Fresh water tank crud

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stiggy

stiggy

Veteran Member
Joined
Apr 28, 2019
Messages
95
Location
USA
Vessel Name
Bailey
Vessel Make
46' Grand Banks Motor Yacht
Starting a refit of our tank monitoring and the first tank I’m working on is our aft fresh water tank. After I drained it I figured I’d have a look inside, kind of a good idea, kind of a bad idea! The tanks are aluminum. Am I looking at calcium deposits or something else?
So here’s some clips from the horror show:
 

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Right there, that's why we never drink the tank water. I don't care how clean it smells and tastes or how clean I think the tanks are or whether there's an inline filter. Even if those deposits are purely mineral, eew.
 
Hi Stiggy,

While no chemist, my investigations into exactly your issues with my aluminum water tanks has lead me to my personal opinion: aluminum fresh water tanks are absolutely wrong for pleasure boat use. Again, while no chemist, it is my belief that all marine-grade alloys commonly used for freshwater tanks react negatively to the presence of generic chlorine in most city water systems commonly used to fill our tanks when pierside. The presence of chlorine negatively reacts with the alloying elements in the tank walls (manganese, magnesium, chromium, etc.) to leach out what I call "crusticles" (see attached picture from one of my tanks). Are they aluminum oxide? Aluminum chloride? Only a mass spectrograph will tell for sure.

While I disagree that these crusticles pose a significant health risk, they DO form corrosion pockets under each and every one of them. These pits can, and absolutely WILL cause aluminum water tanks to eventually fail. My best, but educated guess is that aluminum water tanks aboard pleasure boats have a 20-year useful life. Beyond that, they're on borrowed time.

Unfortunately, I know of no "fix" for the situation, as few water tanks in production boats are designed for routine inspection, which MAY allow routine cleaning, and possible mitigation of the crusticles before they fail the tank. "Mouse milk" (liquid sealers, for instance) have limited viability, primarily because of the inability to access 100% of the tank's interior on a routine basis.

In my opinion, the only option for tanks in your condition is to remove them, and replace them with either custom made stainless (316-L) tanks, or plastic. In my case, I've been unable to locate plastic tanks with sufficient volume or correct form factor to allow me to use plastic tanks for my replacements, and have used custom welded 316-L tanks as my solution to the exact situation you find yourself in. And yes, I've looked through the Ronco catalog (huge, but not huge enough for my purpose!) until my teeth hurt to no avail. And yes, I've investigated custom welded plastic tanks as well. Believe it or not, custom stainless tanks are, in fact, cheaper and WAY more available, at least here in the Pacific Northwest.

Regards,

Pete
 

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jungpeter said:
Hi Stiggy,

While no chemist, my investigations into exactly your issues with my aluminum water tanks has lead me to my personal opinion: aluminum fresh water tanks are absolutely wrong for pleasure boat use. Again, while no chemist, it is my belief that all marine-grade alloys commonly used for freshwater tanks react negatively to the presence of generic chlorine in most city water systems commonly used to fill our tanks when pierside. The presence of chlorine negatively reacts with the alloying elements in the tank walls (manganese, magnesium, chromium, etc.) to leach out what I call "crusticles" (see attached picture from one of my tanks). Are they aluminum oxide? Aluminum chloride? Only a mass spectrograph will tell for sure.

While I disagree that these crusticles pose a significant health risk, they DO form corrosion pockets under each and every one of them. These pits can, and absolutely WILL cause aluminum water tanks to eventually fail. My best, but educated guess is that aluminum water tanks aboard pleasure boats have a 20-year useful life. Beyond that, they're on borrowed time.

Unfortunately, I know of no "fix" for the situation, as few water tanks in production boats are designed for routine inspection, which MAY allow routine cleaning, and possible mitigation of the crusticles before they fail the tank. "Mouse milk" (liquid sealers, for instance) have limited viability, primarily because of the inability to access 100% of the tank's interior on a routine basis.

In my opinion, the only option for tanks in your condition is to remove them, and replace them with either custom made stainless (316-L) tanks, or plastic. In my case, I've been unable to locate plastic tanks with sufficient volume or correct form factor to allow me to use plastic tanks for my replacements, and have used custom welded 316-L tanks as my solution to the exact situation you find yourself in. And yes, I've looked through the Ronco catalog (huge, but not huge enough for my purpose!) until my teeth hurt to no avail. And yes, I've investigated custom welded plastic tanks as well. Believe it or not, custom stainless tanks are, in fact, cheaper and WAY more available, at least here in the Pacific Northwest.

