Citric all the way, it's bio-degradeable, down the sink no worries. It is the main ingredient of many 'eco' cleaners. Mix up a 10% solution, stick your hands in and swish it about, no worries. It also does the job, converts the O in FeO into CO and the Fe falls off. Fun to watch! Get mine from Hexeal by the kilo (work with a lot of rusty stuff!).