What Causes A Hot Water Tank To Deteriorate?

[Desert_Sasquatch]

What are the factors that determine the life of a water tank? Obviously the quality of the water and the materials used to construct the tank are important. But beyond that I'm fuzzy. Is it just the number of gallons of water that pass through it? The number of "fresh" gallons of water that pass through it (ie so water in a hydronic system that was recirculating and being drained just periodically would cause less deterioration than water in a domestic supply situation)? Does the temperature of the water play a role? Does the process of electrically heating water somehow contribute to deterioration of the metal? Or is it all just down to how many years a surface is submerged in water?

I'm kind of trying to tease out whether there's a way to prolong the life of the water tank that gets heated through an active solar thermal system. And I'm trying to understand if the way active solar heating systems are designed is because that's really the best way to do it or because it's the cheapest way to do it in the short term but maybe there's a better way if we look at long term costs.

For instance, would it be better to get a tank without any kind of electric heating and install an on-demand hot water heater downstream of the tank instead? Would that storage tank last longer? But also I'm just trying to understand

 

[Desert_Sasquatch]

OK, here's some info I found. It seems that for Stainless Steel tanks (which seem to generally be suggested for hydronic heating applications, since that requires temps around 82 C or 180 F) will corrode if exposed to too much chloride. And that the potency of this effect goes up exponentially with an increase in temperature.

I'm not really clear what the normal level of chloride is in treated water in the USA...I see that 250 PPM is a maximum but it seems like usually it's much lower. My local area sets 0.2 PPM as the minimum and 4 PPM as the maximum allowed in the drinking water supply.

From what I can tell from the above chart, water at 80 degrees C should be expected to form"crevice corrosion" in SS 316 (food grade, used in some--many/all?--storage tanks) when the Chloride concentration is above about... I'd say 70 PPM.

So it seems like the risk of corrosion from chloride in stainless steel tanks is at least not too bad?