The storage tank commonly made from Carbon steel and is a large container for storing crude oil, water or other corrosive liquids. The effects of corrosion on storage tanks include premature failures and disruptions in service during repairs.
Corrosion is a naturally phenomenon commonly defined as the deterioration of metal because of a reaction with its environment. Corrosion occurs as a result of an electrochemical reaction driven by a potential difference between two electrodes, an anode and a cathode, connected by an electrolyte Water or Soil.
If corrosion isn’t mitigated, dangerous and expensive damage can be the result.
Cathodic protection is the most common electrochemical technique used to prevent corrosion on buried metallic structure coating or not to the soil.
Protection is accomplished by applying a direct current to the surface of the metal, which causes the electrochemical potential of the structure to shift from a corroding state to a polarised and non-corroding state. Cathodic protection intent to polarize our structure to a minimum potential of -850 mv, for carbon steel.
The Cathodic protection of the tank refer to Internal and External Cathodic protection. External cathodic protection refer to buried tank or bottom tank cathodic protection.
- Protection is achieved by connecting the protected structure to a sacrificial anode, which is placed close to the protected structure.
- Sacrificial anodes are made from active metals such as zinc, aluminum, or magnesium.
- CP current is created by the potential difference between sacrificial anodes and the protected structure.
- The type of anode used depends on electrolyte resistivity and the chemical compositions of the electrolyte to which the substrate is exposed.
- Aluminum anodes preferred if Solution resistivity is less than 2 Ohm*m and up to .
- Zinc alloy cannot use if the resistivity of the electrolyte is higher than30 Ohm*m
- Magnesium alloy should not be used if the resistivity of the electrolyte is higher than 150 Ohm*m
- Wirth ICCP we can protect our structure by connecting to an anode bed through a transformer rectifier (TR). The anode bed is buried anodes that are electrically connected and surrounded by backfill material trying to reduce their resistance to the earth. The anode bed placed below the tank our surround the tank
- The main component of ICCP is the TR, which forces the current to flow from the anodes to the protected structure.
- For tank protection where access is limited for anodes replacement after construction, non-soluble anodes made of mixed metal oxide, or polymer material commonly used
- The type of anodes used depends on the chemical composition of the electrolyte, to which the substrate is exposed and the area to be protected.
- TR install near the tank or on Safe areas.
- Anode junction boxes install near TR or to our tank.
- Anodes for impressed current system are made of MMO
- Reference electrode selected to install as far as we can from the anodes
- Test station used to measure potential polarization. Test station installed external of tanks and at close proximity
- Test result can monitored remotely.
External Cathodic Protection (ECP)
External corrosion of tank bottoms is a significant problem for tank owners. Corrosion professionals tasked with protecting these structures should consider multiple factors. One thing is clear: proper installation of an impressed current cathodic protection system plays an important role in reducing corrosion and extending the service life of the tank bottom.
For new tank construction (and many tank bottom replacements), state-of-the-art corrosion prevention requires a clean, chloride-free sand bedding combined with a dedicated impressed current anode cathodic protection system directly under the tank. The use of oil sand, crushed stone, asphalt or other materials directly under the tank bottom should be avoided, as these hinder effective cathodic protection. The vast majority of new tank construction projects utilize mixed metal oxide (MMO) impressed current anode systems.
Two primary cathodic protection configurations are used in tanks worldwide.
The field-erected anode grid configuration utilizes MMO ribbon anodes field cut into strips and laid out in parallel. Titanium ribbon conductor bars are also field cut and laid perpendicular to the MMO anodes. The MMO ribbon and titanium conductor bars are field tack welded at the intersections. Separate power cables are then tack welded to the titanium conductor bars and the cables are fed back to a junction box.
The other system based on linear anodes in concentric rings. With concentric ring anode system minimize the field cutting or welding, the linear anodes supplied factory assembled, tested and ready to be laid into position prior to backfilling. The anode is typically backfilled within a pre-packaged tube filled with a high quality carbon backfill. This enhanced backfill protects the anode during installation, keeps the anode weighted and in place, improves performance and reduces system resistance. For tank replacements where there is very little space between the tank bottom and the anode, a sand backfill can replace the carbon backfill to help ensure that the anode does not short due to contact with the tank bottom. Linear anodes reduces installation time and helps ensure a long, reliable life.
Also a galvanic anodes protection system can use but need to take special concern to ensure design life of system to be equivalent with the design life of the tank. Anodes replacement it is only available only on the perimeter of the tank after tank installation.
Internal Cathodic Protection (ICP)
Internal cathodic protection is a type of cathodic protection that is applied to internal structures like process vessels and storage tanks. It can be used to evaluate the levels of corrosion protection on almost all types of vessels or tanks, such as on grade above ground tanks for storage and process drums.
Thus, internal cathodic protection cannot be utilized to protect structures from atmospheric corrosion, but can be highly efficient in preventing stress corrosion cracking (SCC).
The structures that benefit from internal cathodic protection include interior surfaces of water circulating and storage tank systems.
Corrosion control on tanks internal can be achieved, by galvanic anodes mainly use Zinc and Magnesium anodes or ICCP systems with non soluble anodes (MMO and Platinized).
A variety of configurations are available for suspending, hanging or attaching anodes to the tank for internal protection. Several factors influence the CP system design including water quality, tank coating quality, desired anode system design life, the availability of power, freeze protection/potential and more.
Testing CP Systems
Tanks need to testing potential level against a reference electrode. For internal protection a portable reference electrode may suspended from the roof of the tank to take potential measurements or permanent reference electrodes can be installed at select locations.
For tank bottoms systems, permanent Copper-Copper sulfate reference electrode calibrated and installed in strategic locations below the tank. In some cases, dual reference electrodes such as both zinc and copper-copper sulfate may install.
In addition to the fixed reference electrodes where life may not extend to CP system design life due to Copper Sujphate substitute, it is strongly recommended to provide a reference electrode perforated conduit below the tank bottom to allow sliding of a calibrated reference electrode inside the tube and take potential readings.
Problems Created by Cathodic Protection
For coated tanks, where the applied coating quality is poor, cathodic disbondment may occur due to high CP levels. Higher temperatures may also promote cathodic disbondment. High pH environments are also a concern in terms of stress-corrosion cracking. In such cases, the polarized potential of the tank must be kept at a minimum value of -850 mV.
SYCHEM SA. working more than 30 years in Cathodic protection doing design, supply materials, installation and commissioning’ trying to deliver some of most complex project in the world, on budget and on time for our clients.