The mystery of the white ring

The classic sonic screwdriver has always featured what appears to be a white ring around the middle section.

As can be seen here:

This 'white' band has been consistently seen on the prop since it's introduction and for most of this period has also featured what appears to be a gouge or paint chip.

There are several theories as to what it can be.

1) White Paint - This is the obvious and easiest answer however it does not explain how in some shots this white area is clearly reflective silver yet displays the telltale flake mark. Such as here:

It therefore seems highly unlikely it is white paint. However that brings me onto the second theory

2) Chrome Paint - This is actually a pretty sound idea. When the prop, as built by Tony Dunsterville, first appears in Thunderbirds, the middle section is indeed a very shiny chromed finish:

What lends this idea added credence is that Chrome paint contains real metal (usually nickel or aluminium) and I have been told quite reliably, has a tendency to turn white and cloudy over time due to oxidation. This would explain why the prop goes quite visibly from silver during Thunderbirds and during Pertwee's time with the prop, to white during Davison's era.

However, I have doubts. Chrome paint is not particularly durable and would have flaked off almost entirely during the 15 years the prop was in use, not just exhibited the relatively minor flaking seen. For this reason alone I believe that while it is entirely possible, it is not very probable.

3)  Polished Finish - It has been suggested that the chromed finish on the middle section is down to the metal being turned at a higher speed, effectively polishing it into a mirror shine. I don't buy that at all. This would make the finish intrinsic to the material itself. This is intuitively wrong as the wear pattern can only be the result of something being remove from the base material: ie: electroplate or paint. This brings me onto my final theory and it's one I believe holds the answer.

4) Galvanic Corrosion - I believe that the prop was turned from aluminium and the ring section was plated in nickel to give it a bright chrome finish. However, it is widely known that nickel plating on aluminium is problematic because of the adhesion problems with nickel on aluminium (a quick Google will show the extent of flaking of nickel plate) and also, the danger of galvanic corrosion as a result of bad adhesion. Instead of me explaining what this process is, let me quote John Demakis of http://www.duro-chrome.com/:

"Galvanic corrosion refers to the damage induced when two dissimilar metals are coupled in a corrosive electrolyte. When this occurs, the less noble (less able to resist this type of corrosion) of the metals in the reaction becomes the anode (positive) and corrodes more quickly than it would by itself, whereas the more noble metal becomes the cathode (negative) and corrodes more slowly than it would alone.

What Is It?

Try to think of it in these terms. Consider the way a battery works. Typically in most batteries there are three things that must be present to create the desired reaction.

•A positive electrode that receives electrons from the external circuit when the cell is discharged. With respect to galvanic corrosion, this refers to the more noble of the two metals.

•A negative electrode that donates electrons to the external circuit as the cell discharges. This refers to the less noble metal or the anode.

•An electrolyte that provides a mechanism for a charge to flow between positive and negative electrodes. This would be the water in the solution for plating or the type of plastic used in the application or any other substance used. Even moisture in the air could act as a catalyst for the reaction.

Once all three of these elements are present, the battery creates energy in which galvanic corrosion is entirely responsible. When this sort of reaction occurs outside of a battery setting, for instance in a mold shop, the energy created by galvanic corrosion manifests itself between the two dissimilar metals and eats away at the less noble of the two.

 Aluminum has very little nobility and consequently is much more likely to corrode more quickly. Applying a nickel-based coating on your aluminum parts may be what is necessary to protect them from galvanic corrosion.

...suppose that you are using a nickel solution to plate an aluminum part. Now, if there were any imperfections in that aluminum part, such as pitting, there is a possibility that during the plating process some of the solution or a water molecule could get trapped in that deformity in the part. That water molecule ultimately could become the electrolytic component needed to complete a galvanic cell. The aluminum is less noble than the nickel and so in the plating process used to prevent corrosion and to prolong the life of your part, one could actually be promoting corrosion, and lessening the life of your tool.

So what do we have here? Essentially the above says that nickel can protect aluminium from corrosion IF the aluminium surface has been adequately prepared for the plating process. If it hasn't, galvanic corrosion can be quite rapid.

Here's the rub: the sonic prop was made from aluminium. The surface wasn't prepped properly prior to the nickel plating of the middle section, as can be evidence by the subsequent flaking and wear, which caused the less noble aluminium underneath (that was exposed via the microscopic pitting) to corrode at an accelerated rate in the presence of an electrolyte. In this case, the salt and moisture from sweat is the electrolyte, and this results in the formation of white aluminium oxides, or in this case, aluminium hydroxide - an opaque white substance.