Regards,

Pete
Click to expand...
Pete,
Thanks for the reply. Our aft tank is 145 gals and under our generator. I guess I'll need to start looking at replacement options in the near future.
Do you mind sharing some of the places you contacted up here in your search for replacement?
 
When replacing my water tanks I discovered a full size rubber mallet that had been left inside during fabrication. Forgotten during lunch break I suppose.
 
That looks like biomass to me. I work in the corrosion industry and aluminum is a great choice for many things, unless the pH of the water is high. Aluminum is resistant to corrosion if the pH is slightly acidic. Many municipal waters are alkaline (pH>7.0, and many times, >8.0). That's not normally aggressive water, but not ideal for aluminum.

I'd be curious if you ever add chlorine to this tank or if the water you fill with is chlorinated. If the water sits stagnant more than a few days, the chlorine will dissipate and not be there to kill "bugs" (bacteria, etc.). I normally add a small amount of chlorine to storage tanks at each filling. About 1/4 ounce bleach per 100 gallons of water.

But, 316L is the best, by far, IMO. The only common thing that attacks stainless is chlorides (not the same as chlorine, by the way) and only then above 200 ppm or so.

If you can upload or send me a higher-resolution picture, maybe we can tell, for sure, what it is.
 
H2O_Doc said:
That looks like biomass to me. I work in the corrosion industry and aluminum is a great choice for many things, unless the pH of the water is high. Aluminum is resistant to corrosion if the pH is slightly acidic. Many municipal waters are alkaline (pH>7.0, and many times, >8.0). That's not normally aggressive water, but not ideal for aluminum.

I'd be curious if you ever add chlorine to this tank or if the water you fill with is chlorinated. If the water sits stagnant more than a few days, the chlorine will dissipate and not be there to kill "bugs" (bacteria, etc.). I normally add a small amount of chlorine to storage tanks at each filling. About 1/4 ounce bleach per 100 gallons of water.

But, 316L is the best, by far, IMO. The only common thing that attacks stainless is chlorides (not the same as chlorine, by the way) and only then above 200 ppm or so.

If you can upload or send me a higher-resolution picture, maybe we can tell, for sure, what it is.
Click to expand...
I personally never add any but we’ve only had this for 10 months out of 25 years. I do not pre-filter into the tanks as I’ve been told to let the city water keep things clean.

I’ll try and get another picture next time it’s emptied. Do you have any recommendations on maybe reversing some of this?

If it isn’t blatantly obvious, I have no idea what I’m doing hahaha.
 
I'm of no help on your tank situation, but that first picture looks like you are growing oysters in the tank.

Ted
 
If you cannot find an AL or stainless tank replacement, you can get one or two "plastic" tanks to fill the space with a cross over line to allow for automatic leveling.
You may lose a bit of volume but nothing significant.

On my AT has a plastic holding tank and AL water tank.
 
Last edited:
stiggy said:
I personally never add any but we’ve only had this for 10 months out of 25 years. I do not pre-filter into the tanks as I’ve been told to let the city water keep things clean.

I’ll try and get another picture next time it’s emptied. Do you have any recommendations on maybe reversing some of this?

If it isn’t blatantly obvious, I have no idea what I’m doing hahaha.
Click to expand...

I'd need to do a little more research to be 100% sure, but I'd think about using peracetic acid (PAA) diluted in water. Breweries use this to clean tanks. It has the advantage of being an acid (so it can clean mineral scale/deposits and it's compatible with aluminum) plus it's a disinfectant, so it kills bacteria/algae/mold/fungus and those mussels you have growing in there!

If you want to move forward with that, I'll do more research for you. Without knowing exactly what it is, though, it's a bit tough to pick the best cleaning method.
 
I saw the photo before I read your post. My first thought was “ that looks like an aluminum water tank”.

Aluminum is a horrible water tank material. Replace them.

The same thing happens in water heaters with aluminum tanks. I always use stainless or glass lined water heaters.

Buy plastic replacement tanks from Ronco Plastics. They make high quality tanks with a heavy wall thickness. They will put the fittings where ever you want them. They make almost 500 different size and shapes of tanks.
https://ronco-plastics.com/
 
kthoennes said:
Right there, that's why we never drink the tank water. I don't care how clean it smells and tastes or how clean I think the tanks are or whether there's an inline filter. Even if those deposits are purely mineral, eew.
Click to expand...

If that bothers you dont ever work on you supplu lines in a house.
 
We had custom welded plastic tanks made for a previous boat. Worked well but they weren’t cheap. But that way I could maximize the usage of the available space.
 
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