Pitting corrosion of AA1050 sheets produced by DC casting and twin-roll casting techniques

Aluminum alloy 1050 (AA1050) sheets produced by direct-chill casting (DCC) and hot-rolling process and the same alloy produced by the twin-roll casting (TRC) are compared for their susceptibility to pitting corrosion in this study. To determine any differences between the pitting susceptibility of the DCC and TRC alloys, electrochemical experiments were conducted in deaerated and naturally aerated NaCl solutions with different concentrations (1 M to 0.001 M). The experiments were cyclic potential polarization (CPP) measurements, potentiostatic tests for the repassivation potential determination(Erepas), galvano-staircase cyclic polarization (GSCP) measurements, metastable pitting measurements under potentiostatic conditions and electrochemical noise measurements. Furthermore, immersion tests were performed in 0.01 M NaCl for 7 days. The microstructure investigations showed that larger intermetallics were present in the DCC alloy, while the number of intermetallics in the TRC alloy was larger. The particles contained Al, Fe and Si in both alloys. The pitting potentials (Epit) from the CPP experiments were generally more positive for the DCC alloy sheets than TRC alloy sheet. However, the differences between the two production techniques were found to be less than 30 mV. The results from Erepas potentials determined under potentiostatic condition revealed that the highest difference between the two production techniques was 50 mV and the DCC alloys have more positive potentials. Breakdown (Eb) potentials as well as the protection potentials (Eprot) for the DCC and TRC alloys were found to be close to each other according to the GSCP technique. At 0.01 M concentration, the formation rate of metastable pits in the AA1050 sheet produced by the DCC was found to be higher than that of the AA1050 sheet produced by the TRC. The change in localized corrosion activity of the alloys over time was measured by electrochemical noise measurements. The SEM and optic profilometry investigations after immersion tests showed that pits of larger diameter formed in the AA1050 DCC alloy.