Corrosion in the quench and cutting water system is fairly common. Oxygen dissolved in the water is one of the reasons and that leads to deep, sharp pits in the top of the pipe or any connection that comes off the top of the pipe. In addition to oxygen there is H2S dissolved in the water. At ambient conditions, H2S, oxygen and liquid water will form elemental sulfur which can be corrosive.
I am aware of the use of duplex stainless steel, 2205, for branch connections and areas that have had repeated corrosion failures with good success. Austenitic stainless steel, either 304 or 316 (I think both have been used), has been used in the decoking equipment to reduce corrosion. I believe that constructing the entire decoking water system from those materials would be too expensive and difficult to justify.
Reliance in India published an article in Hydrocarbon Processing in May 2008 on this subject. They tried an oxygen scavenger in the decoking water system and that lowered the corrosion rate significantly. Even so, the corrosion rate was higher than desired so they added a second chemical and the corrosion rate dropped to acceptable levels.
The primary cutting water quality concern is the amount of coke fines as they will cause erosion in the cutting equipment and will deposit in areas of low flow and inhibit performance of the decoking valve and cutting tool. A suspended solids content of less than 500 ppm is considered to be good andn lower is better. Routine analysis of the cutting water should be done and analyzed for pH, Fe, H2S, NH3, Cl, suspended solids. Weekly analysis is adequate.