Effect of Heat Treatments on the Corrosion Resistance of a TRIP Steel and Its Evaluation by Non-Destructive Testing

The development of advanced high-strength steels (AHSS) for the automotive industry requires optimizing the balance between mechanical properties and durability in aggressive environments. This study investigates the effects of two heat treatment routes on the microstructure and corrosion resistance of a transformation-induced plasticity (TRIP) steel (Fe-0.2C-1.75Mn-0.5Si-1Al). Route A includes a full austenitizing step at 1000 °C prior to intercritical annealing, whereas Route B omits this step and begins directly with intercritical annealing at 800 °C. Microstructural characterization (SEM/XRD), electrochemical assays, and eddy current tests were employed. The results revealed that Route A yields a homogeneous microstructure with 12.7% retained austenite, higher than the 7.7% obtained with Route B. Electrochemically, the steel from Route A exhibited the greatest resistance, with the lowest corrosion current density (icorr) of 3.72 µA/cm2 and a more noble corrosion potential (Ecorr) of −743 mV compared to SCE. The improvement mechanism is that the homogeneity induced by complete austenitization minimizes the formation of internal galvanic cells between phases; likewise, the higher austenite fraction provides superior chemical stability, which favors denser passivation. Finally, Route A exhibited the lowest loss of electrical conductivity (16%), validating the use of eddy currents for monitoring the integrity of advanced steels.