Chemical plant, anodic protection

Anodic Protection of Chemical Plant 21.4.1 Special Features of Anodic Protection... 

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. 

Cathodic protection is commonly employed in many chemical plants depending on the compatibility of the process medium with the CP system. Sacrificial anodes may produce unwanted corrosion products or permanent anodes may generate chlorine and other gases and thus affect the quality of the medium stored or transported in the vessels, piping and so forth. Cathodic protection of the most... 

For these reasons, the cathodic corrosion protection of chemical plant parts is restricted to neutral solutions, service water, and alkalis. Various types of anodes are available for the protection of container interiors. Platinized disk electrodes, bar anodes, and titanium basket anodes are used. They are now supplied ready for installation and are designed in such a way that, in continuous operation, they can emit 8-10 A at a current density of 6-8 A/dm. ... 

Anodic inhibition of stress corrosion cracking was first provided on an industrial scale in a large plant for the production of hydrogen by electrolysis of potassium hydroxide solution. After preliminary trials on a laboratory scale, the chemical industry s first anodically protected large-scale plant, a sodium hydroxide solution evaporator with a capacity of 142 t, was put into operation in 1968. Since then, plants have been equipped in the same way (Grafen 1971). 

The potential control in anodic protection installations has two functions. First, the potential must be measured and compared to a desired preset value. Second, a control signal must then be sent to the power supply to force the dc current between the cathode and vessel wall. In early systems, this control function was done in an ON-OFF method because of the high costs of electronic drcuitiy. The recent progress in power electronics has resulted in all systems having a continuous proportional-fype control. Packaging these electronic components occasionally involves special requirements because most of the installations are made in chemical plants. Explosion-proof enclosures are sometimes required, and chemically resistant enclosures are necessary in other installations. 

Several examples of cathodic protection of aluminum equipment in chemical plants, as well as a preference for saoificial anodes of zinc or alununum-zinc alloy, are discussed in Ref 17. Such protection is most successfiil in electrolytes in the pH range of 4 to 8.5— the so-called neutral range (refer to Chapter 2). The cathodic protection of aluminum stractures is reviewed in Ref 18, which supports general experience that cathodic protection is effective in preventing or greatly reducing several types of corrosion attack. 

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