Temperature dependence of the formation of the passivation layer on carbon steel in high alkaline environment of concrete pore solution

- Different temperatures and cooling methods, water- and air-cooled, were used.
- The passive layer was a highly disordered n-type semi-conductor.
- The increase in temperature decreased the thickness of the passive layer.
- More protective passive layer was formed on the air-cooled specimens.
- The passive layer on the 850 °C specimens did not form or it was very weak.

In this study, the formation and the strength of the passive layer on carbon steel in concrete pore solution were investigated for temperatures ranging from room temperature to 850 °C. Two sets of carbon steel specimens were first exposed to different temperatures for an hour. One group cooled to room temperature in the air and the other one in the water. The steel specimens were then immersed in concrete pore solution and the passive layer formed on the steel was investigated. Results showed that in the extreme conditions, i.e. very high temperature, and water-cooled condition, the passive layer rarely formed on the steel. Increasing the exposure temperature led to a decrease in the thickness of the passive layer. This decrease was more apparent when steel was cooled in water compared to air cooled specimens. Results from this study contribute to the knowledge base that may be used to advance the state of the art in predictive models for steel reinforcement in concrete exposed to high temperatures (e.g., fire, power plants).