How to Protect Pipelines from Corrosion?
Pipelines are an essential and versatile structural unit used in water, wastewater treatment, and petrochemical processing. Most of these applications employ steel pipes which are highly susceptible to corrosion.
Most of these pipelines are part of underground infrastructures that make the pipe exposed to the environments of moisture, gases, and oxygen that promotes corrosion.
The article explains the theory behind the active corrosion protection of pipelines used in the construction industry.
1. Active Corrosion Protection
Active corrosion protection involves a set of methods to neutralize the corrosion -causing a reaction in the structure. The technique has an active role in inhibiting the corrosion on the surface of the metal.
For example, a special chemical compound can be added to fluids passing through the pipelines that make it inert to the corrosion reactions. This method of active protection is with inhibitors.
Another method is to connect a sacrificial metal to the pipeline. This method is called cathodic corrosion protection, in which the sacrificial metal is corroded instead of the real surface.
The use of corrosion inhibitors and cathodicprotection for protecting the pipelines are explained below:
Corrosion Inhibitors to Protect Pipelines
The corrosion process happening in the pipes is dampened by using corrosion inhibitors. These corrosion inhibitors alters the properties of the fluid in the pipeline using specific chemical reactions. The process makes the material inert and increases the resistance against corrosion.
The inhibitors used can be:
i) Scavenging Inhibitors
Scavengers or environmental conditioners when added to the fluid removes the aggressive corrosive species present in it. The inhibitors reduce the aggressiveness of the medium. The cathodic reaction happening here is oxygen reduction. The oxygen scavengers reduce the concentration of dissolved oxygen from the solution.The two common scavenging inhibitors are hydrazine and sodium sulfite.
ii) Interface Inhibitors
Interface inhibitors control corrosion at the metal or environment interface. The interface inhibitors can be classified into:
a. Anodic Inhibitors
The anodic inhibitors facilitate the formation of oxide films or passivating films. These films prevent the anodic dissolution of the metal. These inhibitors are a good choice for pipes that are exposed to neutral solutions.
b. Cathodic Inhibitors
The cathodic inhibitors interrupt the corrosion reaction in the corrosion sell by decreasing the rate of the reduction reaction. It precipitates on specific cathodic areas so that it creates a barrier to increase the surface impedance.
c. Mixed Inhibitors
These are chemical compounds that reduce the anodic and cathodic reaction. The most commonly used mixed inhibitors for water treatment industries are silicates and phosphates. In oil and gas industries, inhibitors can be injected directly into the pipeline using special valves at regular intervals.
2. Cathodic Protection of Pipelines
The cathodic protection can be mainly performed by two methods:
- Passive cathodic protection
- Impressed current cathodic protection (ICCP)
i) Passive Cathodic Protection of Pipelines
In this method, the metal is protected by using sacrificial metal anodes. These can be connected either directly or indirectly. The two dissimilar metals would create a potential difference, thus forming an electrochemical cell.
This method of passive cathodic protection is ideal to protect smaller pipeline sections.
a) Corrosion Protection By Epoxy Coating:
Using epoxy coating is one of the easiest ways to protect pipes against corrosion. Coatings and linings can be used on the pipes that are above or underground. Coatings are effective because pipelines are usually made of high-quality steel. And when they are exposed to a natural environment, such as warm temperatures, water, or soil, corrosion occurs. So, if we protect the steel surface from coming into direct contact with the water, moisture, or soil, then pipes will last a lot longer.
This method is very useful for gas pipelines. Coatings can be frequently used in combination with cathodic protection. Nowadays, the majority of new pipelines are protected with fusion bonded epoxy coating (FBE). Sometimes zinc or urethane are also used. The coatings must be properly applied and cured to be effective.
ii) Impressed Current Cathodic Protection (ICCP)
This method of active corrosion protection is employed for large pipelines. The system with a sacrificial anode is connected to an external power source that provides additional power to increase the rate of the electrochemical process.
The pipelines located in longer distances can be protected by this method.
The use of active corrosion protection help to protect the pipelines thus avoiding costly repair and maintenance operations. They can also be combined with passive corrosion protection techniques like coatings to reduce the probability of corrosion in aggressive environments.