NoSeeUm

This DTR Thread got closed for good reason. But sorry, this was a good discussion.

My experience with galvanic corrosion has been just like HOHN quoted from WikiPedia

A few things to note:
1) Galvanic corrsosion is an electrical / chemical process
2) Dissimilar metals contact in physical will develope a voltage potential
3) An electrical process needs an applied voltage and complete path for current to flow
4) An electrolyte is a solution that can conduct electricity
5) Corrosion is commonly called rust which is the oxidation of a metal

A good example of galvanic corrosion is a lead-acid car battery. And yes, juice from a battery is accomplished by corrosion. A battery which we all know produces voltage (2 volts per cell) and is able to supply significant current to start our trucks. Inside a battery typically are two dissimilar lead base metals in physical contact all emersed in an electrolytic solution. A battery is galvanic corrosion at its finest.

Just as a side note, a very cool thing about a most batteries is that the corrosion process is reversable. Most would call it "charging the battery". This is true for all galvanic reactions, just not really practical in most cases. But to charge any battery, you have to force the reaction backwards by applying a greater voltage than that being created by the battery cells. This also something we all know to be true. Essentially, "un-corroding" the metal in the cell.

Just about any chemical process deals with the swapping, sharing and stealing of electrons between adjacent atoms. It also deals with every atom wanting its fair share of electrons, no matter what. The force is quite powerful and each atom is willing to swap, share and steal electrons from an adjacent atom. Sometimes with a great deal of desire sometimes more like "Free Love". This all depends upon the chemical state of the atom and the chemical state of the atom adjacent to it. (circa Chemistry - High School)

The flow of electrical current deals with the electrons from one atom moving to an adjacent atom powered by the force of voltage.

Now notice "Galvanic corrosion is an electrochemical process". This means electrical - chemical. Also notice that electro (electrical) deals with electrons and chemical deals with electrons. OK so it is all about electrons and adjacent atoms.

So how does this work?

The metals in contact produce a voltage, but there is no path for current to flow. Therefore there is no movement of electrons. As soon as you dip the metals in an electrolyte you now have completed the electrical path. Now current can flow, which as I said above is the movement of electrons.

What happens with electrons is that they leave the first metal and migrate to the second metal. So you end up atoms of the first metal missing electons and an abundance of electrons in the atoms of the second metal. It is the llack electrons in the first metal that now causes corrosion problems. A metal atom missing some electrons wants to react with another atom and gets its electrons back allot more than a metal atom with a normal amount of electrons wants to react. Let say, as an example the metal could possibly react with oxygen, because there might be some of that around at times. Hence the rate of corrosion or rust is accelerated.

OK so now as an example, lets consider two metals in contact. Say allumimum and iron. If we spray the two metals with a electrolyte, say salt water we have everything we need to create galvanic corrosion. In this case the aluminium with will rust away. Now by this process, in theory, we have prevented the iron from corroding. The aluminium is corroding in place of the iron because the iron is now getting more than its fair share of electrons (from the aluminuim) and does not need to corrode (react with another atom) to get them. In reality, likely the iron will still corrode but at a much slower rate. This rate depends largely upon the level of electrical current flow and basically how wetted the alumiminium - iron contact area remains through out the process.

Now consider the same example as above, but remove the water and leave the salt. Now the iron and aluminium will corrode at the same rate that they would individually, because each is reacting individually with the salt.

It is the electrolyte solution that makes galvanic corrosion work. And another really bad part about many electrolytes is that they contain some of the very atoms that like to corrode metals all and about themselves. Throw in the electrical process and the the whole rust process gets accelerated. In some cases this is done intentionally, every hear of electro-plating?

Now say you have a boat. In this boat are iron and alluminum. Leave the boat in your drive way all year and the iron and aluminium all corrode at their normal rates (if the boat does not get wet). Now put the boat in an electrolytic solution such as a lake, a river or the ocean. Of course, now the aluminum rots away faster. BUT, if we attach a zinc anode only the zinc will corrode. By the same process I have described above we now have two metals in physical contact times three, all in contact with an electrolyte. As before, the aluminium gets electrons stolen the by the iron and would like to corrode. However, now the aluminium is able steal electrons from the zinc. Not to mention that the unlucky zinc has the iron stealing electrons from it as well. The zinc has no electrons to steal from a less apt metal on the pecking order and so it rusts away.

Lets take another example of water being pumped through some very expensive pipes. We don't want to rust the pipes and we could use zinc if we wanted too. But lets say it is a pain to change zincs all the time. So what is done is that we essentially charge our pipes just like we would charge a battery. A voltage is applied that basically is equal and opposite to the that voltage being applied by the process of galvanic corrosion. We have stopped any current flow. So now we get to keep our expensive pipes rust free for many years.

Allot of typing, I hope at least one person reads this.

Jim

Russ Roth

I read it so you got your one person.

PChouinard

I replied to the original thread. It was a good one because corrosion is an often misunderstood phenomenon. I disagreed with the idea of "baring" the steel and then bolting aluminum to it. Preventing corrosion is best done by protecting the steel from the corrosive. In the case of a vehicle, I believe it would be best done by paint, undercoating, spray-on bedliner material etc.
Good restart on the subject though...

NoSeeUm

I am a fan of innovation. Somestimes all that is required is more information. But I agree a proper coating has been a proven choice for a corrosion inhibiter. But in the spirit of exploration I applaud any effort.

Jim