G'day all,
I do a bit of hand tool and parts refinishing, utilising a Molasses de-rusting bath, and a traditional 'rust blacking' process.
This involves deliberately forming a uniform coat of red rust, Fe
2O
3, on the surface, and then converting it to 'black rust', aka magnetite, Fe
3O
4.
There are a couple of simple techniques to 'passivate' a reactive steel surface, after it comes out of any de-rusting bath.
The simplest is to treat the surface with
boiling water - but you
must use high purity water.
Distilled or demineralised water is best, followed by rainwater. Tap water just doesn't cut it here; bore water even less so.
Pouring the water over the surface will work OK, if it's done straight away. The residual heat will ensure that the workpiece dries off quickly, too.
A related technique involves a 'steam cabinet' - tap water is fine to use in this one, as the workpiece never contacts the liquid water.
As I often work with longish, skinny workpieces, I've built a 'steam pipe apparatus' based on an old leccy frying pan, and some PVC DWV pipe and fittings. Pic of this setup attached; also a couple of refinished tools I've done.
Rust blacked, then given a couple of coats of Boiled Linseed Oil - this is a good old-time anti-rust coating, and it doesn't stink fishily...
For an average thickness workpiece, 10-15min of steaming is all that is needed.
Some of the 'black rust' is not bound to the metal, so this is removed in a process termed 'carding', using a soft wire brush or degreased steel wool.
