Well, it depends on which Mac knobs you're referring to . . . in the mid-seventies they started using plated knobs with black-anodised centers, and if there's corrosion on these, then you pretty much need new knobs (i.e. Audio Classics).
But they can be cleaned - Windex and a toothbrush is a very effective, safe methodology for just about any electronic knob of any material, with the exception of some knobs that have painted/silkscreened nomenclature (i.e. Fender knobs), sometimes the paint can start to come off. For really grungy ones, harsher chemicals can be used, but test for compatibility with the material on an inconspicuous spot, and follow all safety precautions for breathing fumes, inflamability, etc.
The earlier Mac knobs were actually raw aluminum, and with these, you may be able to polish out some corrosion. Since they have an annular machining pattern on the face, the best way is to spin them while polishing . . . try making a drill adapter from the shaft of a discarded potentiometer, or by filing a flat spot on a piece of 1/4" round metal stock from a hardware store. Put the adapter in your drill, attach the knob, spin the drill, and use a Scotchbrite pad for polishing.
