The behaviour in SO 2 at high temperatures of FeB coatings on Fe obtained by ion beam, laser or pack-cementation techniques

Abstract

Boron was incorporated in the near surface region of iron samples by three different methods: ion implantation, laser alloying and pack cementation. The behaviour of these different surface alloys was studied thermogravimetrically between 400 and 600°C in static SO 2 atmospheres at the pressure of 133 hPa. Implanted samples were obtained near room temperature at a dose of 10 17 B + cm −2. The boron amount (∼ 2 μg cm −2) and the thickness involved (∼ 300 nm) although small, induced a noticeable reduction of the corrosion rate. At low temperatures (T ≤ 450°C), the rate law was paralinear and the only product formed was iron (II) sulphate in good agreement with thermochemical provisions. In this temperature range, boron was thought to have a catalytic effect as, when absent, iron (II) sulphate did not appear. At higher temperatures a duplex layer containing iron oxide and sulphide grew parabolically. Surface alloys obtained by laser alloying were 50–100 μm in thickness and exhibited compositions lying in the FeFe 2B region of the phase diagram. Different microstructures were observed, depending on the irradiation conditions. The kinetic laws were parabolic or cubic, showing a greater protection when compared to implanted samples. Eutectic surface alloys exhibited the best behaviour for high temperatures or long times of oxidation. Pack-boronization at 950°C for 2 h in a powder mixture containing boron, ammonium fluoride and alumina led to a layer consisting mainly of Fe 2B. Kinetic experiments showed that such a coating exhibited the best protection at the lowest temperatures.