Crystal Structure Control of Calcareous Deposit Films Formed by Pulse Electrodeposition Process in Seawater and Their Properties

해수 중 펄스 전착 프로세스 의해 제작한 석회질 피막의 결정구조 제어 및 특성 평가

Abstract

As an anti-corrosion method in seawater, cathodic protection is widely recognized as the most effective and technically appropriate corrosion prevention methodology for marine structures against harsh corrosive environment. When applying the cathodic protection in seawater, the surface of the metal facilities the formation of compounds of $CaCO_3$ and $Mg(OH)_2$. These mixed compounds are generally called 'calcareous deposits'. This layer functions as a barrier against the corrosive environment and functions to further inhibit the corrosion process and then leading to a decrease in current demand for cathodic protection. However, calcareous deposit films are partially formed on the surface of the cathode and there are some difficulties to maintain both a corrosion resistance for a long period of time and a strong adhesion between deposits and base metal. In this study, the pulse electrodeposition process was applied to improve adhesion and corrosion resistance of the calcareous deposit films, and to solve the problem of hydrogen embrittlement at high current density. The uniform and compact calcareous deposit films were prepared by pulse electrodeposition process, and their properties were characterized using various surface analytical techniques together with electrochemical methods.

Fig. 1. Experimental schematic diagram of electrodeposition process

Fig. 2. Schematic diagram of taping test and electrochemical anodic polarization test

Fig. 3. Photographs of electrodeposited calcareous films by the application of various direct and pulse current densities for 24 hours in seawater

Fig. 4. Weight gain of electrodeposition films formed by direct and pulse current

Fig. 5. Composition analyses of electrodeposited calcareous films for 24 hours in seawater with applied current density

Fig. 6. Crystal structure analysis of electrodeposition films by direct and pulse current

Fig. 7. Surface morphology observation in accordance with the direct and pulse current after 24 hours

Fig. 8. Taping test for adhesion properties of electrodeposition films formed by direct and pulse current

Fig. 9. Electrochemical anodic polarization behavior curves of electrodeposition films and Fe substrate measured in 3% NaCl solution

Table 1. Chemical composition and mechanical properties of specimen

Table 2. Average concentrations of the 11 most abundant ions and molecules in seawater

Table 3. Experimental condition of electrodeposition process according to the direct and pulse current