Hull cell cathode

The Hull cell cathode has a continuous variation of current density along its length, and there are equations which give the primary current density at any point not too near the end. If the local thickness is measured at two points for which P is known, Tcan be calculated. The real current distribution is a function of cathode and anode polarisation as well as of the resistance of the electrolyte. The metal distribution ratio will be... 

Explain the basis for the operation of the Hull cell in other words, why it is expected that different points on a slanted cathode will be electroplated to different degrees. 

Hull cell — The Hull cell is a trapezoidal cell (see Fig. 1) and is used for screening purposes of electrolytes. Due to the asymmetric shape the current varies over a wide range from the high current density (hed) to the low current density (led) edge. Thus the effect of the current density can be checked by visual inspection of the cathode after the plating process. A typical cell volume is 250 ml. A useful formula for the estimation of the local current density is i/im = -2.33 log(z) - 0.08, where im is the average current density and z the normalized distance from the hed edge. 

Current density distributions in electrochemical systems are governed by Laplace equation (Newman, 1991) with linear/nonlinear boundary condition at the boundaries (electrodes). Consider a Hull cell (see Fig. 6.13) in which a metal is deposited at the cathode (Subramanian and White, 1999). 

The Hull-cell (fig. 3.19) is one of the standard cells used in conventional plating industry to control the process parameters. As can be seen on the figure, the current density along the cathode varies in a wide range, from zero to infinity for a primary distribution, such that with one experiment the influence of the current density on the process can be investigated. 

Fig, 3.195 Hull-cell geometry and primary distribution along the cathode. 

Electrodeposits may show an uneven thickness or localized excessive current density effects such as dendrite formation, (i iV) or oxide/hydroxide formation due to localized pH increases. (These effects may be examined in devices such as the Hull cell, (section 8.1.1) which exhibits a deliberate variation in cathode current density.)... 

The inclined cathode of the Hull cell enables a controlled variation of the current density over the cathode surface to be achieved in a single experimenL Typically, a constant cell current in the range 2 to 5 A is used for chromium plating or high-speed processes. 10 A may be preferred. A nomogram is often used to estimate the local cunent density which varies with the distance x along the cathode (measured from the end nearest the anode) in the following... 

With an applied current of 2 A, the variation in cathode current density is from approximately 2,5 to 85mAcm which covers the working range of most electroplating baths. The Hull cell is also useful as an empirical development tool, e.g. for rapid screening tests on addition agents. The fundamental cell has been modified, for special purposes, in numerous ways. e.g. a bottomless cell may... 

Fig. 8.2 The Hull cell. Various standard sizes are available, a common version being the 267 cm cell a 2 g addition of chemicals corresponds to 1 oz/US gallon in the process bath, (a) Plan view showing the internal dimensions of the American version, (b) Current density ranges along the cathode.

Electrochemical protection can be achieved by forming an electrolytic cell in which the anode material is more easily corroded than the metal it is desired to protect. This is the case of zinc in contact with iron (Fig. 16.11) in this example there is a sort of cathodic protection. Protection of ship hulls, of subterranean pipeline tubings, of oil rigs, etc. is often done using sacrificial anodes that are substituted as necessary. The requisites for a good sacrificial anode are, besides its preferential corrosion, slow corrosion kinetics and non-passivation. Sacrificial anodes in use are, for this reason, normally of zinc, magnesium, or aluminium... 

The steel hulls of ships are subject to corrosion by the reaction of water and oxygen with iron to form rust. Although painting the surface of the steel can provide some protection, even a small scrape that removes paint can allow corrosion to start. Many ships use blocks of zinc attached to the hull to protect the steel from corrosion. The zinc becomes the sacrificial anode of a cell, losing electrons, and going into solution, while the iron in the steel acts as a cathode, gaining electrons as water is reduced. As the cathode, iron does not corrode. 

Figure 11. Illustration of two alternative designs for the rotating cylinder Hull (RCH) cell, which allows the study of non-uniform current distribution on the cathode, under controlled mass-transport conditions. A anode, C cathode, IC insulating cylinder. Reproduced from Ref. 150 with kind permission of Springer Science and Business Media, and with permission from Ref. 95, Copyright (1996) The Electrochemical Society.

Microorganisms are more likely to cause localized corrosion than general corrosion because of the differential oxygen cell. In most cases the localized attack was observed beneath macrofouling layers. Corrosion of copper, steel, and aluminum anodes occur when connected to cathodes on which biofilms grow. Unexpectedly, rapid localized corrosion of steel bulkheads in marine harbor environments and of ship hull plating of several tankers has been observed (22). 

Galvanization is one example of cathodic protection, the process by which a metal is protected by being made the cathode in what amoimts to a galvanic cell. Another example is the use of zinc or magnesiitm bars to protect underground storage tanks and ships. When a steel tank or hull is connected to a more easily oxidized metal, corrosion of the steel is prevented. 

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