Do not use any kind of metal scrubbing pad in a keggle or in any other stainless steel pot. You risk cutting through the protective chromium oxide layer, which encourages rust. Use those green scrub pads or similar, and Bar Keeper's Friend or a similar oxalic acid based cleaner.
Here is the reference you need to read: Appendix B - Brewing Metallurgy
. I recommend you read the entire appendix. John Palmer's How to Brew is a must-read for all new brewers, IMO.