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

Mar 26, 2011
2,909
Corsair F-24 MK I Deale, MD
:plus::plus:
Cause is loss of passivation of the metal under a deposit.
or on...
No, a concentration cell is different, and he does not talk about them. I know of them from oil tanks, but there are other cases. The second link is NASA.

 
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dLj

.
Mar 23, 2017
1,758
Belliure 41 Snug Harbor, Lake Champlain
No, a concentration cell is different, and he does not talk about them. I know of them from oil tanks, but there are other cases. The second link is NASA.

@thinwater you are kinda splittin' hairs here... A concentration cell can happen under deposits, can happen in other geometries also.

The definition - according to NACE - of a concentration cell is:

concentration cell—an electro-chemical cell, the electromotive force of which is caused by a difference in concentration of
some component in the electrolyte. (This difference leads to the formation of discrete cathodic and anodic regions.)


Compare that to deposit corrosion (what we used to call under deposit corrosion, but that is a non-standard term apparently).

deposit corrosion—localized corrosion under or around a deposit or collection of material on a metal surface.[also called
poultice corrosion] [See also crevice corrosion.]


So really, deposit corrosion is a special form of a concentration cell. The term concentration cell is more useful in - as you point out fuel tanks - but more frequently in pipelines, especially buried pipelines as the conductivity of the soil can give notable differences in corrosion behavior, without the presence of deposits...

dj
 
Oct 19, 2017
6,850
O'Day 19 Littleton, NH
Is this deposit corrosion or concentration cell related to what Collier was saying about single metal corrosion with anode and cathode areas within the same material due to imperfections?

Also,

"Concentration cell corrosion is most prevalent in the presence of oxygen. When pure oxygen comes into contact with a wet metal surface, corrosion action is enabled. However, the corrosion is most severe in areas that have minimal oxygen contact."

There seems to be some double talk about oxygen being necessary, yet there are circumstances where a lack of oxygen encourages corrosion? I'm guessing that increased rates of corrosion due to water movement is because more oxygen is presented to the surface as fresh oxygenated water moves past.

-Will (Dragonfly)
 

dLj

.
Mar 23, 2017
1,758
Belliure 41 Snug Harbor, Lake Champlain
Is this deposit corrosion or concentration cell related to what Collier was saying about single metal corrosion with anode and cathode areas within the same material due to imperfections?
Not really. What he was trying to explain is the nature of a general corrosion - that on the surface of a material there can be artifacts, be them microstructural, grain boundaries, surface blemishes etc. that can cause an anode, an area where electrons are leaving the surface, and a cathode, an area where electrons are being attracted to the surface, to be created. Recall, corrosion happens one atom at a time... That's a tough scale to get your mind around, at least for me that's the case...

Also, "Concentration cell corrosion is most prevalent in the presence of oxygen. When pure oxygen comes into contact with a wet metal surface, corrosion action is enabled. However, the corrosion is most severe in areas that have minimal oxygen contact."
So here, I think, there is mixing together of some concepts that apply to different materials. In general, plain carbon steel alloys will corrode faster in the presence of oxygen. While stainless steels require a certain presence of oxygen to maintain their passive state.

In the first case of plain carbon steel, if you lower the oxygen, you slow or stop corrosion. This is easily seen in like hot water closed-loop heating systems. You can take plain water, run it into a close-loop iron piping and radiator set-up (closed-loop means you aren't adding in new water, at least in any significant amount) and there will be an initial corrosion happening, but as the oxygen in the water is consumed through that corrosion process, the corrosion process stops.

Now, take that same closed=loop system and now make the piping out of stainless steel. Now, as oxygen is being used up, the passive oxide layer can't maintain itself. You may get small areas where the passive oxide layer can't reform. This causes a very small area that become active, and you have an electro-chemical cell that has now formed. You likely have enough oxygen, although small, to allow that small area to corrode, there isn't enough oxygen to reform the passive oxide layer and you are off to the races, so to speak, on developing pitting in your stainless steel, and eventually possibly into creating what is called crevice corrosion.

I'm being a little simplistic above, but that's the idea. Now, different alloys, aluminums, coppers, etc, all have their own particular "needs" in this realm...

There seems to be some double talk about oxygen being necessary, yet there are circumstances where a lack of oxygen encourages corrosion? I'm guessing that increased rates of corrosion due to water movement is because more oxygen is presented to the surface as fresh oxygenated water moves past.

-Will (Dragonfly)
So the question of oxygen levels I've addressed above. But here now you add in the "moving water" aspect. So the effect of moving sea water is actually interesting. At velocities above about 3 ft/second and fouling organisms don't tend to attach themselves to the metal. They tend to protect the metal. But now, if you don't allow any fouling, then as sea water velocity increases, indeed the rate of corrosion increases. It seems to top out at somewhere in the 15 to 20 feet per second range for steels. Copper based alloys just get destroyed at those speeds or above...

