What all fuss about aluminum is about
Although there are rare exceptions (there are rare exceptions to EVERYthing), an aluminum--or any metal--holding tank will typically start to leak at a seam or a fitting within 2-5 years. By the time that happens, the inside of the tank has begun to pit...and once it starts there's no way to stop it...eventually the tank will turn into a collander. The average lifespan is about 10 years, even for 316 stainless...I've seen it happen in less...occasionally seen one go a year or two longer. Although you're right that metal is odor-impermeable (provided there aren't any holes in it), a good quality thick-walled (at least 3/8") plastic tank will last for literally decades without permeating OR corroding. The tank on my own last boat--under the v-berth--was 16 years old when I bought the boat, 21 years old when I sold her, and was in such good shape that it was the only part of the sanitation system I didn't replace. Never a trace of odor...nor will there be in another 10 years. Nor did we ever have a single odor permeation complaint for the entire 15 years we sold tanks. Thin-walled plastic tanks, otoh, may permeate, but few of them last long enough to do it...the average age of the cheapest tanks in the marine catalogs is even shorter than metal because they flex and crack under the weight of the contents. Early plastic tanks were even thinner...after enough bad experiences, boat builders switched to metal, believing it to be a superior material. And compared to those early plastic tanks, it is. But not to today's good quality, thick-walled plastic tanks. Fred, your aluminum tank is 10 years old now...you've done your best to minimize the corrosive effects of urine, and so have managed to extend the life of it considerably. But it's living on borrowed time, so I suggest you keep a very close eye on it and the area around and under it for any sign of moisture seepage, and keep a "weather nose" in its direction. 'Cuz sooner or later it has to start leaking, prob'ly on the bottom.
