Understanding pitting corrosion behavior of AZ91 alloy and its MAO coating in 3.5%NaCl solution by cyclic potentiodynamic polarization

The pitting corrosion behavior of AZ91 alloy before and after micro-oxidation treatment in 3.5%Na Cl solution was investigated by cyclic potentiodynamic polarization(CPDP)and optical and SEM observations of corroded surfaces at different polarization potentials.The CPDP results show that both the alloy and the MAO-coated alloy suffer from pitting corrosion and it is difficult for pits to stop growth once the pits initiates.It is revealed that the air-formed Mg O film on AZ91 alloy,the MAO coating along with corrosion products(mainly Mg(OH))formed during CPDP can significantly influence the kinetics of the redox reactions of Mg,and further influence the propagation behavior of pitting corrosion.The optical and SEM images show that the corrosion products on AZ91 alloy are dense and protective but on the MAO-coated corrosion products are very loose.Such observations support the analyses of CPDP results that pits on AZ91 alloy spread to the width whereas pits on MAO-coated alloy propagate to the depth.Overall,pitting corrosion on the MAO-coated alloy can be very severe once the coating ruptures and post-treatments are necessary to provide a promising corrosion protection for the Mg alloys.

Enhancing corrosion resistance of 7150 Al alloy using novel three-step aging process

The effects of a novel three-step aging process （T76＋T6） on the electrochemical corrosion behavior of 7150 extruded aluminum alloy were evaluated and compared with those of the conventional retrogression and re-aging process （T77）. The open circuit potential （OCP）, cyclic polarization and electrochemical impedance spectra （EIS） of the A1 alloys were measured after treatment in three solutions （3.5% NaCl （mass fraction）; 10 mmol/L NaCl ＋ 0.1 mol/L Na2SO4; 4 mol/L NaCl ＋ 0.5 mol/L KNO3 ＋ 0.1 mol/L HNO3）. The parameters including the corrosion potential, pitting potential, pit transition potential and steepness, and potential differences were extensively discussed to evaluate the corrosion behavior of the Al alloys. The electrochemical and scanning electron microscopy （SEM） data show that compared with the 7150-T77 Al alloy, the T76 ＋ T6 aged 7150 A1 alloy exhibits better resistance to pitting corrosion, inter-granular corrosion （IGC） and exfoliation corrosion, which is attributed to further coarsening and inter-spacing of the grain boundary particles （GBPs） as revealed by transmission electron microscopy. Furthermore, the hardness tests indicate that an attractive combination of strength and corrosion resistance was obtained for the 7150 Al alloy with T76 ＋ T6 treatment.

Corrosion behavior of cold-rolled and post heat-treated 316L stainless steel in 0.9wt%NaCl solution

The effect of cold rolling and post-rolling heat treatment on the microstructural and electrochemical properties of the 316L stainless steel was investigated.Two-pass and four-pass cold-rolled stainless steel specimens were heat-treated by annealing at 900℃followed by quenching in water.During the cold rolling,the microstructure of the as-received specimen transformed from austenite to strain-inducedα′-martensite due to significant plastic deformation that also resulted in significant grain elongation(i.e.,~33%and 223%increases in the grain elongation after two and four rolling passes,respectively).The hardness of the heat-treated as-received specimen decreased from HV 190 to 146 due to the recovery and recrystallization of the austenite grain structure.The cyclic polarization scans of the as-rolled and heat-treated specimens were obtained in 0.9wt%NaCl solution.The pitting potential of the four-pass rolled specimen was significantly increased from 322.3 to 930.5 mV after post-rolling heat treatment.The beneficial effect of the heat treatment process was evident from~10-times-lower corrosion current density and two-orders-of-magnitude-lower passive current density of the heat-treated specimens compared with those of the as-rolled specimens.Similarly,appreciably lower corrosion rate(3.302×10^(−4)mm/a)and higher pitting resistance(1115.5 mV)were exhibited by the postrolled heat-treated specimens compared with the as-rolled 316L stainless steel specimens.