Glossary
Wash and disinfection of the Trunk says of brewery

Anodizing

Aluminum anodizing consists of depositing a film of pure aluminum on an aluminum alloy to enhance its resistance to corrosion. Many aluminum drums have been treated in this way and have given complete satisfaction, with one restriction: avoid damaging this protective layer when piercing with a removable plunger. Aluminum does not tolerate alkaline washing, which is why it has been virtually eliminated from the manufacture of brewery kegs and containers.

Stainless steels

Stainless steels are iron-chromium alloys with a minimum chromium content of 10.5%, and may contain other additives such as manganese, silica, nickel, sulfur, phosphorus, carbon and titanium.

These alloying elements are added in variable quantities to modify and improve certain properties of stainless steels....