Demineralized Water Corrosion on Carbon Steel 6

[Xeros]

Hi, I'm Xeros.

I heard that Stainless Steel shall be used on Demineralized water service because it is severe corrosive for carbon steel.

I want to know why Demineralized water is severe corrosive for carbon steel.

and i heard that the reason is that Demineralized water is very active with ion such as CO2, O2 because it is very pure and unstable.
is it right?

if above reason is right,
is Demineralized water corrosive on carbon steel when it blended with fresh water?

 

[SJones]

If the water contacts carbon steel it will take up more ions than if in contact with stainless steel and, therefore, will not be (so) demineralised anymore; which may not be tolerable. Water corrosivity will be driven by several factors other than TDS. Try consulting some corrosion texts such as Shreir - Corrosion or Uhlig's Corrosion Handbook.

Steve Jones
Materials & Corrosion Engineer

http://www.pdo.co.om/pdoweb/

 

[strider6]

Demineralized water has low salt content and this will reduce the conductivity of the water and decrease the corrosion rate. Another effect of the low salt content is that the water is less protective and will not form any layer of CaCO3 or other salt that will protect the steel from corrosion.
The corrosion rate of the demi-water depends also on the oxygen and CO2 content, if your water is a closed circuit and is not in contact with atmosphere the corrosion rate will be low.
If you blend the waters than i'll use the corrosion rate of fresh water on carbon steel as a reference.

"In distilled water, which cannot form a protective scale to reduce the access of oxygen to the zinc surface, the attack is more severe than in most types of domestic or river water, which do contain some scale-forming salts."

http://www.key-to-metals.com/Article40.htm

So this could be one reason someone state that demineralized water is more aggressive than fresh water.

S.

http://www.corrosionist.com

 

[EdStainless]

It isn't so much that the corrosion rate of the CS will be so high as to cause failures, but the corrosion that does take place will contaminate the water. In most high purity applications a few ppb of Fe is the limit.

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Plymouth Tube

 

[rmw]

Demin and DI water is often referred to as "hungry water" in that it wants to be in equilibrium and will pull ions out of the surrounding metals trying to satisfy its "hunger." Not a technical description I know, but it helped me understand the problem many years ago before I had enough real education to know the real reason why.

rmw

 

[Xeros]

Everyone,
Thanks for your help!!!

 

[davefitz]

There are some exceptions to the above rules of thumb.

Modern supercritical steam plants use carbon steel tubed feedwater heaters, and add 50-200 ppb O2 to the demin water to promote the formation of hematite ( at pH of 8.0-8.5)- this is a much more stable oxide scale that does not erode off the tubes, and will not form a corrosion cell as long as the water is flowing . However, stagnant oxygenated water will lead to rapid corrosion , so outages much be preceeded by altering the feedwater chemistry to traditional 4 ppb O2 with no trace hydrazine and pH to 9.2. The DA must be burped occasionally to remove noncondensables including CO2.

So, I think the stainless steel may need to be reserved for the applications , or sectionof the steam cycle, where the noncondensibles have not been removed yet.

 

[bimr]

Stainless Steel or plastics are used on demineralized water service because oxygen is normally present. If you deaerate the water to remove the oxygen, the demineralized water will not be severely corrosive for carbon steel service.

The absence of calcium and magnesium from demineralized water makes demineralized water more aggressive than fresh water. Most fresh waters contain calcium and magnesium. It is also true that a fresh water lacking calcium and magnesium will be aggressive.

The langelier index was created to predict corrosion resulting from a lack of calcium and magnesium in water.

There is no CO2 present in demineralized water, since the demineralized water has a pH of 8-10 pH units. That means demineralized water is basic not acidic.

It is impossible to predict whether demineralized water blended with fresh water will be corrosive on carbon steel service without knowing the water quality parameters and whether oxygen is present.