ON THE CORROSION RESISTANCE OF SOME P/M FERROUS MATERIALS AFTER STEAM TREATMENT

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 materials 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 H₂SO₄ solutions at different  concentration; salt fog spray tests, according to the corresponding standard. The steam treated  samples have been compared with only sintered parts and fully dense low-carbon steel. The materials have been characterized by measurement of density and hardness, by evaluation of pore features by image analysis, by EDAX analysis of oxide distribution. It has been found that steam treatment improves corrosion resistance in NaCl solution while the effect is modest in H₂SO₄ 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. As to weight loss, steam treated sample are competitive with fully dense low-carbon steel. The salt fog spray test evidenced a dramatic difference between steam treated and untreated materials. The positive results attributable to a steam treatment effect are weakened by the presence of some Fe₂O₃, 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. By and large, the results confirm the well-accepted idea that steam oxidation moderately improves corrosion resistance.