The typical reaction that occurs during any oxygen attack on the surface of the steel follows general reactions as follows:
2Fe + O2 → 2FeO
4Fe + 3O2 → 2Fe2O3
3Fe + 2O2 → Fe3O4
Fe + H2O ↔ FeO +H2
Fe + CO2 ↔ FeO +CO
It can be seen from the above reactions that oxygen can be sourced in many forms, including air, moisture, water and carbon dioxide.
As temperature is applied to steel in an open atmosphere, the oxygen present in air will begin to attack the surface of the steel. This attack can be seen in the form of color changes from steel color, light straw, straw color, dark brown, purple, blue, pale blue or to colorless.
The colors that form are indicative of the temperature that the steel has been subjected to, and surface oxides are forming on the steel surface. The thickness of the formed oxide is dependent on time and temperature. It should be further noted that the steel surface chemistry is changing. This means that the immediate surface mechanical properties are changing because of the oxygen attack.
Once the steel temperature reaches approximately 900°F, the oxide will continue to form, but it will begin to have mass, which we understand as scale. In addition to this, a further occurrence will commence and that is the loss of carbon from the surface of the steel. This is known as decarburization.
The steel is trying to achieve equilibrium with the atmosphere that surrounds it. The steel surface chemistry is continuing to change, now with the loss of surface carbon, thus a further change in the steel surface properties. In order to prevent surface oxidation, it will be necessary to protect the steel surface for the oxygen attack. This can be accomplished by the use of the following furnace atmospheres:
- Hydrogen (in low-pressure furnaces or gas-tight furnaces after being purged with nitrogen prior to the introduction of the hydrogen). Hydrogen is generally used at low temperatures in a blended form.
- Nitrogen/Hydrogen (generally at 95% N2 + 5% H2. The blend can also be changed to 90% N2 + 10% H2)
The oxide formation on the surface of the steel can begin to reduce the steel surface-corrosion activity. It will not eliminate it but will reduce the rate of corrosion.