6. Corrosion of pipeline
6.1 TYPES OF CORROSION
Basically, there are four ways corrosion can occur. Corrosion can occur through a chemical reaction or three general types of electrochemical reactions. The three general types of electrochemical reactions that occur depend on the cause of the potential difference between the anode and the cathode. This potential difference can be caused by differences in the environment, differences in the metal, or by external electrical sources of DC current. Understanding this principle leads to an understanding of the principles of operation of cathodic protection systems. Each of these three types of corrosion will be explained in detail, with examples of each. These three types are general corrosion, concentration cell corrosion (electrochemical cell caused by differences in the electrolyte), galvanic corrosion (electrochemical cell caused by differences in the metal), and stray current corrosion (electrochemical cell caused by external electrical sources).
6.1.1 General Corrosion.
This type of corrosion is chemical or electrochemical in nature. However, there are no discrete anode or cathode areas. This form of corrosion is uniform over the surface of the metal exposed to the environment. The metal gradually becomes thinner and eventually fails.
The energy state of the metal is basically what causes this reaction. Referred to as the “dust-to-dust” process, high levels of energy are added to the raw mmaterial to produce the metal. This high energy level causes an unnaturally high electrical potential. One law of chemistry is that all materials will tend to revert to its lowest energy level, or its natural state. After high levels of energy are added to the metal, when it is exposed to the environment (an electrolyte), it will tend to revert to its natural state. This process is normally extremely slow, and is dependent on the ion concentration of the electrolyte that it is exposed to. Only under very extreme conditions (acidic electrolyte) can this form of corrosion be significant. The corrosion rate for steel climbs drastically at a pH below 4, and at a pH of about 3 , the steel will dissolve.
General corrosion tends to slow down over time because the potential gradually becomes lower. Failures of pipelines or tanks would not quickly occur from this type of corrosion since no pitting or penetration of the structure occurs, just a general corrosion over the entire surface (except under very extreme circumstances where the metal could dissolve in an acid electrolyte). However, in nature, the metal is not completely uniform and the electrolyte is not completely homogeneous, resulting in electrochemical corrosion cells that greatly overshadow this mild form of corrosion.
6.1.2 Concentration Cell Corrosion.
This type of corrosion is caused by an electrochemical corrosion cell. The potential difference (electromotive force) is caused by a difference in concentration of some component in the electrolyte. Any difference in the electrolyte contacting the metal forms discrete anode and cathode regions in the metal. Any metal exposed to an electrolyte exhibits a measurable potential or voltage. The same metal has a different electrical potential in different electrolytes, or electrolytes with different concentrations of any component. This potential difference forces the metal to develop anodic and cathodic regions. When there is also an electrolyte and a metallic path, the circuit is complete, current flows, and electrochemical corrosion will occur. Soil is a combination of many different materials. There are also many different types of soil, and even the same type of soil varies greatly in the concentration of its constituents. Therefore, there is no such thing as truly homogeneous soil.
These soil variations cause potential differences (electromotive force) on the metal surface resulting in electrochemical corrosion cells. Liquids tend to be more uniform, but can vary in the concentration of some components such as
oxygen varies by depth and flow rates. Biological organisms are present in virtually all-natural aqueous environments, these organisms tend to attach to and grow on the surface of structural materials, resulting in the formation of a biological film, or biofilm. These films are different from the surrounding electrolyte and have many adverse effects. Following are examples of common forms of concentration cell corrosion.
18.104.22.168 Dissimilar Environment.
Pipelines tend to pass through many different types of soils. The metal exhibits different electrical potentials in different soils. The electrical potential in those soils determines which areas become anodic and which areas become cathodic. Since both the anode and cathode are electrically continuous and the electrolyte is in contact with both, current flows, resulting in oxidation and reduction reactions (corrosion and protection). The area of the pipeline or tank, which is the anode, corrodes. Since the ground tends to consist of horizontal layers of dissimilar soils, pipelines that traverse several layers of soil tend to be affected by this type of corrosion frequently. Water and oil well casings are prime examples of this type of electrochemical corrosion cell. Other examples are pipelines that go through areas of generally different materials such as rock, gravel, sand, loam, clay, or different combinations of these materials. There are over 50 general types of soil that have been characterized for corrosion properties. Each of the different types of soils has different soil resistivity values. In areas where the soil resistivity values vary greatly in relatively short distances, dissimilar environment corrosion cells are formed. These types of electrochemical corrosion cells are most serious when the anode is relatively small, soil resistivity is the lowest and the electrical potential difference is the greatest. Examples of corrosive soils are Merced (alkali) silt loam, Montezuma (alkali) clay adobe, muck, and Fargo clay loam.
