The debate between cast iron and steel could last until the end of time. Both have desirable properties depending on the situation.
I think the trick to keeping steel from corroding is two fold: a tight closed system and water treatment. It is an area no one seems to deal with much. When I was looking into it, no local boiler techs used water treatment of any kind and looked at me with a funny stare when I brought up the subject. The local supply house had nothing in stock for closed systems. Why should they care? It only prolongs the life of the boiler! Those guys are no where near the scene of the crime when the leaks are detected seven or so years down the road. No one blames the technician that far in the future, though they should!
There is little information on it on the web. I used an Air Force and an ASHRAE guidance paper on water treatment. If done correctly it will keep the boiler from corroding. I used softened water treated with sodium nitrite, borax, and washing soda. It was the simplest/cheapest to obtain and least likely method for me to screw up.
This link is broken, either the page no longer exists or there is some other issue like a typo.
1. Nitrite-Borax Treatment Program
a. For the initial dosage, add approximately 18 lbs of sodium nitrite-borax for
every 1000 gallons of water in the system. Enough sodium nitrite-borax should be added to
sustain a nitrite residual of 1000 ppm as nitrite (NO2).
b. Maintain a nitrite residual of 600 to 1000 ppm.
c. Add sodium carbonate to maintain the pH at 8.5 to 9.5.
d. Check pH and analyze the system water to determine the nitrite content 24
hours after initiating treatment.
e. Make necessary adjustments, if needed.
No one locally had a stock mixture so I found the US Army boiler treatment program had proportions by weight: 68% Sodium nitrite, 10% Borax, 17% soda ash, 5% copper corrosion inhibitor. I left out the last ingredient since I had no idea what it was specifically and the Air Force program didn't mention it. If anyone knows what it is, let me know please!
ASHRAE indicated it was better to err on the over saturation side, so I did. It is critical that the pH remain between 8.5 and 9.5 for it to work effectively. I believe that the theory is that it actually forms a film on surfaces by controlled oxidation. Once filmed over (with magnetite), the surface oxidation stops. I could be wrong on this. The engineers on this site will correct me as needed-hopefully!
Does anyone here treat their boiler water or monitor pH?