I've been experimenting with de-rusting of 'stuff', initially using 'electro', then chemical with Deox-C and now citric acid.
So, not sure where to start concentration-wise I mixed 1kg of citric acid power with about 20 l of hot water and then put my test parts in to 'soak'. A fairly rusty screwdriver was pretty clean 24 hours later so I added and removed various bits from then on with generally very good results (nice clean steel / iron).
However, I left a few bits in for a few days and today I've noticed that there is a distinct 'crust' on the uppermost surface of the objects, *possibly* the citric acid that has re-crystallised (if that's what it could do)? It can be scraped off with a suitable tool to varying degrees of ease (easy from chrome, less easy from more rough / pitted parts).
So, remembering my 'O' level 'science', is it possible I made a 'super-saturated solution' when I initially used hot water and now it has all cooled down that some of the citric acid has re-crystallised out?
Would it do so (mainly) on the horizontal surfaces of objects in the bath?
The second question, is there anything I could use to chemically remove this buildup but that wouldn't dissolve my fingers or the various metals please (and be ideally readily available and cheap)? If citric acid is an acid are we looking at an alkali of some sort? A quick Google suggests caustic soda (sodium hydroxide) but if so what concentration (roughly) would I be looking at please? Would 'washing soda' do?
The third question is of 'hydrogen hardening'. I think I understand this is the penetration of hydrogen into the material, (eventually) wreaking it to some degree (to varying levels depending on the material).
How likely is this to affect the strength or reliability of everyday items? (ie, not aircraft / F1 components). ;-)
Cheers, T i m