Exam API-571 topic 1 question 196 discussion

Oxidation of carbon steel begins to become significant above about 1000 °F (540 °C). Rates of metal loss increase with increasing temperature. At temperatures above about 1000 °F (540 °C), the oxidation of carbon steel becomes significant. This is because the iron oxide layer that forms at these temperatures is less protective than the oxide layer that forms at lower temperatures. The higher temperature oxide layer is also more likely to crack and expose the underlying steel to further oxidation.