In article ,
wrote:

I was making another clone of my miniature sundials. On this occasion
I thought I would try a few things differently. Amongst others I
decided to soft-solder the brass 0.020" foil to the steel substrate.
Normally I glue them together with Goop.

The soldering went smoothly (except for having to buy a bigger
soldering iron). I proceeded to machine the piece in the normal way
yesterday and today I put it together:

http://www.flickr.com/photos/2768312...in/photostream

The etching is a bit raggedy due to high current density (I was also
trying a new bath) but otherwise things looked normal until I noticed
the fresh rust on the inside of the ring:

http://www.flickr.com/photos/2768312...in/photostream

I interpret this as caused by the acid-based flux leaking onto the
freshly exposed steel. This raises the question of what to do about
this in future. Clealy no amount of thorough cleaning will get rid of
the flux that is hidden until exposed by turning/boring. Maybe I shall
go back to using rosin dissolved in alcohol as flux.

If I understand, you machine after soldering the brass to the steel. I
have a lot of trouble believing that the acid flux would penetrate solid
mild steel so far that peeling 0.050" off wouldn't solve the problem.

Use of rosin flux has been suggested, but that probably won't work very
well for steel. The traditional dodge is to tin the steel using acid
flux, wash the flux residue off with hot water and bicarbonate of soda,
and then solder brass to steel with plumbers flux (intended for
soldering copper pipe). Clean the flux off with acetone followed with
hot water.

I have also soldered cadmium-plated steel by mechanical cleaning
followed by soldering with plumbers flux.

Joe Gwinn