Sailboat Owner's Guide to Corrosion - Galvanic and Stray Current Corrosion (Collier 6 & 7)
- Thread starter Will Gilmore
- Start date
I peeked in at that thread. I would GUESS, the pin hole was caused from the inside, but too many chimed in with more info.
Jim...
The photos you posted in your thread on your tank showed corrosion initiation from the outside surface of the tank rather than from the inside of the tank. However, your images from the inside of the tank, while not very clear due to the geometric limitations, did show a fair amount of what appeared to be deposits on the inner surfaces. That can be a bit sketchy to evaluate without more detailed examination but the conservative approach is to simply replace the tank.Hey! That's my tank!
for reference, the tank spent time sitting in a small amount of water right in that corner. the water was in a fiberglass basin with nothing else, no grounds or anything.
You will need to address the conditions that caused your current problem - be that make a cradle to pick your tank up so it's not sitting in water, or somehow getting rid of that problem so water can't sit on the tank. A cradle sounds like a good option to me, but it's your boat, your choice...
dj
I'm going with isolation and an additional barrier coat on the bottom of the tank combined with rubber standoffs and a drain drilled into the cradle so that if any water does get in there, it gets out immediately. The corrosion was outside -> in for sure.
Here's an excerpt from an abstract to a research study on AC stray current corrosion:
Study on the Influence of AC Stray Current on X80 Steel under Stripped Coating by Electrochemical Method
"Goidanich et al. [15] obtained an experimental result that the AC corrosion rate is twice the self-corrosion rate of the undisturbed current sample at a current density of 10A/m2; when the AC current density exceeds 30A/m2, the corrosion rate increases exponentially."
I hope I'm not taking this out of context and completely misrepresenting the conclusion, but it looks like the AC stay current has a definate negative effect on corrosion rates.
-Will (Dragonfly)
Study on the Influence of AC Stray Current on X80 Steel under Stripped Coating by Electrochemical Method
"Goidanich et al. [15] obtained an experimental result that the AC corrosion rate is twice the self-corrosion rate of the undisturbed current sample at a current density of 10A/m2; when the AC current density exceeds 30A/m2, the corrosion rate increases exponentially."
I hope I'm not taking this out of context and completely misrepresenting the conclusion, but it looks like the AC stay current has a definate negative effect on corrosion rates.
-Will (Dragonfly)
Sure - at 30 amps per square meter and above! That's big current! The article you are referencing is dealing with railways and high power transmission lines, especially inthe effect on the corrosion of pipelines that may be crossing or running parallel to those high power transmission sources. And then at those power levels, the corrosion rate is doubled from the corrosion rate if that were not present. So that rate is not a lot, and the current required is quite large. So, don't park your boat on an electrified railroad track...
I'm not saying that AC current doesn't have any effect on stray current corrosion. It's simply not the major player in what we are talking about - boats. Of course one must install AC power correctly...
dj
Ok but we are talking perhaps even a trickle of Amps.
I am trying to avoid reaction RATES.
MilliAmps / Zinc anode surface areas.
Is BIG...
So...
Avoid All Stray Currents to reduce Corrosion rates...
Jim...
With all due respect, you are mixing up concepts and terms. Galvanic couples are NOT stray current corrosion.
From the NACE dictionary I posted earlier:
stray current—current flowing through paths other than the intended circuit.
stray-current corrosion—corrosion resulting from stray current.
When you couple two dissimilar metals you are creating an intended circuit. Whether or not you are intending the resultant corrosion from it or not, you have indeed made an intended circuit.
Stray current, as talked about in H.H. UHlig's "Corrosion Handbook" states the following:
The effects of AC current on corrosion is also mentioned in that above reference:
Note in the above, it says "commercial frequencies". If you go into the RF range, then this changes things quite a lot, but it is unimaginable how you could have that happening on your boat, and have you, the human, still be alive...
Galvanic corrosion, which you allude to in your post, is quite a different matter. Again, from the NACE dictionary I posted earlier:
galvanic corrosion—accelerated corrosion of a metal because of an electrical contact with a more noble metal or
nonmetallic conductor in a corrosive electrolyte.
Galvanic couples in the marine environment are quite important. From the book "The Corrosion of Metals in Marine Environments" by Fink and Boyd:
It is my understanding that these threads are aimed at bringing greater clarity to to the subject of corrosion as it applied to our boats, I feel strongly that mixing up terminology, something Collier does way too much in his book, should be avoided.
Just my 2 cents worth...
dj
Yes, that is the point. Clearly defined definitions and terms are important to that end. I hope to have the time to compile a list of important key terms to define for each section. I will edit them into the introduction post at the beginning of the thread and list, perhaps, in their entirety in the Resource thread. I look to you guys for any help in all of this.
