Fairy Liquid and similar products with salt in have never had any adverse effects on any of my bikes over the years, whether its on protected or unprotected metal. My normal routine these days is to use cheap neat supermarkets own brand washing up liquid to not only clean the bike but also clean and degrease the chain, cassette and chainrings and I've never seen any salt damage to any of those parts. My bikes probably get exposed for a longer time to more salt from the roads riding them in one winter than the total amount of washing up liquid they see in a lifetime, but by washing it off after every ride, I never see any damage caused by it.
End of the day, we are just cleaning our bikes in it and then washing it off, not leaving it to soak for days in the stuff. If washing up liquid is so corrosive to metal, how come all my cutlery and pans, whether its made from, stainless steel, steel or aluminium etc has never corroded away in the washing up bowl, especcially when it can be left for long periods of days. I even have an all metal butter knife, my first real knife when I was a child to eat my meals with still in use, how many gallons of washing up liquid has it seen over the past 50+ years and its still in perfect condition.
And even if you use it to clean the frame, it only needs a squirt or two in a bucket of water which dilutes the salt content to almost nothing.
