The Chemistry of Pee

[oldvarnish]

Does anyone know which of the chemical constituents of urine attacks the welds in stainless steel holding tanks? And if so, what might be best to flush through a s/steel holding tank to neutralise the effects?

The obvious answer is to use plenty of sea water. But I wonder if that is sufficient?

(And before anyone suggests changing the tank for a plastic one, which would be my choice, my suspicion is that this boat started life as a tank around which the rest of the boat was built).

 

[alant]

Does anyone know which of the chemical constituents of urine attacks the welds in stainless steel holding tanks? And if so, what might be best to flush through a s/steel holding tank to neutralise the effects?

The obvious answer is to use plenty of sea water. But I wonder if that is sufficient?

(And before anyone suggests changing the tank for a plastic one, which would be my choice, my suspicion is that this boat started life as a tank around which the rest of the boat was built).

In another life, whilst working for ICI, I used to visit an Oxfordshire hospital, which centralised all the laundry for other hospitals nearby. Many of these were caring for elderly apparently, who tended to wet themselves/bedclothes, requiring lots of laundry.

The hospital engineer, reported to me, that the entire sewage works was needed to be rebuilt, because high levels of Urea were rotting away the reinforced concrete tanks/chambers of the works.

Stainless Steel, usually corrodes due to lack of oxygen, which it needs to repair its skin.
Should be OK against pee & as you said, its diluted anyway.

 

[vyv_cox]

Does anyone know which of the chemical constituents of urine attacks the welds in stainless steel holding tanks? And if so, what might be best to flush through a s/steel holding tank to neutralise the effects?

The obvious answer is to use plenty of sea water. But I wonder if that is sufficient?

(And before anyone suggests changing the tank for a plastic one, which would be my choice, my suspicion is that this boat started life as a tank around which the rest of the boat was built).

Seawater alone will attack the area immediately adjacent to welds under some conditions. Stainless steel's corrosion resistance is due to the presence of chromium in concentrations of at least 13%. Welding grades of stainless steel have a very low carbon content. If the metal is not the low carbon grade (304L or 316L), or is welded with the incorrect filler, then chromium in the immediate area of the weld will combine with carbon to produce chromium carbides. This means that the chromium content of the stainless in this region will have a reduced chromium content, and therefore will be susceptible to corrosion, sometimes very rapidly.

That said, urea can be very aggressive, dependent upon the base metal and filler composition. I doubt if there is anything that will flush away corrosion.

 

[Lakesailor]

I understand that it is the glenfiddich constituent of pee that causes the problems

 

[DownWest]

I re-welded a couple of tanks in Iberia, it would have helped if I had know it was a north of the boarder problem......

I was asked about the design criteria I used for terrace railings for an enormous plantation villa in Quinta de Lago. I discribed the idea of a couple of Glasgies having a fight after numerous brews, then careening into the railings. .Guess where the owner came from? His reply.. "You must have met some of my guests".

A

 

[Bilgediver]

I understand that it is the glenfiddich constituent of pee that causes the problems

Nah........Can't be that! Our friend the Headmistress said this was a big problem across the pond and over they they drink that Burbon rubbish

 

[halfway]

Not enough Oxygen.

Black waste will absorb all available Oxygen and produce a few "gasses" instead. This does not happen in sewage works as they can not afford to let the waste become deoxygenated. The locals complain.

So the welds, even if they have chromium available, the chromium will be useless.

Basically you cut the top out and tar/epoxy/rubber coat the inside.

 

[Nealo]

Hi - don't know anything about yachts but I spend a fair amount of time sorting out urine "problems"... There's nothing inherent in urine that could cause a problem, it's basically harmless and, believe it or not, odourless. What causes the problem is when it reacts with other substances and/or the atmosphere to form bacteria and, commonly, ammonia. Depending on the concentration, ammonia can be highly caustic and might be causing the problem you have. I remember one local authority in the North West some years ago where a lamp post had collapsed because the dog wee had gradually rotted away the metal base, no I'm not joking, they ended up having to replace loads at huge cost.
You can neutralise ammonia but to be honest prevention is better than cure and as suggested some form of inner coating would be the answer.

 

[DownWest]

An aside: In the article on the AirHead composting toilet for boats, the urine is collected separately. A bit of sugar is then added to neutralize it before disposal (over the side) Sounds like a good clean and epoxy liner is in order, if you can get access.
A

 

[TradewindSailor]

Not enough Oxygen.

Black waste will absorb all available Oxygen and produce a few "gasses" instead. This does not happen in sewage works as they can not afford to let the waste become deoxygenated. The locals complain.

So the welds, even if they have chromium available, the chromium will be useless.

Basically you cut the top out and tar/epoxy/rubber coat the inside.

The trouble is that there's hardly any coating that sticks to Stainless Steel. I've had some success with sanding wet epoxy onto stainless steel plate ...... but I have also seen black water tanks with the coal tar epoxy peeling cleanly off with no effort.

