posted 05-08-2001 11:04 AM               

My experience has revealed that very few corrosion engineers feel the slightest need to understand the basics of corrosion mechanisms, and fewer still will appreciate the fundamental inconsistencies that exist in the accepted conventional theories of electrochemistry. To the rest of you, few though you may be, I just want to say “Don’t take my word for it. Check it out with your physicist friends.”

The electric field can be expressed as a vector, E, which has a given value at every point in space. The value of E at a specific location is defined as the force experienced by a unit charge at that location (expressed in units of force per charge). From the definition of E as the force per unit charge we see that (E dl) is equal to 1/q (F dl) and (F dl) is of course the energy gained by moving the charge q along the infinitesimal path dl (energy is equal to force times path). Incidentally, this equation also shows us that the vector E at a given point is equal to the negative gradient of the electric potential field at that point.
Mathematicians have shown us that there are two distinctly different types of vector fields
the irrotational (or conservative) field
and
the non-irrotational (or non-conservative) field (This could possibly be called the rotational field, but I’m sticking to the terminology used in my textbook on electric fields).

The field E is irrotational at a certain point if the curl of the vector field (curl E) is zero at that point. As a consequence of Stokes’ theorem in mathematics, if we take the integral of (E dl) around a closed loop in an completely irrotational field (where the curl of E is zero everywhere) the result will be zero. This is equivalent to stating that the energy of a charged particle at any location is uniquely defined independent of any pre-history such as the path the charge travelled to get to that location. The gravitational field is probably the best known example of an irrotational field. The gravitational energy of a mass at any location is independent of how the mass got there.

The field is non-irrotational at a certain point in space if the curl of E is non-zero at that point. If we take the integral of (E dl) around a closed loop in a non-irrotational field the result is not zero, energy is gained during transfer through a closed loop, and therefore the energy of a charge in such a field is not uniquely defined, it will depend on the pre-history of the charge.

So, if the we move a charge around a closed loop in an irrotational field, the total energy gained or lost, which is equal to the loop integral of (F dl), is seen to be the integral of [q (E dl)] which in turn is equal to zero as per the definition of irrotationality. Thus if the electric field is irrotational then no energy is gained (or lost) in a closed electric circuit. Because of this, it is generally recognised in classical electrical theory that irrotational fields are incapable of sustaining stationary electrical currents because energy is continuously depleted in the form of ohmic heat during current flow. If we have a stationary electric current, therefore, the circuit has to pass through an area where the electric field is non-irrotational.

In the book ”Classical Electricity and Magnetism” by Panofsky and Phillips we can read in section 7-2:

”Stationary currents are only possible if there are present sources of electric fields known as electromotive forces which produce non-irrotational fields.”

To accommodate electrochemical currents, Panofsky and Phillips conclude in the last sentence of the same section that chemical potentials create non-conservative (or non-irrotational) fields. If we accept this statement at face value, however, a new problem is introduced, because we can deduce from Maxwell’s equations that the only irrotational electric fields possible are those created by time-varying magnetic fields and chemical potentials should, therefore, not be capable of creating any non-irrotational electrical fields. We are consequently left with two options: either we have to conclude that Maxwell’s equations are not complete or we have to find a mechanism whereby electric currents are transported by forces other than electrical fields.

The first option is out of the question. Maxwell’s equations are considered by scientists as one of the most perfect laws of physics and any modification would even entail the need for changes in e.g. the Lorentz transformations in the theory of relativity. We are therefore left with the latter option.

The problem at hand is analogous to making a river flow in a closed loop. The water will flow downhill under the influence of gravity, but gravity (which is an irrotational field as mentioned previously) can not make the water flow back to the starting point. In order to have a river flow in circles, we have to introduce a device into the loop that pumps or moves the water uphill against the gravitational force.

The same is true for currents in an electrochemical system. In part of the electrical circuit the charges will flow under the influence of the electric field, but in order to have a sustained current loop, there has to be some non-electrical mechanism or other that transports the charges against the electric field.

Such a mechanism does exist. It is well known that metal ions in aqueous solution surrounded by water dipoles are thermodynamically very stable in the sense that they have low free energy. As a consequence of this, the metal atoms will have an inherent tendency to shed electrons and jump from the metallic phase into the electrolyte phase as an anion and thereby establish a charge separation, which in turn sets up an electric field. This electric field is conservative (non-irrotational) as is any field set up by charge distribution. The field is oriented from the positively charged ions to the negatively charged metal adjacent to the ions and it will have this direction across the electrode interface as well as through the electrolyte. Note that the field has a direction which tends to pull the ions back into the metal phase and impede the ion forming process. If the charged ions are prevented from migrating or diffusing, the charge will eventually build up to the point where the electric field exactly balances the tendency to dissolve and the process would eventually come to a complete stop. But, because the metal ions are free to move and because the same electric field that they establish also tends to pull positive ions towards the cathode and negative ions towards the anode, the electric field opposing the diffusing process is continuously degraded and it never achieves this maximum value and the dissolution process is therefore allowed to continue more or less indefinitely.

But in the final analysis it is the chemical dissolution process caused by the reduction in thermodynamic free energy that establishes the electric field that drives the currents, and therefore this must be the driving force of the overall corrosion process. The contribution of the electric current is simply to ensure that the electric field at the anode interface does not build up to the point where it stops corrosion and thus it has a depolarisation function..

posted 05-01-2001 06:24 AM               

“Note that, as the current I flows through the solution, there appears an additional potential difference IR, between the two electrodes of an actual cell owing to the passage of current through the resistance of the solution. This acts as the driving force for the ionic motion required in the process, i.e. that force required to give the ions in solution a preferential drift between electrodes.” (Section 8.2.8)

Basically, these two authorities on the subject are asking us to accept that the currents create their own driving force. They’re trying to camouflage this obvious contradiction by using terms such as “ionic motion” and “preferential drift” instead of electrolytic current, but we all know better, don’t we?