I'm new to kegging, but I was made aware that I shouldn't use sodium metabisulfite when cleaning a keg.
So what cleaners can I use?
More specifically, can I use OxyClean or Diversol?
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Oxyclean is a good cleaner, and you can use that on kegs. Diversol on the other hand is a bleaching product (chlorine-based), and like sodium metabisulfate that you were warned not to use will corrode the kegs if left too long. (EDIT: just to clarify, Oxyclean's active ingredient is sodium percarbonate, and this is safe to use. Sodium metabisulfate, mentioned in the original question, is not safe to use as it's corrosive.)
As well as oxyclean, I recommend PBW (Powdered Brewery Wash) - it's very good at getting underneath any organic material and lifting it off the surface and keeping it suspended in the water. You can clean 4-5 kegs with the same batch of PBW - I've not had the same reusability with oxyclean. Both oxyclean and pbw are non-corrosive and it's safe to leave kegs soaking overnight.
Before cleaning a used keg, fill with half a gallon of warm water, swirl a little and leave for 15 mins. (Use warm, but not hot water - it can melt the glue that holds the rubber handles in place.) This brief soak will remove most of the gunk (yeast, trub) that's stuck to the bottom. The keg will then be much cleaner already, and more kegs can be cleaned in the same PBW solution before it becomes too soiled.