[optimisticrealist]

I am unable to change out the sacrificial magnesium anode in my hot water tank so I am thinking of just dropping a bunch of pieces of magnesium into it instead. Anyone see any problems with this being that the magnesium would be sitting in the bottom of a lined tank, not touching metal? Someone told me that it needs to be attached to the metal that it is protecting. Is this true? Why? I don't see why it wouldn't corrode and do its job. If it works less efficiently, then I could just use more material... Thanks

[Borek]

Someone told me that it needs to be attached to the metal that it is protecting. Is this true?

Yes.

Why?

Because that's how the https://en.wikipedia.org/wiki/Cathodic_protection works.

[Enthalpy]

Second opinion identical to Borek's.

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Has someone here first-hand experience with sacrificial anodes aka cathodic protection?

My trials told "very little efficient if any".

My opinion until I change it is that the theory about potentials, corrosion and cells works with batteries made of very pure metals but fails with construction alloys as these contain already corrosion couples before building any explicit cell of them.

Also, even pure magnesium has a bad reputation of corroding without any sort of couple or cell.

Sure, Mg is commonly used to protect steel. But so are radioactive lightning rods despite zero evidence suggests they are better.

[optimisticrealist]

Someone told me that it needs to be attached to the metal that it is protecting. Is this true?

Yes.

Why?

Because that's how the https://en.wikipedia.org/wiki/Cathodic_protection works.

It seems to me that the electrolyte could form part of the circuit to connect the anode to the metal it is protecting. I could be wrong... I will find out if my new stainless steel element corrodes to failure inside a month. As it is, the element is insulated from the metal of the tank and is corroding fast... the electrolyte apparently making the electrical connection.

[Borek]

It seems to me that the electrolyte could form part of the circuit to connect the anode to the metal it is protecting.

Sure it does, but it is not enough to complete the circuit, it is just half of the circuit. That's electrochemistry 101.

Take any battery. It has two contacts, + and -, they are internally connected through the electrolyte, yet the current doesn't flow. It will start to flow when you close the circuit putting something between the contacts.

[optimisticrealist]

OK. Then the magnesium I drop into the lined tank shouldn't corrode much at all if there is no circuit.  I found a way to ground to the tank making the issue moot.  Thanks

[Borek]

OK. Then the magnesium I drop into the lined tank shouldn't corrode much at all if there is no circuit.

No - it will still corrode. It doesn't need to be a sacrificial anode for that. Being a sacrificial anode it will corrode faster though.

[Enthalpy]

Could zinc replace magnesium here?

The potentials should suffice to protect iron alloys.

Zinc won't corrode as fast as magnesium does, especially in hot water. It's more widely available too.

Also: what is the use of the hot water in the tank? If the sacrificial electrode does its work, it will inject ions in the water, which may be undesirable or go against some regulation.