Early stages of zinc corrosion and runoff process induced by Caribbean sea water

Abstract

Flat samples of electrolytic zinc were immersed for periods of 8, 10, 30 and 90 days in Caribbean Sea water for further analysis of the corrosion behavior and runoff process using different techniques. The free corrosion potential (o.c.p.) were monitored and correlated with the runoff rate. The Zn2+ ions released from anodic sites interact with the OH- ions formed at the cathodic sites, giving origin to the slightly soluble Zn(OH)2 precipitated during the experiment. X-ray diffraction analysis revealed the formation of several zinc hydroxides. The main corrosion product was simonkolleite [Zn5(OH)8Cl2.H2O], and as minority phases two zinc carbonate hydroxides, [Zn5(CO3)2(OH)6] and [Zn4CO3(OH)6.H2O], and later stages Mg9Zn4(SO4)2(OH)22.8H2O and Zn(OH)2. The changes of pH in the interface zinc/substitute ocean water were monitored in situ with scanning electrochemical microscopy (SECM) technique and the local anodic and cathodic sites were mapped. Due to the corrosion reaction, the initial pH of sea water was diminished to 4.4 at the anodic sites as a consequence of metal ion hydrolysis. This fact leaded to the formation of a complex variety of zinc corrosion products, consuming at least a fraction of the produced OH- ions.