sulfiric acid/dilute in steel tank.?

Question

why can concentrated sulfuric acid be stored in mild steel tanks, but not dilute sulfuric acid?

Answer 1

Fe + H2SO4 -----> FeSO4 + H2

This only works if there some H+ ions, and there are only some H+ ions if there is a lot of water to dilute the acid.

Answer 2

Almost every process plant, power plant and water treatment plant has one or more concentrated (70% to 99.5%) sulfuric acid storage tank. It may surprise you to learn that concentrated sulfuric acid can be stored in uncoated, unlined carbon steel tanks at ambient temperature because, concentrated sulfuric acid forms a protective iron sulfate film on the carbon steel that prevents carbon steel metal loss. They key word here is concentrated. Concentrations less than 70% or greater than 99.5% will be very corrosive to carbon steel and these sulfuric acids cannot be stored in unlined carbon steel vessels. It is extremely important that these tanks be properly designed and maintained throughout their lifetime. Concentrated sulfuric acid has a tendency to absorb water causing it to become diluted and hence corrosive. Unlike most corrosive substances, sulfuric acid becomes more corrosive as it is diluted by water. Thus a very minor leak in a sulfuric acid system can rapidly become a major leak when the acid absorbs water from the air.