Sailboat Owner's Guide to Corrosion - Self-Corrosion (Collier 5)

[Will Gilmore]

Welcome to Section 2 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 will cover one or more chapters and be posted in its own thread. 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)

 

[Davidasailor26]

Chapter 5 is really where things start to get interesting to me from a marine standpoint, beyond the chemistry of chapters 1-4. I’m a few pages into it now and it’s raised a few thoughts/comments for me:

- Collier associates the corrosion that happens around things like stern tubes, keel bolts, and coated lifelines as pitting corrosion. Traditionally I’ve heard those referred to as crevice corrosion, but he has separate examples of that.

- in the stray current corrosion section he uses the example of wire exposed to bilge water. But then he says the place where the current leaves into the electrolyte is deteriorated, while the place where the current returns is protected. Can someone help me understand the electron flow involved, and how this type of current flow is often associated with wasting of things that are connected to DC ground like propellers?

- In the dealloying section he talks about dezincification as the loss of zinc, since it’s less noble than copper, which makes sense to me. But then at the end of the section he says that in an alloy of graphite and iron it’s the graphite that is dissolved. Why would the graphite dissolve if it is the more noble part?

 

[thinwater]

Examples of pitting corrosion, all from inside fuel tanks (steel and aluminum). The normal cause is a concentration cell resulting from sludge deposits. It is by far the most common cause of failure in fuel tanks.

 

[Will Gilmore]

But then he says the place where the current leaves into the electrolyte is deteriorated, while the place where the current returns is protected. Can someone help me understand the electron flow involved,

Good question. As you might have read in my most recent post in Section 1, I was confused by the direction of current flow with regards to ground. In a couple of different sources on electricity, they refer to the convention of describing electrical current as moving from positive to negative, but electrons are negative in charge and physically flow towards positive charge (an absence of electrons).

The Earth is negatively charged, a giant reservoir of electrons, but the ground is positively charged during thunderstorms. So, while normally electrons physically flow from the ground, lightning actually is the reverse flow of electrons.

These two concepts came about in the early days of electrical science and remained in use by convention, even after the error of directional flow was discovered.

For me, the biggest confusion comes in when reading a text such as Collier, where the author obviously knows what he's talking about, but doesn't leave me clear on which standard he's referring to. When he says current or electrical flow, is he talking about the actual movement of electrons or the conventional standard of looking at current moving from positive to negative?

-Will (Dragonfly)

 

[JamesG161]

[Pitting Corrosion]The normal cause is a concentration cell resulting from sludge deposits.

Cause is loss of passivation of the metal under a deposit.
or on...
Stainless Steel standing rigging , cheap grade of SS.
_____

I’ve heard those referred to as crevice corrosion

No exposure to Oxygen and a long long time for Stainless Steel. Metal keel bolts should all be made with a Stainless that has no crevice Corrosion potential, then no worries.

_____

stray current corrosion

Just means not accounted for or unexpected.
Usually fast loss of metals.

Kind of like a short circuit, battery drain.

Jim....

 

[DArcy]

The Earth is negatively charged, a giant reservoir of electrons, but the ground is positively charged during thunderstorms. So, while normally electrons physically flow from the ground, lightning actually is the reverse flow of electrons.

The Earth is not negatively charged. It could be considered neutral however should not be confused with AC supply neutral although they are mostly at the same potential. You can have a positive or negative potential difference to ground. For instance, Telco systems historically operated at -48 volts DC. In this case ground is more positive than the working voltage. Earth, or ground, is just a reference.

 

[Will Gilmore]

The Earth is not negatively charged. It could be considered neutral

The literature I have read says the Earth overall, is considered neutral, but at ground level, there is normally a negative charge. Your point about relative charge, however is an important one.

Here's a link to some lecture notes that address this.
The Feynman Lectures on Physics Vol. II Ch. 9: Electricity in the Atmosphere

-Will (Dragonfly)

 

[JamesG161]

Why would the graphite dissolve if it is the more noble part?

Actually all of the Noble chart, is eventually be Oxidized. [to the Stable Oxidation state]

Graphite is pure Carbon, not the composite Carbon in a PSS type mechanical seal.

