The notion of a country in the world without a large and complex network of pipelines is similar to a human body with no arteries. Pipelines that transport and distribute oil, gas, chemicals, water, steam, petroleum products and other substances are of critical significance for the economy. And the health of these critical assets is severely endangered by electrochemical deterioration, or corrosion.
Pipeline corrosion is the deterioration of pipe material and the related system due to its interaction with the working environment. It affects pipeline and accessories made of both metals and non-metals. Pipeline corrosion—and the related catastrophic failures that it can cause—cost billions of dollars to the economy. A study released by the U.S. Federal Highway Administration in 2002 estimated that the direct cost of corrosion in the United States was $276 billion each year, or about 3% of the nation's GDP.
In other words, corrosion is a big problem. It predominantly affects pipelines made of metals such as copper, aluminum, cast iron, carbon steel, stainless steel and alloy steel pipes used for buried, underground, submerged or other pipelines. That makes designing and selecting the best available systems and materials for pipelines and their corrosion protection systems an extremely important issue for the oil and gas industry. Federal regulations also require that all high-risk pipelines carrying oil, gas or other hazardous substances incorporate reliable and effective pipe materials and coatings, as well as cathodic protection. Here we'll take a look at the key types of corrosion that affect pipelines, and some of the methods that are used to protect this infrastructure. (For more on this topic, see the 21 Types of Pipe Corrosion & Failure.)
The Corrosion Process
Corrosion of most pipelines occurs due to an electrochemical reaction in the presence of an electrolyte. The electrochemical nature of the process also facilitates the detection and mitigation of this deterioration, which is accomplished by monitoring the voltages and the currents associated with the corrosion rate.
The rate of corrosion of a piping system is generally related to external and internal factors. External factors include a working environment of pipes, soil chemistry and moisture for buried pipes or water chemistry in the case of submerged pipes. (Take a deep dive with the article Industry Experts Discuss Subsea Pipeline Corrosion Management.)
Internal factors that contribute to corrosion may include:
- The oxygen content or reactivity of liquids and gases carried
- The use of dissimilar metals within the piping system
- The temperature, flow rate and pressure of the fluids and gases
Types of Pipeline Corrosion
There are a few different types of corrosion. Here we'll look at how they occur.
Uniform Pipe Corrosion
As the name indicates, uniform pipe corrosion causes uniform loss of the material along the surface of the pipe, resulting in a continuous thinning of its solid structure. The rate of reaction is measured by the depth of penetration of the surface in millimeters per year. By selecting a suitable piping material and a combination of corrosion protection methods such as cathodic protection as well as surface coatings, it is possible to prevent this type of deterioration.
Pitting corrosion is the severe, localized deterioration of a limited surface area, leading to cavity formation, or pits, on a pipe's surface. In some cases, these pits may puncture the pipe. The reasons for pitting corrosion include:
- Pipe material defects or surface defects
- Mechanical damage to the protective passive film
- Penetration by an aggressive chemical species, such as chlorides
This type of corrosion is frequently found in passive metal alloys and metals, such as aluminum or even stainless steel. Pits normally vary in shape and depth. Improper material selection for piping can be one of the causes.
This corrosion can be prevented by:
- Selecting a pipe material for the specific service environment, such as the temperature and chemical concentration of a reactant (resistant to pitting)
- Devising cathodic or anodic protection
Selective leaching, or graphitic corrosion, occurs when a noble metal and a more reactive element form an alloy. This may result in the loss of the reactive element from the pipeline's surface, causing loss of strength and premature failure. A typical example of this is the removal of nickel, cobalt or zinc from copper alloys. This can result in color changes or changes in density in the affected material. The addition of aluminum or tin can, in some cases, provide protection from leaching.
Galvanic corrosion occurs when dissimilar alloys or metals of different corrosion potentials are connected electrically. In this case, only the metal working as an anode with respect to the other will deteriorate. This reaction can be prevented by using a combination of metals that are closer in the galvanic series, and by placing insulation between the two. Coating of the cathodic surface will also help.
Crevice corrosion is caused by an accelerated reaction at joints and other crevices of a pipeline due to differential oxygen availability. The surfaces starved of oxygen become the anode in an electrochemical reaction. Replacing riveted joints with welded joints can help overcome such problems.
Intergranular deterioration refers to the selective deterioration at a surface's grain boundaries (due to high temperature) when the grain boundary reaches high activity, which is prone to corrosion. Heat treatment and welding heat can cause this transformation, leading to corrosion. This problem can be prevented by selecting extra-low carbon stainless steel materials. (Quick read: Why is Stainless Steel Corrosion Resistant?)
Cavitation and Erosive Corrosion
Cavitation damage occurs in a pipeline when the fluid’s working pressure drops below its vapor pressure, leading to the formation of vapor pockets and vapor bubbles that collapse at the internal surface of the pipeline. This can also lead to erosive corrosion. Parts of pipelines such as pump suctions, discharge pipes, elbows, tees or expansions or fitments at heat exchangers—even valve seats—may be extremely prone to this damage under certain operational conditions.
