g'day,a guy at my work uses molasses bath's to remove rust and it does a great job.has anyone used this method?
Yep. It is slow [days to weeks, depending on thickness of rust], so you have to be patient. It will corrode sheetmetal parts if left in too long, too. And it does get pretty stinky.
From my understanding molasses is sensational and can be used mixed down with water but has three shortfalls. One is it takes longer. Two is the cost of molasses, even from a stock feeder and thirdly while molasses does dissolve rust first it will also dissolve good metal whereas electrolysis will only dissolve rust and then stop no matter how long your piece is left in the bath.
It really has to be mixed with water. Anywhere from about 1+7 to 1+10 molasses:water ratio will work well.
Tip: Use hot tap water when mixing it up - speeds things up. A 5kg bucket of black molasses was about $13 from the local stock supplies joint, last time I bought some.
Picked up some SODA ASH from Bunnings today, active ingredient being SODIUM CARBONATE, the same stuff the americans use (ARM & HAMMER).. cost a couple $ more compared to Bi-Carb SODA, will see how it goes.
Soda Ash is also available in bigger packs from swimming pool chemical suppliers, and that works out cheaper than bicarb soda [aka Sodium Hydrogen Carbonate]. I would not use any chloride-containing electrolyte solution, like salt water - the metal absorbs some of it, and perpetual after-rust problems are likely.
For mechanical rust removal, a 'twist knot cup type' wire brush on a 4" angle grinder is about the best method I've used. As freebird mentions, good PPE is essential - eye protection [preferably full face shield], long welders' gloves and a leather apron. The flung off bristles
will penetrate normal clothing.
One other thing I noticed, the last piece I cleaned up with electrolysis, I hosed off and left to dry, only to come back to find surface rust.. any ideas how to prevent that from happening ?
Rapid after-rusting is a problem with any of the chemical [including electrolytic] de-rusting methods.
One of the best ways to passivate the surface is to rinse it off thoroughly, and swab it with diluted Phosphoric Acid solution before it dries. This will form a thin protective grey coloured Iron Phosphate surface coating, which is also a good base for the primer coat. Nearly all of the commercial 'rust remover' products are Phosphoric Acid based - if it doesn't show on the label, the MSDS will say. Try a dilution of about 1+4 to start with; if the metal surface doesn't turn grey pretty quickly, add a bit more rust remover.
For small steel parts, just boil them for 5 minutes or so in clean water to passivate them.