The rate of Oxidation depends on a few factors, that all material transport or change undergoes.

I suspect it has to with Fe to C bond, in his example.

The worst is the Graphite impregnated packing for shaft sealing.
Jim...

 

[Witzend]

I just changed my graphite packing after 10ys, was still good.

 

[Will Gilmore]

I just changed my graphite packing after 10ys, was still good.

What is the environment? You're on saltwater, in a crowded and electrified marina?

-Will (Dragonfly)

 

[dLj]

Chapter 5 is really where things start to get interesting to me from a marine standpoint, beyond the chemistry of chapters 1-4. I’m a few pages into it now and it’s raised a few thoughts/comments for me:

Chapter 5 is where I start having a lot of objections to how he wrote this book as he mixes way too many terms in and does so incorrectly, in my opinion, causing a lot more confusion than if he'd used the correct terms from the get go... But no matter, this is the book we are discussing...

- Collier associates the corrosion that happens around things like stern tubes, keel bolts, and coated lifelines as pitting corrosion. Traditionally I’ve heard those referred to as crevice corrosion, but he has separate examples of that.

Pitting corrosion - see the NACE data dictionary I attached in the previous thread. Here is the definition of pitting: localized corrosion of a metal surface that is confined to a small area and takes the form of cavities called pits. However, it is commonly stated if there is lots of corrosion, and it is manifested with lots of pitting, to be called pitting corrosion. But really, it's just general corrosion exhibiting pitting. There are many other forms of general corrosion...

Crevice corrosion on the other hand, is a defined term in corrosion studies. See the data dictionary:

crevice corrosion—localized corrosion of a metal or alloy surface at, or immediately adjacent to, an area that is shielded
from full exposure to the environment because of close proximity of the metal or alloy to the surface of another material or
an adjacent surface of the same metal or alloy.

Note the main difference between crevice corrosion and pitting due to general corrosion, is that in crevice corrosion, there is a surface that is shielded due to close proximity to something else. Also notice that Collier talked about "Poultice Corrosion", and gives an explanation of the type of material that may be shielding the metal.

The difference between crevice corrosion and pitting is that in crevice corrosion, which typically starts as a pit, there is lack of exposure to the environment, hence there is a reduction of available oxygen. That does not allow fo the passive oxide film to re-form at the bottom of the pit, hard to do in any case, but more difficult when there are restrictions in oxygen flow. The bottom of the pit becomes more acidic, corrosion generates hydrogen, recall, how many hydrogen ions is the definition of how acidic something is. This then promotes more corrosion at the bottom of the pit, which then sets up a vicious cycle and you get an ever deepening pit, or in other words, it grows into a crevice. So you're correct, most of the time in the areas you've mentioned, there is a propensity to find crevice corrosion, although you can find just pitting also.

- in the stray current corrosion section he uses the example of wire exposed to bilge water. But then he says the place where the current leaves into the electrolyte is deteriorated, while the place where the current returns is protected. Can someone help me understand the electron flow involved, and how this type of current flow is often associated with wasting of things that are connected to DC ground like propellers?

Stray current corrosion is a unique form of corrosion. It is the only form of corrosion that has a source external to the affected structure. I have to say, I have heard 100's of boaters say they have a problem with stray current corrosion, but rarely is that actually the case...

Stray current corrosion has quite unique patterns which usually make it pretty easy to identify. The anodic and cathodic areas in stray current corrosion are usually widely separated. The metal deterioration occurs at the anode, or where the current leaves the structure.

- In the dealloying section he talks about dezincification as the loss of zinc, since it’s less noble than copper, which makes sense to me. But then at the end of the section he says that in an alloy of graphite and iron it’s the graphite that is dissolved. Why would the graphite dissolve if it is the more noble part?

Note, he states this is the case for grey iron. In fact, this process is only for grey iron. However he is wrong. The graphite remains in place and the iron around it corrodes away, indeed, as you suggest due to the more noble nature of the graphite. This process is not called degraphitization as Collier states, but rather it is called graphitization as the ferrite surrounding the graphite is corroded away leaving only the graphite, a very weak structure.

dj

 

[Witzend]

What is the environment? You're on saltwater, in a crowded and electrified marina?

