Strong Acid System - Acid Cooling - Shell and Tube - Anodic Protection
September 21, 2001

Anodic Protection Reference Electrode Controller Power Unit Cathodes

Associated Links

---

Anodic Protection

Anodic protection depends on the formation of a passive film on a metal immersed in an electrolyte by the application of an anodic potential to the metal.  The passive state is defined as a state of increased corrosion resistance of metals and alloys caused by control of the anodic process through anodic polarisation.

The tubes and shell of the exchanger that are in contact with the sulphuric acid are maintained at the required potential in the passive region by measuring this potential versus a reference electrde and transmitting the value to the potential controller.  The controller has a set value corresponding to a chosen potential in the passive range.  Any difference between the set value and the measured potential causes the controller to signal the direct current power source to supply enough current to bring the vessel to the required passive potential.

An individual power source is provided for each control unit and this is sized to meet the requirement for the initial passivation procedure.   This may take approximately 4 hours on the drying tower acid system and 1 to 2 hours on the absorber acid system.  After reaching the passive state, the current requirements are very low, 1 to 2 amps on the absorber acid system and somewhat higher on the dry acid system.

The anodic protection system consists of the following components:

 

Reference Electrode

The reference electrodes are submerged in the acid.  These electrodes are the sensing devices for obtaining the metal potential of the acid wetted metal surfaces.  Usually it is the electrode at the acid inlet that is used for control purposes.

 

Controller

The controller displays and monitors the potentials of the reference electrodes.  The potential from the reference electrode is compared to the set point and a signal is transmitted to the power unit adjusting its output in order to maintain the reference electrode at the control point.

 

Power Unit

The power unit supplies the necessary current to the cathodes for passivation.  The output is controlled by the Controller.

 

Cathodes

The main and pin cathodes are submerged in the acid to provide the electrical contact for current to be supplied through the acid to the acid wetted surfaces of the cooler.