Following up with the theme of what a surveyor can not see but can only summarize or, although not meant to do, surveyors can speculate in a wordy way; better check the bottom of your tank as I can't see it, therefore, neither can you.
Tankage comes in many forms, and our leading example is fuel.
In this photo, we see the bottom section of an aluminum tank approximately eight years old. It received some saltwater exposure during its installation period and was removed once for repair.
The photo shows more than one section of the tank developing corrosion and having been smeared with some form of epoxy to prevent this from deteriorating further or leaking. Still, the leaking is and was only one step away. The epoxy was no longer holding the fuel.
Surveyors need help seeing below, around, or under most fuel tanks. Help would mean removing the tank to allow full access. You are lucky to see even a tiny percentage of what should be viewed. The most robust material must be considered by placement, history, and knowledge base factors, with deduction or recommendation.
Our study photo came from a catamaran whose tank was constructed and CE-approved. Its placement was within the aft cabin and below a bunk; therefore, you may assume it would be high and dry. This was not the case, and the other tank from the other hull was in a further state of ‘leak’ with pronounced corrosion. The background or history tells a different story, as this was a hurricane-damaged vessel. The vessel had received salt water in this section of the boat, and the tank sat in it just enough time to create issues. When this style of tankage is placed in an engine compartment, the exposure increases as the water-tight deck access is only sometimes that water-tight to prevent intrusion.
For a while, this particular builder also constructed holding tanks from aluminum. These, too, had their issues. Internally, they clogged with calcification, and externally, they did exactly what you see in the picture and corroded, except this time, it was not fuel leaking.
Is this normal? Well, even aluminum can be broken down with enough exposure. The outcome becomes the photo when salt water is deposited to aluminum and left.
What about steel or plastic? I hear you ask, but you didn’t ask, so I will tell you anyway.
All materials have their faults. A modern plastic water tank can eventually split, most commonly around a seam or where it has to bridge an internal stringer.
Depending on your location, steel can suffer from internal and external condensation. Condensation makes corrosion.
As boat owners all around the world will tell you. It is a boat. No extended guarantees exist on any item that has to live around the ocean. Only inspection and preventive treatment can stop the incurable or a replacement promptly. For this tank, the answer was to replace the entire bottom section, and it came back looking like new and should, now the thickness has increased, be double or more in its future life expectancy and be trouble-free.
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