-Will (Dragonfly)

On a mooring in fairly fresh water with solar charged batteries. I recognize this is a favorable environment, but I go through my zincs every year. I even need to put zincs on my chain or it needs replacing every 2 years, so it's more active than you would think.

 

[JamesG161]

I just changed my graphite packing after 10ys, was still good.

I go through my zincs every year.

Shaft Zincs should protect your shaft that has Graphite packing.

It is the shaft , not the packing, that is the issue, normally.

So if your shaft is still has low corrosion...

Good job on Zinc maintenance.
Jim...

 

[dLj]

I just changed my graphite packing after 10ys, was still good.

It would be very surprising if not. The graphite would likely only breakdown from mechanical processes - e.g. wear out...

Look where it is on the EMF scale:

It is more noble than platinum! Your graphite is not going to "corrode" away in sea water...

dj

 

[dLj]

On a mooring in fairly fresh water with solar charged batteries. I recognize this is a favorable environment, but I go through my zincs every year. I even need to put zincs on my chain or it needs replacing every 2 years, so it's more active than you would think.

I would look carefully at all the wiring - make sure you don't have any breaks in insulation that may be leaking current. When you say "fairly fresh water" - what exactly do you mean by that? Your info says you are in Elk River, is that Elk River Minnesota? Are you in fresh water? What makes it "fairly fresh"?

dj

 

[Witzend]

I would look carefully at all the wiring - make sure you don't have any breaks in insulation that may be leaking current. When you say "fairly fresh water" - what exactly do you mean by that? Your info says you are in Elk River, is that Elk River Minnesota? Are you in fresh water? What makes it "fairly fresh"?

dj

It has to be a very dry August to get any salinity this far up the Chesapeake. A rare event in the last 10yrs.
Wiring is in good shape, main is off most of the time. Would have to be somewhere in the solar charging system which I installed and don't suspect to be a problem.

 

[Witzend]

It would be very surprising if not. The graphite would likely only breakdown from mechanical processes - e.g. wear out...

Look where it is on the EMF scale:
View attachment 175445

It is more noble than platinum! Your graphite is not going to "corrode" away in sea water...

dj

Precisely my point to the previous post.

"The worst is the Graphite impregnated packing for shaft sealing.
Jim... "

 

[JamesG161]

Stainless Steel becomes the electron source to Graphite, instead of your Without the Zinc.

Good job on your Zinc maintenance.
Jim...

 

[dLj]

It has to be a very dry August to get any salinity this far up the Chesapeake. A rare event in the last 10yrs.
Wiring is in good shape, main is off most of the time. Would have to be somewhere in the solar charging system which I installed and don't suspect to be a problem.

So there is a bit of data on the water quality where you are (see below attached links). Frankly, you are in a fresh water system that is pretty clean. The region you are in will have well aerated fresh water, which in fact can be quit corrosive! For example, the fresh water system for New York City is considered one of the best water sources among large metropolitan regions. The water is so clean, it is actually quite corrosive to the aqueducts used to transport the water from the Catskills to the city. Hence, they add in controlled amounts of P at the source, which acts as a corrosion inhibitor so they don't have corrosion problems in the aqueduct. Some of the piping systems in New York City date from the early 1800's and are still viable... Pretty amazing actually... So in your case, you may find that you are in a region with really good clean water, but is rather corrosive.... So, bottom line - as @JamesG161 says - keep up the good application of your zincs!

Water Quality Analysis of Lead in Northeast River, Cecil County, Maryland

An official website of the State of Maryland.

mde.maryland.gov

Water Quality Analysis of Lead in Northeast River, Cecil County, Maryland

An official website of the State of Maryland.

mde.maryland.gov

https://mde.maryland.gov/programs/Water/TMDL/DocLib_Northeast_02130608/wqa_ne_pb_dr.PDF

 

[JamesG161]

The region you are in will have well aerated fresh water

@Witzend might try the special Alloy with Aluminum in place of a Zinc. Those special Aluminum anodes will protect your galvanized anchor chain too, when in the water.
Shaft Aluminum Anodes - Standard (X-1AL thru X-17AL)

Mil-Spec in that links means Military Specification. [not direct from the MILL]
Jim...