Corrosion behaviour of hafnium diboride in aqueous solutions

A dense HfB₂ ceramic material was prepared by hot pressing in the presence of 2.5 wt.% Si₃N₄, as a sintering aid. The microstructure consisted of a fine HfB₂ matrix with refractory secondary phases (BN, HfO₂, Hf(C,N,O)) and Si, mainly located at the triple points. The material corrosion behaviour in acid and alkaline solutions of chlorides and sulphates was investigated by means of several experimental techniques, that is potentiodynamic polarization curve recording and potentiostatic tests coupled to solution chemical analysis, scanning electron microscope (SEM) observations and X-ray photoelectron spectroscopy (XPS) surface chemical characterization. Sintered HfB₂ undergoes an electrochemical corrosion attack of the HfB₂ phase and a chemical corrosion attack, mainly localized on the secondary phases. In acid sulphate solution a strong corrosion attack on the diboride phase occurs which leaves a secondary phase skeleton. In acid chloride solution and in alkaline chloride and sulphate solutions, the rate of secondary phase dissolution is quicker than the dissolution rate of the primary diboride phase and an intergranular corrosion attack morphology occurs. The Si phase dissolves relatively fast only in alkaline solutions. In analogy with the corrosion behaviour of ZrB₂, corrosion of HfB₂ phase is thought to produce both soluble and insoluble (hafnium and boron oxides) corrosion products. In acid sulphate solution the soluble corrosion products are mainly boric acid and a sulphate complex of Hf(IV). In the other solutions they are mainly constituted by boric acid (or borates, depending on the solution pH). The high corrosion rates detected in acid sulphate solution are due to the marked stability of the soluble sulphate complex of Hf(IV).