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Can't hold a pH at 8.5 or below in an old steel piping system

Can't hold a pH at 8.5 or below in an old steel piping system

Can't hold a pH at 8.5 or below in an old steel piping system

(OP)
I am installing a boiler with an aluminum heat exchanger in an old steel piping system. I am having a lot of trouble getting the old system clean, which is causing my pH to creep. The pH must be kept between 6 and 8.5 in order to protect the aluminum. I know that this is different from the desired condition for the mild steel piping. I'm told that 9 or 10 is a common pH level.

We have filled and flushed the system numerous times and circulated cleaners. I don't know exactly what was used, but we have hired one of the local leading water treatment companies to do this for us. They seem perplexed, which is causing me a lot of stress. I understand that the cleaners are typically alkaline based, so I suggested (and I know next to nothing about chemistry) that they try an acid-based cleaner to "shock" the system a little and knock off any scaling. They gave me the impression that I had a good idea ----- so why didn't they think of it???

The problem is that we have achieved 8.5 pH, but it then creeps up. I'm told that's from the pipe reacting to the new pH level after being exposed to much higher levels for many years. This reaction is driving the pH back up.

Anyone ever come cross this? In my mind, I would just keep introducing acid until the pH comes down, but I'm sure there is more to it than that.

Any thoughts?

RE: Can't hold a pH at 8.5 or below in an old steel piping system

Do you have any of your other water quality readings? Dissolved oxygen, conductivity, etc... That might be useful information as well.

RE: Can't hold a pH at 8.5 or below in an old steel piping system

If you have oxygen present, your pH may be creeping up due to oxygen reduction generating OH- ions, or hydrogen evolution which makes H+ ions leave (increasing pH).

Eliminate the oxygen and your pH will stabilize and corrosion will stop. Fail to eliminate the oxygen and you will never be able to keep the pH low.

RE: Can't hold a pH at 8.5 or below in an old steel piping system

What type of occupancy do you or did have in the building?

RE: Can't hold a pH at 8.5 or below in an old steel piping system

You could have piping encrusted with deposits from the old system.

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