Something that might have contributed to the rivet heads shearing off? The amount of tensile force needed would be significantly reduced under these circumstances...
"Ferrous metals placed in contact with moist salt air will rust if not properly protected. Nonferrous metals, those without an iron base, do not rust, but a similar process known as corrosion takes place. The salt in moist air (found in the coastal areas) attacks the aluminum alloys. It is a common experience to inspect the rivets of an aircraft which has been operated near salt water and find them badly corroded.
If a copper rivet is inserted into an aluminum alloy structure, two dissimilar metals are brought in contact with each other. Remember, all metals possess a small electrical potential. Dissimilar metals in contact with each other in the presence of moisture cause an electrical current to flow between them and chemical byproducts to be formed. Principally, this results in the deterioration of one of the metals.
Certain aluminum alloys react to each other and, therefore, must be thought of as dissimilar metals. The commonly used aluminum alloys may be divided into the two groups shown in figure 6-32.
Aluminium groupings
Group A Group B
1100 2117
3003 2017
5052 2124
6053 7075
figure 6-32
Members within either group A or group B can be considered as similar to each other and will not react to others within the same group. A corroding action will take place, however, if any metal of group A comes in contact with a metal in group B in the presence of moisture.
Avoid the use of dissimilar metals whenever possible. Their incompatibility is a factor which was considered when the AN Standards were adopted. To comply with AN Standards, the manufacturers must put a protective surface coating on the rivets. This may be zinc chromate, metal spray, or an anodized finish."
Now Ric's rivet is a AN455 or AN456 3/32 inch Brazier head made of
2117 T4 aluminium, Group B.
The skin is a sheet of 0.032 Alclad which is a 2024 alloy? Group A doesn't include 2024 aluminium but, on checking the aluminium alloy composition limits (% weight) table it is of remarkably similar composition to series 1100 and series 3003 aluminium alloys.
http://avstop.com/ac/apgeneral/rivets.htmlThe rivets may well have been weakened significantly by the environment the aircraft ended up in.