Pipelines are one of the most essential and versatile types of underground infrastructure. They are used in a broad range of industries from water and wastewater treatment to petrochemical processing. Unfortunately, most of these piping networks consist of steel pipes, which are susceptible to corrosion damage. Water, acids, acidic gases and oxygen are just some of the fluids that create environments that promote or accelerate the corrosion process. This issue can be further exacerbated in industries where pipelines are required to operate under high-temperature and high-pressure environments.
The oil and gas industry, for example, relies heavily on pipeline networks and infrastructure. The United States alone contains thousands of miles of piping that is responsible for transporting crude oil and other hydrocarbon-based fluids.
The Cost of Corrosion Protection and Quality Control
However, corrosion is by far the leading cause of piping failure in this industry. According to studies conducted by the National Association of Corrosion Engineers (NACE), corrosion damage costs the oil and gas industry over USD 7 billion annually. The same study also found that the water and wastewater industry spends over USD 36 billion per year on corrosion. In both cases, more than half of this expenditure is directed towards corrosion protection methods and services. (For more on this topic, see Corrosion Costs & Recommended Practices for the Water Industry.)
What is Active Corrosion Protection?
Corrosion protection methods generally fall into one of two categories: active or passive. Active corrosion protection describes a range of techniques that halt or neutralize corrosion-causing reactions. In other words, it takes an active role in inhibiting corrosion on the surface of the metal. For example, chemical compounds can be added to fluids transported in pipes to reduce their reactivity (make them more inert).
Another method of active corrosion protection is the use of cathodic corrosion protection. This method involves connecting a more reactive metal (directly or indirectly) to the metal to be protected. Cathodic protection then works by "diverting" corrosion to the sacrificial metal, while the other surface remains protected.
Active corrosion protection is probably best understood by contrasting it with passive corrosion protection. Passive corrosion protection does not actively prevent corrosion at the surface. Instead, this method involves isolating the metal from corrosion-causing elements. With passive protection, a protective coating, for example, may act as a barrier that prevents air and moisture from coming into contact with the underlying steel substrate. With these two elements out of the picture, corrosion cannot occur on the surface of the metal.
In a nutshell, during active corrosion protection, the metal may remain exposed to corrosion-causing elements while various processes actively counteract its formation. Passive corrosion protection, on the other hand, involves separating the metal from the corrosive environment, i.e., corrosion-causing elements never come into contact with the surface.
Active Corrosion Protection as Applied to Pipelines
As mentioned previously, active corrosion protection can be achieved by either neutralizing corrosion-causing reactions (with inhibitors) or by diverting corrosion to a more reactive, sacrificial metal (via cathodic protection). Both methods are explained in more detail below.
Corrosion inhibitors are chemical compounds that dampen the corrosion process. They do this by undergoing various chemical processes that alter the properties of the fluid in the pipeline, thus making them inert and less prone to cause corrosion. Inhibitors used in pipelines generally fall into two categories:
1. Scavenging inhibitors (scavengers)
Scavengers, also known as environmental conditioners, operate either by removing aggressive corrosive species from the transported medium or by reducing their aggressiveness. In alkaline solutions, oxygen reduction is a common cathodic reaction. In this case, oxygen scavengers control corrosion by reducing the concentration of dissolved oxygen from the solution. Hydrazine and sodium sulfite are two well-known scavenging inhibitors.
2. Interface inhibitors
Interface inhibitors are so-called because they control corrosion at the metal/environment interface. These types of inhibitors are further classified as:
- Anodic inhibitors – These inhibitors facilitate the formation of oxide films, also known as passivating films, to prevent the anodic dissolution of the metal. These inhibitors are most effective in near-neutral solutions.
- Cathodic inhibitors – Cathodic inhibitors work by decreasing the rate of reduction at the cathode, thus interrupting the reactions in the corrosion cell. These inhibitors precipitate selectively on cathodic areas to create a barrier that increases surface impedance.
- Mixed inhibitors – Mixed inhibitors are simply chemical compounds that reduce both anodic and cathodic reactions. Silicates and phosphates are examples of mixed inhibitors used in the water treatment industry to prevent the occurrence of rusting.
In the oil and gas industry, inhibitors can be pumped directly into well formations using a macaroni or kill string, where completions allow. In some instances, they may also be inserted directly into the pipeline using special valves that continuously inject the inhibitor chemicals at regular intervals.
Cathodic protection can be classified as either passive or impressed current protection systems. While they both have similar characteristics, each method has distinct features that lend themselves to different applications. (Get an introduction in the article The Basics of Cathodic Protection.)
1. Passive cathodic protection
In this system, sacrificial metal anodes are connected directly or indirectly to the protected metal. The current generated by the potential difference between the two dissimilar metals is enough to form an effective electrochemical cell. Passive cathodic protection systems are ideal for small pipeline sections.
2. Impressed current cathodic protection (ICCP)
Impressed current cathodic protection (ICCP) systems are used on larger pipelines where the current generated by passive methods is insufficient. (Watch the Video: When to Use Impressed Current Cathodic Protection.) In these systems, sacrificial anodes are still used; however, they are connected to an external power source that delivers additional current to drive the electrochemical process. When vast pipeline distances need to be protected, a flange isolation kit (FIK) is typically used to break up the electrical connectivity between adjoining pipes.
Active corrosion protection is a must for protecting pipelines and avoiding costly repair and maintenance operations. While active methods by themselves are reasonably effective, they can also be combined with passive corrosion protection techniques (e.g., coatings) to significantly reduce the probability of corrosion in aggressive environments.