Pipelines are used for transporting water, petroleum products, natural gas, and other utilities. There’s a huge network of piping systems used in every country all over the world. Pipelines may be onshore or offshore and are subject to corrosion in both cases.
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 pipelines where the applied coating has failed or been damaged exposing bare pipeline metal 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.
Galvanic Anode Cathodic Protection (GACP)
- In this type of CP, protection is achieved by connecting the protected structure to a sacrificial anode, which is placed close to the protected structure.
- Sacrificial anodes for onshore pipeline are made from active metals such as zinc 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.
- 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
Impressed Current Cathodic Protection (ICCP)
- Wirth ICCP we can protect our structure by connecting to an anode bed through a transformer rectifier (TR).
- The main component of ICCP is the TR, which forces the current to flow from the anodes to the protected structure.
- The anode bed is a series of buried anodes that are electrically connected for onshore application surrounded by backfill material trying to reduce their resistance to the eart. The anode bed should be placed remotely from the protected structure.
- The now days two main types of anodes commonly used
- semi-soluble anodes made of high silicon cast iron (HSCI),
- non-soluble anodes made of platinum, mixed metal oxide, or polymer material
- 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.
- Junction boxes used to connect our anode with TR
- Test station used to monitor structure potential level.
Cathodic Protection on Pipelines
- Cathodic protection intent to polarize a pipeline to a minimum potential of -850 mv, for carbon steel.
- Anode ground bed located remotely on different arrangement Horizontal / vertical and depth Deep well ground bed.
- Anodes connected with Transformer rectifier through junction boxes
- Reference electrode used to monitor the potential level.
- The polarized potential is to be measured through test stations, which are to be installed at the following locations along the route of pipeline:
- At frequent intervals
- At crossings with foreign structures
- At points of electrical isolation
- At some galvanic anode locations
- At casings
- Near sources of electrical interference
- At the location of stray current discharge to earth
Problems Created by Cathodic Protection.
In large pipeline networks, there are a lot of crossings, parallelism, and approaches, wherein the pipeline has its applied CP system. DC interference may occur between pipelines, accelerating corrosion. In order to overcome this problem, pipelines can be electrically coupled, either directly or through resistance.
For coated pipelines, 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 pipeline 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.