Coating electroaccretion of galvanized iron and aluminum in seawater

Coating electroaccretion on galvanized iron and aluminum 1100 under cathodic polarization in artificial and natural seawaterwas investigated through electrochemical tests and optical imaging techniques. Biofilm affects the current density and the morphologies of gas evolution, particularly the maximum size of the gas bubbles and the interaction between gas evolution and calcareous deposit. Coating mineral composition is related to the type of metallic material and can be different according to growth in natural or artificial seawater. On galvanized iron in ASTM and natural seawater at potential <−1.2V versus Ag/AgCl, coating is composed of aragonite and brucite as calcareous deposits on pure iron, aragonite forming before the growth of brucite. Even when coupled to a magnesium anode, the zinc layer can corrode and large aggregates of brucite and aragonite form on the bare steel. Coatings are composed of zinc hydroxychloride Zn5(OH)8Cl2·H2O and aragonite without brucite if electroaccretion is performed in natural seawater at potential >−1.2V versus Ag/AgCl. Coatings grown on aluminum 1100 are different from those on galvanized iron. In ASTM seawater, the coating on aluminum 1100 is composed of aluminum oxide and Mg4Al2(OH)14·2H2O; in natural seawater, only of aluminum oxide. On specimens coupled with magnesium anode, the coating does not contain brucite and is composed of aragonite with Mg6Al2(OH)18·4H2O islands.