Anodic Protection is an effective technique to protect steel tanks from corrosion and acid from iron contamination.
System Operation/Theory
Carbon steel in concentrated sulfuric acid exhibits solution potentials in the ACTIVE CORROSION zone. An external source of direct current moves the solution potential from the ACTIVE CORROSION zone into the PASSIVATION zone where corrosion rates are an order of magnitude lower.
The film formed and maintained by an anodic protection system limits the corrosion rate. Anodic protection is also effective in the wet/dry zone since the protective film remains for a finite amount of time after the acid level falls.
Initially a high level of current is required to form the passive film on the immersed vessel walls. Once the vessel is passive, the current required to maintain passivity is typically 1% to 5% of the passivation current.
Benefits of a Corrosion Service Anotection® system.
A properly applied anodic protection system will minimize general corrosion and prevent hydrogen grooving of all immersed surfaces. The added benefit of reduced Iron pick up will help maintain acid purity.
Fully automatic operation eliminates the need for operator intervention.
Maintenance is minimal and installation does not require vessel entry.
Major Components/Installation
Current Source:
The controlled current source can be installed next to the vessel or inside a near by control room. It rectifies and transforms single phase or three phase AC into low voltage DC current. The output is used to maintain the feedback reference electrode at a predetermined set potential. The current source is interlocked with a tank level sensor to turn the unit off at low levels in order to prevent sparking. Size of the current source is proportional to the size of the vessel and acid characteristics.
Cathodes:
Cathodes are used to return protective current from the immersed vessel surfaces. The number of cathodes required to protect a tank depends on the current source and vessel size. The cathodes utilized for our systems are solid metal cored with a maximum diameter of 0.5” and offer the following features:
Designed with a patented braid to prevent possible sparking.
Inserted into the vessel through nozzles on the roof (i.e. may be installed/replaced with the tank on line).
Light weight and may not require the use of a crane/hoist to install.
Entry into and welding inside of the vessel are not required for installation.
References:
References are used to measure the electrochemical solution potential and enable closed loop control of the protective current being discharged. The Anotection® anodic protection system utilizes a platinum reference electrode which is generally installed through the roof of the vessel and therefore requires no personnel entry for installation. At least two references are supplied per vessel but more may be necessary depending on the size of the vessel.
Remote Monitoring:
Anodic protection is not the primary focus of a plant. As plant personnel assignments and priorities change, the operation of the anodic protection system may be forgotten. Each system is therefore provided with an optional inexpensive telephone/internet based remote monitoring system that permits Corrosion Service to ensure valid system operation. We will contact the system weekly and notify the owner immediately on system fault.
Piping
The potential of stainless steel sulfuric acid piping can oscillate in and out of the active corrosion zone at elevated temperatures. An Anotection® anodic protection system can keep the electrochemical potential in the passive range & thus reduce corrosion. Piping system components are similar to that of a storage tank.
