Sacrificing Sacrificial Anodes

[Charles Erwin]

Ralph I think this is right.

OH- + HCO3- > CO3-- + H20

CO3-- + Ca++ > CaCO3

Anyhow - you will know it when you see it for sure.

Charles

 

[Ralph Johnstone]

Charles, so far so good as far as the bicarbonate to carbonate and finally to calcium carbonate.

I'm just not clear on why the CaCO3 should choose to preferentially precipitate out on the Zn. With the Zn going to Zn++ and 2e- I don't see (or maybe just missing) any dissociation of the water to locally produce an excess of OH- and get the ball rolling for coating the anodes.

I guess the question I have here is 'how do you know this is specifically CaCO3" on your zincs and not just a collection of random crud. Granted, it doesn't matter what it is, it still stops the reaction.

Guess I'm just lucky that my three zincs are corroding (and sloughing off) just enough to keep them clean for a couple of years without any surface buildup.

 

[FourPoints]

I've been following this thread with curious interest, but had nothing to add until now.

My dinghy motor, a Honda 2HP, has a rather large zinc on it. It's the same one specified in the owners manual so I know it wasn't done randomly by the original owner. I've noticed that it builds up what looks like a calcium crust over it when I've left the engine in the water for a day or two. I scrape it off a couple times a year when it starts to cover a significant portion of the zinc, but otherwise haven't worried about it. My auxiliary engine is a Tohatsu 8HP 2 stroke. I finally this year went and bought the OEM anode, instead of using an oversized pancake zinc that more or less fit, when I was doing work on it, and the OEM anode is far far too light feeling to be zinc. It looks and feels like it's an AL alloy of some sort.

Does this mean that on the dinghy motor it should be a smaller anode, and given that it's an aluminum engine, should have one of the AL alloy anodes on it instead of a zinc one?

This is a picture of the OEM anode from Tohatsu.

 

[fruivita]

My boat is in the Chesapeake Bay (brackish) but the marina is off a creek (more fresh) so the zinc/aluminum decision has me wondering if I should use both? It seems to me depoending on the conditions either the zinc or aluminum would get eaten away and I wouldn't have to worry about whether I had chosen the right annode. Anyone have experience with this? Any chemistry geniuses out there who can tell me whether my logic flies?

 

[jibes138]

It seems the latest logic is to outlaw zincs because they are environmentally hazardous and tell everyone that aluminum is better anyway.