I started to write a second, lengthy comment but decided to make it a post instead. See Denethor and the Palantír for more information.

It would appear so, but Tolkien says somewhere (again, I apologize for not being able to put my finger on the book) that Denethor’s will – being so nearly pure Númenorean – was too strong to be overthrown by Sauron; the same was true with Aragorn. Still, Sauron deceived Denethor into despair by carefully selecting what he allowed the Ruling Steward to see in the palantír.

Similarly, Saruman would be too strong for Sauron to overthrow and corrupt. If anything, Saruman was corrupted by studying the arts of the enemy too deeply – which may have been Tolkien’s rebuke of C.S. Lewis’ The Screwtape Letters, of which Tolkien strongly disapproved and believed to reflect the unprofitable (to JRRT) close friendship of Lewis with Charles Williams (whom Tolkien never liked very well).

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