[Charles CL]

Dear All,

Problem:
In normal condition, corrosion occur with the presence of oxygen, however there is a special case that often occur in our daily life which is corrosion in the absence of oxygen.

Anaerobic corrosion:
4Fe + SO₄^2- + H⁺ + 3H₂O --> FeS + 3 Fe(OH)₂ + OH^-

Water and metal is the main factor of corrosion to occur.
Corrosion occur with the presence of FeS

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In another study says the oxygen acts as the terminal electron acceptor for corrosion to occur by sulphate reducing bacteria in a mixed aerobic/anaerobic system.

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Is the above statement means if the inside of the tank (anaerobic condition) has corrosion occur, will it induce the corrosion of outside of the tank (aerobic condition)?

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Sulfate reducing bacteria is commonly known to bring the corrosion to occur. This bacteria is presence in neutral pH 7 which then reduce the pH and thus making the environment more susceptible to corrode.
Any method to prevent this bacteria to corrode from inside of the tank?

Appreciate for any idea.

Thanks.

[Arkcon]

There's a lot happening here.  Worst of all, I don't know where exactly the question belongs.  Lets see if we can get somewhere taking the questions point by point.

Dear All,

Problem:
In normal condition, corrosion occur with the presence of oxygen, however there is a special case that often occur in our daily life which is corrosion in the absence of oxygen.

Anaerobic corrosion:
4Fe + SO₄^2- + H⁺ + 3H₂O --> FeS + 3 Fe(OH)₂ + OH^-

Water and metal is the main factor of corrosion to occur.
Corrosion occur with the presence of FeS

---------------------------------------------------------------

OK, that's clear.  Note that there are many corrosion processes that don't require atmospheric oxygen, even if they're called 'oxidation'.  So lets be sure we keep this in mind at all times. 

Thing is, I don't really understand your reaction.  Its possible, but may not be common, or plausible.  Also, how is it describing what you're talking about?  Your reaction describes an iron tank, full of sulfate ion and acid, reacting with water to produce iron hydroxide and hydroxide anion?

That just doesn't seem likely.  I know dilute sulfuric can't dissolve iron like it can for a more active metal, but hydroxide forming?  No, I don't see that happening.

In another study says the oxygen acts as the terminal electron acceptor for corrosion to occur by sulphate reducing bacteria in a mixed aerobic/anaerobic system.

Sure.  But that likely works by a different mechanism than the one you've written above.

Is the above statement means if the inside of the tank (anaerobic condition) has corrosion occur, will it induce the corrosion of outside of the tank (aerobic condition)?

I suppose it depends on the thickness of the tank, but for anything thinker than a capacitor, no that doesn't seem likely.

Sulfate reducing bacteria is commonly known to bring the corrosion to occur. This bacteria is presence in neutral pH 7 which then reduce the pH and thus making the environment more susceptible to corrode.

Well, it'd be best if we had the reaction written out.  Is that the one at the top?  You should include the bacteria as a "reactant" or maybe as a catalyst to be clear.  I'll go look up sulfate reducing bacteria, so that we're all on the same page.  Someone in the biochemistry sub-forum may be able to help more.

Any method to prevent this bacteria to corrode from inside of the tank?

Appreciate for any idea.

Thanks.

Yes.  An antibacterial that's compatible with your process will prevent them from surviving to perform the metabolism that may be corroding your tank.  I have to assume this is not for food or medicine, so you can use anything: phenol or other powerful organic sterilizer, high temperature steam injection, heck, even short uv or gamma irradiation, if you have the money.  Maybe someone in Chemical Engineering sub-forum faces this sort of problem.

[Enthalpy]

Corrosion is difficult to predict because it's not a simple consequence of redox couples and ions. It depends on the heterogen composition of the alloy, where every detail counts. For instance, at welded joints of 18-10 stainless steel, <0.02%C is markedly better than <0.06%C and deserves a separate standardized alloy name.

On an existing tank, besides acting on the liquid, you can try to cover steel with a film of better metal like nickel. Some treatments work on parts too big to fit in a bath.

[Charles CL]

Dear Arkcon,

Following describe the conditions i referred to:

A tank has two portion, half if covered with liquid (contain water), another half covered by H2S and other gases but no oxygen is present.

therefore, in material selection of the tank. The material of tank wall for the portion covered by liquid would be mild steel, while the portion covered by H2S and other gases would be stainless steel.

We are considering the necessary of application of stainless steel for the particular portion of tank as no oxygen present in the tank.

My thinking is that the water molecule is another main factor that cause this corrosion to occur, as the O molecule can come from water too.

However, we can't see any corrosion for the portion of liquid which contain water, so should we afraid of corrosion at portion of contact with H2S and other gases?

My consideration is that the water content in the liquid portion can be evaporated at any temperature. this will cause the corrosion to occur, but i can't figure out how of this can be happen.

What cause of the water molecules ionized and O separated and combine with Iron and forms the corrosion factor?

Additional information:
1. temperature: 50 degree C (max.)