Photoshop selections not protected from eraser tool.

I was hoping someone could tell me why selections aren't protected. For example, I shot a chromed item, then after inverting the selection to remove the surrounding part of the image, I see that the delete key or the eraser tool also encroaches upon the selected part of the image I want to keep, but begins to erase it also. It's very slight, but the more I erase from the area I want to be sure is cleared, the selected area degrades further and further. This isn't noticeable on a white background, but it is painfully obvious on a black background, as well as a transparent background. I suspect this may have something to do with the chrome being so light in saturation as to appear nearly white, but I also feel there must be a way to accomplish the task. Any tips would be greatly appreciated! Thank you!