Corrosion and lead keels

[Bill Roosa]

Just been going over my new copy of "Boatowner's Mechanical and Electrical Manual" by Nigel Calder. Great book BTW. In it, I noticed that lead is a very noble metal. Since there is a large hunk of lead hanging by SS bolts to my keel which are attached to my "common ground point" why does this not cause ALL my other metal items to be cathodic and rust or at the very least eat my zincs for lunch and then dine on my prop?The only thing I can think is that the lead is isolated from the SS bolts by some sort of insulator. I've never heard of such a thing but then not too many folks drop their keel either.Any insight would be appreciated.Bill

 

[Don S/V ILLusion]

it;s electrons are too heavy Bill

Bad joke = the real answer is in the table below> notice how close lead is to stainlessThe Galvanic Table Active (Anodic) Magnesium Mg alloy AZ-31B Mg alloy HK-31A Zinc (hot-dip, die cast, or plated) Beryllium (hot pressed) Al 7072 clad on 7075 Al 2014-T3 Al 1160-H14 Al 7079-T6 Cadmium (plated) Uranium Al 218 (die cast) Al 5052-0 Al 5052-H12 Al 5456-0, H353 Al 5052-H32 Al 1100-0 Al 3003-H25 Al 6061-T6 Al A360 (die cast) Al 7075-T6 Al 6061-0 Indium Al 2014-0 Al 2024-T4 Al 5052-H16 Tin (plated) Stainless steel 430 (active) Lead Steel 1010 Iron (cast) Stainless steel 410 (active) Copper (plated, cast, or wrought) Nickel (plated)

 

[Bill Roosa]

Nobody uses 430 stainless!!!!!

Where did you get your info?The table I have lists lead really high (Cathodic wise) above most of the common marine construction materials. Since I did not see bronze in your list I assume you got it from some chemistry book and not a list of "metals commonly use in marine applications"To wit:most anodicmagnesiumzincaluminum alloyscadmiummild steel wrought ironcast ironSS 41018-8 SS type 304Ni=resist18-8 type 316 SSInconelAluminum bronzenaval brassyellow brassred brass Muntz metaltin copper50-50 lead/tin solderAdmiralty brassAluminum brassmanganese bronzesilicon bronzebronze composition Gbronze composition MSS type 410LEAD --------------------------------------Nickel18-8 SS type 316titaniumplatinum graphitemosts cathodicI'm just trying to understand and not trying to muddy the waters. how come my lead keel does not rot my boat? By the numbers it should eat up my zincs at a very high rate of speed but it does not. there has to be a reason!

 

[Don S/V ILLusion]

Regardless

Lead is way down there on either list. Chenistry is chemistry, regardless of the application. Since there are numerous permutations of alloys such as zinc and bronze, it's sometimes misleading to include them in any comparison. As far as the answer to your question, the less active metals (e.g., cast iron) are essentially inert even in an electrical field

 

[Brian M H23]

Notice

Bronze, Lead, SS, nickel (used to plate currency), platinum, all down there as cathodic. Zinc is way up there. The mass of these more noble metals doesn't change the rate of oxidation. It just means that, until all the zinc is gone, the lead won't really oxidize. Not that lead oxide really hurts much - in fact they used to paint wood boats with it. (And mercury oxide bottom paint..mmmm) The presence of salt, stray current, etc etc promotes this oxidation. The metals will just rot their way down the table. But if theres no corrosion going on (lead and zinc in a dry room, touching each other), don't expect to see the lead eating the zinc! The zinc won't be sacrificed until the electrons want to get away (oxidize)..and then they come out of the most anodic metal they can.

 

[Don12364]

Its a Question of POTENTIAL

Previous posts show only the ORDER of the galvanic series.The attached shows the potential (Voltage) at which metals sit when in seawater. The text above the table advises that minimal corrosion will occur under water if the potential difference is not greater than 0.15 volts. Out of water but in a wet atmosphere this can be 0.25 volts.Over the years I have designed various odd items for our military and never had any trouble when using this data.Note:- The potential difference between lead and zinc is 0.55 so your keel is eroding your zinc. When the zinc is much reduced in area the keel will carry on the good work of protecting your prop. Only snag is that we put many coats of paint on our keels so the active surface area is much reduced plus the potential difference between lead and bronze is only 0.3 volts so there won't be much activity.Best thing is to make sure the zinc is in good shape.