Sailboat Owner's Guide to Corrosion - Protection (Collier 13 - 15)

[Will Gilmore]

Welcome to Section 5, the Sailboat Owner's Guide to Corrosion - Protection (Collier 13- 15 & 18), of our SBO Book club study of Everett Collier's book, 'The Boatowner's Guide to Corrosion".

We are reading and posting, in sections, as we work our way through the subject of corrosion in the marine environment. Each section covers one or more chapters and has been posted in their own threads. As the new threads are started, I will tag any interested participants and create a linked table of contents to make it easier to follow along and participate.

A rough outline of the sections to come are as follows. They may be edited and updated as the current of our discussion requires.

Sections

1. Sailboat Owner's Guide to Corrosion - Fundamentals (Collier 1-4)
2. Sailboat Owner's Guide to Corrosion - Self-Corrosion (Collier 5)
3. Sailboat Owner's Guide to Corrosion - Galvanic and Stray Current Corrosion (Collier 6 & 7)
4. Sailboat Owner's Guide to Corrosion - Metals Aboard (Collier 8 - 12)
5. Sailboat Owner's Guide to Corrosion - Protection (Collier 13 - 15)
6. Sailboat Owner's Guide to Corrosion - Hull and Motor (Collier 16 - 17)
7. Sailboat Owner's Guide to Corrosion - Electronics and Plumbing (Collier 18 - 19)
8. Sailboat Owner's Guide to Corrosion - Topsides (Collier 20 - 21)
9. Sailboat Owner's Guide to Corrosion - Resource

Let me know (@Will Gilmore) if you want your name added to this list. It will appear at the beginning of each new thread to let interested members know when the new thread has opened. You can then post something or click the [Watch] button to follow the thread.

Tagged participants:
@Will Gilmore
@DArcy
@mermike
@jssailem
@dlochner
@dLj
@JamesG161
@ontherocks83
@rgranger
@Mark Maulden
@Davidasailor26
@LeslieTroyer

I'll be happy to edit/update this list at any time.

Some basic rules for maintaining useful and topic focused discussion:
SBO is a public forum, and as such, anyone interested, is welcome to participate, ask questions and express their opinions. There are limitations to the form these expressions can take and the culture of participation. All the basic forum rules of decorum and good manners apply. As a participant in this series of discussions, we would ask that participants remain on topic and refrain from derogatory language or remarks. The express purpose of this series of threads will be to understand corrosion in the marine environment using Collier's book as a guide. We therefore, expect participants to make an honest effort to read and stay up on the material under discussion. We are a group of congenial sailors with a sense of humor that often can run astray. A playful comment on occasion is expected, but anyone of us will feel free to firmly redirect anyone back to the subject at hand if it looks like it is in danger of derailing the discussion. A moderator will be asked to intervene, edit out any inappropriate comments, and possible ban an offending participant, should the group find their continued presence a serious distraction. Posting in this thread will be considered agreement to these terms. Thank you so much for your understanding and cooperation. I look forward to being part of the amazing community that is developing around this subject. I know we will all have a great time.

-Will (Dragonfly)

 

[dLj]

Collier uses two terms in the beginning of Chapter 13 under cathodic protection; passive protection and active protection. I'm not sure where those two terms came from, I've never heard them. They do not exist in the NACE definitions and according to H. H. Uhlig, cathodic protection is cathodic protection, irrespective if there is an impressed current or not. Impressed current systems and sacrificial anode systems are both correct terms, not sure why he didn't just use those two terms in the intro to this section, he certainly goes on to explain them both. The rest of Chapter 13 is nicely written.

dj

 

[Will Gilmore]

A question that I've been turning around and around in my head, since first reading about Impressed Current System, is the idea of cathodic demand verses anodic supply.

Often, we are taught concepts in electricity through a water movement analogy. I recognize this is an imperfect analogy, but when imagining electrical flow, I think of low and high pressure movement the way gravity might affect water over a dam. There is a height difference that expresses the potential energy. This means a supply of water at the top of the dam might represent the anode while the pool at the bottom represents the cathode. Gravity and height difference is the voltage difference.