I'm not sure I know the fundamental reason for this, but in copper alloys it's because of erosion. The higher velocities just physically remove metal. I'm guessing that for steels and such, it may have more to do with moving away corrosion products, keeping the surface "cleaner" so it is more likely to corrode. But I'm not really sure...

dj
 
Feb 14, 2014
5,560
Hunter 430 Waveland, MS
@jssailem asked me in a PM something like this..
"What happens to the Zinc atom that has given up its free electrons by sacrifice?"

He was wondering about the now more positively charged Zinc atom.

The simple answer is...
Zinc forms with other sea water crap to become stable or even with H2O to form a ZnOH [Zinc Hydroxide]

So, some one mines, refines, and reduces Zn . Then it is alloyed, formed, and sold as shaft Zinc.
So we Sail Boat owners then disperse the Zn all over the oceans again.:oops:
______
there will be an initial corrosion happening, but as the oxygen in the water is consumed through that corrosion process, the corrosion process stops.
Thank you dj.
Titanium is a good example.
_____
if you don't allow any fouling, then as sea water velocity increases, indeed the rate of corrosion increases
Velocity may also remove the depleted electron Zinc from its molding Alloy. Cleaning, so to speak.
A Navy Study noted higher Dissolved Oxygen and Velocity increases corrosion rates.

The higher velocities just physically remove metal
Could be, but I suspect the Zinc slowly depletes, but can form an Oxide on its surface.

A simple test is to take an Old Zinc, file the surface till you get shiny metal. Leave it outside for a few days.
It will not be shiny anymore. Zinc Oxide.

Velocity can cause Chemical Reactions to increase at faster rate too.
Jim...
 
Oct 19, 2017
6,850
O'Day 19 Littleton, NH
Shall we move on to the next section, Galvanic Corrosion (Collier 6)?

-Will (Dragonfly)
 

dLj

.
Mar 23, 2017
1,758
Belliure 41 Snug Harbor, Lake Champlain
Shall we move on to the next section, Galvanic Corrosion (Collier 6)?

-Will (Dragonfly)
fine by me...

@JamesG161 - You've brought up Titanium a couple times, IIRC, in these discussions. That's an interesting material w.r.t. corrosion. It's not used much in boats, at least not in the older boats I'm familiar with. i'm guessing in some of the high end racing boats it may be finding uses. I haven't talked about this alloy in this context, but if you're interested we can.

You've also brought up zinc corrosion above in this thread. Zinc corrosion and its function in sacrificial anodes, both as a primary constituent or secondary constituent in anode development/use is a very complex subject. It's probably beyond the scope of this discussion group. Certainly using zinc, especially as anodes, is very much a topic to cover. It's coming up soon in the upcoming chapters.

dj
 
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Feb 14, 2014
5,560
Hunter 430 Waveland, MS
You've brought up Titanium a couple times
The Russians have Titanium hull submarines.

Many Titanium knives and tools are being made for marine service.
It is expensive, but essentially no corrosion for Sailing.
West Marine

I have one of these for stubborn knots and tightening shackles, etc.
CUDA Titanium Marlin Spike Folding Knife | West Marine

No stress cracks, no Zinc anodes needed for Titanium shaft or props.
Its Strength to Density ratio is best of all Metals. [ meaning strong and light weight]

I will would love a Titanium mast.:cool:
A Titanium heat exchangers for our engines and salt water.

Jim...

PS: As you might tell, I have a lot of experience equipment made of Titanium.
 
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Likes: Will Gilmore

dLj

.
Mar 23, 2017
1,758
Belliure 41 Snug Harbor, Lake Champlain
The Russians have Titanium hull submarines.

Many Titanium knives and tools are being made for marine service.
It is expensive, but essentially no corrosion for Sailing.
West Marine

I have one of these for stubborn knots and tightening shackles, etc.
CUDA Titanium Marlin Spike Folding Knife | West Marine

No stress cracks, no Zinc anodes needed for Titanium shaft or props.
Its Strength to Density ratio is best of all Metals. [ meaning strong and light weight]

I will love a Titanium mast.
A Titanium heat exchangers for our engines and salt water.

Jim...

PS: As you might tell, I have a lot of experience equipment made of Titanium.
Give me a sailboat with a titanium hull - that would be awesome! But lord knows I can't afford it! I wonder how a titanium mast would compare to a carbon fiber mast. High performance masts are bent into shapes while sailing to produce better sail shape. I wonder how a titanium mast would work in that case. Given the cost, you'd need a whole lot of advantages somewhere in addition to simply corrosion resistance and strength to weight ratio...

I'd like to try out a titanium knife, never had one.

Sure, you don't get stress cracks in titanium because as soon as one might appear, the part breaks completely!

Not sure I'd want a heat exchanger made from titanium - its a poor heat conductor. I don't need the weight reduction there. The strength to weight ratio is irrelevant, and some of the really good copper based alloys in this application are more than sufficient for corrosion resistance.

