Deposits calcareous

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. 

Kalk-echtlieit. /. fastness to lime, -einlagerung, /. calcareous deposit, -eisengranat, m. lime-iron garnet, andradite. -eisenstein, m, ferruginous limestone, kalkempfindlich, a. sensitive to lime. Kalkempfindlichkeit, /. sensitiveness to lime, kalken, kalken, v.t. lime. 

The corrosion rates for both maraging steel and the low alloy steels in seawater are similar initially, but from about 1 year onwards the maraging steels tend to corrode more slowly as indicated in Fig. 3.32. The corrosion rates for both low alloy and maraging steel increase with water velocity . During sea-water exposure the initial attack was confined to local anodic areas, whereas other areas (cathodic) remained almost free from attack the latter were covered with a calcareous deposit typical of cathodic areas in sea-water exposure. In time, the anodic rust areas covered the entire surface. ... 

This value will decay to less than 0-05 A/m if the calcareous deposit forms. 

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. 

This analysis is far from exact since it assumes a remote groundbed, uniform soil resistivity and uniform defect density in the coating. At best it demonstrates that attenuation is likely to follow an exponential decay and that it will be less severe for larger diameter pipes than for smaller. The problem is more difficult to solve for more complex structures (e.g. congested pipeline networks) and especially so for marine installations where the development of the calcareous deposit introduces the possibility of temporal variations in attenuation. 

This section is not intended to deal with those environmental factors which influence cathodic current demand (e.g. oxygen availability or the presence of calcareous deposits) but those which directly affect the performance of the anodes. 

The build-up of calcareous deposits is a complex topic. Very high current densities will not necessarily result in the most protective scale. In the extreme, hydrogen evolution may rupture the scale resulting in reduced protection. An optimum current density will exist, and this should be recognised. 

Attack on the substrate by contact with Mg(OH>2 and Ca(OH)2 (calcareous scale) can also cause deplatinisation to occur. Anodes located close to the cathode or operating at high current densities can lead to a rapid build up of calcareous deposit, the major constituents of which are Mg(OH)2 and Ca(OH>2. The alkaline conditions so generated can lead to rapid dissolution of the platinum. The calcareous deposit can be removed by washing with dilute nitric acid. 

In alkaline solutions approaching pH 10, PbOj is unsuitable for use, and for this reason it should be mounted clear of any calcareous deposit which may be formed on a cathodic area close to the anode this deposit indicates the formation of alkali which may have a detrimental effect on the PbOj deposit. 

Continuous Anodes Consist of considerable lengths of relatively flexible copper-cored material which can be contoured to suit restricted spaces or to distribute current in a localised fashion. Typically they may be used in water boxes at a non-ferrous tubeplate/ferrous water box junction. Anode terminations pass through the water box via insulating entry points and the anodes are supported on insulators within the box. Anode/cathode distance must be such as to prevent the anode becoming engulfed in calcareous deposit that forms on the cathode. 

Fig. 3.11 shows the position of the sensor in a bypass pipe in the bottom area of the drum. This ensures that the sensor is always immersed in the suds and the pipe can always drain at the end of the program, which helps to avoid the pipe clogging up with calcareous deposits. 

It will be apparent that if normal extracellular fluids were subjected to an isotonic resorption of sodium and chloride ions by the process, the net effect would be to concentrate other ions and precipitate minerals. This suggestion was made613 to explain one of the methods of forming deposits in the calciferious glands of earthworms. It was proposed that the posterior glands received blood directly from the intestine. Fluid was formed in these glands by a process of filtration and saline was then resorbed by the epithelial cells. This resulted in the formation of calcareous deposits (Fig. 5). 

SCALE. 1. A calcareous deposit in water tubes of steam boilers resulting from deposition of mineral compounds present in die water, e g., calcium carbonate. 

In another 13-week study, rats that were fed the compound up to a maximum level of 252 mg tin/kg/day as stannous chloride, showed increased relative kidney weights (Dreef-van der Meulen 1974). The protein-like droplets and calcareous deposits which are common in the rat strains used were present in the controls but were not seen in the tin-fed animals. The absence of calcareous deposits in the females confirms the observations of DeGroot et al. (1973), but the relevance of these finding to compound toxicity is unclear. The organ weight change itself, in the absence of histopathological or other effects, cannot be considered a toxic effect. 

