s an alkyl group having from 1 to 6 carbon atoms such as methyl, n-propyl, n-butyl, t-butyl, t-pentyl, t-hexyl, R3 is an alkyl group having from 1 to 12 carbon atoms, and n is an integer of 1 to 4 and (b) at least one metal phosphonate represented by the formula ##STR4## where R is as before, R4 is hydrogen or alkyl having from 1 to 6 carbon atoms, R5 is an alkylene radical having from 1 to 4 carb