Metalert

Metalert was an enhanced form of dalekanium created by Zeg to overcome the problems Daleks would have during space travel. It was stronger and capable of withstanding great heat, acid, and mercury, but was not immune to extreme cold, such as liquid oxygen. Zeg was accidentally coated with metalert. (COMIC: Duel of the Daleks)

Dalek Sec's casing was also made of metalert. (PROSE: Birth of a Legend)

Behind the scenes
According to the Doctor Who DVD Files, the Dalek Emperor seen in TV: The Parting of the Ways was reinforced with metalert.