The analogy falls apart when considering the idea of a secondary "potential" anode. With a waterfall or dam, the force pulling any height water supply, to fill the pool below, is unaffected by any other supply of water. The idea of a sacrificial anode or an impressed current anode satisfying the electrical pressure between it and the cathodic metal such that there is no longer any force to strip electrons from other potential anodic metals, is hard to get my head around.

Why does a single source supply of electrons remove the force to pull electrons from all other potential anodic metals?

-Will (Dragonfly)

 

[dLj]

Why does a single source supply of electrons remove the force to pull electrons from all other potential anodic metals?

-Will (Dragonfly)

If I'm following you correctly in your question, you are not falling apart with your analogy. But typically with the size of our recreational sailboats, there is not sufficient distance to worry about. Now, on larger steel hull boats for example, you do have that issue. So you have to design the number, size and placement of anodes such that the whole submerged structure is protected. Getting into pipelines, those are much longer distances, plus they are gong through different soils, hence varying conductivity, so those require a lot of planning and work to cathodically protect correctly.

Am I understanding your question?

dj

 

[JamesG161]

Why does a single source supply of electrons remove the force

You are charging your Galvanic battery.

Or...

Water pressure analogy is true if...
Water is flowing over the top of the dam, or constant pressure potential.
Jim...

PS: We call that Steady State Potential or no tendency to change over time.

 

[DArcy]

@Will Gilmore you are stretching the limits of the water analogy. People like to use water flow as an analogy for electrical current so we can visualize the flow. Don't think about open surface flow, only inside a pipe, so you can keep the concept of pressure which is a corollary for electric potential difference (voltage difference).
Breaking down your analogy, current flows when you have a pressure difference. Electrical pressure is caused by unequal voltage at either end of a conductor. If you have two different metals which are electrically connected and immerse them in an electrolyte then you get an electric potential difference due to the different metals having different affinity to electrons. This potential difference can be represented by the electromotive series https://patentimages.storage.googleapis.com/6e/a8/e8/89e4c330f869f0/imgb0001.png
Put copper and iron together in an electrolyte and you get a difference of 0.34 - (-0.44) = 0.78V. That is enough to create a current which causes an exchange of electrons.
Put nickle and tin together and you get a difference of (-0.14) - (-0.28) = 0.09V. This potential difference is not really high enough to cause current to flow so you won't exchange electrons. There just isn't enough pressure.
So think of the cathode and anode as being the pressure source at either end of a pipe (conductor) with the anode at a higher pressure than the cathode.
The sacrificial anode does not fit well in the analogy of water pressure. Put some zinc in contact with the copper and iron and it looses electrons more readily than the iron so there are enough electrons available that the iron doesn't loose many - until the anode is depleted. This assumes there isn't anything restricting current from the zinc, at least no more than from the iron. Maybe think of the iron as an intermediate pool between the top of the dam (zinc) and bottom (copper). As long as the pipe from the pool above is big enough, the pool in the middle doesn't loose water, it just flows from the top pool to the bottom pool.

 

[Will Gilmore]

Am I understanding your question

Not quite, I think.
The battery analogy comes close.

What I'm asking, in my clumsy way is,
Let's say you have a pool that is 100 meters below a dam. We can name the pool, Iron Pond.
There's a given potential for water over the top of that dam to flow into the pool. That flow is depleting the reservoir behind the dam. We'll name the reservoir Aluminium Reservoir. There's a certain rate of flow based on the length of the lip of the dam. If left to follow over that lip, it will completely drain the reservoir because it's a shallow reservoir.
Now, let's say there's a cliff on the opposite side of Iron Pond from the dam. It's 1000 meters tall and much longer than the dam separating Aluminium Reservoir. On top of that cliff is another body of water we can call Zinc bog. The engineers concerned that Aluminium Reservoir will be emptied before the town can use the water for flushing their toilets, decide to put a stop to the water flowing over the edge of the dam by cutting down the earth along the top of Zinc cliff and let Zinc Bog drain into Iron Pond instead.
It seems that by feeding Iron Pond from Zink Bog, the flow out of Aluminium Reservoir has naturally stopped. What is going on here?
Is it that I have my geography wrong? Maybe Zink Bog isn't dumping into Iron Pond directly, but rather into Aluminium Reservoir first so there is no noticeable drop as the excess water flows into Iron Pond. Or am I looking at it the other way, Zinc Bog is positioned between Iron Pond and Aluminium Reservoir? From an anode to cathode physical view, that doesn't make sense.

