Process for making an oxide-layer

A process of producing an oxide layer consisting predominantly of Fe.sub.3 0.sub.4 on an iron surface in an oxidizing atmosphere and at a temperature between 500.degree. to 600.degree. C. whereby the oxidizing atmosphere of combustion gases is produced by understoichiometrically burnt natural fuel gas or other fuel gases producing CO when burnt, comprising the following process steps:

• (a) controlling the combustion of the fuel gases by cooling the combustion chamber to maintain an apparent reaction temperature of from 500.degree. to 800.degree.C. in the combustion chamber to form the combustion gases;
• (b) without cooling to remove water from said combustion gases subsequently conveying the latter into a heated blackening chamber while maintaining constant the mass of the combustion gases and maintaining the temperature substantially above the dew point of the combustion gases;
• (c) producing a blackened oxide layer on the iron surface in the blackening chamber by contacting the iron surface with the combustion gases;
• and
• (d) monitoring the proportion of CO in the combustion gases after burning of the gases producing CO with a sensing means and controlling the air and fuel supplies by a control means in accordance with the measured values of the CO concentration.