Corrosion of aluminum in sea water

by Pete Johnson

My day time job is at Sikorsky Aircraft (think helicopters) and my specialty is in special processing. This includes all types of aluminum treatments (anodize, heat treat, alodine, paints & coatings).

Things to keep in mind in the case of aircraft are:

1) They are made from Different alloys of aluminum. 3 of the most popular alloys are 2024, 6061, and 7075. All three have different corrosion potentials. Now to complicate things, you can heat treat them to different hardnesses or heat conditions. These hardnesses will impart different corrosion rates to the individual alloys. Finally, many aircraft manufacturers will use Al-clad aluminum sheet for the exterior of an airframe. Al-clad is a coating of pure aluminum on the alloy sheet. The pure aluminum is much more corrosion resistant then alloy. But many times it is only on the exterior of the sheet, not both sides. So, you can have two different corrosion rates for the same sheet.

2) Anodize finishing - Most, but not all, exterior aluminum will get an anodize coating. The main types of anodize are (best to worst for corrosion) Hard Coat Sulfuric, Sulfuric, Chromic Acid, Phosphoric Acid. Another anodize is Boric Sulfuric Acid anodize, but it has only been around since the early 90's and would not be on a historic aircraft yet and Phosphoric anodize is used chiefly for bonding.

The corrosion rates between all of these differ greatly and some manufacturers, Boeing, do not use any anodize on exterior surfaces. Boeing likes Al-clad.

3) Primers and Paints - Just about every aircraft since the late 30's has gotten some type of chromate primer on the interior. Up to the mid-late 80's Zinc Chromate was a favorite Since then Epoxy chromates are used. Almost any time you see an aircraft painted it will have some type of chromate coating.

Now put all this into salt water add corrosion modifiers for temp and galvanic effects and you have only a basic guess as to what the actual rates will be. Not to mention what corrosion may have occurred prior to loss of the aircraft.

So, any real rates will be individual to the aircraft type and manufacturer.

Pete Johnson
Wreck Diver and Project Engineer
Special Processing
Sikorsky Aircraft