Operational Area

.

Anodic Corrosion Protection

The anodic corrosion protection method uses passivity as corrosion protection. Passivity is an electrochemical surface quality, identifies by a highly reduced reactivity inside an existing medium. Metallic materials can be protected against corrosion by means of anodic polarization, provided they are passivatableinside the attacking electrolyte and that their corrosion potential is located in the area of active dissolution. This is why the anodic corrosion protection is applicable only to passivatable systems.

The following figure shows the typical current density-potential curve for anodic corrosion protection, in combination with the influence of alloying elements on the passivation behaviour of Cr-Ni steels in concentrated sulphuric acid at temperatures up to 160C.

typical current density-potential curve for anodic corrosion protection

UR  = aktives Ruhepotential
UP  = Passivierungspotential
UA  = Aktivierungspotential
UD  = Durchbruchspotential
iP    = Passivierungsstromdichte
iA    = passive Auflösungsstromdichte

Application - Examples:

 

 

  • For sulphiric acid heat-exchangers and air-coolers
  • For caustic soda evaporators
  • For caustic soda tanks

 


For Sulphuric Acid Heat-Exchangers

By means of the anodic internal protection, acids of various concentrations at temperatures of up to 160°C from the production of sulphuric acid, can be  treated, including heat recovering and dressing of sulphuric acid waste. Without the anodic corrosion protection, corrosion damages occur, for example in sulphuric acid coolers, therefore the availability of such plants is highly limited. These damages in coolers and heat exchangers are avoided when using the anodic corrosion protection. Highly precise and permanent potential measuring is also required for corrosion protection units. For this purpose we have designed a special reference electrode, ensuring the highest level of measuring precision. As constantly alternating operating conditions, i.e. concentration and temperature, must be presumed in the sulphuric acid production, we are using potential controlled prtective current units only which are keeping up the protective potential, which was fixed at the start-up operation, during all operating conditions. With today`s advanced technology, shell and tube heat exchangers, with acid flow around the tubes of any length, can be protected against corrosion without any problems. Air-coolers for sulphuric acid can also be protected in the anodic way, however, their tube length are limited. Sulphuric acid pipes, connected with the heat exchangers, can also be integrated into the protection without any problems. It is certainly of great interest for the plant operator to have the opportunity of using cheaper austenitic materials in connection with the anodic corrosion protection.

Air-Cooler

Air-Cooler - anodic internal corrosion protection


Test-Samples

Test-Samples Left side without anodic protection and right side with anodic protection Left side without anodic protection and right side with anodic protection.
The time of install about 1 year at 100°C / 98,5% H2S04.


Shell and Tube Heat-Exchangers Pipe Cathodes and Reference Electrodes

 

Shell and Tube Heat-Exchangers Pipe Cathodes and Reference Electrodes - 1

 

Shell and Tube Heat-Exchangers Pipe Cathodes and Reference Electrodes - 2


For Caustic Soda Evaporators and Tanks

The application of anodic corrosion protection in the aluminium industry both for storing and like use during concentration of caustic soda has brought important economic advantages to the plant operators, some of which are listed below:

The anodic corrosion protection avoids pitting and stress corrosion with considerable reduction of surface corrosion.

Likewise anodic corrosion protection permits the use of commercial carbon-steels as well as subsequent weldings on protected objects while omitting unstressed heat treatment. With older objects, anodic corrosion protection stabilizes pre-cracks, stress-cracks and roll-in surfaces by oxydic incorporations. Even surfaces for apparatus, tanks and pipes are created and crystal growth is inhibited. By means of the anodic corrosion protection, the growth of aluminium silicate deposits is crucially reduced, simultan-eously the time intervals for acidifying are extended, and heat transmission is improved. Further advantages are the improved quality of the caustics through reduction of oxydic contents, the omission of nondestructive material tests by activation of the surfaces and weldings.

Finally maintenance and inspection test intervals are reduced to a minimum. Anodic corrosion protection offers at the same time maximal operational safety, and a saving in operation costs to the plant operator.

Caustic Soda Evaporator with Potential Mesuring

 

anodic corrosion protection for Caustic Soda Evaporator with Potential Mesuring

Caustic Soda Tanks with Pipe Cathodes

 

anodic corrosion protection for Caustic Soda Tanks with Pipe Cathodes

 

anodic corrosion protection for Caustic Soda Tanks with Pipe Cathodes - 2


.
.

xxnoxx_zaehler