Local electrochemical impedance measurements on inclusion-containing stainless steels using microcapillary-based techniques

Pitting Corrosion of Hot-Dip Galvanized Coatings

Lead (Pb) addition to hot-dip galvanizing (HDG) baths affects the physical characteristics of zinc coatings and is also useful to protect kettles. The influence of lead additions on both corrosion rate and morphology as well as on structure of zinc coating is less investigated. In this paper, three different additions, (Pb = 0.4–0.8–1.2 w/w) were chosen for three series of steel substrates, plus references without lead. The three steels chosen as substrates contained silicon (Si) = 0.18, 0.028, 0.225 w/w, respectively. The experimental part included both macro- and micro-electrochemical measurements, weight loss vs. time plots, Glow Discharge Optical Emission Spectroscopy (GDOS) and SEM/EDX microanalysis of both surface and cross-section of samples. Lead concentration is responsible for evident bimetallic coupling in the surrounding of lead inclusion with consequent increased dissolution rate, chunk effect, and rougher surface morphology.

Influence of pH and Chloride Concentration on the Corrosion Behavior of Unalloyed Copper in NaCl Solution: A Comparative Study Between the Micro and Macro Scales

The effects of pH and chloride concentration on the electrochemical corrosion of copper in aqueous sodium chloride (NaCl) media were studied at the micro scale using a microcapillary droplet cell and at the macro scale using a conventional large scale cell. Using an experimental design strategy, electrochemical response surface models of copper versus pH and NaCl concentration were constructed with the minimum number of experiments required. Results show that the electrochemical behavior of copper under corrosive media shows significant differences between the micro and macro scale experiments. At the micro scale, the pit initiation of copper occurs at more negative potentials for high NaCl concentrations and alkaline pH values. Also, the micro scale potentiostatic measurements indicate higher stabilised passive currents at high NaCl concentrations and low (acidic) pH values. At the macro scale, the pH is shown to have a greater influence on the corrosion potential. The chloride concentration is the most significant factor in the passive current case while at the micro scale the effect of these two factors on the passive current was found to be the same. The surface morphology of the formed patina on the corroded copper in both micro and macro systems reveal a more significant role of the chloride concentration on the structure and the grain size of the patinas. Finally, micro and macro electrochemical impedance spectroscopy of copper at various NaCl concentrations and pH values demonstrates a different behavior of copper after several potentiodynamic polarization cycles.