5 days prior to the annual inspection cycle of your plant, an iron pipe transpor

Question

5 days prior to the annual inspection cycle of your plant, an iron pipe transporting 1M NaOH solution at 500°C to a storage tank ruptured.

Hearing of the failure, the chief chemist in your team runs in and states "this shouldn't have happened! The hematite should have protected the inside of the pipe! We inject 10^-10 molar (FeO4)^-2 into the system for a reason!"

You decide to do a walk-down along the section upstream of the pipe, and you notice 2 things:
1) the O2 inlet valve has been closed off, and
2) an in-line O2 detector shows 10^-26 atm O2.

You make a note of this value, then return to your window-less cubicle to write up the report.

What happened? What should the inlet valve have been set at? As the new-hire corrosion engineer, explain and justify your findings in a way that your supervisor would understand. Use Ellingham diagrams and the Nernst equation if necessary.

Explanation / Answer

molar = mol per liter:

10^-10 mol of FeO4)^-2 per liter expected

Apply:

a)

Clearly Oxygen is now not flowing, therefore it is decreasing in oxidation (FeO4)-2 number, which will NOT avoid the oxidation of the material, that is, the pipe will corrode

b)

For O2

10^-26 atm O2

P V= nRT

P = n/V*RT

P = (10^-10)(0.082)(50+273)

P = 2.6486*10^-9 atm

the value should be set at 2.6486*¨10^-9 atm of O2, not 10^-26 atm

which makes sense, since O2 inelt valve is closed

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