Cavitation can be prevented at the design stage by reducing fluid pressure gradients and excessive pressure drops in the range of the vapor pressure of the liquid, as well as ensuring zero air ingress. Coatings can also reduce the rate of material loss.
Erosion corrosion is due to the relative movement of fluid and the inner surface of the pipe. Fluid turbulence can result in a rapid rise in erosion rates. Poorly finished internal pipe surfaces or pits that may form can disturb smooth fluid flow, leading to localized fluid turbulence. This can result in a high erosion rate. A combination of cavitation, erosion and corrosion—at a high temperature or high pressure—can lead to very severe pitting corrosion. (Learn more in Combating Cavitative Corrosion and Erosive Corrosion.)
The addition of chromium or molybdenum to steel can improve the corrosion protection in this case.
Stray Current Corrosion
Stray current corrosion is caused by the flow of stray currents through pipelines. This can cause localized pitting and pin holes on metal surfaces at the precise points where stray currents leave the surface.
Sources of stray electricity include:
- High voltage overhead or buried lines nearby
- Electric railways
- Electric welding machines
- Earthed DC source
- Cathodic protection
Damage can be reduced by controlling electricity leakages, bypassing stray current to an earthing station or the use of an additional protective system.
Stress Corrosion Cracking
Stress corrosion damage is the growth of pre-existing cracks in corrosive conditions. It can lead to the sudden failure of ductile metal pipes under tensile stress, particularly at higher temperatures. Cracks grow rapidly in the case of alloys, but failure occurs only if the amount of stress exceeds a specific threshold level. Therefore, preventive measures for stress corrosion cracking include limiting the loads to ensure they are below threshold stress levels.
Different types of coatings designed to mitigate corrosion include plant-manufactured systems as well as the tapes applied at a construction site. Here are some of the key types of pipeline coatings, along with their advantages and disadvantages.
Coal Tar Enamel Coatings
- Pro: These have fair adhesion to steel
- Con: There are severe health concerns associated with their use
Plant-Applied Tape Systems
- Pro: These have good adhesion to steel
- Con: They may face blistering problems under stress
Polyolefin (Crosshead Extruded) with Butyl Adhesive
- Pro: This coating is non-polluting
- Con: It has low adhesion strength with steel pipes
- Con: It may suffer disbondment due to cathodic protection
Polyolefin (Dual-Side Extruded) with Butyl Adhesive
- Pro: This coating has a good adhesion to steel
- Pro: It can resist disbondment due to cathodic protection
- Con: It can’t be removed for pipe repair
- Pro: This coating has excellent adhesion to steel
- Pro: It can resist disbondment
- Con: It needs high-temperature application
- Con: It has poor abrasion resistance
Multi-Layer Extruded System of Polyolefin or Multi-Layer Epoxy Systems
- Pro: This coating has excellent adhesion to steel
- Pro: It resists disbondment due to cathodic protection
- Pro: It has high abrasion resistance
- Pro: It has resistance to hydrocarbons
- Con: There's a high initial cost
- Con: Stringent application parameters must be followed
Metallic Spray Coatings
The application of sprayed thermal coatings, such as zinc and aluminum, is advantageous in submerged piping systems. These coatings offer corrosion protection under varying environmental conditions at a low cost.
For very-high temperature pipes, refractory linings, polyurethane linings, metallic coatings and concrete-mortar linings are used.
Cathodic Protection (CP)
Cathodic protection is an electrical method of reducing the corrosion rate of the pipe's metal surface by converting it into the cathode of an electrochemical cell. This is achieved by moving the potential of the pipe metal into the negative direction by impressing the necessary voltage through an external power source (in case of an impressed current CP), or by providing a sacrificial anode in the system (in a galvanic CP).
In the case of an impressed current protection system, a calibrated current is superimposed on the piping structure by means of a dedicated power supply consisting of a rectifier-transformer connected to a local utility power source. This is connected to an anode buried in the ground.
In the case of a galvanic CP (sacrificial anode) system, the galvanic hierarchy between a sacrificial anode metal, such as zinc, and the pipe metal, is used to supply the required protective current.
Sacrificial anodes are made of different alloys of aluminum, zinc or magnesium. For very large pipelines, sacrificial anodes cannot deliver enough protective current to ensure complete pipe protection. An impressed current cathodic protection system is selected under such conditions.
Protecting Pipelines from Corrosion
Studying the mode of corrosion damage of a particular pipeline aids in the determination of the root cause of corrosion, and a suitable solution. Corrosion can be minimized by selecting suitable systems and materials at the design stage. Cathodic protection systems facilitate the continuous monitoring of pipelines. While planning new pipelines, the advanced methods of monitoring and protection should be incorporated.