THE BEHAVIOR IN SO2 AT HIGH-TEMPERATURES OF FE-B-COATINGS ON FE OBTAINED BY ION-BEAM, LASER OR PACK-CEMENTATION TECHNIQUES

Boron was incorporated in the near surface region of iron samples by t hree different methods: ion implantation, laser alloying and pack ceme ntation. The behaviour of these different surface alloys was studied t hermogravimetrically between 400 and 600-degrees-C in static SO2 atmos pheres at the pressure of 133 hPa. Implanted samples were obtained nea r room temperature at a dose of 10(17) B+ cm-2. The boron amount (appr oximately 2 mug cm-2) and the thickness involved (approximately 300 nm ) although small, induced a noticeable reduction of the corrosion rate . At low temperatures (T less-than-or-equal-to 450-degrees-C), the rat e law was paralinear and the only product formed was iron (II) sulphat e in good agreement with thermochemical provisions. In this temperatur e range, boron was thought to have a catalytic effect as, when absent, iron (II) sulphate did not appear. At higher temperatures a duplex la yer containing iron oxide and sulphide grew parabolically. Surface all oys obtained by laser alloying were 50-100 mum in thickness and exhibi ted compositions lying in the Fe-Fe2B region of the phase diagram. Dif ferent microstructures were observed, depending on the irradiation con ditions. The kinetic laws were parabolic or cubic, showing a greater p rotection when compared to implanted samples. Eutectic surface alloys exhibited the best behaviour for high temperatures or long times of ox idation. Pack-boronization at 950-degrees-C for 2 h in a powder mixtur e containing boron, ammonium fluoride and alumina led to a layer consi sting mainly of Fe2B. Kinetic experiments showed that such a coating e xhibited the best protection at the lowest temperatures.