。
Rust is slowly corroding the metal parts.
Literal
Rust [subject-が] metal [attributive-の] part [object-を] a-little-at-a-time is-corroding-[continuous-ている].
少しずつ ('a little at a time, gradually') uses ずつ, the distributive particle for 'each, per.' 侵食する ('to erode, to corrode') is used both literally (physical erosion) and figuratively (gradual encroachment). ~ている marks the ongoing process.