Carbon steel doesn't have a protective passive layer like stainless does. This pasive layer protects it from most media, but can be broken down locally by chlorides. ( This normally occurs in places around inclusions where the chromium content is lower.)
Carbon steel will just corrode over the entire surface in chlorides, as the chlorides provide a nice electrolyte for the corrosion mechanism. With stainless steel, the chlorides chemically react with the chromium oxide passivation layer, causing it to break down in localized spots, exposing unprotected material to the electrolyte. The stainless steel cannot re-form a protective layer, because the chlorides prevent this from occurring as well, and the pitting mechanism gets started.
So this brings up another issue I have struggled with. Which material is better in a condensing type natural gas fired boiler where the water is condensed using the flue gas like a heat exchanger application inside the boiler for better efficiency? The exhaust contains chlorides (bad for stainless) but is also a corrosive environment (bad for carbon). Looks as though stainless would "pit" and carbon steel would have even wear. From my experiences in steam systems even corrosion is preferred over pitting if you have to choose between the two. So if the carbon steel used is thicker (allowing more corrosion to take place before failure) would it last longer than the stainless in a condensing boiler application?
I have limited personal experience in this area and manufacturers stick by their product so its hard to get a straight answer...
If you cannot find a grade of stainless which will resist pitting under these conditions, and I don't think there is one in hot, acid, chloride filled media, then you should not use stainless
The use of stainless is not indicated unless it can remain passive in the given environment. The exception is when pitting and perforation are unimportant to the desgn and function, which is not the case with boilers.
Why is the corrosion rate of carbon steel lower in pH 4 HCl solution than in pH 4 aceti acid/sodium acetate buffer solution? and assume that no oxygen in both cases.
KeItSiSt,
Some manufacturers of condensing furnaces use 29-4C SS in the condensing portion of their heat exchangers in NG furnaces. There is little if any welding in these though.
In one application where the flue gas was being exhausted through a water bath, the project was scrapped because the corrosion issues were so bad. The material used was 316L.
29-4C and Seacure are great superferritic stainless steels, but because they lack toughness they can't be used in thicknesses above about 0.060 inches. This rules them out of a lot of applications where their corrosion resistance would permit their use.
