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

[jssailem]

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...

 

[JamesG161]

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.
_____

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...

 

[jssailem]

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.

 

[Will Gilmore]

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)

 

[Will Gilmore]

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)

 

[DArcy]

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.

 

[Will Gilmore]

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)

 

[Davidasailor26]

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.

 

[thinwater]

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.

 

[jssailem]

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.

 

[JamesG161]

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.

Titanium does this very well. Never paint Titanium, since paint stops the Oxygen exposure.
Jim...

 

[Will Gilmore]

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)

 

[dLj]

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.

@thinwater I was just skimming through this thread as someone was asking me about Collier and I saw your comment how you'd like to see more about passivation. I realize this comes from several years ago, but in any case, here comes some info on passivation. My apologies for missing your comment before.

There are two general definitions of passivity in metals:

1) A metal active in the EMF Series, or an alloy composed of such metals, is considered passive when it's electrochemical behavior becomes that of an appreciably less active or noble metal.

2) A metal or alloy is passive if it substantially resists corrosion in an environment where thermodynamically there is a large free energy decrease associated with its passage from the metallic state to appropriate corrosion products.

If you'd like, I can send you the chapter from H. H. Uhlig's "Corrosion Handbook" - one of the best resources of corrosion information I'm aware of. He has a very interesting discussion of passivity and the theories surrounding it. It may be a bit "heavy" for this discussion.

I'm sure one of the things that is remarkable is that the stainless steels have both an active and passive state as shown on the Galvanic series (graph taken from ASTM G82).

I've highlighted the austenitic alloys found in this chart. The active potential is circled in red and the passive potential is circled in blue.

The when/where/how/why these alloys pass from passive to active is still an unknown. Also of note, the EMF potential of these alloys is nearly the same as titanium when they are in the passive state. I've spent a number of years working in the practical arena of passivating stainless steels, titaniums, and other alloys.

Sticking to the stainless steels (titanium gets a bit more complicated) there are accepted processes that help push the SS alloys to be passive - called passivation in the practical world. Typically nitric or citric acid baths are used to impart the passivity. In practical terms, the acid baths perform two functions - A) remove exogenous iron (in any manufacturing environment there is a lot of microscopic iron particles that get deposited on the surface of the metal). B) Help form a passive oxide layer.

The functional standards surrounding passivation, ASTM A380 and ASTM A967 have tests to determine if the passivation process was "successful". All of those tests are aimed at detecting free iron (exogenous iron) on the surface of the material. It's impossible to know if B) occurred - it is assumed that it was successful if no exogenous iron is found left on the surface. It is not actually necessary to use an acid bath to form that passive oxide, but having exogenous iron on the surface is disruptive to that formation, hence that is the main focus of the acid baths and what is checked. All these alloys will naturally fully passivate in air if no exogenous iron is present on the surface.

If you still are interested in this and would like more conversation, let me know.

dj