Non-chromate conversion coatings

A conversion coating solution which comprises an aqueous solution of from about 0.2 g/l to about 45 g/l of free H.sub.2 SO.sub.4, from about 1.5 g/l to about 58 g/l of H.sub.2 O.sub.2, from about 3 g/l to about 33 g/l of SiO.sub.2, from about 0.15 g/l to about 10 g/l of at least one of the organophosphorus compound promoters specified below, and from about 2 g/l to about 20 g/l of at least one secondary promoter selected from the group consisting of ascorbic acid, boric acid, gluconic acid, glycolic acid, tartaric acid and salts of said acids, wherein the organophosphorus compound is one having the general formula:

• [X(R.sub.1).sub.m ].sub.n .multidot.[R.sub.2 ].sub.p .multidot.[X(R.sub.1).sub.m ].sub.q,
• wherein
• X is a group of the formula ##STR3## in which Z.sub.1 and Z.sub.2 independent from each other are hydrogen, sodium or potassium;
• m is either 0 or 1;
• p is either 0 or 1;
• n+q is either
• (a) 1 when P=0, or
• (b) equal to the number of available
• bonds provided by R.sub.2 when p=1;
• R.sub.1 is a
• (a) C.sub.1 -C.sub.4 alkyl or a C.sub.1 -C.sub.4 hydroxy-substituted alkyl and p=0;
• and
• (b) C.sub.1 -C.sub.4 alkylene or a C.sub.1 -C.sub.4 hydroxy-substituted alkylene and p=1;
• R.sub.2 is selected from
• (a) N.ident., m=1
• (b) .dbd.N(CH.sub.2).sub.r N.dbd., m=1 and r is an integer from 2 to 6 ##STR4## (d) a C.sub.1 -C.sub.4 alkylene or a C.sub.1 -C.sub.4 hydroxy-substituted alkylene m=0 or 1.