-Will (Dragonfly)

 

[Will Gilmore]

Thanks DArcy, I hadn't seen your response before my 4am brain was able to refresh my phone. You said exactly what I was thinking, yet...
There exists the same pressure variance between dissimilar metals regardless of any intermediary pool and since the cathode pool is never full and potential is never satisfied, why does it not just pull from all anode sources according to their respective potentials?
A zinc anode located at the nut end of a propeller shaft will satisfy the pressure pull between the ss. shaft behind the brass propeller without being physically between the two.

-Will (Dragonfly)

 

[DArcy]

Now you are getting into the molecular or chemical realm in which I am not well versed. Maybe the iron gains electrons from the zinc as fast as it looses them to the copper or maybe the copper just pulls electrons from the highest potential difference.

 

[JamesG161]

@Will Gilmore there will be a tendency to react with all metals until
Steady State
Then all electrons will flow to the highest potential metal , no short cuts then.
The idea of a pool of electrons is a bit off course.

Remember it is a Galvanic Cell or chemical reaction.
Jim...

 

[Will Gilmore]

The idea of a pool of electrons is a bit off course.

I know the analogy isn't perfect. I just mean a source and a dearth. I also know there is a circular flow. It may not be complete, but the forces act in a circuit.
I could rephrase my analogy in terms of pipes and tanks, valves and pumps, but those concepts have limited analogous application as well.

I'm just thinking out loud. Since the actual existence of electrons is known only through theory and inference. There are some questions that may remain a mystery.

-Will (Dragonfly)

 

[Will Gilmore]

Chapter 14 discusses strategies to avoid or reduce corrosion before employing an anode to reduce corrosion.
Collier makes the point that the least nobel metals are first to corrode but also talks about the proportions of anodic metal to cathodic metal. He used the example of s.s. screws in an aluminum fixture. By making the anode metal larger than the cathode metal, the rate of corrosion can be vastly reduced. The aluminum fixture still corrodes, but it does so at a rate inversely proportional to its mass ratio to the cathodic metal.

There is a relationship here that somewhat addresses my question above. It seems there is a limited attraction force in the cathode that if satisfied at a certain rate, reduces or eliminates its attractive forces outside the anode/cathode couple.

It seems as though, there may be a timing factor or something going on. The longer an anodic metal is subject to the attractive force, the more likely it will give up its electron. The more loosely held electrons have a reduced time interval. If, while an electron is in transit, the force is satisfied, the time interval resets, in a sense.

Could this be a driven harmonic system?

-Will (Dragonfly)

 

[dLj]

I know the analogy isn't perfect. I just mean a source and a dearth. I also know there is a circular flow. It may not be complete, but the forces act in a circuit.
I could rephrase my analogy in terms of pipes and tanks, valves and pumps, but those concepts have limited analogous application as well.

I'm just thinking out loud. Since the actual existence of electrons is known only through theory and inference. There are some questions that may remain a mystery.

-Will (Dragonfly)

Corrosion is current limited. Voltage provides the push of electrons to participate in the corrosion process, but the process itself is current limited. That's why @Maine Sail's example in an earlier thread showed tremendous damage in a very short length of time, basically because the amount of current available was driven by the direct connection to the battery - a ton of stored current! It is the value of the current applied to corroding specimen that drives the corrosion, not the voltage potential.

dj

 

[Will Gilmore]

I am enjoying chapter 14. He talks about designing around corrosion considerations by choosing appropriate materials, working with design elements that help eliminate conditions that are conducive to corrosion and coating or treating materials to prevent corrosion. He also mentions changing the environment. I haven't read exactly what he means by that. We sail where we sail because we live there or that's where we want to sail. moving from salt to freshwater doesn't seem a reasonable response. However, I haven't read what he has to say on that, so maybe he's talking about reducing moisture content of the air and keeping bilges and such dry.
He does talk about physical structures designed to avoid standing water collection, eliminating crevices and small places that catch water and hold it, by changing shapes of elements like stringers, protruding gaskets, that sort of thing. Large clear limber holes to drain bilge sections, seems like an important feature. There was a thread earlier about the corrosion of an aluminum fuel tank that was collecting water between it and the hull.

