The Characteristic Analysis of Calcareous Deposit Films Formed on Steel Plate by Cathodic Current Process in Marine Environment

Abstract

Cathodic protection is widely recognized as the most cost effective and technically appropriate corrosion prevention methodology for the port, offshore structures, ships. When applying the cathodic protection method to metal facilities in seawater, on the surface of the metal facilities a compound of calcium carbonate(CaCO 3 ) or magnesium hydroxide(Mg(OH) 2 ) films are formed by Ca 2+ and Mg 2+ ions among the many ionic components dissolving in the seawater. And calcareous deposit films such as CaCO 3 and Mg(OH) 2 etc. are formed by the surface of the steel product. These calcareous deposit film functions as a barrier against the corrosive environment, leading to a decrease in current demand. On the other hand, the general calcareous deposit film is a compound like ceramics. Therefore, there may be some problems such as weaker adhesive power and the longer time of film formation uniting with the base metal. In this study, we tried to determine and control the optimal condition through applying the principle of cathodic current process to form calcareous deposit film of uniform and compact on steel plate. The quantity of precipitates was analyzed, and both the morphology, component and crystal structure were analyzed as well through SEM, EDS and XRD. And based on the previous analysis, it was elucidated mechanism of calcareous deposit film formed in the sacrificial anode type (Al, Zn) and current density (1, 3, 5 A/m 2 ) conditions. In addition, the taping test was performed to evaluate the adhesion.

Electrochemical Characteristics of Aluminum Alloys in Sea Water for Marine Environment

“…In the past, films were formed in advance before anodic protection to reduce the effective protection area with a view to reducing the required current density [7]. Recently, anticorrosion techniques to adjust supply currents so that the calcareous deposit is evenly precipitated on the surface have been studied [9,10]. Thereafter, a turning point at which the current density rapidly increased as the applied potential decreased was observed.…”

“…(2) the absorbed atoms are combined with each other to become hydrogen molecules. Thereafter, when the hydrogen molecules are sufficiently formed, they are gathered so that hydrogen bubbles are formed [6][7][8][9]. Meanwhile, when the applied potential has been dropped further so that the hydrogen reactions occurring of the surface of specimens led to the threshold current density, the water is directly reduced and hydrogen bubbles are formed:…”

“…(3) (Period III). In this section, all specimens showed quite similar tendencies [6][7][8][9]. In this case, attention should be paid to the hydrogen (ion or atom) generated in the areas adjacent to the surface of specimens by the above three different reactions because the hydrogen may penetrate into the material for have negative effects.…”

“…This can be attributed to the fact that parts of the films were damaged due to the formation of the calcareous deposit and fast hydrogen generation occurring simultaneously in the activation polarization section. The excessive current densities supplied to the damaged areas leads to rapid increases in the generation of hydrogen bubbles so that surface damage developed [9]. The calcareous deposit formed on the surface consists mainly of Mg(OH) 2 and CaCO 3 and is precipitated through the following reactions.…”

“…Therefore, calcareous deposit composed of calcium compounds forms the films on the cathodic surface [7][8][9][10].…”