Electrochemical and Corrosion Fatigue Behavior of FeAlMn Alloys in NaCl Solution

The electrochemical polarization and corrosion fatigue behavior of the austenitic Fe-8.25Al-29.95Mn-0.85C and Fe-9.33Al-25.94Mn-1.45C alloys in NaCl solutions have been investigated. Potentiodynamic polarization tests in aqueous solution containing 0.005 to 10.0 wt% NaCl in the pH range of 2 to 6 have been conducted. Experimental results showed that the electrochemical parameters, such as corrosion potential and anodic current density, as well as the passivation properties, depended upon the pH and the chloride ion concentration of the solution. In 3.5 wt% NaCl solution, no passive region was observed if the pH was <3. Meanwhile, the pitting potential of the FeAlMn alloys increased with decreasing chloride ion concentration.

Fatigue crack growth (FCG) rates of the FeAlMn alloys, either in air or in 3.5 wt% NaCl solution at 25 C, were measured using center-cracked specimens. Sinusoidal load at a R ratio of 0.1 was applied with a frequency of 1 or 5 Hz. The experimental results showed that the FeAlMn alloys had better fatigue resistance than AISI⁽¹⁾ 316 stainless steel (SS) in air. The FCG rates at $ΔK=20MPam$ and at 5 Hz ranked in the following order: AISI 316 SS > Fe-8.25Al-29.95Mn-0.85C (aged at 550 C/4 h) > Fe-9.33Al-25.94Mn-1.45C (solution annealed) > Fe-8.25Al-29.95Mn-0.85C (solution annealed). Environmental enhancement on the FCG rates for both solution-annealed FeAlMn alloys tested in 3.5 wt% NaCl solution at 25 C was observed. The environmental effect became more evident as the testing frequency decreased from 5 to 1 Hz. For the aged Fe-8.25Al-29.95Mn-0.85C alloy, however, the FCG rates in 3.5% NaCl solution were higher at 5 Hz than at 1 Hz.