He also talks about surface treatment and small design elements, such as hard vs rounded corners and how they affect protective coating integrity. Rough surfaces make it hard to obtain even coatings and encourage chipping. Protective coatings wear off of hard corners faster and go on thinner than they do over rounded corners. Collier also mentions the inherent problems with right angle turns in piping. The internal turbulence can exacerbate corrosion. Two forty-five angles in place of a right angle would reduce a lot of that effect.

I'm interested, especially in this chapter, because I have ideas and dreams to design and build a world cruiser out of steel. There are a lot of decisions this chapter would affect. Knowing some of the things to consider would make design work more successful without adding complexity.

-Will (Dragonfly)

 

[Will Gilmore]

I just had a great conversation with JamesG161 regarding my question about how adding an anodic current or sacrificial anode stops the less nobel metal from corroding when the potential between that metal and the cathodic metal still exists.
Thanks Jim, I feel like I'm slowly coming around.

To return to my earlier analogy of Iron Pond being fed by Aluminium Reservoir; adding Zinc Falls from Zinc Bog to satisfy the needs of Iron Pond can to preserve the water in Aluminium Reservoir because Zinc Falls not only dumps water into Iron Pond, but it also dumps water into Aluminium Reservoir.

What Jim has been trying and trying to hammer into my head is that the potential between the anodic metal and the cathodic metal doesn't go away because of a greater potential between the new anode and the cathodic metal. It's just that the new anode also has potential between it and the old anodic metal that now becomes another cathodic metal between it and the new anode. While it may still lose electrons to the more nobel cathodic metal, it is also being fed electrons from the new anode to replace them.

Whew!!! I think I'm finally understanding some important stuff here.

-Will (Dragonfly)

 

[JamesG161]

It's just that the new anode also has potential between it and the old anodic metal that now becomes another cathodic metal between it and the new anode. While it may still lose electrons to the more nobel cathodic metal, it is also being fed electrons from the new anode to replace them.[]

Top down source or Corrosion Potential or Electron Source or Volts [ the driving force]
So...
Mg>Zn>Al>Bronze> Stainless Steel>Graphite

Jim...

PS: The energy flow or flux is the Amps or the energy flow in the Galvanic Circuit.

 

[DArcy]

To return to my earlier analogy of Iron Pond being fed by Aluminium Reservoir; adding Zinc Falls from Zinc Bog to satisfy the needs of Iron Pond can to preserve the water in Aluminium Reservoir because Zinc Falls not only dumps water into Iron Pond, but it also dumps water into Aluminium Reservoir.

Just to be clear, Aluminum will be the anode when put in a system with Zinc. Referring back to the electromotive series https://patentimages.storage.googleapis.com/6e/a8/e8/89e4c330f869f0/imgb0001.png the more positive metals will be the cathode, the more negative will be the anode. That's why Aluminum hull boats don't use Zinc anodes.

 

[Will Gilmore]

Thanks DArcy. I haven't seen that chart before. Where did it come from? Chart 4-1 on page 20 of Collier's book only lists Aluminum alloys.
Perhaps there are no industrial applications for pure aluminum.

-Will (Dragonfly)

 

[DArcy]

If you search for electromotive series you will find quite a few versions of this table but they all provide a list of various metals with their potential to give up electrons. Some tables have better information on alloyed metals such as stainless steel. I have a better one in a text book but it is at work and I'm working from home these days.

 

[Will Gilmore]

Rereading my previous post, I forgot to add that Aluminum alloys are listed as more noble than Zinc. The table includes many alloys and only a few pure elements.

-Will (Dragonfly)