Outside mirrors corroding... how can i replace?

[m_bisco]

Hey guys,

My outside mirror casings on both sides are starting to corrod. The triangular shaped pieces that are attached to the door have started corroding and is driving me nuts..... I'm starting to see some white underneath it and don't know exactly what it is.

Has this happened to any one else? If so is there a piece that i can buy to replace it or do i have to purchase the whole mirror piece?

I've attached a pic of the part that in talking about. Its not from my car but i'll try to take a few pics tonight and post them up a little later.

Any help would be great!