Ever wondered why the aluminium shelled Ludwigs suffer from pitted chrome? Read on:

The bimetallic driving force causing galvanic corrosion was discovered in the late part of the eighteenth century by Luigi Galvani in a series of experiments with the exposed muscles and nerves of a frog that contracted when connected to a bimetallic conductor. The principle was later put into a practical application by Alessandro Volta who built, in 1800, the first electrical cell, or battery: a series of metal disks of two kinds, separated by cardboard disks soaked with acid or salt solutions. This is the basis of all modern wet-cell batteries, and it was a tremendously important scientific discovery, because it was the first method found for the generation of a sustained electrical current.The volt, a unit of electrical measurement, is named for Volta.

Galvanic corrosion (also called 'dissimilar metal corrosion' or wrongly 'electrolysis') refers to corrosion damage induced when two dissimilar materials are coupled in a corrosive electrolyte. When a galvanic couple forms, one of the metals in the couple becomes the anode and corrodes faster than it would all by itself, while the other becomes the cathode and corrodes slower than it would alone. For galvanic corrosion to occur, three conditions must be present:

1. Electrochemically dissimilar metals must be present.
2. These metals must be in electrical contact.
3. The metals must be exposed to an electrolyte.

The relative nobility of a material can be predicted by measuring its corrosion potential. The well known galvanic series lists the relative nobility of certain materials in sea water. The galvanic table lists 92 metals in the order of their relative activity in seawater environment. The list begins with the most active (anodic) metal (magnesium) and proceeds down the to the least active (cathodic) metal of the galvanic series (graphite). In general, the further apart the materials are in the galvanic series the bigger the potential difference and the higher the risk of galvanic corrosion as a result. This should be prevented by design (or not by design in Ludwig's case). The amount of potential voltage in the couple is expressed in the Anodic Index and is determined by the difference in the voltage potentials between the two metals.

Galvanic Compatibility:

* For harsh environments, such as outdoors, high humidity, and salt environments fall into this category. Typically there should be not more than 0.15 V difference in the "Anodic Index". For example; gold - silver would have a difference of 0.15V being acceptable.

* For normal environments, such as storage in warehouses or non-temperature and humidity controlled environments. Typically there should not be more than 0.25 V difference in the "Anodic Index".

* For controlled environments, such that are temperature and humidity controlled, 0.50 V can be tolerated.

You'll notice how humidity controlled environments can tolerate the problem much more, underlining the importance of keeping your Supra / Super dry when not in use!

If we assume a Ludwig shell is triple plated with a layer of copper, nickel and chrome then the copper plating with an Anodic Index of 0.35V will be in electrical contact with aluminium alloy. I'm not sure exactly what alloy Ludwig used in their shells but here are a few values of aluminium alloys as an example:

Aluminum, wrought alloys of the 2000 Series - 0.75V
Aluminum, wrought alloys other than 2000 Series aluminum, cast alloys of the silicon type - 0.90V
Aluminum, cast alloys other than silicon type, cadmium, plated and chromate - 0.95V

At best we are looking at a 0.40V difference and therein lies the problem. Once the platings fail and the aluminium is open to the elements it will start to corrode and push against the chrome plating causing the brittle chrome to snap off giving the characteristic pitting! So the bottom line is if you got a corroded shell with exposed aluminium, connected one wire to the aluminium base metal, one wire to the chrome and soaked it in concentrated salt water you'd have just created a crude Ludwig battery! Let's now consider a brass shell:

Brass - 0.40V
Copper- 0.35V
Nickel- 0.30V
Chrome- 0.60V

You can instantly see how much more suited brass and copper are and given how closely they tie up there is unlikely to be much of a reaction at all. This clearly shows why the brass drums from the early 60's fared so much better than the later aluminium shelled ones.