Diluted Sulfuric acid

[essenwood]

Hi all,
It is commonly known that diluted sulfuric acid is very corrosive. What is actually the mechanism behind it?



Thanks,

 

[cloa]

Very corrosive to steel- there is a range of metals, alloys, plastics and ceramics that handle dilute sulfuric quite well.
Acid plus metal gives hydrogen plus metal ions. The sulfate ions are in dilute sulfuric are spectators.

 

[bigghassan]

Corrosion is two electrochemical reactions (anodic and Cathodic) on metal surface when immersed in common electrolytes (water, soil, solution, acid … etc.).
Due to many reasons one portion of metal surface will transfer electrons during the metal body to another. The first portion will be anode while the other is cathode.
Corrosion control is to eliminate at least one of these reactions usually the cathodic reaction.
In the water, cathodic reaction needs oxygen to take place while in the acids; the cathodic reaction will carry by excessive hydrogen ions.
All acids are corrosive but severe corrosion will take place rapidly in strong acids rather than diluted acids

 

[cloa]

bighassan is not considering passivity effects (protection layers formed) on steel in concentrate sulfuric acid which reduce the rate of corrosion rather low levels.

 

[essenwood]

Guys,
Thanks for the replies.
Diluted sulfuric acid is more corrosive than concentrated sulfuric acid (70-99%). Below is extract from numerous writings on the matter:

<<<"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.">>>

My question is; why at lower H2SO4 concentration eg 70% ; iron sulfate is less effective in protecting against metal loss. Would prefer mechanism explained in chemistry terms please.

Thanks.

 

[EdStainless]

At lower concentrations there are a different set of compounds formed on the surface (various compounds of Fe and S). If the compounds formed are either porous, or soluble, or both, then they will not provide any corrosion protection. In fact they may greatly accelerate the attack.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[ash9144]

When dilute the protective iron sulfate will dissolve in water. At higher concentrations it forms on the surface and does not dissolve in the solution.

There are many other factors to consider when designing a sulfuric acid tank.

 

[moltenmetal]

Above 93% there is precious little actual "water" around. Thinking of conc acid as a solution of acid in water is thinking about it the wrong way 'round!

The passive film formed on carbon steel in conc sulphuric acid is not only removed (or not formed in the first place) in the presence of excess water, but also by fluid movement (ie even modest fluid velocities), hydrogen bubbles produced by corrosion etc. as ash9144 suggests.