Passivation

[mekanik]

Had a little look into this and it seems that Citric Acid can be used instead of Nitric Acid(link) but you would still have to put a protective coat over that so it looks like the etch primer would be a better solution,Just had a check in the shed, the Manor Paints is the Chromate(yellow base) that i prefer but Chromadex do a none Chromate etch if you prefer that.I got this for refurbishment of Spilka H type window hardware as they are made from mild steel then passivated and clear coated but this deteriorated after about 10 years, just out of interest where are you located, i am near Ulverston.
Hope that helps.
Regards
Mike


https://www.amazon.co.uk/Citric-acid.../dp/B00GOT02C2