Stress Corrosion and Its Mechanism of Hot-dip Galvanized Coating on Q235 Steel Structure
The corrosion behavior of hot-dip galvanized coating on the Q235 steel plates commonly-used in grid equipment by applied bending stress was studied via immersion test in 0.05 mol/L NaCl solu-tion while applied bending stress with a home-made three-point bending stress loading device.The re-sults showed that by the applied bending stress,the corrosion of the hot-dip galvanized coating on Q235 steel plate was a process of repeated formation and spallation of corrosion products,of which the former involves apparently the occurrence of corrosion pits,while the later does not.The corrosion products were mainly composed of ZnO,Zn(OH)2 and Zn5(OH)8Cl2·H2O.As the applied stress increased,the Ecorr was decreased,but the Icorr and the electrochemical impedance were increased for the hot-dip galvanized coating on Q235 steel plate.A corrosion model was established to illustrate the corrosion process and the relevant mechanism for the corrosion of the hot-dip galvanized coating/Q235 steel plate.That is,the corrosion of the hot-dip galvanized coating was speeded by the applied bending stress to form more cor-rosion product Zn5(OH)8Cl2·H2O,which induced the formation of cracks at the stress concentrated sites beneath the corrosion product,i.e.,the corrosion pits in η-Zn layer.The cracks then penetrated through the η-Zn layer,and extended along the interface ζ-FeZn13/η-Zn.As a result,electrochemical corrosion of the galvanized coating was accelerated.