An Initial Investigation of Calcareous Deposits upon Cathodic Steel Surfaces in Sea Water

Abstract

The influence of potential and nominal sea water velocity upon the nature of calcareous deposits formed upon a cathodic steel surface has been investigated. Specific attention was focused upon change with exposure time of the current density to maintain a prescribed cathodic potential and upon thickness of the calcareous film. Equations have been developed whereby pH of the electrolyte adjacent to both a filmed and film-free surface can be rationalized in terms of cathodic current density and character of the electrolyte flow. Structure of calcareous deposits was investigated by scanning electron microscopy. Results are discussed within the frame of current understanding of marine cathodic protection and cathodic protection practices.

Natural Deposit Coatings on Steel during Cathodic Protection and Hydrogen Ingress

“…After the initial 100 h less than 25% of the current required to polarize the steel in 3.5% w/v NaCl was required to polarize in the synthetic seawater due to the deposition of the calcareous coating. The current density decay over the experiment for this coating is comparable with previous findings for a mixed Mg(OH)2 and CaCO3 deposit [9,33]. Figure 6a,b show pH measurements for the two solutions 3.5% w/v NaCl and synthetic seawater.…”

“…Calculations for the pH required for the deposition have yielded values of pH 7.3-8.7 for the CaCO3 polymorphs (calcite and aragonite) and pH 9.3-11.25 for brucite (Mg(OH)2 [6,7,[9][10][11] and the composition achieved dependent on the potential applied, temperature, and flow of the electrolyte (ASTM D1141 [12] in our case) [9,[13][14][15]. In previous studies it has been established that the deposit has a two layer structure made up of a fine layer of Mg-rich brucite followed by a Ca-rich polymorph [7,14,[16][17][18][19][20][21].…”

A Study of Calcareous Deposits on Cathodically Protected Mild Steel in Artificial Seawater

“…Currently, the presence and formation of a thin inner layer of magnesium hydroxide on the steel, under cathodic protection, is attributed to a high level of alkalisation (above 10) near the surface due to the reduction of oxygen and at very low potentials, due to the process of water reduction [2,5,6,15]. This statement is very well justified for cathodically protected steel, which has no evidence of corrosion.…”

“…When steel is under cathodic protection in seawater, the pH near the steel surface slowly becomes more alkaline due to oxygen reduction [2] and the deposition of calcium carbonate is expected to occur first. In previous studies [3,5,6,17] it was demonstrated that the compound that is initially precipitated next to the iron-containing corrosion products is possibly brucite (Mg(OH)2), and not calcium-rich aragonite (calcium carbonate, CaCO3).…”

“…Additionally, like paints, they are thought to have an increased ionic resistance and to afford some degree of resistance inhibition. A considerable amount of work has already been done in understanding the mechanism formation of calcareous deposits and factors influencing this formation [2][3][4][5][6][7][8][9][10][11][12][13][14]. However, the effects of corrosion and the development of corrosion products under cathodic protection are reported sporadically [8,9].…”