Current density requirement

Certainly a thermodynamically stable oxide layer is more likely to generate passivity. However, the existence of the metastable passive state implies that an oxide him may (and in many cases does) still form in solutions in which the oxides are very soluble. This occurs for example, on nickel, aluminium and stainless steel, although the passive corrosion rate in some systems can be quite high. What is required for passivity is the rapid formation of the oxide him and its slow dissolution, or at least the slow dissolution of metal ions through the him. The potential must, of course be high enough for oxide formation to be thermodynamically possible. With these criteria, it is easily understood that a low passive current density requires a low conductivity of ions (but not necessarily of electrons) within the oxide. 

Table 10.5 Estimates of the current density required to protect bare steel in various corrosive...

Cathodic Current Densities for Protecting Steel Examples of current density requirements for the protection of steel (to achieve a steel potential of —0-8 V vs. Ag/AgCl/seawater) are given in Tables 10.13 and 10.14. It should be realised that the current demand of a structure will be influenced by, inter alia, temperature, degree of aeration, flow rate, protective scales, burial status, presence of bacteria and salinity. 

It is important that the correct current density requirement is assigned for design purposes. If too high a value is used the structure may be waste-fully overprotected, whereas a value too small will mean that the protection system will underprotect and not achieve its design life. 

Estimate of current required The surface area of the structure is calculated and the current density required for the particular environment is selected (Table 10.26). In the case of an existing structure the condition of the coating may be unknown and the application of a temporary cathodic-protection system may be necessary to determine the amount of current required for protection, as established by the potential. Such a test to determine the... 

Environment Current density required for adequate cathodic protection based on superficial area (mA/m )... 

Current density requirements depend on the environment, galvanic effects, velocities and other factors influencing polarisation. In the absence of galvanic influences or other secondary effects 30mA/m may be sufficient in sea-water to maintain adequate polarisation for protection once it has been achieved it is however normally necessary to apply 100-150 mA/m to achieve initial polarisation within a reasonable period and if rapid protection is required, current densities as high as 500 mA/m may be applied. 

Because these variables have a very pronounced effect on the current density required to produce and also maintain passivity, it is necessary to know the exact operating conditions of the electrolyte before designing a system of anodic protection. In the paper and pulp industry a current of 4(KX) A was required for 3 min to passivate the steel surfaces after passivation with thiosulphates etc. in the black liquor the current was reduced to 2 7(X) A for 12 min and then only 600 A was necessary for the remainder of the process . From an economic aspect, it is normal, in the first instance, to consider anodically protecting a cheap metal or alloy, such as mild steel. If this is not satisfactory, the alloying of mild steel with a small percentage of a more passive metal, such as chromium, molybdenum or nickel, may decrease both the critical and passivation current densities to a sufficiently low value. It is fortunate that the effect of these alloying additions can be determined by laboratory experiments before application on an industrial scale is undertaken. 

The high current density requirement means that the bubbles must be moved out of the way so that current can pass from the electrolyte into the anode base. The vertical channels/grooves in the anode face provide a low-energy path for the bubbles to move from the surface to the vapor space and exit the cell. 

Figure 15 Applied current densities required for different applied potentials for an active-passive material in acid. If there is a dissolution rate of 1 pA/cm2, cathodic protection to Ec would require an applied current density of 10,000 pA/cm2, while anodic protection to E4 would require only 1 pA/cm2. (After Ref. 21.)...

An optimal use of the applied current density requires one further condition the flow of organic pollutants toward the anodic surface must be sufficient to consume the produced hydroxyl radicals on the same surface. The maximum flow of organic pollutants toward the anode can be expressed as limiting current density (iiim), and this further condition assumes the form (9.2). 

The two ways of protecting a system are (i) to define the protection potential Ep and maintain the potential of the structure and (ii) the current density required to protect the structure along with the area are used together with the output of the anode materials and use the calculated weight of the anode material. The amount of anode material required for protection of a structure. 

It is important to note that passive behavior of the metal also depends upon the electrolytic environment. As an example mild steel is passivated in pure nitric acid, but not in dilute aqueous nitric acid. The current density required to passivate pass can be high and the current density to maintain the film r fiim, may be small. 

Fig. 4. Effect of temperature on the critical current density required to start electropolishing silicon in 5% HF.

The effect of viscosity on the critical current density required to start electropolishing a horizontal silicon electrode in 5% HF solutions at 25° C is presented on a log-log plot in Fig. 6. Glycerine was used to increase the solution viscosity. 

In Chapter 10 it is stated that corrosion of a surface may be related to the velocity of the corroding fluid across the surface the higher the velocity the higher the rate of corrosion. For this reason the magnitude of the current density required to maintain a particular cathodic potential will increase vith electrolyte velocity. Due to changing cross-sectional area for a given volumetric flow, the necessary current is likely to vary from location to location within a heat exchanger. 

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... 

The technique may be understood in terms of metallic passivity, i.e. the loss of chemical activity experienced by certain metals and alloys under particular environmental conditions as a result of surface film formation. Equations 15.2 and 15.3 suggest that the application of an anodic current to a metal should tend to increase metal dissolution and decrease hydrogen production. Metals that display passivity, such as iron, nickel chromium, titanium and their alloys respond to an anodic current by shifting their polarisation potential into the passive regon. Current densities required to initiate passivity are relatively high [Uhlig and Revie 1985] but the current density to maintain passivity are low, with a consequent reduction in power costs [Scully 1990]. 

The current density influence on the process of hydrochloric acid electrolysis seems to be varied. On the one hand, the rise of current density i followed by increase in both expenditure of electric power and chlorine cost, - since it will cause an increase in operation expenses, in particular, expense of cold for cooling HCl before electrolysis. On the other hand, the production rated at the work at increased current densities requires lesser investments. It should be taken into account also that while increasiiig the current density from 1 to 4 kA/m. the chlorine yield rises from 95,6 to 97% [4). 

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