Removing/Preventing Aluminum Oxide in Water Tanks

[eianm]

Originally Posted by benethridge

Hi, all.

A previous owner replaced the original water tanks with aluminum tanks. Since we have both been using city water which is chlorinated, it appears to be corroding the tanks slowly, i.e. creating aluminum oxide and pitting of the aluminum walls. (See attached pictures.)

I am trying to figure out the best way to:

(1) Disinfect the tanks and
(2) Remove the current aluminum oxide on the walls and
(3) Stop any further corrosion.

Unfortunately, the tanks are baffled with no access ports to the baffled areas. Also, whoever installed them built a new wood floor over them and apparently glued and screwed the floor in place (see attached picture), so it would probably be a big deal to rip up the floor, remove the tanks and take them out to a specialty shop for a thorough cleaning.

For disinfecting, I'm thinking to simply shock the tanks with chlorine using the standard procedure one can google.

For removing the current aluminum oxide, I'm thinking of putting a gallon of vinegar in each 50 gallon tank, filling with freshwater and letting it sit overnight.

For stopping any further corrosion, I'm thinking about going to a pool supply store and buying some dechlorinator. I will add the appropriate amount of the dechlorinator each time I fill the tanks with city water.

Thoughts?

Ben Ethridge aboard Driftwood in Miami FL

I wrote an article for Cruising World magazine a few years ago on refurbishing aluminum water tanks- i believe it is still online- fully detailed DIY instructions.

[john manning]

Aluminium oxide forms rapidly on aluminium surfaces and is the coating that prevents corrosion. Aluminium itself is a reactive metal.

Small levels of cloride ions will not affect the aluminium oxide coating unless the acidity increases for some reason.

Your problem may not be chemical. You may have a stray electrical current in the tank (eg. level gauge fault). I would also recommend complete electrical isolation. ie. Remove bonding wires to the tank.

[benethridge]

Originally Posted by eianm

I wrote an article for Cruising World magazine a few years ago on refurbishing aluminum water tanks- i believe it is still online- fully detailed DIY instructions.

I'm interested. Do you have the link or remember its title?

[benethridge]

Originally Posted by john manning

Your problem may not be chemical. You may have a stray electrical current in the tank (eg. level gauge fault). I would also recommend complete electrical isolation. ie. Remove bonding wires to the tank.

I've already checked that and had removed the bonding wires early on. I don't think that's the problem. See early on in this long thread where we all discussed that.

[thinwater]

magnesium anodes sold for boats are not pure Mg, but rather an alloy with a potential similar to Zn, and are used to protect copper alloys from dezinc. A different problem.



What you want is a Mg water heater anode. The potential is greater.



Note: Have not tested this in an aluminum tank.

[benethridge]

Originally Posted by thinwater

magnesium anodes sold for boats are not pure Mg, but rather an alloy with a potential similar to Zn, and are used to protect copper alloys from dezinc. A different problem.

What you want is a Mg water heater anode.

Not sure I agree. Here are my reasons:

1. Martyr makes magnesium alloy anodes specifically for general galvanic protection on boats in freshwater. I bought these Martyr anodes for my water tanks. The full article from the Martyr website:

'The reason why Mg anodes work better than Zn or Al in fresh water has to do with the higher driving voltage available when using Mg anodes. Fresh water is much less conductive to flow of ionic current, i.e. it has notably higher resistivity than salt water. Thus, the higher driving voltage of Mg is able to deliver more current than Zn. In other words, it is easier to overcome the resistance of the current path in fresh water with Mg anodes than with the same size, shape, and number of Zn anodes. Furthermore, with Zn anodes there is a risk of potential reversal at elevated temperatures in certain waters. With Al anodes, there is a risk of “passivation” in fresh waters, i.e. if the driving voltage is adversely reduced, insufficient current will be delivered for complete cathodic protection.'

https://martyranodes.com/about-freshwater-anodes/

This article from Fisheries Supply backs up the Martyr article:

https://www.fisheriessupply.com/unde...ificial-anodes


2. After a month of using them, the Martyr magnesium alloy anodes APPEAR to be working, i.e. there appears to be far less corrosion than before I installed the Martyr anodes. I will need to open the tanks again and inspect in another 6 months or so to confirm.

Bottom Line: Thus far, (and this is not a very scientific on my part - just trying various ideas to see what works) the problem appears to have been simple galvanic corrosion caused by small amounts (measured in parts per billion/million) of copper and/or other relatively noble metals in the city water.

I'll update this post when next I check the tanks.

Thanks for all your input.

Ben

[benethridge]

Hi, everyone.

