Formation of Calcareous Deposits Under Different Modes of Cathodic Polarization

Abstract

Calcareous deposits that formed in natura) seawater upon ASTM'' A710 (UNSI2) K20747) steel polarized either potentiostatically, galvanostatically, or by a mixed-mode technique (initially galvanostatic followed by potentiostatic) have been studied. Parameters investigated included variation of current density (potentiostatic control) or potential (galvanostatic control) with time, total charge transfer, deposit thickness, thickness-to-charge ratio, deposit morphology, and deposit composition. Based upon these parameters, the mixed-mode technique was judged to provide the deposit with greatest protective properties within the time frame of the experiments. This may be related to the nature of the initial, magnesium-rich film established during he preliminary experimental period; the mechanism of calcareous deposit formation is discuseed within this context.

Natural Deposit Coatings on Steel during Cathodic Protection and Hydrogen Ingress

“…The layers (region 1 in Figure 8) are clearly Mg-rich with the lighter contrast areas (region 2 in Figure 8) and Ca-rich with the fine layer at the metal surface a combination containing both Ca and Mg. The coating consists of a fine layer of a mixture of Ca and Mg, followed by a predominantly brucite coating; this is contrary to others findings of the two layer structure of a fine brucite layer followed by CaCO3 [7,14,16,17,[19][20][21]. Figure 9.…”

“…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].…”

“…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]. Leeds and Cottis [22] found that a CaCO3 only deposit occurred at a potential of −0.7 V (SCE) and Barchiche et al [19] suggested an aragonite deposit would be formed at a potential −0.9 ≤ E ≤ −1.1 V (SCE), a combination of aragonite and brucite at −1.2 V (SCE) with brucite the only phase forming at a potential more negative than −1.3 V (SCE) in the 10-30 °C range with rotating electrodes.…”

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

“…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].…”

“…More recently, publications in this area mainly deal with the examination of calcareous deposits formed on steel at constant potentials. This probably reflects the conditions employed practically for steel structures in seawater [5][6][7][8][9][10][11][12][13][14], and only a few publications consider exploiting constant current, pulse or mixed modes of polarisation [3,4,15].…”