A Cornelius keg or Corny keg is a type of keg originally manufactured by the Cornelius company for use in the soda industry. Now obsolete, they are easy to find used and popular with home brewers.
About Cornelius Kegs
These kegs are generally preferred by home brewers because they are well suited for storage in converted refrigerator or freezer kegerators and hold the typical 5 gallon batch of beer. These kegs can be found in used condition (often filled with residual CO2 pressure and a bit of old syrup) for a few dollars more than their worth in stainless steel scrap. 2 and 3 gallon versions are also available, although they are rarer than full-sized 5-gallon kegs.
Two different disconnect/coupling systems (ball-lock and pin-lock) were widely used and are NOT interchangable.
Using Cornelius Kegs
At the very minimum, all 5 O-rings should be replaced and the kegs should filled with hot water/detergent over night (followed by sanitizer) before the first brewing use of a Cornelius keg that has previously been used for soda.