I had my parts resting on plastic in my tank when I did mine. It did not affect the plastic. However, rust particles did accumulate on it and stain it as the plastic was white. I am sure it could have been cleaned off but I didn’t try. The caustic Soda is in fact caustic so if left in long enough it might damage it but if it were me and I couldn’t get the plastic part off I would just go for it.