Effect of steam treatment on the corrosion resistance of some P/M ferrous material | Effetto del trattamento a vapore sulla resistenza a corrosione di componenti in acciaio sinterizzato

A commonly accepted opinion is that steam treatment increases the corrosion resistance of P/M ferrous materials. To confirm the scanty literature data, the behaviour of steam treated material has been investigated. The test samples have been pistons for refrigerator compressors, namely P/M parts typically steam treated. The corrosion resistance has been checked by different methods: evaluation of corrosion resistance by means of potentiodynamic tests and corrosion potential measurements in different aggressive environments; weight loss measurements in H2 SO4 solutions at different concentration; salt fog spray tests, according to the corresponding standard. The steam treated samples have been compared with only sintered parts. The materials have been characterized by measurement of density and hardness, by evaluation of pore features by image analysis, by EDXS analysis of oxide distribution. It has been found that steam treatment improves corrosion resistance in NaCl solution while the effect is modest in H2 SO4 solutions. The weight loss test indicated that corrosion products, which remain entrapped inside the inner porosity, hide the actual loss of material originated by corrosion. The collapse of particles, when the sintering necks fail, is the reason of a “jumping” course of the results. The salt fog spray test evidenced a dramatic difference between steam treated and untreated materials. The positive results attributable to steam treatment effect are weakened by the presence of some Fe2 O3, on the outer surfaces. This unwanted oxide frequently appear on steam treated P/M parts in case of not-perfect equipment or incomplete process control, at any stage. The different tests show that steam treatment is an effective method to improve the corrosion resistance of ferrous porous materials. However, the improvement may be weakened by the presence of a Fe2 O3 layer. This harmful oxide may form if H2O/H2 ratio exceeds a safety limit. By and large, the results agree with the few data existing in the literature. Corrosion potential measurements and mass loss tests in acidic solution show at first a low corrosion resistance for steam treated material. This behaviour can be ascribed to the preferential dissolution of a thin and porous hematite outer layer formed on the outer surfaces of the parts, as evidenced by SEM and X-ray analysis. Once this layer is removed, the good properties of Fe3 O4 layer appear. In case of simply sintered samples, the corrosion products remain entrapped inside the inner porosity, thus hiding the actual effect of the aggressive environment. The steam oxidation process caused some transformation of MnS into FeS and Mn and Fe oxides. The so formed constituents remain frequently embedded inside the oxide layer in the pores. This last point should be investigated more deeply, because the positive influence of MnS addition on material’s machinability may decrease.