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
www.corrosionpedia.com
