Corrosion Rate In Environments Research Articles

Effect of Grain Size on the Anodic Dissolution of Lean Duplex UNS S32202 Austenitic-Ferritic Stainless Steel

The effect of grain size on the anodic dissolution of lean duplex UNS S32202 dual-phase austenitic-ferritic stainless steel was evaluated. Grain coarsening was achieved by heat treatment, and grain size and grain boundary densities determined by automatic image analysis after etching. Potentiodynamic electrochemical testing in acidic chloride medium allowed isolating the anodic dissolution behavior of the crystallographic phases of the material. A relationship between grain boundary density (for grain sizes in the micrometer range) and dissolution rate has been found, showing that reducing grain size enhances active corrosion rates in environments that promote active behavior. This leads to new possibilities of industrial adjustment of the corrosion behavior of duplex stainless steels via grain size control.

Composition and protective ability of rust layer formed on weathering steel exposed to various environments

The compositional change of rust (corrosion products) layer formed on weathering steel exposed to atmosphere with different amount of air-borne sea salt particles in Japan have been investigated by the X-ray diffraction method. The mass ratio (α/γ) of crystalline α-FeOOH to γ-FeOOH, in the rust layer formed on the weathering steel exposed in an industrial environment, increases with an increase in exposure duration. The α/γ is closely related to the corrosion rate in environments when the amount of air-borne salt is less than 0.2 mg NaCl/dm 2/day (2.31 × 10 −7 g NaCl/m 2/s). However this is not the case in seaside environments with a higher amount of air-borne salts. The mass ratio (α/γ ∗) of crystalline α-FeOOH to the total mass of γ-FeOOH, β-FeOOH and Fe 3O 4, in the rust layer formed on the weathering steel is related to the corrosion rate even in seaside environments certainly more than 0.2 mg/dm 2/day (2.31 × 10 −7 g/m 2/s) of air-borne salt particles. When the α/γ ∗ is more than 1, a higher corrosion rate more than 0.01 mm/year (3.17 × 10 −13 m/s) is not observed. The α/γ ∗ is a protective ability index of rust formed on weathering steel.

Effect of resistivity on the corrosion mechanism of mild steel in sodium sulfate solutions

Electrochemical impedance spectroscopy (EIS) was used to study the corrosion behavior of mild steel samples immersed in 1, 0.1, 0.01 and 0.001% Na2SO4 aqueous solutions at room temperature in order to analyze the corrosion mechanism and obtain representative values of corrosion rates in environments with different resistivity. The EIS technique was used to measure corrosion current densities in 1 to 0.01% Na2SO4 solutions, and the measurement corresponding to 0.001% concentration gave a scattered Nyquist plot, whereas in the Bode representation a response associated with solution resistance was obtained. Other electrochemical techniques such as linear polarization resistance, LPR, Tafel extrapolation method and electrochemical noise (EN) were used in order to compare the results obtained by EIS. The ‘charge-transfer resistance’ (Rct) parameters obtained in the EIS technique agree well with the corresponding values of ‘polarization resistance’ (Rp) values obtained by the LPR technique, whereas the ‘electrochemical noise resistance’ (Rn) parameters obtained by the EN technique gave the highest values for all concentrations. These parameters generally tended to increase as the concentration of the solution decreased.