Renal Effects. Some changes in renal histopathology were seen in several short to intermediate-duration studies of stannous chloride. However, these affects were not present in either rats or mice during the NTP investigations of this compound. The absence of renal calcareous deposits in females and protein droplets in males exposed to high doses of stannous chloride despite their presence in the controls and at low doses cannot be clearly categorized as an adverse effect. 

Own experiments in divided cells using Nation membrane separators and hypochlorite solutions in the ppm range of concentration resulted in current efficiency values for active chlorine reduction of a few percent. Shifting the pH to higher values complicated the experiments. A buffer stabilised the pH but the relatively high concentration of buffer ions hindered the electrochemical reaction. Thus, quantification is difficult. Kuhn et al. (1980) showed reduction inhibition when calcareous deposits were precipitated on the cathode, but practical experiments showed the decrease of chlorine production in this case. 

In affected cells (Fig. 7.20a), deposits can be removed by changing electrode polarity. At low pH calcareous deposits dissolve (7.70)... 

Barchiche, Ch., Deslouis, C., Festy, D., Gil, O., Refait, Ph., Touzain, S. and Tribollet, B. (2003) Characterization of calcareous deposits in artificial seawater by impedance techniques. 3-Deposit of CaCCE in the presence of Mg(II). Electrochim. Acta 48, 1645-1654. 

Kraft, A., Blaschke, M., Kreysig, D., Sandt, B., Schroeder, F. and Rennau, J. (1999b) Electrochemical water disinfection. Part II Hypochlorite production from potable water, chlorine consumption and the problem of calcareous deposits. J. Appl. Electrochem. 29, 895-902. 

The majority of cherts from the inland site of Fiais were also from calcareous deposits. Figure 4 shows that Al and Ti increase at the interior site, which is buried in a clay-rich soil. It is admittedly difficult to determine whether the increases in such elemental concentrations are caused by di-agenetic factors or by inhomogeneity of the materials. These changes stand out in sharp contrast to the majority of cherts from Samouqueira, which on the basis of thin-section analyses, are of a meta-volcanic origin. These cherts are less susceptible to weathering and appear to have experienced little... 

Petrographic comparison of the Mexico City majolica with Teotihuacan sherds shows that except for the secondary deposition of carbonates, which is present in the majolica but absent in the Precolumbian sherds, the mineral composition of both sets of specimens is very similar. Both notably include hornblende and similar feldspars as inclusions, and both are low in quartz. Similarly, except for the calcite in the majolica, both sets of sherds show similar x-ray diflFraction patterns. The mineralogical evidence, therefore, strongly suggests that both sets of sherds were made from closely related clays and that the compositional diflEerences that exist between them are primarly the result of the accumulation of a secondary calcareous deposit within the majolica sherds during burial in the wet soil of Mexico City. 

A so-called porcelain liver should be clarified by differential diagnosis. (188) This condition is characterized by focal calcareous deposits in the liver capsule, which can be detected by X-ray examination as well as by sonography and CT. It may be caused by phleboliths, pseudolipoma, histoplasmosis, calcified larvae, small capsule haematomas, small abscesses, etc. 

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 time constant with calcareous deposits was found to be the same as for a bare surface in the low-frequency range with a slight shift cf diagrams to lower reduced frequencies when the deposition time increases. At high frequency (above p — 1), a second time constant appeared. Both these features characterized the presence cf a partially blocked surface with a moderate value of the active fraction cf the electrode. The slight separation of the EHD diagrams observed in the low-frequency region, when the deposition time is... 

Figure 15.11 Calcareous deposit formed at —1.2 V(SCE) and 1200 rpm, I/Iq = 0.7. (Taken from Deslouis and Tribollet and reproduced with permission of Elsevier, Inc.)...

Electrohydrodynamic impedance characterization of calcareous deposits showed mainly partially blocked electrode behavior and allowed the estimation of the average size of characteristic sites of the interface. These results have been confirmed by ex-situ SEM images. 

C. Deslouis, D. Festy, O. Gil, G. Rius, S. Touzain, and B. Tribollet, "Characterization of Calcareous Deposits in Artificial Sea Water by Impedance Techniques I. Deposit of CaCOs without Mg(OH)2," Electrochimica Acta, 43 (1998) 1891-1901. 

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