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As a professional marine surveyor, I’m used to seeing a wide range of vessel conditions, from well-maintained labor-of-loves to neglected disasters. I know I've partly covered this topic in a past article, but recently I came across something that left me genuinely shocked. While performing an inspection on a sailing catamaran that had been meticulously cared for by a very competent sailor, I found extreme corrosion on a new through-hull and ball valve seacock—fittings that were only six months old!
The corrosion was concentrated around a single seacock below the waterline, and all the surrounding metal in the starboard bilge showed signs of corrosion as well. Curiously, the corrosion diminished the farther I moved from the seacock. This presented a mystery: if a stray DC current in the marina or onboard had been the cause, the vessel’s zinc anodes would have been heavily worn, and more fittings would have shown similar damage. Yet, the zincs were intact, and the corrosion seemed localized.
Uncovering the Cause: A Leaky Washing Machine and Laundry Detergent
Upon further investigation, I discovered that this heavily corroded seacock was the discharge point for a recently installed washing machine. The mystery deepened until I noticed a critical detail: the discharge line had been leaking, allowing laundry detergent to spill into the bilge. This was the smoking gun.
Laundry detergents, commonly contain ammonia, and ammonia is highly corrosive to metals. Ammonia can cause metals to break down through a chemical process that accelerates corrosion, particularly in the already harsh marine environment. When ammonia interacts with metals like brass or bronze, it can break down the protective oxide layer, exposing the metal to faster corrosion. The combination of moisture, salt, and ammonia in the bilge created a perfect storm for this damage.
The Science of Corrosion: How Ammonia Eats Away Metal
Ammonia is a compound of nitrogen and hydrogen that can disrupt the protective oxide layers on certain metals, leading to pitting and corrosion. In a marine setting, ammonia intensifies the corrosive effects of seawater and humidity. When ammonia mixes with water, it forms ammonium hydroxide, which is highly aggressive to brass, bronze, and other common marine metals. Once these protective barriers are broken, corrosion sets in, and without immediate intervention, structural integrity can be compromised.
Another process of ammonia induced corrosion is call Ammonia Stress Corrosion Cracking or SCC. SCC occurs when the metal is exposed to both mechanical stress (or residual tensile stresses) and ammonia in the environment. This process affects carbon steels as well as copper and zinc alloys.
In this case, the laundry detergent accelerated corrosion in the starboard bilge, attacking not just the seacock but also the surrounding metal. Luckily, the seacock’s threads were still intact, though it required a special inspection.
Taking Action: Emergency Inspection and Cleaning
The vessel was scheduled for a 1,500-mile delivery along the eastern seaboard just a few days after my survey. To ensure the boat’s safety, I made a rare exception to the standard marine survey protocol and conducted a light destructive test. After cleaning the threads just behind the through-hull nut with a wire brush, I was relieved to find that the integrity of the fitting had not been compromised.
Generally, surveyors avoid destructive tests, but in this case, the boat’s owner and manager requested a more detailed inspection. Thanks to this extra step, we avoided condemning the through-hull and delaying the delivery.
The Lesson: Keep Corrosive Cleaners off Your Boat!
This situation serves as an important reminder: be extremely careful with the cleaners and detergents you bring aboard. Products containing ammonia, bleach, and other harsh chemicals can do significant damage to your vessel over time. Ammonia, in particular, can corrode metal fittings, and bleach can damage polyester gelcoat finishes. Here’s a closer look at how these chemicals affect boats:
- Ammonia and metal: As we saw in this case, ammonia breaks down the protective oxide layer on metals, accelerating corrosion and weakening essential fittings. It is also highly destructive to aluminum, here is a link to some info on that.
- Bleach and polyester gelcoat: Bleach can react with polyester resins found in gelcoat finishes, leading to discoloration and weakening of the surface. Bleach is a strong oxidizing agent, which means it can break down the chemical bonds in polyester resins and gelcoats. Polyester materials are typically composed of cross-linked polymers, and the oxidation caused by bleach can disrupt these bonds. This leads to:
Loss of mechanical strength: Over time, the structural integrity of the resin weakens, and the surface of gelcoat gains more and more porosity.
Cracking and brittleness: The surface may become more brittle, making it susceptible to cracking, particularly under stress or environmental exposure.
Chalkiness: The surface may lose its smooth finish and become rough or chalky as the top layer degrades.
To avoid these issues, always read the labels of any cleaning product before bringing it aboard your vessel. When in doubt, it’s better to throw it out (not into the water, of course!). If you hire a cleaning service, make sure they have extensive experience with boats and use marine-safe products. And always ensure they are bonded and insured, so you have recourse if something goes wrong.
Conclusion: Safe Sailing Starts with Smart Choices
Corrosion is a silent killer in the marine environment, and even well-maintained vessels can fall victim to hidden dangers like this one. By paying close attention to the products you use on board and working with a professional marine surveyor, you can catch potential issues before they turn into costly repairs or, worse, boat sinking hazards. This situation was a close call, but thanks to a thorough inspection and quick action, the vessel was able to proceed on its journey without delay.
Remember, keeping harsh chemicals like ammonia and bleach off your boat is essential for maintaining its longevity and integrity. When it comes to cleaners, always choose marine-safe options, and when in doubt, consult a knowledgeable marine surveyor.
Safe sailing!
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