So…

My mate has his RSV in my garage to clean it and get it sparkling so he can put it on sale..
Before Xmas he was there cleaning it and left some bits in degreaser to clean…the problem is that we went to my garage at lunch time tpday and the bits are all funny.. Looks like the degreaser has corroated the alloy?
Don't know what type of material it is, but it's the foot pegs, rider and pillion…

Any way to bring it back to the normal state?

Brush or rub any loose furring off the parts and spray or soak in light oil (wd40/gt85/3in1/duck oil or similar) should help. With the damp uk air I protect metal parts and tools by the above or wrap in an oily tissue/rag I also keep the little bags of silica gel in my toolboxes, for the same purpose.

Some degreasers will corode alloy even if it's anodised. What colour has the metal gone? I fear there'll be no easy way to restore them.

Bead blasting will give the best results if you can find a local place to do it. They'll be almost impossible to polish by hand.

they were silver, it's the footpegs out of an RSV Mille

JP I meant what colour are they now?

they are like a dark grey, almost black

B*gger!

Bead blast & laquer, replace or paint!

I made a really good job of the forks on a bike that had stood outside for 2 years. They were covered in fur but when that was cleaned off they were grey.

I did it with Scotchbrite - you know the green nylon-ish things you get for washing up? Like the ones with a spong square, but without the sponge - and washing up liquid in warm water. It's hard work but can be done! Then cleaned off all remnants of the detergent with meths, and put on several layers of thin laquer with a brush.