An interesting Unclassifed report by Du Pont de Nemours & Co.:
Spot Tests for Contaiminants on Aluminum- DP-88 Chemistry by H. S. Hilborn & R. C. Pugh- Savannah River Laboratory, Atomic Energy Division (November 1954).
This is an example of corrosion/filiform on two "different" aluminum products... but none the less, interesting to say the least. Maybe there is a possibility that your Airstream could be converted into a Nuclear Power Plant when you are not traveling the country... talk about being green... yellow or whatever.
"Spot Tests for Contaminants on Aluminum-
Corrosion of aluminum components that are commonly used in a nuclear reactor may be both dangerous and expensive. Metallic inclusions in aluminum may give rise to galvanic attack with resultant deterioration of the component. The identification of harmful inclusions is essential if they are to be traced to their source.
The technique employed both by Caley (1) in the analysis of ancient coins and by Alcoa (2) in the identification of certain impurities in aluminum was based on electrographic sampling combined with colorimetric spot tests of the type described by Feigl (3). Many of the inclusions encountered direct application of tests previously described in the literature, so that some adaptation and extension was necessary. This report contains a compilation of tests as finally used to identify inclusions found in aluminum and to ascertain whether the pits in aluminum cladding penetrate to the substrate metal.
Procedures were adapted to detect and identify minute quantities of the following metals which may be present as inclusions or alloying constituents-
1. Iron inclusions in aluminum
2. Lead inclusions in aluminum
3. Magnesium on aluminum
4. Mercury on aluminum
5. Molybdenum on aluminum
6. Nickel plating on aluminum
7. Tin on aluminum
8. Copper flashing on stainless steel or Copper inclusions in aluminum
9. Uranium in uranium-aluminum alloy or Uranium substrate at corrosion
pits or pin holes in aluminum sheath
The methods described are simple, fast and capable of revealing extremely small quantities of contaminants. The necessary equipment is sufficiently portable to permit its use in the field."
"The equipment which consists of clips, wire, a six volt dry cell, cathodes and filter paper is compact and portable. Tests in the field are easy and convenient, facilitating inspection in the field."
I know this is just ONE EXAMPLE of an actual scientific test on aluminum, but there could a correlation of an inclusion within the aluminum or on (which I think is most likely salt (NaCl) and water. This paper goes into the process and how to determine the inclusion... in 1954
. At least this is a report paid by the Atomic Energy Commission Contract At (07-2)-1. There might be a way to access this entire report through the Library of Congress using the author/title/and paper DP-88.