poultice corrosion will be briefly discussed below since they pose a potential safety hazard to afflicted vessels.
Stress Corrosion Cracking
In stress corrosion cracking of stainless steel, corrosion, which can begin as a microscopic pit in the surface of the metal, migrates into the meat of the metal along or through grain boundaries which are highly susceptible to corrosion. The conditions which contribute to this form of corrosion are: time (in years), tensile stress on the component (a stress which is trying to pull it apart), and a corrosive environment (like saltwater).
Examples of commonly affected items are stainless steel chain plates on sail boats. These plates are used to attach a sailboat’s rig to the vessel. The damage usually starts where the chain plate is embedded through a deck where moisture can accumulate and remain due to breakdown of the sealing system. The problem with this type of corrosion is that the chain plate can look fine from the outside on deck, but it’s essentially rotting on the inside. The result can be catastrophic failure of the component causing a rig to come down. The appearance of this type of failure is that of rotting wood on the inside of the material. The probability of damage is dependent on the magnitude of the factors discussed above. We recommend that chain plates be completely inspected every 10-12 years. To inspect these plates they have to be completely exposed, a cost that should be factored into the decision to purchase a particular vessel. If you see any discoloration weeping from the chain plates, or see any signs of pitting or surface cracking, this plate must be replaced and the remaining plates considered suspect. Here are a few pictures.
This form of corrosion can attack all metals, but aluminum is the most highly susceptible metal found in most pleasure boats. When moisture is held adjacent to something made of aluminum, and there is an absence of a supply of air at the interface, a chemical reaction takes place at the surface of the metal. The aluminum is transformed into an aluminum hydroxide substance which looks like a thick, white, pasty substance. It has no strength at all. Large pits can be eaten into the material in a relatively short time period (months) and the result is usually a complete compromise of the material.
Poultice corrosion most commonly manifests itself in the damage of aluminum fuel tanks. Up until very recently, boat manufacturers typically stabilized fuel tanks with materials that were moisture absorbent (like spray-in foams) or with supports where moisture could be trapped between the support and the tank (like rubber placed between the tank and the securing straps). As long as the areas stayed perfectly dry there were no problems. But low in the boat where water can accumulate and get splashed around, fuel tanks have been affected.
The first sign of trouble is usually the smell of gasoline in the compartment holding the fuel tank. Repairs can be very costly, and the dangers associated with leaking gasoline are significant. If you must replace a tank due to this type of damage, the method of securing the tank must be modified to protect the new one. One method is to encapsulate the whole tank in coal tar epoxy (following the manufacturers surface preparation guidelines precisely) and then ensuring that neoprene rubber is placed between any securing hardware to prevent compromising the epoxy coating. Here’s a couple of pictures.