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Old 14-03-2012, 16:34   #1
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Phosphoric Acid Bad for Steel Bilges?

We have just returned to our yacht to find about a gallon of phosphoric acid has leaked from a plastic container into the bilges of our steel boat. We don't know how long the acid would have pooled as we were away from the boat for 6 months. Also not sure how quickly it would have evaporated. The boat was in a marina pen in Bocas del Toro which is hot and humid. Does anyone know if this is likely to have caused structural (or other) harm to our hull?
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Old 15-03-2012, 02:38   #2
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Re: Phosphoric acid bad for steel bilges?

#1 Get a good torch(flashlight or whatever), a screwdriver and go and take a long, careful look.

My senior chemistry is long over, but I suspect that it would not harm paint. Once you look you'll know more than me.

My experience with phosphoric acid is that it mostly acts on rust, but once the rust is gone it leaves a black coating that is very tenacious.

I'd dig into any black spots with the screwdriver and if the metal is not corroded I'd do a serious clean up with a water soluble degreaser, then rinse and get the whole lot totally dry.

Then I'd put a on a good coating of POR 15 and topcoat.

If your inspection reveals any corrosion it would be best to get a surveyor to have a look.
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Old 15-03-2012, 03:34   #3
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Re: Phosphoric acid bad for steel bilges?

How long does the acid stay active?? maybe use something to neutralize it first.
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Old 15-03-2012, 04:26   #4
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Re: Phosphoric acid bad for steel bilges?

Considering phosphoric acid is also a food additive (E338 IRCC) and also used to whiten teeth I wouldn't be too concerned. Of course the concentration is important but this is not a tough acid.

Dilute it down, wash it out a few years and have a good look. The upside: any rust should be held at bay for awhile
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Old 15-03-2012, 10:21   #5
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Re: Phosphoric Acid Bad for Steel Bilges?

Thanks for your replies; very helpful and re-assuring.
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Old 16-03-2012, 05:19   #6
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Re: Phosphoric Acid Bad for Steel Bilges?

just be careful, strong phosphoric acid will burn skin,it is a strange one, slow acting, but i spilled some on my jeans a few years back, a few hours i kept getting this itching on my leg, i removed my jeans later and there was a large scab/burn mark, i still have a scar to date from it, looks like a black mark on my leg.
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Old 16-03-2012, 06:24   #7
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Re: Phosphoric Acid Bad for Steel Bilges?

It's a weak acid but still an acid and it will corrode any metal if given enough time. It has to be flushed out of the boat, but I would not count on just water being enough. If is was my boat I would be filling the bilge up with a caustic water mixture and allow it it to soak to be sure any acid is neutralized.
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Old 16-03-2012, 06:28   #8
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Re: Phosphoric Acid Bad for Steel Bilges?

Phosphoric acid is the ingredient in rust inhibitors like Ospho...

So, if you had rust in the bilge in can't too bad a thing!
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Old 16-03-2012, 06:42   #9
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Re: Phosphoric Acid Bad for Steel Bilges?

Quote:
Originally Posted by sidmon View Post
Phosphoric acid is the ingredient in rust inhibitors like Ospho...

So, if you had rust in the bilge in can't too bad a thing!
Yes but it works by leaving behind a phosphate coating that plates out on the metal. It is the acid part that does the cleaning and you still have to get the acid off the metal.

I'm not going to go into a a chemistry lesson, but I do corrosion control treatment for a living.
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Old 16-03-2012, 06:48   #10
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Re: Phosphoric Acid Bad for Steel Bilges?

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Yes but it works by leaving behind a phosphate coating that plates out on the metal. It is the acid part that does the cleaning and you still have to get the acid off the metal.

I'm not going to go into a a chemistry lesson, but I do corrosion control treatment for a living.
Didn't finish the thought on what to do with too much of a good thing...

Since its your day job...Now what?
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Old 16-03-2012, 06:50   #11
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Re: Phosphoric Acid Bad for Steel Bilges?

Quote:
Originally Posted by sidmon View Post
Phosphoric acid is the ingredient in rust inhibitors like Ospho...

So, if you had rust in the bilge in can't too bad a thing!
Don't quote chemistry you do not know. I won't quote English lit.

The product you are quoting is a surface prep that must be removed and rinsed. Phosporic acid is also not the only ingredient. Don Lucas is closer to the point. It must be neutralized.

There are many corrosion inhibitors that include phosphoric acid--old-school engine coolants used to list phosphoric acid as an ingredient--but it is always in a neutralized form; the engine coolant formulators included matching amounts of sodium hydroxide to neutralize it. This approach, along with other ingredients creates a buffer system with a stable and slightly alkaline pH, which is good for the steel. Phosphoric acid by itself will eat until it is satisfied, turning iron into ferric phosphate, which isn't steel.

Additionally, phosphoric acid does not evaporate anymore that salt does; it simply dehydrates and becomes more concentrated and more corrosive.

Rinse a lot and then wash with an alkaline cleaner.

(I have developed corrosion inhibitors for cooling systems, including a patent on one of the new long-life formulations)
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Old 16-03-2012, 06:57   #12
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Re: Phosphoric Acid Bad for Steel Bilges?

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Don't quote chemistry you do not know. I won't quote English lit.
Never meant to intimate the OP need not take some action...

Merely stated that phosphoric acid is used in products like Ospho, and inferred that its not an immediate and critical catastrophe...

I can see why you would want to stay away from quoting English Lit (not that I framed my statement well in all fairness).

But thanks for the discourse on corrective action.

Have a nice day.
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Old 16-03-2012, 07:00   #13
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Re: Phosphoric Acid Bad for Steel Bilges?

I've worked 4 years in a phosphoric acid plant and I can write a book on it but will make the long story short:
Industrial grade phosphoric acid (used in large quantities in chemical fertilizers) contains some fluoric and sulpuric acid and at concentration above 30-35% kills every metal and alloys including high grade stainless steel easily. Very special alloys are used. Nonetheless, GRP (yr hull material I assume) holds pretty well as well as most of polymers like PVC, PE. Polypropylen and teflon are the best.
The one you had on the boat should be pharmaceutical grade and rather diluted (10-15%) If so, definately no problem for yr hull but you should carefully check the keel and the bolts which are normally in ordinary carbon steel. If stayed in contact long enough, it is very likely that you already had some sort of corrosion.
In short, phosphoric acid is not the worst but the impurities in it coming from the production process can be a killer. (Commercial phos. acid is brownish whilst the pharm. grade is colorless and limpide.)
Good luck
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Old 16-03-2012, 07:10   #14
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Re: Phosphoric Acid Bad for Steel Bilges?

The OP's boat hull is steel FWIW.
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Old 16-03-2012, 08:06   #15
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Re: Phosphoric Acid Bad for Steel Bilges?

I should have known better than to get into this thread.
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