Effect of pH on Black Iron pipe for steam

[ports394]

What kind of effect will a pH drop in RO product water have on regular black iron pipe used to move 125 psi steam?
The original pH was 6.5. It's dropped to 5.4 - 5.7.

Would this much of a pH drop accelerate the rusting/breakdown of black iron pipe used to move regular cold water?

 

[blacksmith37]

Depends mostly on what /how much ,species is causing the pH . I assume RO water is pretty clean , then a very small addition can cause a large pH swing. And, yes a pHof 5.5 could be aggressive.
I once worked at an R&D facility with a distribution system for distilled water; the pH could be below 5, yet people who knew about it said it was no problem for corrosion tests, it still had very low impurity levels.

 

[ports394]

blacksmith37,

Without paying an outside lab to do a complex analysis of the water, this is only a guess. I know that the RO membrane is blocking alkalis. But it isn't able to block lactic acid, so eventually the pH will drop because the alkali's are coming out.

Like you said, the pH of 5.5 could just be because the water is so pure that it is easy to drop.

I guess there's more information to provide to get a better answer on this. Unfortunately I won't have it until I send the water samples out.

The old RO was letting a bit of the alkalis through too, and so the water came out much closer to what the feed was.

 

[racookpe1978]

I'm missing something: If the pipe is carrying steam, then it must be pure water (regardless of source water pH) since all the impurities (acids) would be left behind when boiled, right?

And, if the black iron is transferring condensate, then where would the acidity come from?

 

[mjpetrag]

some acids (ie formic) can carry overhead since a lot of them have boiling points only slightly above water.

ports394 - grab a sample of the condensed water and let it dry out in open air. check to see if there's some stuff left behind. that might be your acid.

-Mike

 

[unclesyd]

Probably the biggest acid producer in such cases would be CO2.

 

[ports394]

Right... CO2 + H2O is carbonic acid.. but i thought that was extremely unstable and it broke down when it wasn't under pressure.

 

[steamdog]

Don't most use an amine in the boiler water to regulate the condensate pH? Most condensate is pretty pure once it is formed, and it will dissolve most anything because it does become acidic without treatment.