Cathode current density

The reaction mixture is filtered. The soHds containing K MnO are leached, filtered, and the filtrate composition adjusted for electrolysis. The soHds are gangue. The Cams Chemical Co. electrolyzes a solution containing 120—150 g/L KOH and 50—60 g/L K MnO. The cells are bipolar (68). The anode side is monel and the cathode mild steel. The cathode consists of small protmsions from the bipolar unit. The base of the cathode is coated with a corrosion-resistant plastic such that the ratio of active cathode area to anode area is about 1 to 140. Cells operate at 1.2—1.4 kA. Anode and cathode current densities are about 85—100 A/m and 13—15 kA/m, respectively. The small cathode areas and large anode areas are used to minimize the reduction of permanganate at the cathode (69). Potassium permanganate is continuously crystallized from cell Hquors. The caustic mother Hquors are evaporated and returned to the cell feed preparation system. 

Metal Anode Diaphragm Cathode Cell feed g/L Electrolyte, g/L Temperat ure, °C CeU voltage, V Cathode current density, A/m Energy requirement, kWh/kg Current efficiency, %... 

The fall in reduction of area and the occurrence of internal cracks are a measure of the corrosion damage. There exists a clear correlation with cathodic current density in which a slight inhibition due to O2 and stimulation by CO2 can be recognized. The susceptibility is very high in the range of cathodic overprotection and is independent of the composition of the medium. 

The action of effects in the environment and cathodic current densities on ac corrosion requires even more careful investigation. It is important to recognize that ac current densities above 50 A m can lead to damage even when the dc potential is formally fulfilling the protection criterion [40]. 

The anodic and cathodic current densities are inversely proportional to the length of the anode bed or the well casing. Since the length of the casing is usually greater by a factor of 10 than the length of the anode bed, the current density of the... 

In uniform corrosion the superficial or geometrical area of the metal is used to evaluate both the anodic and cathodic current density, although it might appear to be more logical to take half of that area. However, surfaces are seldom smooth and the true surface area may be twice to three times that of the geometrical area (a cleaved crystal face or an electropolished single crystal would have a true surface area that approximates to its superficial area). It follows, therefore, that the true current density is smaller than the superficial current density, but whether the area used for calculating /, and... 

Fig, 1.24 Tafel lines for a single exchange process. The following should be noted (a) linear f-log I curves are obtained only at overpotentials greater than 0-052 V (at less than 0-052 V E vs. i is linear) b) the extrapolated anodic and cathodic -log / curves intersect at tg the equilibrium exchange current density and (c) /, and the anodic and cathodic current densities... 

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. 

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. 

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 ). 

The most recently developed anode for the cathodic protection of steel in concrete is mixed metal oxide coated titanium mesh The anode mesh is made from commercially pure titanium sheet approximately 0-5-2mm thick depending upon the manufacturer, expanded to provide a diamond shaped mesh in the range of 35 x 75 to 100 x 200 mm. The mesh size selected is dictated by the required cathode current density and the mesh manufacturer. The anode mesh is supplied in strips which may be joined on site using spot welded connections to a titanium strip or niobium crimps, whilst electrical connections to the d.c. power source are made at selected locations in a suitably encapsulated or crimped connection. The mesh is then fitted to the concrete using non-metallic fixings. 

Features The rate of deposition is low compared with that of vacuum evaporation, and is affected by variables such as pressure and temperature of coating-chamber atmosphere, arc voltage, cathode current density and geometry of cathode and A/j. A high vacuum is not essential, particularly where an inert gas can be used. The A/, disintegration rate is affected by the atomic weight of coating-chamber gas. 

Deposition of nickel at rates up to 1 mm/h in the concentrated solution is described by Kendrick . If pure nickel anodes are operated at a current density between 0-5 and 1-OA/dm in suiphamate solutions, a substance which behaves as a stress reducer is produced continuously in sufficient quantity that the stress in deposits can be varied at will from compressive to tensile by adjusting cathode current density and solution temperature. This finding is exploited with the concentrated suiphamate solution in the Ni-Speed processand in a further development cobalt is added to give deposits of... 

The reaction rate per unit area i. For a corroding metal the partial anodic and cathodic current densities cannot be determined directly by means of an ammeter unless the anodic and cathodic areas can be separated physically, e.g. as in a bimetallic couple. If the metal is polarised a net current 4 for cathodic polarisation, and for anodic polarisation, will be obtained and can be measured by means of an ammeter. 

Skold and Larson" in studies of the corrosion of steel and cast iron in natural water found that a linear relationship existed between potential and the applied anodic and cathodic current densities, providing the values of the latter were low. However, the recognition of the importance of these observations is due to Stern and his co-workerswho used the term linear polarisation to describe the linearity of the rj — i curve in the region of E o , the corrosion potential. The slope of this linear curve, AE — AJ or Af - A/, is termed the polarisation resistance, / p, since it has dimensions of ohms, and this term is synonymous with linear polarisation in... 

Electrolyte-sulphuric acid (5% wt.%) plus an inhibitor (0-5kgm ) such as diorthotolyl thiourea, quinoline ethiodide or /3-naphthol quinoline. The temperature should be 75°C, the cathode current density 2000 Am and the time of cathodic polarisation 3 min. The anode should be carbon or lead. If lead anodes are used, lead may deposit on the specimens and cause an error in the weight loss. If the specimen is resistant to nitric acid the lead may be removed by a flash dip in 1 1 nitric acid. Except for this possible source of error, lead is preferred as an anode, as it gives more efficient corrosion product removal. 

Anode Shield protective covering of insulating material placed on a painted structure in the immediate vicinity of the anode to reduce the cathodic current density in that area, thus preventing the development of excessive alkalinity and stripping of the paint see Saponification). 

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