22.214.171.124 Oxygen Concentration.
Pipelines or tanks that are exposed to an electrolyte with a low oxygen concentration are generally anodic to the same material exposed to an electrolyte with a high oxygen content. This is most severe when a pipeline or tank is placed on the bottom of the excavation, then backfill is placed around the remaining part of the structure. The backfill contains a relatively high amount of oxygen during the excavation and backfill operation. This can also occur when the metal is exposed to areas that have different levels of oxygen content.
126.96.36.199 Moist/Dry Electrolyte.
Pipelines or tanks that are exposed to areas of low and high water content in the electrolyte also exhibit different potentials in these different areas. Generally, the area with more water content becomes the anode in this electrochemical corrosion cell. This is most severe when a pipeline passes through a swampy area adjacent to dry areas or a tank is located in dry soil, but the water table in the soil saturates the tank bottom.
188.8.131.52 Non-Homogeneous Soil.
Pipelines or tanks that are exposed to an electrolyte that is not homogeneous exhibit different electrical potentials in the different components of the soil. This can occur in any soil that is a mixture of materials from microscopic to substantially sized components. The area(s) with the higher potential becomes the anode in this electrochemical corrosion cell. This is most severe when a pipeline or tank is placed in an electrolyte with components that cause large potential differences or where there are small anodic areas and large cathodic areas.
184.108.40.206 Concrete / Soil Interface.
Pipelines or tanks that are in contact with cement and exposed to another electrolyte exhibit different potentials in each area. The area not in contact with cement becomes the anode in this electrochemical corrosion cell. A pipeline or tank that is in contact with concrete and soil (or water) may be a very severe corrosion cell, because of the high potential difference of the metal in the two different electrolytes.
220.127.116.11 Backfill Impurities.
This is similar to the non-homogeneous soil concentration cells, except that the “backfill impurities” are materials that do not normally occur in the soil, but are foreign materials mixed into the electrolyte during or between the excavation and the backfill process. This can be any material that forms anodic or cathodic areas on the structure. It can also be an isolating material that forms different conditions in the electrolyte, or a metallic material which actually becomes an anode or cathode when in contact with the structure (galvanic corrosion).
18.104.22.168 Biological Effects.
Biological organisms may attach to and grow on the surface of a metal, causing a different environment that in some cases may be extremely corrosive to the metal. Most bacteria that have been implicated in corrosion grow best at temperatures of 15 oC to 45 oC (60 oF to 115 oF). These bacteria are generally classed by their oxygen requirements, which vary widely with species, and may be aerobic or anaerobic. Their metabolism products influence the electrochemical reaction by forming materials or films (slime) that act as a diffusion barrier, or change ion concentrations and pH. Some bacteria are capable of being directly involved in the oxidation or reduction of metal ions and can shift the chemical equilibrium that influences the corrosion rate. Aerobic bacteria form oxygen and chemical concentration cells, and in the presence of bacteria capable of oxidizing ferrous ions, further accelerate corrosion. Many produce mineral or organic acids that may also breakdown structure coatings. The breakdown products are then sometimes usable as food, leading to accelerated corrosion.
6.1.3 Galvanic Corrosion.
This type of corrosion is caused by an electrochemical corrosion cell developed by a potential difference in the metal that makes one part of the cell an anode, and the other part of the cell the cathode. Different metals have different potentials in the same electrolyte. This potential difference is the driving force, or the voltage, of the cell. As with any electrochemical corrosion cell, if the electrolyte is continuous from the anode to the cathode and there is a metallic path present for the electron, the circuit is completed and current will flow and electrochemical corrosion will occur.
