Corrosion Behavior of Austempered Ductile Iron Used in the Aeronautical Industry Evaluated on Acid Solutions

The austempered ductile iron (ADI) is a material which has great potential when talking about applications relating wear resistance, high strength, toughness, and ductility. This material has a unique microstructure, consisting of a matrix of high carbon austenite and a mixture of ferrite and carbides. This microstructure provides mechanical properties much higher than other iron castings. Usually ADI is used on automotive industry, such as suspension parts, arrows, and crankshafts. It is also used on the aeronautical industry, in the building process of aircraft landing gear. In certain applications where ADI is used, its corrosion resistance must be a transcendental property; nonetheless owing to its structure, the ADI doesn’t possess pretty noticeable resistance to corrosion. Despite this, it’s of huge interest to know how different parameters on heat treatment (applied to ADI) may affect the ADI microstructure, and it can disturb its behavior due to those variables, and in consequence its application areas could be reduced. In this paper we analyze the corrosion resistance presented by ADI irons, after having been subjected to different austempered treatments (different temperatures within the austenitizing muffle furnace and salt bath). The austenitizing temperatures evaluated were 815, 870, and 930 °C, where the waiting time used was 90 min. On the other hand, the salt bath was used at a temperature of 250 and 400 °C, having the waiting time of 60 min. The corrosion resistance was evaluated on two different solutions (NaCl and H₂SO₄), supporting this with two different electrochemical techniques, the linear polarization resistance (LPR) and potentiodynamic polarization curves (CPC). The outcomes show that the temperatures of the salt bath play a crucial role in evaluating the corrosion resistance of these irons.