Steel facing is a form of electroplating that deposits a molecularly thin layer of pure iron onto a copper plate. If done correctly, no loss of detail occurs to the plate or image.
The benefit to steelfacing is the ability to print bright colours (such as yellow or white) without oxidation and the ability to print larger editions from fragile plates (such as a drypoint or mezzotint).
Below is the printing sequence from a series of steelfaced plates. Without steelfacing the colours printed muddy and grey or green due to the oxidation that occurs to the colour from the copper plate. Here you can see that the colours stay vivid and clean when printed from steelfaced plates.
The colour orange and a transparent blue/grey being printed from a steel faced aquatint plate.