Corrosion is usually defined as the degradation of metals due to an electrochemical process. Corrosion is a natural process that can deteriorate metal structures and cause costly damage to your business.
An electrochemical process is defined as a chemical reaction involving the transfer of electrons between two electrodes in an electrochemical cell.
So, to understand the corrosion of a metal, it is essential to know about electrochemical cell or corrosion cells.
An electrochemical cell is a device that converts chemical energy into electrical energy or vice-versa. In an electrochemical cell, electron liberated by oxidation of a species at one electrode (anode) flows through an external conductor to a second electrode (cathode) where they are consumed by a reduction reaction. A common example of an electrochemical cell is a standard 1.5-volt cell which is used to power many electrical appliances such as TV remotes and clocks. Such cells capable of generating an electric current from the chemical reactions occurring in them care called Galvanic cells or Voltaic cells. Alternatively, the cells which cause chemical reactions to occur in them when an electric current is passed through them are called electrolytic cells.
The fundamental components in an electrochemical cell are Anode, Cathode, Metallic Path & Electrolyte. All these four components must be present for corrosion to occur.
Anode(-) – Metal at the anode is oxidized causing it to lose mass or corrode. Metal atoms (M) loses an electron to form Metal ions (M+)
Metallic path: The electron produced due to this reaction travelled to the cathode through the metallic path.
Cathode(+) – Reduction takes place at the cathode and electrons are consumed. M(+) is attracted to the negative charged electrons e(-)
Electrolyte: It is the electrically conductive solution (eg. Salt solution) that must be present for corrosion to occur. An electrolyte contains positively & negatively charged ions called cations & anions respectively. It is surrounded by the anode and cathode. Electron flow from anode to cathode through an electrolyte.
For eg. Let say one electrode is of Fe & it produces 2 electrons as it oxidized. Fe(s) —Fe(2+) +2e(-). Let other electrodes be of Cu. This electron then travels to the metallic path given (let in this case it is metallic wire). The electron then finds the Cu(2+) in the solution and the Cu is reduced to Cu metal. During the reaction, the Fe electrode will shrink in size, while the Cu electrode will become larger due to the deposited Cu that is being produced.
So, if these conditions & elements are present they accelerate the corrosion and are collectively referred to as the electrochemical cell. SO, corrosion occurs due to the formation of an electrochemical cell.
Now, natural gas having specifications as in Acess code of PNGRB is non-corrosive and hence internal corrosion in Natural gas pipeline is not a problem, however internal coating can improve the friction factor by smoothing the surface. The external corrosion of steel pipe is mitigated by the cathodic protection method.
Cathodic Protection: Cathodic protection is a method for preventing corrosion on submerged and underground metallic structures. Cathodic Protection (CP) is an electrochemical process where DC current is applied to a metal to slow or stop corrosion currents. When properly applied, CP stops the corrosion reaction from occurring. Cathodic protection works by placing an anode or anodes (external devices) in an electrolyte to create a circuit. Current flows from the anode through the electrolyte to the surface of the structure. Corrosion moves to the anode to stop further corrosion of the structure.
Cathodic protection is commonly used to protect numerous structures against corrosion, such as ships, offshore floaters, subsea equipment, harbours, pipelines, tanks; basically all submerged or buried metal structures. The technique is based on converting active areas on a metal surface to passive, in other words making them the cathode of an electrochemical cell.
External corrosion of the steel pipeline is mitigated by a combination of passive and active protection techniques. The passive protection technique consists of a suitable coating on the pipeline system. The active protection technique consists of a sacrificial anode type cathodic protection system. Anode for this purpose is Zn or Mg anode. Permanent Cathodic protection system along with all associated facilities would be designed for a life of 40 years. During the construction, the line would be protected by sacrificial anodes (temporary cathodic protection) for one year or till the commissioning of Permanent cathodic protection.
In essence, cathodic protection connects the base metal at risk (steel) to a sacrificial metal that corrodes in lieu of the base metal. The technique of providing cathodic protection to steel preserves the metal by providing a highly active metal that can act as an anode and provide free electrons. By introducing these free electrons, the active metal sacrifices its ions and keeps the less active steel from corroding.By supply of current, the potential of the metal is reduced, the corrosion attack will cease and cathodic protection is achieved. Cathodic protection can be achieved by either:
- Sacrificial anode cathodic protection
- Impressed current cathodic protection, often referred to as ICCP
- Sacrificial anode CP or Galvanic anode system: This makes use of a corrosive potential for different metals. Without CP, one area of the structure exists at a more negative potential than another, & corrosion results. If, however, a much less inert object is placed adjacent to the structure to be protected, such as pipeline, & a metallic connection( an insulated wire) is installed between the object and the structure, the object will become the anode and the entire structure will become cathode. That is, the new objects corrode sacrificially to protect the structure. Thus, the galvanic CP system is called a sacrificial anode CP system because the anode corrodes sacrificially to protect the structure. Galvanic anodes are usually made of either magnesium or zinc because these metals have higher potential compared to a steel structure.
- impressed current system: Impressed current cathodic protection uses the same elements as the galvanic protection system, only the structure is protected by applying a current to it from an anode. the anode and the structure are connected by an insulated wire, through a battery or rectifier such that the current flows from the anode through the electrolyte onto the structure, just as in the galvanic system. The main difference between the galvanic and impressed current system is that the galvanic system relies on the difference in potential between the anode and structure, whereas the impressed current system uses an external source to drive the current. The external power source is usually a rectifier that changes input AC power to the proper DC power level. The rectifier can be adjusted so that the power out can be maintained during the system life. Impressed current cathodic protection system anodes typically are high-silicon cast iron or graphite.
Limitation of CP: Cathodic protection has been used for years to protect structures that suffer long-term exposure to corrosive environments. But the installation of cathodic protection itself can be costly. The specific details of how structures are constructed can also add to the complexity and therefore cost of cathodic protection. In addition to this cost, the system also requires routine maintenance, including a periodic visual inspection. In the case of impressed current cathodic protection there is also the ongoing cost of electricity. Sacrificial anodes, in particular, have a limited amount of current available, are subject to rapid corrosion, and therefore have a limited lifespan. In summary, cathodic protection is a commonly used method of protecting steel structures, yet can be costly and require routine maintenance and replacement.
CGD has special problem for CP as compared to transmission pipeline because most of the city areas are refilled and reclaimed ones. The refilling gives different resistivity at different location thereby making selection of anodes or impress current difficult. With this kind of problem being faced in CP design in CGD. Therefore net effect is that the CP margins for cross country pipeline is reducing in general but the same is on increase in CGD.