That's a common misunderstanding. The Palantir never corrupted anyone. They only became dangerous to use once Sauron got his hands on one. You know, that immortal demon god who always uses mind control to get what he wants? If you use a Palantir he’ll notice and start working you over. If he is stronger than you are then he can force your Palantir to show you things of his choosing.

When Denethor used Gondor’s Palantir he saw orc armies marching and pillaging, foundaries forging weapons, Southrons marching north with Oliphants, corsairs raiding the coast, wildmen pillaging Rohan, etc, etc. Sauron never let him see allies coming to his aid, or his own troops winning battles.