[curiouscat]

What is the influence of aeration i.e. dissolved Oxygen on corrosion rates of metals? Intuitively I'd think that less O2 means less corrosion  but just recently I remember reading that extremely pure water tends to be more aggressive on metallic piping system components.

Is there any truth in this? Or does it depend on a case by case basis on the alloy concerned.

I'm only concerned with common metals of construction e.g. Mild Steel, Stainless Steel grades, etc. & not exotic cases.

[Hunter2]

Oxygen is the stuff who does the corrosion. The metal get oxidized. Pure water has a slight acidic pH what also increase corrosion.

[Arkcon]

The more pure the water, the less likely there's a mini electrochemical cell to attack the metal.  Maybe the real issue is the temperature?  As water approaches super-critical temperatures and pressures, it becomes very aggressive against metal.  Great for cleaning, if you don't mind losing some metal.

[Corribus]

Agree with Arkcon. I'm not seeing the link between "purity of water" and oxygen content. Only, "purity of water" and electrochemical activity. Do you have a link to where you read that pure water tends to be more aggressive on metallic piping system components?  (What do we mean by "aggressive" here, too? The only thing I can think of is that pure water might have a higher driving force for dissolution of metallic components in contact with it, but it's a stretch.)

[curiouscat]

I think you guys are right. Maybe I misremembered the factoid.

Maybe it was a specific corner case. e.g.

"....stainless steels have poor corrosion resistance in low-oxygen and poor circulation environments"

http://cdn.intechopen.com/pdfs-wm/33625.pdf

That article hints as to why too..

Also:

"...Copper alloy components in power plants require single digit ppb DO concentrations whereas iron alloys can benefit from the passivation effects of higher concentrations in the 30 to 150 ppb range."

[curiouscat]

"....Copper will resist most natural waters within certain velocity limitations (except soft, aggressive waters),"  (think I was stupidly mistaking soft for pure)

Materials Engineering for the Chemical Process Industries---Prof.dr.ir. Walter BOGAERTS



Softened water is made potentially aggressive to metallic piping, a properly sized and
maintained softener will produce a water with zero hardness that can be corrosive to home
plumbing. Water with zero hardness will also attack glassware resulting in a hazy surface
sheen or “rainbow etching”.

http://www.ct.gov/dph/lib/dph/environmental_health/private_wells/29_Hardwater-Softeners_Facts_and_Issues.pdf

[curiouscat]

I was mistaking DO / purity when I think what I'd read was about soft n hard water differences....

" The softer the water, the lower it is in carbonate; therefore, soft water is more corrosive than hard."

[Borek]

" The softer the water, the lower it is in carbonate; therefore, soft water is more corrosive than hard."

Not exactly, water hardness is about amount of Ca²⁺ and Mg²⁺, not about counterions (ie they don't have to be in form of carbonates/hydrogencarbonates - although they often are).