I take it mike_gb that you've been looking at the infamous P&G "Science In The Box" website?
The site claims to provide details of the key ingredients in P&G products, but the reality is it hides almost as much information than it provides using the dubious excuse that the full formulations are trade secrets.
Fairy liquid does contain a fair amount of salt and yes, it is sufficient to cause corrosion to bare metal. I know one or two people in P&G and I'll make some enquiries tomorrow about the exact percentage it contains.
Salt is used as a thickening agent in shampoos, other cosmetic products and washing up liquids and it works by combining with other ingredients to increase the viscosity of the finished product. It's not unusual for a shampoo to contain as much as 5% salt for this purpose and bearing in mind the viscosity of Fairy Liquid, it probably contains at least a similar amount.