alloy: A homogeneous mixture or solid solution of two or more metals, the atoms of one replacing or occupying interstitial positions between the atoms of the other.
the "alloy" tanks you are refering to are an aluminum based alloy. ever hear of chrome-moly or stainless steel?? both alloys. the reason some "alloy" tanks dont rust is because they are an aluminum alloy. most of us know aluminum doesnt rust.
take a look at arrons bsa tank in the pics section of the caferacer web site. its alum alloy.
good luck on your triton quest. if youre ever serious about finding one, youre in the right place.