Knowledge for the Sulphuric Acid Industry
Sulphuric Acid on the Web
Introduction
General
Equipment Suppliers
Contractor
Instrumentation
Industry News
Maintenance
Acid
Traders
Organizations
Fabricators
Conferences
Used
Plants
Intellectual
Propoerty
Acid
Plant Database
Market
Information
Library
Technical Manual
Introduction
General
Definitions
Instrumentation
Plant Safety
Metallurgial
Processes
Metallurgical
Sulphur Burning
Acid Regeneration
Lead Chamber
Technology
Gas Cleaning
Contact
Strong Acid
Acid Storage
Loading/Unloading
Transportation
Sulphur
Systems
Liquid SO2
Boiler Feed Water
Steam Systems
Cooling Water
Effluent Treatment
Utilities
Construction
Maintenance
Inspection
Analytical Procedures
Materials of Construction
Corrosion
Properties
Vendor Data
DKL Engineering, Inc.
Handbook of Sulphuric Acid Manufacturing
Order
Form
Preface
Contents
Feedback
Sulphuric Acid
Decolourization
Order Form
Preface
Table of Contents
Process Engineering Data Sheets - PEDS
Order
Form
Table of Contents
Introduction
Bibliography of Sulphuric Acid Technology
Order Form
Preface
Contents
Storage/Loading/Unloading -
Storage Tanks - Anodic Protection
April
2, 2007
Anodic Protection Theory of Operation Cathodes Reference Electrodes Controller Power Stage |
Associated Links |
Early in 1976, Magna Corporation and Chemetics recognized that it would be
beneficial if a tank protection system could be designed that would reflect the
significant field experience gained by Chemetics from the anodically protected
acid cooler. The result was a tank protection system to achieve a minimum
corrosion allowance of less than 3 mpy.
Anodic protection is effective in minimizing corrosion and preventing hydrogen
grooving of steel. At the same time, the quality of the acid is improved
with respect to iron content which is an important criterion for some acid
purchasers.
The anodic protection system consists of the cathodes, reference electrodes, controller and power stage.
Anodic protection depends upon the formation of a passive film on a metal immersed in an electrolyte by the application of a positive potential to the metal. The passive state is defined as a state of increase corrosion resistance of metals and alloys through anodic polarization. The inside of the tank that is in contact with the sulphuric acid is maintained at a predetermined potential against a reference electrode, and transmitting the potential value back to a controller. The control circuit compares this potential with a preset value and actuates the power stage to supply current to the tank via cathodes suspended in the acid from the tank roof. In this manner the potential of the tank is maintained within the passive range.
Tests show that the passive film will remain essentially intact when the tank is
drained of acid. Consequently, when the tank is refilled the full
protection of the original passive film is very rapidly recovered. It is
when the tank is washed with water that the passive film is completely
destroyed.
The cathode is basically a hollow tube which extends down to within 30 mm (1 ft)
of the tank floor. The cathodes are inserted through special flanged
openings in the roof of the tank. The cathode is electrically insulated
from the flanged connection. The number of cathodes required depends on
the capacity and the diameter of the storage tank. The cathode is
internally pressurized and a pressure gauge is used to indicate any perforation
of the cathode wall.
Reference electrodes are either inserted through nozzles in the roof or onto the
tank side wall. The roof mounted electrodes consist of a short reference
electrode welded to the end of a support pipe. The support pipe is flanged
and mounted onto a nozzle on the roof of the tank.
Side wall mounted electrodes are mounted using an acid seal similar to the ones
used on Chemetics acid coolers. This method of installation is
mechanically simpler and easier to install.
There are at least three reference electrodes installed in a tank. The are
positioned so that they will indicate when the tank is uniformly passivated.
The electrode will also cause a alarm to sound and shutdown the system is if the
electrode should malfunction for any reason. The electrode will also
shutdown the system if a low level is reached in the tank.
The function of the controller is compare the tank potential fed back from the
controlling reference electrode with a preset value and to control the current
supply from the power stage to the cathodes. The controller indicates the
current flow to the cathodes and the potential of the tank wall measured by the
control reference electrode and the two auxiliary electrodes. In addition
various fault conditions are indicated on the display panel of the controller.
The power stage consists of a step down transformer and a silicon rectifier
The unit is normally immersed in oil for cooling and also as a means of
protection against the corrosive atmosphere often found near acid plants.
The power output to the cathode is an appropriate DC current for the tank size,
acid strength and temperature. Once passivation has been achieved in the
tank, the power consumption is only a few hundred watts.