Minima in the strength of MnZn ferrites

Like many other oxides, MnZn ferrite shows a considerably decreased strengt h in a humid environment. Conventionally one explains this behavior by subc ritical crack Growth, i.e., a kinetic effect affecting the oxygen-metal bon d breaking rate at the crack tip via a reaction with water. The dissociatio n of water in a proton and hydroxyl ion is considered as crucial. However, it appears that also other gases have a detrimental effect on the strength. In this paper the effect of H2O, H2S, NH3, NO, and CO on the strength of M nZn ferrite have been studied experimentally. For water a clear minimum in strength occurs at relative low partial pressure. For the other gases the d ata also indicate a minimum. An adsorption model taking into account the pr esence of competitive adsorption mechanisms, involving dissociative and non dissociative adsorption of the adsorbate and the (non)dissociative adsorpti on of the ever present N-2, is presented. This model can explain the experi mentally observed features well. Although kinetic effects induce subcritica l crack growth in MnZn ferrites, it is concluded that adsorption is an impo rtant strength lowering effect for MnZn ferrite when exposed to active gase s.