Method for producing doped alkali silicate protective layers on metal and glass substrates

A method for producing a vitreous protective layer on a metal substrate or a glass substrate or a pre-coated metal substrate or a pre-coated glass substrate, wherein the method comprises

• (a) mixing with NaOH and KOH one or more silicon compounds of formula (I)
• RnSiX4-n (I)
• in which R is independently alkyl, aryl, alkaryl or arylalkyl, X is independently alkoxy or acyloxy, and n is 0, 1 or 2, with the proviso that at least one silicon compound of formula (I) in which n is 1 or 2 is used,
• (b) adding water to a mixture obtained in (a) to hydrolyze the silicon compound(s) of formula (I),
• (c) adding at least one compound of formula (II)
• MYm (II)
• in which M is Pb, Ti, Zr, Al or B, Y is independently alkoxy, and m corresponds to a valence of M, wherein one or more alkoxy groups may be replaced by one or more chelate formers and/or two alkoxy groups may be replaced by an oxo group,
• to a hydrolyzed mixture obtained in (b), a molar ratio M/Si being from 0.01/1 to 0.04/1, M being a molar amount of M in the at least one compound of formula (II) and Si being a molar amount of Si in the at least one silicon compound of formula (I), to obtain a coating sol,
• (d) applying the coating sol obtained in (c) to the metal substrate or glass substrate or the pre-coated metal substrate or the pre-coated glass substrate, and
• (e) thermal densification of the coating sol applied in (d) at a temperature of from 300° C. to 500° C. to form the glass like vitreous protective layer.