Air compressor tank cathodic protection

[taiwanluthiers]

I notice that tanked water heaters have cathodic protection, basically a sacrificial anode made of magnesium attached to the tank to prevent it from rusting... as you know water heaters are always in contact with water.

Is there a reason why air compressor tanks aren't protected this way as it's always going to have water in the tank no matter what. Then there's the fact that it rusts inside out and that a brand new looking tank can basically be covered in rust inside and probably rusted through and ready to explode at any time...

Wouldn't attaching a piece of magnesium to the tank solve that problem?

 

[Flynth]

Personally I don't think so. It is just my thoughts, I may be wrong.

In a compressor tank you have water on the bottom, also often (depending on temperature) you get condensation drops forming on the top surface where they sit until they frow large enough to run down the side or fall.

A cathodic protection works only when all elements are electrically connected in vicinity of corrosion site. So if the sacrificial metal sits on the bottom I don't think it would protect the top giving you a false sense of security. In some way it is better to have a rusting tank and know about it rather than have a slower rusting one, but have no idea...

This makes you think why they don't put a zinc layer on the inside. That would really help.

 

[taiwanluthiers]

This makes you think why they don't put a zinc layer on the inside. That would really help.

Maybe it's just cost... or that galvanized steel doesn't really weld well...

 

[Flynth]

Maybe it's just cost... or that galvanized steel doesn't really weld well...

Oh no, welding galvanised steel is a nightmare (trust me, don't even try, metal vapor poisoning headache is horrible and it's one of the best case scenarios)

However, they could galvanise it after welding. I imagine normal tanks wouldn't work for this, because it would take ages for the liquid zinc to evacuate through tiny holes, but they could heat up the whole thing to zinc melting temp. Put let's say few kg of liquid zinc in, then cool it a bit while sloshing. Eventually draining the excess liquid. That custom procedure is probably why they don't do it.

Isn't there some chemical method to cover mild steel with zinc?

 

[taiwanluthiers]

Oh no, welding galvanised steel is a nightmare (trust me, don't even try, metal vapor poisoning headache is horrible and it's one of the best case scenarios)

However, they could galvanise it after welding. I imagine normal tanks wouldn't work for this, because it would take ages for the liquid zinc to evacuate through tiny holes, but they could heat up the whole thing to zinc melting temp. Put let's say few kg of liquid zinc in, then cool it a bit while sloshing. Eventually draining the excess liquid. That custom procedure is probably why they don't do it.

Isn't there some chemical method to cover mild steel with zinc?

You could plate it... but again it would be a custom procedure.

Why is it screwing a piece of magnesium onto the bottom of the tanks not work to protect the tank? They do this for boat hulls and that's saltwater which is MUCH worse for rust.

I mean for water heaters I think the sacrificial anode is screwed through the top of the tank.

 

[MrWhoopee]

Hot dip galvanizing is a possibility, which is by far the longest lasting. Filling and draining the molten zinc might be a problem. The biggest argument against is probably the same reason they don't allow galvanized pipe for fire sprinkler systems. The zinc flakes off and blocks orifices (orifii?). That, and the increased cost.

 

[B2]

Stainless Steel tank would be nice, but it actually rusts too if exposed to salts. Perhaps not necessarily as physically strong as some steels, but I bet these tanks are not anything complicated, just cheap to make.

It is my understanding that a Hot water tanks come with either a 5 or 10 year warranty. Same tank just more insurance and $$$!

So here is my question. Consider a hot water heater that is not being used and sets with water in it for 10 years, while the water is shut off. Has the rusting process stopped? I recently looked at a building that appeared to be abandon, but it had new hot water heaters put in during a partial remodeling process that occurred almost 30 years ago. On the outside they look like new, just very dusty.

I once had a hot water heating system. The radiators, some pipes and the pumps were just made from common steel or cast iron. However they did not rust out from the inside. I think this is because the water is not changed very often, the Oxygen in the water must be depleted after a while and so there is no more rusting. Is this reasonable?

Are those 30 year old hot water heaters any good?

Dave L.

 

[taiwanluthiers]

Stainless Steel tank would be nice, but it actually rusts too if exposed to salts. Perhaps not necessarily as physically strong as some steels, but I bet these tanks are not anything complicated, just cheap to make.

It is my understanding that a Hot water tanks come with either a 5 or 10 year warranty. Same tank just more insurance and $$$!

So here is my question. Consider a hot water heater that is not being used and sets with water in it for 10 years, while the water is shut off. Has the rusting process stopped? I recently looked at a building that appeared to be abandon, but it had new hot water heaters put in during a partial remodeling process that occurred almost 30 years ago. On the outside they look like new, just very dusty.

I once had a hot water heating system. The radiators, some pipes and the pumps were just made from common steel or cast iron. However they did not rust out from the inside. I think this is because the water is not changed very often, the Oxygen in the water must be depleted after a while and so there is no more rusting. Is this reasonable?

Are those 30 year old hot water heaters any good?

Dave L.

Check the sacrificial anode. There's no way the heater tank will be any good as it's constantly exposed to water and oxygen.

 

[Jake M]

It would seem to me that if the compressor wasn't flooded (as is a hot water heater), you'd have a hard time keeping the galvanic cell. Galvanizing might be more plausible, as the entire coating is sacrificial, so local moisture can keep the ions moving more locally. I'd bet it'd be cheaper to put in a drain valve and replace the tank every 20, 30, 40 years, or what ever the climate allows.

 

[pontiac428]

Sacrificial anodes really only work in a cell with a salt bridge, so they work for immersed systems. I have no freaking idea what the theory is behind anode blocks on dry installations like flagpoles, but I've seen them there. I don't know why manufacturers don't passivate or at least paint the insides of air tanks. Zinc is an ablative coating that can easily be electroplated to the inside of a tank, no molten metal involved. I just retired a beautiful ASME certified 60-gal tank after looking inside with a bore scope helped me realize I was living on borrowed time. Got a nice Quincy with heavy paint all over it, except the bare naked inner surface, already giving me red/brown condensate after my first year. Grrr.