A composite surface having a thickness from 10 to 5,000 microns comprising a
spinel
of the formula MnxCr3-xO4 wherein x is from 0.5
to 2 and oxides of Mn, Si selected from the group consisting of MnO, MnSiO3,
Mn2SiO4 and mixtures thereof which are not prone to coking
and are suitable for hydrocarbyl reactions such as furnace tubes for cracking.