 

[Heckler]

Seawater alone will attack the area immediately adjacent to welds under some conditions. Stainless steel's corrosion resistance is due to the presence of chromium in concentrations of at least 13%. Welding grades of stainless steel have a very low carbon content. If the metal is not the low carbon grade (304L or 316L), or is welded with the incorrect filler, then chromium in the immediate area of the weld will combine with carbon to produce chromium carbides. This means that the chromium content of the stainless in this region will have a reduced chromium content, and therefore will be susceptible to corrosion, sometimes very rapidly.

That said, urea can be very aggressive, dependent upon the base metal and filler composition. I doubt if there is anything that will flush away corrosion.

Vyv
on my last platform we had quite a hi level of H2S, all the pipework and control valves had to be designed to cope with this, a lot of the engineering ethos was, make it feckin hefty! I was told that it helped to contain the inevitable corrosion from the H2S.
Would that be a prob to the SS?
Stu

 

[vyv_cox]

Vyv
on my last platform we had quite a hi level of H2S, all the pipework and control valves had to be designed to cope with this, a lot of the engineering ethos was, make it feckin hefty! I was told that it helped to contain the inevitable corrosion from the H2S.
Would that be a prob to the SS?
Stu

Dry H2S isn't a problem in natural gas, hence platforms I worked on had molecular seives to dry the gas before export. Once there is water there as well things can get difficult, as sulphuric/sulphurous acid may be present. Austenitic stainless steel copes moderately well with it, as does carbon steel. The big problem arises if the hardness of the steel exceeds the magic 22 Rockwell C. Carbon and stainless steels then become susceptible to sulphide stress corrosion cracking, which can be catastrophic. Flanges and valves specified for wet H2S have very stringent hardness limits. So in many cases the fittings are made large but soft because they can't be made strong (= hard) but light.

Not enough Oxygen.

Black waste will absorb all available Oxygen and produce a few "gasses" instead. This does not happen in sewage works as they can not afford to let the waste become deoxygenated. The locals complain.

So the welds, even if they have chromium available, the chromium will be useless.

Not an issue. There are many oxygen-free applications in which stainless steel is used, every LNG plant on earth, for a start. Half of a chemicals refinery is piped in stainless steel, no oxygen but loads of corrosive stuff. Pipework lasts for decades. The chromium oxide film on stainless steel forms within microseconds when it is made/formed/welded and is very difficult to remove.

 

[kesh]

i've heard of mild steel holding tanks not corroding because the bacteria suck all the oxygen out. this is from much older narrow boats when stainless steel tanks weren't used so much.

so i'm guessing even if the chromium oxide doesn't form the bare iron can't react with any oxygen because there isn't any.

 

[halfway]

vyv_cox: "no oxygen but loads of corrosive stuff"

Hold on a minute. If it was corrosive to stainless and stainless had no protection from it, they would not use it.
Most chemical plants, that are using things corrosive to stainless, use other materials like GRP filament wound vessels. Oil refineries seem to worry more about wear on bends than corrosion.

We are taking about a water based liquid, with electrolyte, in a oxygen starved environment, that is generally static. All the ingredients for stainless to selectively disappear.

As for covering it. You need to get a good metal primer on first.

 

[halfway]

vyv_cox: "The chromium oxide film on stainless steel forms within microseconds"

Does not matter when it forms. It matters if it has the right environment to reform once damaged. It is very easy to damage. It is selective damage that initiates the corrosion site.

The eating away of stainless through deck fittings is quite well known. First you add a little moisture in the surroundings. Then you flex the metal, or provide sharp thread shaping to initiate the site. Make sure that the water gets in through the tiniest of cracks so the air exchange is limited. Add a little sea water salts and wait in a medium heat for a few years.

There you have it, one mast in the sea. One of the easiest recipes for disaster. Leaks in boats do not normally mean wet bunks, they mean your stainless is washing away. Good sealant is the answer.

I watched a rigger tighten the rigging on a boat, but got confused as he pulled out the chainplate bolts. The stainless had gone!


The only different in tanks is that the weld edges are the points that are most damaged when the tank walls flex and "pant".

 

[vyv_cox]

vyv_cox: "The chromium oxide film on stainless steel forms within microseconds"

Does not matter when it forms. It matters if it has the right environment to reform once damaged. It is very easy to damage. It is selective damage that initiates the corrosion site.

The eating away of stainless through deck fittings is quite well known. First you add a little moisture in the surroundings. Then you flex the metal, or provide sharp thread shaping to initiate the site. Make sure that the water gets in through the tiniest of cracks so the air exchange is limited. Add a little sea water salts and wait in a medium heat for a few years.

There you have it, one mast in the sea. One of the easiest recipes for disaster. Leaks in boats do not normally mean wet bunks, they mean your stainless is washing away. Good sealant is the answer.

