STATISTICAL EVALUATION OF CORROSION OF SIALON IN BURNER RIG SIMULATEDCOMBUSTION ATMOSPHERES

Samples of a sialon with low levels of aluminium substitution were exp osed, for times up to 500 h, to a combustion atmosphere simulated in a low-velocity burner rig at 1300 degrees C. Corrosion was evaluated by metallographic measurement of multiple polished sections. It was foun d that the extent of corrosion varied systematically along the length of each sample, which has bee explained as being the result of creep o f the corrosion scale towards the base of the sample. The data from a number of measurement points along a sample were combined using a norm alizing procedure so that a sufficiently large data set could be acqui red for statistical analysis. Extreme value statistics were used to an alyse the section loss data and the extreme value parameters mu(e) and sigma(e) were determined and correlated with time of exposure. The mo st likely extreme value, mu(e), was found to follow a parabolic relati onship with time which is in conflict with mass change results, where the parabolic rate constant decreased at longer times. The time-depend ent correlation for the extreme value parameters was used, in conjunct ion with the reproductive property of the extreme value model, to pred ict the most likely maximum extreme value at longer times and for larg er sample sizes. The results of this prediction were compared with dat a for superalloys corroded in similar atmospheres at 900 degrees C. Th e behaviour of the sialon was found to be comparable with that of unco ated superalloys. (C) 1997 Elsevier Science Limited. All rights reserv ed.