Bad news: Neither the addition of the magnesium anodes (to eliminate galvanic corrosion of the aluminum) nor the use of the catalytic carbon filter (to remove chlorine/chloramine) has helped. After 2.5 months, I checked the starboard tank and the white slime pitting is as bad as ever.

I wish I could find a chemist to analyze it and tell me what it is for sure.

The magnesium anode appears to have been sacrificing itself for the aluminum. It had lost about 1/4 of its original size.

I'm pretty much out of my Wile E. Coyote ideas now. I'm thinking I'll just have to carve up the floor and replace the aluminum tanks with custom plastic tanks when these (nearly new) aluminum tanks corrode through. Sad.

Ben

[Rapanui]

Our aluminium water tanks are 20 years old and still fine. The water in Croatia is heavy in minerals, mainly limestone and the pump inlet water filter is regularly full of balls of deposited minerals. We do not treat the town water when filling the tanks. Our preventative measures are to fill the tanks treated with Puraclean over winter (freezing is not an issue) drain and refill before we use the water in the spring and drain and refill after each absence in the season.
We have no problems when we follow this regime.
I would suggest to the OP that aluminium tanks have been widely used for many years, and largely used with no problems, mainly because the owners are unaware of what the inside of the tank looks like and hence make no efforts to restore their tanks to a condition they think is right. My own view, is the precipitate that deposits on the tank internals helps protect the aluminium from corrosion, and the only thing that matters is that the water is safe to drink. If the OP is concerned about that, get the water tested and treat accordingly, there are a number of aluminium safe water treatments available.

[benethridge]

Lucky you. My guess is that either (a) the chemical composition of your water is such that it protects your aluminum tank as you say or (b) you never look in your tanks to see how bad the corrosion is, yes?

I think Nigel Calder would disagree with you that it's not going to eventually be a problem for most boats. From his Boatowners Mechanical And Electrical Manual pages 520-521:

'"Aluminum is one of the more reactive metals in the galvanic series table"'...'although the causes of corrosion may differ, the mechanism is the same'...'All it takes to set this corrosion in motion is moisture."
...
Coating aluminum tanks with anticorrosion paints and surface treatments will help prevent corrosion, but they can be a double-edged sword. If the coating is breached and moisture is added, any tendency toward galvanic corrosion is likely to be concentrated at this point...'
...
'...(2.3 mm) plate (the minimum thickness permitted by ABYC for aluminum fuel tank construction) has a service life of 6.5 years in the marine environment...(3.2 mm) plate (commonly used for more heavily built tanks) has a service life of 17.4 years....You cannot assume aluminum tanks will last the life of today's boats.'

Patrick Childress, who was arguable, before he died of COVID, the most knowledgeable Youtube sailor out there, ended up painting his aluminum tanks. His tanks actually looked somewhat better than mine, except for that large corrosion hole in the video at minute 4:15:

https://www.youtube.com/watch?v=bRofFfb37eg

I just googled "aluminum water tank treatment" and found nothing significant (meaning nothing that would stop the corrosion), so what please are these "aluminum safe water treatments available"?

Ben

[rslifkin]

I'll be replacing my aluminum water tank this winter. After 35 years, it's starting to leak. They definitely don't last forever, but they can last a while depending on the grade of aluminum used, tank thickness, how much chlorine it sees, etc.

[Neziak]

The answer to your question can be found in detail in the book Corrosion of Aluminium by Christian Vargel. Page 613-615 in the second edition.

In short, the corrosion is caused by stagnant water which forms localised excesses of H+ and OH- ions. That is why the corrosion is not uniform but in clusters. By moving the water regularly, you eliminate these excesses.

My understanding is that replacing the water regularly (at least once a month) considerably slows down or even halts this corrosion.

The book also has a section on chlorine (pg 605) and concludes that the levels found in tap water are not sufficient to damage the oxide coating. Aluminium can be resistant to corrosion even in swimming pool concentrations of chlorine.

For more detail and the scientific data, along with the properties and resistance provided by the different series, please buy the book at Elsevier dot com and reward the author for his work.

https://www.elsevier.com/books/corro...B&gclsrc=aw.ds

Nez

[benethridge]

Originally Posted by Neziak

In short, the corrosion is caused by stagnant water which forms localised excesses of H+ and OH- ions. That is why the corrosion is not uniform but in clusters. By moving the water regularly, you eliminate these excesses.

My understanding is that replacing the water regularly (at least once a month) considerably slows down or even halts this corrosion.
Nez

So if that is true then why am I getting just as bad corrosion as before when I've been living on this boat all these months (since I last shocked and cleaned the water tanks...and yes, I cleaned off all the white yucky corrosion as well)? I use up and replace all the water in the tanks about every 10 days. Wouldn't that run counter to your and the author's logic?

Ben