I watched a rigger tighten the rigging on a boat, but got confused as he pulled out the chainplate bolts. The stainless had gone!


The only different in tanks is that the weld edges are the points that are most damaged when the tank walls flex and "pant".

Now you are changing the subject. That is crevice corrosion, which happens with very many metals in addition to stainless steel. For example, it happens at the base of older domestic central heating radiators where the two halves are welded together - mild steel. It occurs with brass, aluminium and many others.

 

[ean_p]

I've always understood that a lot of stainless tanks fail because of poor welding practice in that the weld is not gas 'shielded' on the inside of the tank as it is been made as this would be quite expensive in terms of shielding gas costs to flood/ bath the inside of the plates during construction. As a result the internal weld is not as it should be especially at the margins of the weld were constituents are lost and reduced due to heat and oxidation etc.. When you watch quality pipework been put together it always has a shield gas injected into the pipe under pressure so as to protect and help the inner face / result of the weld process form as required.

 

[halfway]

vyv_cox: "Now you are changing the subject."

No. The original question was: "Does anyone know which of the chemical constituents of urine attacks the welds in stainless steel holding tanks?"


To attack the weld you have to corrode it away. The main weld, which is thicker than the sheet, will hardly go first. But the poor piece of sheet just next to it will. The mechanism in tanks is due to the flexing. This happens at the stress points in the tank design: next to the welds. Damage is done in those areas to the passivation of the stainless by mechanical means.

So that covers the "attacks the welds" bit.

If Urine or salt water attacked Stainless uniformly, like unpainted steel tanks, the question would have been different. E.g. "Why has my tank got weeping areas and is all thin?"

Now the other part is the biology bit, that makes urine a precursor to the actual attacking agent, which in this case is a lack of Oxygen. The corrosion of the stainless steel is a symptom of the lack of passivation reformation at points of damage. Once the lack of Oxygen is created then the corrosion process is the same as steel in sea water. Without the passivation reforming process the H2S and CL will do their normal nasty job on the steel.

If it was due to incorrect carbon content or bad weld process etc., where the Chromium content was compromised, it would not need urine to start the process off. It would corrode just with simple sea water. So the question would not highlight the differences with Urine and holding tanks.

I know stainless is used in sewer environments for supports for the H2S gas monitors, but this is generally dry and vented. Stainless is used in our local sewage works for the main tanks, but these are open and aerated. However any stainless wire in an enclosed holding tank will last only a few months.

 

[Allan]

Reading through this thread I thought the the idea of an internal coating sounded good. I then remembered the things I have read, here and before, about excluding oxygen. Could a coating cause problems rather than cure them?
Allan

 

[vyv_cox]

vyv_cox: "Now you are changing the subject."

No. The original question was: "Does anyone know which of the chemical constituents of urine attacks the welds in stainless steel holding tanks?"


To attack the weld you have to corrode it away. The main weld, which is thicker than the sheet, will hardly go first. But the poor piece of sheet just next to it will. The mechanism in tanks is due to the flexing. This happens at the stress points in the tank design: next to the welds. Damage is done in those areas to the passivation of the stainless by mechanical means.

So that covers the "attacks the welds" bit.

Utter technobabble. How does a tank fail in fatigue? Even by stretching the imagination to the limits, assuming some resonance that applied itself to a badly designed tank wall for several million cycles, the contents would damp it. In 30 years of failure diagnosis I have never even heard of a fatigue failure in a tank.

If Urine or salt water attacked Stainless uniformly, like unpainted steel tanks, the question would have been different. E.g. "Why has my tank got weeping areas and is all thin?"

Now the other part is the biology bit, that makes urine a precursor to the actual attacking agent, which in this case is a lack of Oxygen. The corrosion of the stainless steel is a symptom of the lack of passivation reformation at points of damage. Once the lack of Oxygen is created then the corrosion process is the same as steel in sea water. Without the passivation reforming process the H2S and CL will do their normal nasty job on the steel.

If it was due to incorrect carbon content or bad weld process etc., where the Chromium content was compromised, it would not need urine to start the process off. It would corrode just with simple sea water. So the question would not highlight the differences with Urine and holding tanks.

I know stainless is used in sewer environments for supports for the H2S gas monitors, but this is generally dry and vented. Stainless is used in our local sewage works for the main tanks, but these are open and aerated. However any stainless wire in an enclosed holding tank will last only a few months.

If you were to leave sewage in a tank until it became a stinking mass, you might just generate enough H2S to be a problem, although I doubt it. In reality holding tanks are flushed with aerated seawater every few days.

The mechanism I gave above is the correct one. Its technical word is sensitisation, which you are welcome to Google. Some people carry around seawater plus sewage in stainless steel tanks, but few of us carry seawater alone. If we did, the same failure mode would be more common.