For those of you who have retrofitted VERY OLD hot water tanks (say 20-30+ years old) that somehow lasted throughout the years and perhaps were made long before hot water tanks were cheapened and turned into a commodity - please read:
I'm a big fan of indirect hot water tanks - for those who do not know - an indirect is a tank with a coil inside that you pump hot boiler water into at a 160-180 degrees and it heats the tank using the coil surface (essentially a built in heat exchanger) to whatever temperature you have the aquastat or sensor set at.
Anyways. A LOT of indirect HWT manufacturers use 316L stainless steel, some 316 Ti (Viessmann) for their stainless steel tanks. It is generally regarded as a very high, expensive, grade of stainless steel and 300 series stainless steels are more expensive than 400 series stainless steel due to their nickel content and nickel prices are through the roof (400 series do not contain nickel). Only one manufacturer I know of uses a 400 series stianless steel for their indirect tanks (Bradford White RTV tanks which use 444 stainless steel).
Now the reason I point this all out is the following: If you do some metallurgical reading you'll find that 300 series stainless steel are not immune to chloride (chlorine) attack. The higher the chlorine content in the water, the more degradation occurs overtime on 300 series stainless steel. Yet if you do some metallurgical reading you'll find that 400 series stainless steels are virtually immune to chloride attack.
Here is what I find even more interesting: If you look at fact sheets on the different grades of stainless steel, you'll find common uses of the various grades. Some 400 series grades are listed as being utilized for the manufacturer of hot water tanks. Now perhaps they may be talking presently, but I detect a hint of historical perspective in the mentioning of these in these fact sheets (you can find them on line - look for the A.L. sheets).
Now what I'm trying to figure out is why is there this gravitation towards 300 series stainless steel (316L, 316 Ti) in indirect manufacturing when 400 series stainless steels are cheaper and immune to chloride attack which is a common issue with domestic water, and what I'd like to know from you is how many of you have found some of these very old hot water tanks that finally failed - were they made out of stainless steel? And if so, do you know what grade they were, or perhaps can you provide some info on manufacturer, model, etc. and I'd like to research it to find out what they manufacturered some of these very old hot water tanks out of?
Indirect tanks have been around a fair amount of time, but it's only in the past 10 years that they have really started to spread like wild fire in installations in conjunctin with high efficiency boilers. So many of them haven't been around long enough to see mass failures yet - I would expect them to last at least 15 years or so but until some time passes we have yet to truly find out. I've seen a few fail but not enough to say it's a common problem.
Here is a fact sheet so you can see what I am talking about