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My situation is somewhat bad: I've got sludge in my boiler. I would like to get that situation under control. The reason I got sludge, I think, is because of some years of lazy maintenance as well as a badly leaking pipe at one time. Does controlling pH halt corrosion when it's already started?

Because of this thread, and because of the time of the year here in the States, I made a trip to my friend's house just last night. Her situation is similar to yours, and also perhaps to that of @SlamDunk so maybe what I found can help you both.

She has a reasonably-sized boiler for her house, it's 4 sections and three burners, a Lennox branded Dunkirk design (that is prone to carryover due to its side-supply design) installed in 2007.

The boiler behaves quite well but she tends to listen to her plumber who advised her to drain the boiler monthly (BAD ADVICE). She certainly hasn't done it every month, but she has done it periodically. Her water is generally brown when I look at it, but I've never tried to get the corrosion there under control before. But with her permission I am now.

After last night, and also remembering my very-unmaintained boiler that I used to have, I think it can be a rather long road, but not a hopeless one. I'll tell what I did and what my plan is going forward. See if it matches what you are seeing.

1. Her boiler has run a few times this season, and about a month ago she drained probably 4 gallons. The water was clearish-looking in the gauge glass. First, I released about 1/2 gallon from the boiler drain which started out medium brown but then ran mostly clear and indicated, as expected, about 7ph. Then I flushed the bottom gauge glass port. Then I cleaned the pigtail for good measure. It was dirty but not blocked.
   
2. I added some water to the cold boiler to get it to nominal level again.
3. I added only about 3 ounces of 8-Way the easiest way I could which was the main vent fitting, believe it or not. (Her main returns to the boiler resulting in a 4" wet return right next to the Hartford Loop.)
4. I steamed the boiler for awhile to let the condensate carry the 8-way into the boiler
5. Very quickly the water started to turn brown in the gauge glass. There was a little water coming into the glass from the top port which indicates foaming. Her boiler doesn't have a sight glass on the header (pity) so it's really hard to tell if it was carrying over, but it may have been slightly (the water level was down about 2-3" in the gauge glass).
6. I stopped the boiler after about 10 minutes of steaming and drained about a gallon of what was surprisingly dark brown water which indicated about 10ph (higher than I expected). Some of that color was no doubt from the 8-way purple indicator, but a majority must have been from sedimented "mud" and/or scale that the 8-way freed up. I was reminded at that moment of my own experience in 2017 with my original boiler at my house which was another unmaintained and filthy (inside) Dunkirk-design Utica. In one of my very first boiler videos, you can see me being clueless about it but I already had a sight glass installed at the end of my main which indicated quite clearly that I for sure was carrying over (this was immediately after I started trying to clean my boiler with 8-way). See the 2 minute video here, it's quite shocking to see carryover for the first time!
7. I ran the boiler again, and again it seemed it might be carrying over some (but not catastrophically—steam was getting to her radiators). I stopped it again after 10 minutes and drained another brown gallon that measured about 8-9ph, then slowly added more fresh water. I repeated this step once more.
8. Finally it was getting late. I didn't want to leave her with carryover condition, so I drained to the bottom of the gauge glass and again slowly refilled. The water I drained this last time was still brown, I believe still from the freed-up mud/scale. When I dumped the bucket there were in fact some scale pieces.

So between my experience years ago, and last night, and what @SlamDunk saw, it seems pretty obvious we do have to be careful of carryover when adding 8-way (and I'm sure any of the other treatments) to boilers with scale and/or mud buildup (a new clean boiler has been shown to be able to handle even an extreme overdose without carryover. Not to say that's advisable, just stating the fact). This is interesting and ironic given that all these treatments claim to reduce surging. My own thinking has been flawed regarding this because I missed the idea that even though my clean boiler can take seemingly any amount of treatment, mud/scale in an older boiler can present a completely different behavior.

My recommendation is start SLOW and be ready to drain off the freed up stuff to avoid carryover. As much as it pains me to add fresh water during these steps, I think it's necessary. I feel that the amount of corrosion caused by this fresh water should be much less than the overall benefit of getting the water chemistry to a place where corrosion is greatly reduced over the long haul.

My plan going forward is to revisit this boiler in a week or so and try another couple ounces of 8-way. I believe that eventually the vast majority of mud/scale that is going to break free will do so, then her boiler can be almost as clean as a new one and won't require any cycle of drain, add fresh water, corrosion. Of course I'll report back, thanks!

Was the return too high or just rotted out?

Replaced rotting Peerless’s with a Williamson upgrade main venting and radiator vents.

Still need to go back to install the demineralize filter.

It’s rotted out due to too much make up water. Most likely leaks in the system.

We’ll find out once the heating cranking over next few weeks and we’ll go from there