Anode maintenance

The benefits of anode maintenance are longer tank life, less rust buildup, and savings on costly changeovers. However, a cost-benefit analysis may show the cost of replacing anodes could exceed the cost of increased life expectancy or the cost of water heater replacement. Increased life expectancy without increased cost has not been shown to be technically feasible. It is also not known how much life is gained by the replacement of water heater anodes as the water heater can fail because of other reasons such as heating element failure. 

Figure 7 - Outline of Cell Cleaning and Anode Maintenance Procedures...

Anode maintenance used to take 3 days for 36 cells, but it is now possible to clean 40 cells in 2 days. This practice helped to minimize any significant extension of the overall maintenance cycle. The history of the changes in the anode maintenance procedure is listed below. 

In 1997, the Hikoshima smelter carried out an expansion of the cellhouse, which then was the bottle neck of production, and increased its production fiom 59,200 t/y to 71,000 t/y. The major features of this expansion project were increasing the number of cells, increasing the working efficiency of the cells and raising the current density. We plan to cany out further improvements, such as anode maintenance while energizing the cells, to establish ourselves as a cost competitive smelter in the world. 

Acid Coolers. Cast Hon trombone coolers, once the industry standard (101), are considered obsolete. In 1970, anodically passivated stainless steel sheU and tube acid coolers became commercially available. Because these proved to have significant maintenance savings and other advantages, this type of cooler became widely used. Anodic passivation uses an impressed voltage from an external electrical power source to reduce metal corrosion. 

Hydrogen is involved in cathodic protection with magnesium anodes on account of the high contribution of self-corrosion. This must be considered in its use in closed containers, e.g., boilers. In enamelled boilers there is no danger from deflagration of the oxy-hydrogen gas under normal service conditions [2] however safety requirements must be observed [28,29], particularly with routine maintenance work. 

Protection with impressed current, with galvanic anodes, and a combination of both processes is used for marine structures and offshore pipelines. Their properties, as well as their advantages and disadvantages, are given in Table 16-1. The protective measures must be optimized for every structure. In the impressed current protection of offshore platforms, for example, the difficulties of maintenance and repair will be of major importance, whereas in harbor installations these problems can be... 

New types of anodes have been developed and tested as shown in Fig. 16-5 to improve the possibility of maintenance and repair. They can be lifted onto a ship and repaired. The connecting cables are also replaceable. In shallow water, the anchorage must be accurately calculated because considerable dynamic stressing can occur in heavy seas. The ocean floor must be suitable for long-term anchorage. No supply ships must anchor in the area around the platform. This requirement alone often prevents the installation of impressed current anodes since the operator does not wish or is not able to restrict himself to these conditions. 

Carbon steels can be anodically protected in certain salt solutions. This involves mainly products of the fertilizer industry such as NH3, NH4NO3 and urea. Anodic protection is effective up to 90°C [26]. Corrosion in the gas space is suppressed by control of pH and maintenance of a surplus of NH3. 

Metals immersed or partly immersed in water tend to corrode because of their thermodynamic instability. Natural waters contain dissolved solids and gases and sometimes colloidal or suspended matter all these may affect the corrosive projjerties of the water in relation to the metals with which it is in contact. The effect may be either one of stimulation or one of suppression, and it may affect either the cathodic or the anodic reaction more rarely there may be a general blanketing effect. Some metals form a natural protective film in water and the corrosiveness of the water to these metals depends on whether or not the dissolved materials it contains assist in the maintenance of a self-healing film. 

Magnesium anodes suspended inside a galvanised hot-water tank and in electrical connection with it afford cathodic protection to the zinc, the alloy layer and the steel, at high temperatures as well as in the cold. The magnesium is eventually consumed but it is probable that in the interim a good protective scale will have formed on the inside of the tank, so that the magnesium anode will then no longer be necessary. One of the difficulties of this method, however, is the maintenance of a sufficiently even current distribution over the inside of a tank to protect the whole surface, especially in waters of low conductivity. The method is therefore unlikely to be applicable to soft waters. 

Numerous materials fall into the category of electronic conductors and hence may be utilised as impressed-current anode material. That only a small number of these materials have a practical application is a function of their cost per unit of energy emitted and their electrochemical inertness and mechanical durability. These major factors are interrelated and —as with any held of practical engineering—the choice of a particular material can only be related to total cost. Within this cost must be considered the initial cost of the cathodic protection system and maintenance, operation and refurbishment costs during the required life of both the structure to be protected and the cathodic protection system. 

There are obviously situations which demand considerable over-design of a cathodic protection system, in particular where regular and efficient maintenance of anodes is not practical, or where temporary failure of the system could cause costly damage to plant or product. Furthermore, contamination of potable waters by chromium-containing or lead-based alloy anodes must lead to the choice of the more expensive, but more inert, precious metal-coated anodes. The choice of material is then not unusual in being one of economics coupled with practicability. 

In using metallic Pb as an anode the formation and maintenance of a hard layer of PbOj is essential, since it is the PbOj that is the actual inert anode, the Pb acting both as a source of PbOj and an electrical conductor. PbOj is relatively insoluble in seawater and its dissipation is more usually associated with mechanical wear and stress than electrochemical action. 

Advantages No external source of power is required installation is relatively simple the danger of cathodic protection interaction is minimised more economic for small schemes the danger of over protection is alleviated even current distribution can be easily achieved maintenance is not required apart from routine potential checks and replacement of anodes at the end of their useful life no running costs. 

Nail sickness Nail sickness is chemical decay associated with corroded metals in marine situations. Chemical degradation of wood by the products of metal corrosion is brought about by bad workmanship or maintenance, or unsuitable (permeable) timber species, all of which permit electrolyte and oxygen access which promotes corrosion. Chemical decay of wood by alkali occurs in cathodic areas (metal exposed oxygen present). Softening and embrittlement of wood occurs in anodic areas (metal embedded oxygen absent) caused by mineral acid from hydrolysis of soluble iron corrosion products. 

The potentiostat has supplied an experimental tool for the study of anodic protection. The elucidation of passive behaviour made possible by poten-tiostatic anode polarisation curves allowed investigators to determine the conditions necessary for maintaining a metal in a stable passive condition by provision of a suitable environment, addition of cathodic alloying elementsand/or maintenance of the required potential by means of external anodic polarisation - . ... 

When the boiler is placed back online, certain types of anodic inhibitors (which are generally employed to act as polishing treatments in the maintenance program) also may prove beneficial in further strengthening the passive film. 

In these simpler types of boiler plants, corrosion protection generally is provided solely by the use of chemical treatments containing anodic passivators and alkali. Under conditions in which the (oxygen-containing) MU water requirement is expected to be very low, no specific oxygen scavenger is typically employed. This is often not the case in practice, however, for various reasons such as inadequate preventative maintenance, poor BD practice, the infiltration of air or oxygen, or other such reason. 

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