Formation of nickel phosphate coatings on iron or steel

A process for forming a protective crystalline nickel phosphate coating on an iron or steel surface comprising first contacting the surface with an alkaline alkali metal phosphate solution exhibiting a pH of from 8 to 12 containing 0.5-20 g/l of nickel phosphate suspended therein and thereafter contacting the surface with an aqueous composition consisting essentially of:

• 1 - 10 g/l of hydroxy carboxylic acid selected from the group consisting of salicylic, gallic, lactic, tartaric, citric, malic, glyceric, glycolic, mandelic and tropic acids
• 3-50 g/l of phosphate (as P.sub.2 O.sub.5)
• 2-6 g/l of nickel
• and having a Total Acid value of 10-50 points and a Free Acid value of 1-5 points, whereby a crystalline coating of nickel phosphate is formed on the surface.