I would need a lot more info on props made from titanium. You need a lot of ductility and resistance to damages in props. The difficulty with titanium, especially Ti6Al4V, is that it is very notch sensitive.

But I'd love a sailboat hull made from titanium!

dj
 
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Feb 14, 2014
5,560
Hunter 430 Waveland, MS
I'd like to try out a titanium knife, never had one.
Here is a handy and small one.
Ocean Quest Titanium BC Knife
Especially if you dive your boat.
_____
Sure, you don't get stress cracks in titanium because as soon as one might appear, the part breaks completely!
I meant corrosion stress cracking, like found in some Stainless Steels

Also the mechanical yield of Titanium really doesn't allow cracking. Its main problem is bending, it wants to return to is original shape, not crack.

Titanium conducts heat very well.
You conduct heat by a similar mechanism to conducting Electricity. Atom to Atom versus Electron to Electron.
Jim...
 
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dLj

.
Mar 23, 2017
1,758
Belliure 41 Snug Harbor, Lake Champlain
Titanium conducts heat very well.
With all due respect - you got that one wrong. Ti6Al4v conductivity is 3.8 BTU/hr/sq.ft./F - copper is on the order of 400 (same units). Basically two orders of magnitude better heat conductor...

Now if you need a heat exchanger for a space craft or some other highly weight sensitive application, you might be able to make a case for a Ti based heat exchanger. But not competitive on a sailboat...

dj
 
Mar 26, 2011
2,909
Corsair F-24 MK I Deale, MD
I'm not going to get into a discussion of nomenclature.
With all due respect - you got that one wrong. Ti6Al4v conductivity is 3.8 BTU/hr/sq.ft./F - copper is on the order of 400 (same units). Basically two orders of magnitude better heat conductor...

Now if you need a heat exchanger for a space craft or some other highly weight sensitive application, you might be able to make a case for a Ti based heat exchanger. But not competitive on a sailboat...

dj
In fact, Ti liquid-liquid HEs are not that rare in the chemical industry. The film and fouling factors are often limiting anyway and the corrosion resistance of Ti takes the day. I've speced them a number of times.

 

DArcy

.
Feb 11, 2017
1,168
Islander Freeport 36 Ottawa
But I'd love a sailboat hull made from titanium!
Not me, I would MUCH rather have a carbon fiber hull. Higher strength, lower density, corrosion resistant, easier to repair (well, sort of, with the right equipment). That is why we see so many high end boats built with carbon fiber and none made from titanium.
Same for masts. Carbon fiber, being an engineered material, can be made to be flexible where desired and stiff where needed. Not to mention more cost effective.
 
Feb 14, 2014
5,560
Hunter 430 Waveland, MS
Unfortunately for mono hulled Sailors, weight is not so much an issue as HULL Speed.
Lighter hulls = counter balanced keels.

Carbon fiber, fiber glass fiber, all for strength in a resin.

Smaller auxiliary horsepower though.:biggrin:
Jim...
 
Oct 19, 2017
6,850
O'Day 19 Littleton, NH
Perhaps the ideal hull would be an aluminum hull insulated from any electrolytic contact in a casing of carbon fiber reinforced resin. Strong, light, flexible, impact resistant.

If you paint your metal with white epoxy paint, is there titanium in the paint? Titanium White is a common coloring agent in paints. So is cobalt and other metals. What affect must a coating like those have on the metal substrate?

-Will (Dragonfly)
 

dLj

.
Mar 23, 2017
1,758
Belliure 41 Snug Harbor, Lake Champlain
Not me, I would MUCH rather have a carbon fiber hull. Higher strength, lower density, corrosion resistant, easier to repair (well, sort of, with the right equipment). That is why we see so many high end boats built with carbon fiber and none made from titanium.
Same for masts. Carbon fiber, being an engineered material, can be made to be flexible where desired and stiff where needed. Not to mention more cost effective.
Heck I wouldn't complain about that either... ;)

But I'm sure the real reason is that a titanium hull would be very expensive by comparison, and probably not a lot better. but you have to ask, why didn't the Russians make their subs from carbon fiber instead of titanium?

dj
 

DArcy

.
Feb 11, 2017
1,168
Islander Freeport 36 Ottawa
but you have to ask, why didn't the Russians make their subs from carbon fiber instead of titanium?
Just search for images of Russian engineering :yikes: Some impressive stuff, but it doesn't all make sense.
The ultimate hull is a cored, carbon (or even better, boron) fiber unidirectional layup. Use an aramid fiber (Nomex) honeycomb core for best results. It will be MUCH stronger than aluminum, titanium or steel and WAY lighter. Heck, even cored fiberglass panels can be stronger (and WAY lighter) than monolithic steel or titanium panels. Honestly, composites are just better for boat hulls. Maybe steel is still better for ice breakers. :huh: