AT WL uses Mg as the metal which reacts with air and it also migrates through the first couple layers of AP and rubber. It swells. The reason why Erroretch uses Mg is it combusts more fully in small motors. It ages poorly. The alternative technology we use at U.S. Rockets is low micron Al. On the order of 6-8 micron. It ages 40+ years.

Last I checked AT BT uses copper chromite which does not age quite as badly.

Tech Jerry