22.214.171.124 Dissimilar Metals.
The most obvious form of this type of corrosion is when two different kinds of metal are in the electrolyte and metallically bonded or shorted in some manner. All metals exhibit an electrical potential; each metal has its distinctive potential or voltage (paragraph 2-4). When two different metals are connected, the metal with the most negative potential is the anode; the less negative metal is the cathode. An “active” metal is a metal with a high negative potential, which also means it is anodic when compared to most other metals. A “noble” metal is a metal with a low negative potential, which also means it is cathodic when compared to most other metals.
Dissimilar metal corrosion is most severe when the potential difference between the two metals, or “driving voltage,” is the greatest.
Examples of active metals are new steel, aluminum, stainless steel (in the active state), zinc, and magnesium. Examples of noble metals are corroded steel, stainless steel (in the passivated state), copper, bronze, carbon, gold, and platinum.
One example of this type of corrosion occurs when coated steel pipelines are metallically connected to bare copper grounding systems or other copper pipelines (usually water lines).
126.96.36.199 Old-to-New Syndrome.
This type of corrosion can also be rather severe. Steel is unique among metals because of the high energy put into the process of producing the steel . New steel is more active, than corroded steel.
The potential difference between the high negative potential of the new steel and the low negative potential of the old steel is the driving force, or voltage, of this electrochemical corrosion cell.
A severe and common example of this type of corrosion is when an old bare steel pipeline fails, and a small section of the pipeline is replaced with a coated section of new steel. The new section is the anode and corrodes to protect the large cathode, resulting in failure of the new section.
188.8.131.52 Dissimilar Alloys.
The most obvious example of this type of corrosion is different metal alloys. For example, there are over 200 different alloys of stainless steel.
Also, metals are not 100 percent pure. They normally contain small percentages of other types of metals. Different batches of a metal vary in content of these other metals. Different manufacturers may use different raw materials and even the same manufacturer may use raw materials from different sources. Each batch of metal may be slightly different in electrical potential. Even in the same batch of metal, the concentration of these other materials may vary slightly throughout the finished product. All these differences will produce the electromotive force for this type of corrosion to occur.
184.108.40.206 Impurities in Metal.
No manufacturing process is perfect. Small impurities may be mixed into the metal as it is produced or cooled. Impurities at the surface of the metal may become part of the electrolyte causing concentration cell corrosion, or if metallic, they may be anodic (corrodes and leaves a pit behind), or cathodic (corroding surrounding metal).
220.127.116.11 Marred or Scratched Surface.
A marred or scratched surface becomes anodic to the surrounding metallic surface. This is similar to the old-to-new syndrome, where new steel is anodic to the old steel. This electrochemical corrosion cell is set up by the difference in the electrical potential of the scratched surface compared to the remaining surface of the structure. Threaded pipe, bolts, marks from pipe wrenches and other tools, and marks from shovels and backhoes are common examples of this type of electrochemical corrosion cell. This situation is further aggravated because the metal thickness is also reduced in these areas.
18.104.22.168 Stressed Metallic Section.
Metal that is under stress becomes anodic to metal that is not under stress. Bolts, bends, structural or mechanical stresses, and soil movement are common examples. This situation results in the metal shearing or cracking from the stress long before corrosion has penetrated the entire thickness of the structure.
Metal that is at an elevated temperature becomes anodic to the same metal at a lower temperature. As previously discussed, a more active metal is anodic to a more noble metal. Since elevated temperature makes a metal more active, it becomes anodic to the rest of the metal. This electrochemical corrosion cell may cause accelerated corrosion on metals that are at elevated temperatures.
22.214.171.124 Simultaneous Sources of Corrosion.
Each of these previously discussed types of electrochemical corrosion cells may cause significant corrosion, but in many cases there are a combination of many different types of corrosion simultaneously at work to make corrosive situations even worse on the metal surface. Understanding the actual cause of corrosion is of utmost importance in maintaining a submerged or buried metallic structure, such as a pipeline or storage tank.
When corrosion is noted, or when a corrosion leak occurs, it is essential that
the cause of the corrosion be identified so that corrective action can be taken. Once the type of corrosion is understood, the method of repairing the cause of the corrosion can be easily determined and future leaks can be prevented. In many cases, the location of the anodic area can be predicted by understanding the process of corrosion. These anodic areas tend to be in the worst possible places. Examples are pipeline river or swamp crossings, pipelines entering pits or foundations, pipelines under stress and pipelines at elevated temperatures.
