Natural Deposit Coatings on Steel during Cathodic Protection and Hydrogen Ingress

Natural Deposit Coatings on Steel during Cathodic Protection and Hydrogen Ingress

Type	Journal Article
Author	Wayne R. Smith
Author	Shiladitya Paul
URL	Free Full Text Source:  http://www.mdpi.com/2079-6412/5/4/816
Volume	5
Issue	4
Pages	816-829
Publication	Coatings
Date	2015-11-12
Abstract	The calcareous coating formed during cathodic protection (CP) in seawater reduces current demand by hindering the transport of species required to support the cathodic reactions and, thereby, improve the economic performance of CP systems. However, it is unclear as to whether the coating reduces or enhances hydrogen uptake. To determine this, two sets of samples were polarized at −1.1 V (standard calomel electrode, SCE) in 3.5% w/v NaCl and synthetic seawater (ASTM D1141) at 20 °C and the diffusible hydrogen content measured over a period of 530 h. Under such conditions reports suggest a deposit with two distinct layers, comprising an initial brucite layer followed by an aragonite layer. Contrary to other findings, a fine initial layer containing Ca and Mg followed by a brucite layer was deposited with a few specks of Ca-containing zones in synthetic seawater. The hydrogen uptake was found to occur within the initial 100 h of exposure in synthetic seawater whilst it continued without the benefit of a deposit coating, i.e., in 3.5 wt % NaCl solution.