One of the things that I learnt from my father, who was a chemical engineer, is that halide ions are aggressive towards metals, and steels in particular.  Now bleaches contain a lot of chloride ions, and I have just looked at the bottle of limescale remover in our toilet and it contains hydrochloric acid.  It is not advised for use with stainless steel.  Since DIY toilet seats often come with steel or brass hinges, this implies trouble around the corner.

Now my favourite de-ruster is citric acid, which can remove both rust and limescale without attacking the steel.  So I was gratified just now to come across the following scenario for iron-citrate complexes in Science Codex:

Transition metal catalysts could be key to origin of life, scientists report

If interstellar tourism ever gets under way, will we see signs like Passengers are requested not to flush the lavatory while the spacecraft is in low orbit around a planet ?