In a majority of leak situations, the primary concern is to patch the hole in the pipeline or tank. Without an understanding of corrosion and corrosion control, a bad situation can be made even worse. Even considering the criticality of stopping a gushing leak, it is imperative to fix the cause of the leak. This means taking action to identify and mitigate the cause of the leak. In some situations it may be a failed insulator or broken bond wire which actually caused the leak. Probably the most common cause of corrosion leaks are the methods or materials used from previous leak repairs, breaking or shorting the continuity. An example of many types of corrosion at work simultaneously can be demonstrated by the following figure, which shows most of the different types of corrosion discussed.
6.1.4 Stray Current Corrosion.
This type of electrochemical corrosion cell is caused by an electromotive force from an external source affecting the structure by developing a potential gradient in the electrolyte or by inducing a current in the metal, which forces part of the structure to become an anode and another part a cathode. This pickup and discharge of current occurs when a metallic structure offers a path of lower resistance for current flowing in the electrolyte. This type of corrosion can be extremely severe because of very high voltages that can be forced into the earth by various sources.
The potential gradient in the electrolyte forces one part of the structure to pick up current (become a cathode) and another part of the structure to discharge current (become an anode).
Stray current corrosion occurs where the current from the external source leaves the metal structure and enters back into the electrolyte, normally near the external power source cathode. The external power source is the driving force, or the voltage, of the cell. Stray current corrosion is different from natural corrosion because it is caused by an externally induced electrical current and is basically independent of such environmental factors as concentration cells, resistivity, pH and galvanic cells. The amount of current (corrosion) depends on the external power source, and the resistance of the path through the metallic structure compared to the resistance of the path between the external source’s anode and cathode.
An example of stray current corrosion is caused by impressed current cathodic protection systems, where a “foreign” electrically continuous structure passes near the protected structures anodes and then crosses the protected structure (cathode). This corrosion is usually found after failures in the foreign structure occur. Stray current corrosion is the most severe form of corrosion because the metallic structure is forced to become an anode and the amount of current translates directly into metal loss. If the amount of current leaving a structure to enter the electrolyte can be measured, this can be directly translated into metallic weight loss. Different metals have specific amounts of weight loss when exposed to current discharge. This weight loss is normally measured in pounds (or kilograms) of metal lost due to a current of one amp for a period of one year (one amp-year). For example, if a stray current of just two amps were present on a steel pipeline, the result would be a loss of 18.2 kilo grams (40.2 pounds) of steel in one year. For a coated pipeline, this could result in a penetration at a defect in the coating in an extremely short period of time, sometimes only a few days.
126.96.36.199 DC Transit Systems.
Electrified railroads, subway systems, street railway systems, mining systems, and trolleys that operate on DC are major sources of stray current corrosion. These systems may operate load currents of thousands of amperes at a common operating potential of 600 volts. Tracks are laid at ground level and are not completely insulated from the earth. Some part of the load current may travel through the earth. In the event of a track fault, these currents could be extremely high. Buried or submerged metallic structures in the vicinity (several miles) of these tracks could be subject to stray current effects. Pipelines that run parallel, cross under the tracks, or are located near the DC substation, are especially prone to these stray currents. If there are high resistance joints in the pipeline, the current may bypass the joint, leaving the pipeline on one side of the joint, and returning on the other side. Since the source of the stray current is moving, it may be necessary to monitor the metallic structure over a 24-hour period to see if these currents affect it.
188.8.131.52 High Voltage Direct Current (HVDC) Electric Transmission
Lines. Power distribution systems are another source of stray currents. Most power systems are AC, although sometimes DC systems with grounded neutral may be used. These transmission lines, under fault conditions, may use the earth as the return path for the DC current. Because DC requires only two-wire instead of three-wire transmission, it is sometimes used when large amounts of power needs to be transported large distances.
Conversion units are located at each end of the transmission lines.
Each of these conversion units are connected to a large ground grid. Any unbalanced load would result in a current in the earth between these two ground grids. These unbalanced currents are naturally not constant they vary in direction and magnitude. HVDC line voltages may be 750,000 volts or higher.