I'm actually happy to see that all isn't easy agreement and members capitulating to each other when there is a difference in opinion or understanding. Real learning happens when complacently held ideas are challenged and worked through as a group. One of the reasons I was reluctant to look for and ask the author to join the discussion was I didn't want a single recognized "expert" to be in a position of "final say" on our subject of interest. As much study as this field has undergone, it seems there is still debate and inconsistentcies in knowledge around it.
Thanks guys
-Will (Dragonfly)
Galvanic Corrosion: Corrosion of an anode metal due to its electrical connection to a more noble metal, the cathode, while immersed in an electrolyte.
Stray Current: In the world of corrosion, stay current reffers to an electrical current that promotes corrosion and whose source is external to the galvanic cell created by electrically connected disparate metals in an electrolyte.
What do you guys think? Two working definitions for this section. Any tweaks or additions?
I'll put others up when I can.
Anode - section 1
Cathode - section 1
Electrolyte - secretion 1
Pit Corrosion - section 2
Crevice Corrosion - section 2
I'm taking suggestions.
-Will (Dragonfly)
Stray Current: In the world of corrosion, stay current reffers to an electrical current that promotes corrosion and whose source is external to the galvanic cell created by electrically connected disparate metals in an electrolyte.
What do you guys think? Two working definitions for this section. Any tweaks or additions?
I'll put others up when I can.
Anode - section 1
Cathode - section 1
Electrolyte - secretion 1
Pit Corrosion - section 2
Crevice Corrosion - section 2
I'm taking suggestions.
-Will (Dragonfly)
+1..! Here's what DC stray current can do in less than 24 hours.....
This wire was for a bilge pump on a center console fishing boat. It was soldered into the bilge pump system. It fractured at the solder / wire juncture, then wire then fell into the bilge water, and fed live DC power into the bilge water. That night there was a torrential rain storm and the boat started to sink because the bilge pump was no longer operational. It sunk to the point that the battery terminals of the cranking battery were fully submerged. The owner noticed the boat sitting low, and about to sink, and called the boat yard immediately.
By the time they got to the boat, and he had used it just the day before, every seacock and thru-hull fitting that was in contact with water was destroyed. They had just been installed by the boatyard that spring.
We find just as much DC stray current issues on mooring sailed boat as we do on marina sailed boats.
That is a significant point to make. Kind of a myth busters conclusion. Thanks for chiming in.
-Will (Dragonfly)
The vast majority of DC stray current issues originate on the boat not from off the boat. Unfortunately most boat owners assume all corrosion is caused by being in a marina.
Or a new boat that just entered the marina.
When the time is right, I will tell the story of recently "rewired" boat [that just arrived] that wiped out the nearby boats, Zincs in 3 days.
Jim...
PS: We pin pointed them with a Stray current survey.
Please note the "pitting" on that picture. Plus the Green Copper residue from the Bronze alloy decomposing.
Good example for helping locate the cause.
Jim...
Will, why would you create a new definition for these terms? NACE has published definitions.
galvanic corrosion—accelerated corrosion of a metal because of an electrical contact with a more noble metal or
nonmetallic conductor in a corrosive electrolyte.
stray current—current flowing through paths other than the intended circuit.
You certainly should not put "galvanic cell" in the definition of stray current. Those are two different concepts. Also, the electrically connected disparate metals in an electrolyte has multiple errors within this definition.
More later.
dj
Will,
I've just converted the corrosion dictionary I uploaded previously as a .pdf into a word document. I still have a lot of work to clean it up, those conversions from .pdf to .doc are never clean. Im also going to reach out to NACE and ask if there is any problem with us doing this, I don't think so, it's public document. But it never hurts to check.
What may be more interesting is to convert this document into a kind of dictionary - encyclopedia. You could then annotate the individual definitions with dialog that we are having etc. Sound like a good idea?
dj
Got the original .pdf converted over to a word file. Took a bit of clean-up, but it's not terrible at this point. Not sure wehre it should be put but I'll attach here.
Oh darn - looks like I can't up-load it as a word document. Maybe as a text document? Let me see....Oh boy, that was ugly...
I guess I can only bring it in as a .pdf - if anybody wants a word copy, let me know. At least this .pdf is a lot easier to read than the previous one as I've removed all the headers that weren't needed and over-written text in places etc...
dj
p.s. - put in two calls to NACE, still waiting on a response. Left two voice messages.
Oh darn - looks like I can't up-load it as a word document. Maybe as a text document? Let me see....Oh boy, that was ugly...
I guess I can only bring it in as a .pdf - if anybody wants a word copy, let me know. At least this .pdf is a lot easier to read than the previous one as I've removed all the headers that weren't needed and over-written text in places etc...
dj
p.s. - put in two calls to NACE, still waiting on a response. Left two voice messages.