http://www.eng-tips.com/viewthread.cfm?qid=182988&page=1

A "poison pad" is intended to permit the welding of dissimilar materials without affecting the parent (corrosion resistant) material's metallurgy. A prime example is the welding of carbon steel legs to stainless steel vessels to save cost- doing so directly without a poison pad would potentially introduce carbon into the HAZ of the vessel shell.

http://www.letco.com/engineeringtables.php?value=11
POISON PAD
A pad formed to the shape of the vessel used to separate two dissimilar metals.