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Sailboat Owner's Guide to Corrosion - Basic Theories (Collier 1-4)

Oct 22, 2014
15,695
CAL 35 Cruiser moored EVERETT WA
OK... While drifting, I can see we have addressed chapters 1-3 and in 4 the concepts of a corrosion...

Electrochemical as in what happens to my aluminum mast when it sits exposed to the elements and I scrape the surface layer of aluminum oxide off...

Galvanic as in the Zinc attached to my shaft to protect my prop

and Electrolytic where an electrical current is in the mix and this outside source is flowing between the two metals under my boat in the electrolyte I enjoy sailing.

It is the exchange of atoms that is Corrosion. That the tables help to provide a guide to identify the corrosion potential of various metals in volts. The greater the separation the better the less noble metal will give up it's atoms for the more noble metal...
 
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Likes: Will Gilmore
Feb 14, 2014
5,560
Hunter 430 Waveland, MS
It is the exchange of atoms that is Corrosion.
No! The exchange of Electrons. The atoms or the base metal stays the same.

The Electron chemistry or Electrochemical is normally in hydroxylic acid.:pimp:
_____
Electrochemical as in what happens to my aluminum mast when it sits exposed to the elements and I scrape the surface layer of aluminum oxide off...
That is Oxidation of Exposed Al to eventually form Al2O3 a stable molecule. But not Galvanic which need 2 part Reactions to flow Electrons. note: RedOx

Jim...
 
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Likes: Dave Groshong
Oct 22, 2014
15,695
CAL 35 Cruiser moored EVERETT WA
OK Jim.. I am hoping I am understanding the same thing but with different words.

Reviewing the "Redox" info and identifying the process of Oxidation, as I read the text by Collier, he describes "Simple Electrochemical Corrosion" as a "single piece of metal... in contact with an electrolyte. Electrochemical corrosion can occur in the atmosphere or under immersion. It normally proceeds continuously and at an extremely slow rate."

This appears to to be the type of chemical action that takes place when my Al mast is scratched so that bare metal is exposed. The atmosphere acts as the electrolyte upon the metal and through the process of "Oxidation" the aluminum is converted to aluminum oxide. This oxide has a tight bond to the metal and serves to protect the metal until I scrape or brush it off.

:what:
 
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Likes: Will Gilmore
Oct 19, 2017
6,850
O'Day 19 Littleton, NH
The atmosphere acts as the electrolyte upon the metal and through the process of "Oxidation" the aluminum is converted to aluminum oxide.
I cannot say whether the air can be considered the electrolyte, air is generally thought of as an insulator, a semi-conductor, at best, but the process of electrolysis is defined as one or more of the materials involved, cathode, electrolyte, or anode converting, through the movement of electrons, from one material to another. The oxidation process sounds like a form of electrolysis.
Others with a better understanding will, I hope, chime in, but this should be one of the definitions I need to start compiling.

-Will (Dragonfly)
 
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Likes: jssailem
Oct 19, 2017
6,850
O'Day 19 Littleton, NH
Collier describes basic corrosion in chapter 4. He specifically addressed three types of corrosion. One is a natural single metal corrosion, referred to as electrochemical corrosion. When in the presence of oxygen and nitrogen, a metal will give up electrons to oxidation. It occurs very slowly and the resulting surface of metal oxidation can slow or halt the process.

The second process is called galvanic corrosion. This is the basic concern most sailors have. Two different metals connected through an electrolyte and a low resistance conductor. This can be direct contact or a secondary conductor such as a copper wire. The circuit is then complete and elections can flow from the anode metal to the cathode metal, resulting in accelerated corrosion.

Electrolytic corrosion is the third form of corrosion. Electrolytic corrosion is just like galvanic corrosion except that a outside electrical source encourages the accelerated exchange of elections even more. This is also referred to as stray current corrosion. By adding a current to the process, the action is actually increasing the electrical potential, greater pressure variant of electrical polarization.

I would think, just connecting the system to a ground, would have that effect.

-Will (Dragonfly)
 
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Likes: jssailem

DArcy

.
Feb 11, 2017
1,168
Islander Freeport 36 Ottawa
I would think, just connecting the system to a ground, would have that effect.
Not necessarily. You need to have an potential difference to create what Collier is calling electrolytic corrosion. If everything is connected to the same ground you won't get a potential difference. Of course, if your ground is not at the same potential as your dock neighbor then yes, you can have a problem.
 
Oct 19, 2017
6,850
O'Day 19 Littleton, NH
Of course, if your ground is not at the same potential as your dock neighbor then yes, you can have a problem.
Yes, that's what I meant, but you said it so much better than I did. Thanks.

By "ground", I'm talking about any large electron well, a place starved and greedy for elections, such as 'Earth Ground' is. This can provide the motive force to move elections, stipping them from the anode metal.

-Will (Dragonfly)
 
May 17, 2004
3,374
Beneteau Oceanis 37 LE Havre de Grace
By "ground", I'm talking about any large electron well, a place starved and greedy for elections, such as 'Earth Ground' is. This can provide the motive force to move elections, stipping them from the anode metal.
Strictly speaking ground is the election source, where they come from, not the sink to which they go. It’s a bit confusing because current is said to flow toward ground, but the flow of electrons is opposite that. Collier mentions that dichotomy in chapter 3.

Generally in a galvanic cell the anode is the ground, giving electrons away as current flows in.
 
Mar 26, 2011
2,909
Corsair F-24 MK I Deale, MD
Aw for heaven's sake guys... It's a boating book, not a college text.

I'm skimming through it and I think he's done a pretty serviceable job of making chemistry accessible. The intro chapters are just the view from 40,000 feet, and that is enough for his purposes. Corrosion inspection is part of my day job (refinery equipment), and so far I haven't seen anything that gives me real heartburn, given the focus. Writing a book is hard work.

I would like to see more about passivization.
 
Oct 22, 2014
15,695
CAL 35 Cruiser moored EVERETT WA
I think the writing is good. I simplifies the concepts and states the facts with clarity. Much better than my chemistry books did 55 years ago.
 
Oct 19, 2017
6,850
O'Day 19 Littleton, NH
Strictly speaking ground is the election source, where they come from, not the sink to which they go. It’s a bit confusing because current is said to flow toward ground, but the flow of electrons is opposite that. Collier mentions that dichotomy in chapter 3.
Yes, of course. My mistake. I was thinking of ground, as in a lightning storm. I was taught in my weather lab that positive charged ground follows under a lightning storm. This is what pulls the ions from the air and causes lightning. I have been visualizing that image as I think of electron flow going to ground instead of the other way around.

-Will (Dragonfly)