Abstract
In this work, plain, low carbon steel S235JRG1 was boronized at 1273 K for 45–150 min by using a Durborid® powder. The microstructure, phase constitution and oxidation behavior of the resulting boride layers have been investigated. Layers with an average thickness of 76–123 μm have been produced. The boride layer has a distinct tooth-like microstructure. It is composed of Fe₂B and FeB in unequal amounts. The boride layer oxidation behavior has been investigated by a simultaneous thermal analysis in a flowing synthetic air at 873–1173 K for 21–24 h. A parabolic oxidation of the boride layer has been observed. The rate constants are found between 1.039 × 10^–9 to 3.781 × 10^–6 kg² m^–4 s^–1, depending on temperature and oxidation time. The activation energy of oxidation at temperatures below 1173 K has been estimated to be 93 kJ mol^–1. At 1173 K, two successive parabolic periods have been found, followed by a breakaway oxidation behavior. The oxide scale of the boronized steel is composed of different iron oxides and iron borates. The oxidation mechanisms of boride coatings are discussed and implications towards high temperature stability are provided.

Key words
boronizing, high temperature oxidation, X-ray diffraction, thermogravimetric analysis

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