Cathodic protection calcareous deposits

At the surface, the ocean waters throughout most of the world are 200-500 % supersaturated in calcium carbonates and precipitation of these carbonate scales will influence the corrosion reaction occurring at the metal surface [6]. The buildup of protective calcareous deposits by cathodic protection can lower the subsequent current demand in natural seawater [22-24]. 

As an example. Fig. 20-7 shows potential and protection currents of two parallel-connected 750-liter tanks as a function of service life. The protection equipment consists of a potential-controlled protection current rectifier, a 0.4-m long impressed current anode built into the manhole cover, and an Ag-AgCl electrode built into the same manhole [10,11]. A second reference electrode serves to control the tank potential this is attached separately to the opposite wall of the tank. During the whole of the control period, cathodic protection is ensured on the basis of the potential measurement. The sharp decrease in protection current in the first few months is due to the formation of calcareous deposits. 

An increase in carbonate-ion concentration moves the equilibrium in favour of calcium carbonate deposition. Thus one secondary effect of cathodic protection in seawater is the production of OH , which favours the production of CO, , which in turn promotes the deposition of CaCOj. Cathodically protected surfaces in seawater will often develop an aragonite (CaCOj) film. This film is commonly referred to as a calcareous deposit. 

Thus brucite (Mg(OH)2) is also commonly found on surfaces under cathodic protection in seawater. Because more hydroxyl ions (higher pH) are required to cause magnesium hydroxide to precipitate, the magnesium is virtually always found in the calcareous deposits associated with calcium and its presence is an indicator of a high interfacial pH and thus either high cathodic current densities or relatively poor seawater refreshment. 

A consequence of cathodic protection in seawater is the formation of a protective calcareous scale . The increased local pH at the steel surface caused by hydroxyl production (a product of the cathodic reaction) favours the deposition of a mixed scale of CaCO, and Mg(OH)2. This scale is beneficial since it is protective and non-conducting, thus reducing the cathodic current density. Ensuring a high current density in the early period of operation will encourage calcareous scale deposition and thus reduce the current requirements in the long term (see Section 10.1 Principles of Cathodic Protection ). 

Calcareous deposition (CaCOa and Mg(OH)2) on the metallic surface creates a diffusional barrier toward oxygen and thus decreases the energy needed to maintain efficient protection. Knowledge of the formation time and characteristics of such layers are then essential to improve cathodic protection monitoring. 

The calcareous deposit formed on cathodically protected metal surfaces will also be affected by the presence of micro-organisms and macro-fouling. Physical disruption and alteration of structure, composition and crystal form will influence the response to cathodic protection. The consequent economic implications of changes in current density requirement for protection, and the engineering design implications and associated economics may be significant [Maines 1993]. Work is therefore required to improve the understanding of the relationship between... 

CALCAREOUS COATING OR DEPOSIT - A layer consisting of a mixture of calcium carbonate and magnesium hydroxide deposited on surfaces being cathodically protected against corrosion, because of increased pH adjustment to the protected surface. 

In general, one finds that the material outside the occluded region is virtually unattacked. Most often, the fuUy exposed material dissolves under passive conditions, with penetration rates of less than 1 tA cm (ca. 0.5 mpy). In some cases, evidence of cathodic protection of the fully exposed surface can be observed (e.g. formation of calcareous deposits). Just inside the occluded region, there is also usually minimal attack. The amount of attack increases as one moves deeper into the crevice, with a maximum in the attack depth often occurring between 0.1 and 3 mm from the mouth. At points further into the crevice, the amount of attack is... 

E15.4. During cathodic protection of iron under sea water conditions, the generation of OH leads to the calcareous deposits by reaction with calcium and magnesium ions. 

Due to the fonnation of the calcareous deposit, the diffusion of oxygen decreases and consequently the limiting current for oxygen decreases. Thus, the cathodic current necessary for the protection decreases considerably. 

In soils with low conductivity, a corrosive process that is initially fast corresponds to a high concentration of metal ions in the anodic areas, with the formation of protective films, and a strong alkalinization of the cathodic areas, with the formation of calcareous deposits for which the corrosion process tends to be negligible over time. Low-conductivity soils with a content of carbonates greater than 1% may be protective, and to them, we owe the good state of preservation of many... 

Littauer and Jenningsexamined the cathodic electrolysis of seawater as a means of fouling prevention at various current densities, and obtained partial protection. However, at current densities greater than 1 mAcm , calcareous deposits formed on the electrode. 

Dilute seawater is undersaturated in carbonates, which reduces the likelihood of forming protective ctdcareous films on a metal surface. In deep ocean waters, the calcareous deposits are not spontaneously formed in an ambient environment and are often not precipitated under cathodic protection conditions [6]. In the cold waters of the deep ocean environmental zones, the precipitation and/or dissolution of the calcareous deposits is mainly controlled by the calcium carbonate saturation level, II [25]. 

Fischer, K. P. and Finnegan, J. E "Cathodic Protection Behavior of Steel in Sea Water and the Protective Properties of the Calcareous Deposits, Paper 582, CORROSION/89, NACE International, Houston, 1989. 

Precipitation of Calcareous Deposits. The natural presence of calcium and magnesium in seawater has been advantageously used to coat internal walls of vessels such as ballast tanks with a protective film of calcareous deposits. These films, once they are formed by the cathodic polarization of metal surfaces in seawater, greatly reduce the current density needed to maintain a prescribed cathodic potential. For most cathodic surfaces in aerated waters, the main reduction reaction is described by Eq. (8.4) ... 

This scale is often developed on cathodically protected steel surfaces in seawater, and is often called a calcareous deposit. For this reason, some natural hard waters are less corrosive than softened waters. The addition of zinc sulfate (ZnS04) in alkaline solutions also inhibits corrosion by precipitating insoluble zinc hydroxide (Zn(OH)2) on the cathodic area. 

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