Corrosion of metal is a common issue that causes massive financial losses. Cathodic protection (CP) is a popular method employed to prevent corrosion, where the metal surface is forced to assume the role of the cathode. An example of this is Ballast tanks in ships that store and release water to stabilize the vessel. Due to the highly corrosive nature of sea water, CP is employed to protect the tanks from deterioration. Although, this is a highly efficient method, a major drawback that arises from it is cathodic disbondment.

Cathodic disbondment is delamination of a coating from a surface that has been treated by CP to contain corrosion. This phenomenon represents a critical challenge in industries that rely on metal structures, such as oil and gas, marine, and infrastructure. To fully understand cathodic disbondment, it’s essential to explore its roots in the corrosion process, the mechanisms behind cathodic protection, the causes and consequences of disbondment, real-world examples, methods to mitigate it, and the testing protocols used to evaluate coatings’ resistance.

How Corrosion Begins

Corrosion is a natural electrochemical process that occurs when a metal, especially iron or steel, reacts with its environment, leading to its deterioration. The most common form of corrosion, rusting, happens when iron reacts with oxygen and water to form iron oxide. This process can significantly weaken metal structures, leading to failures in pipelines, bridges, tanks, and other critical infrastructure.

At the core of the corrosion process is the formation of an electrochemical cell, which consists of anodic (oxidation) and cathodic (reduction) regions on the metal surface. In the anodic region, metal atoms lose electrons and form metal ions, which then react with oxygen and water in the environment to produce rust. On the other hand, the cathodic region is where the electrons are consumed, usually by the reduction of oxygen. This movement of electrons drives the current in the formed electrochemical cell of the corrosion process.

The Role of Cathodic Protection

During corrosion, the loss of the metal occurs at the anodic region when the metal atoms lose electrons and form metal ions. In order to protect the integrity of the metal, it is crucial to prevent the formation of these anodic regions on the metal surface. Cathodic protection is a widely used method that achieves this by converting the entire metal surface into a cathode in an electrochemical cell, thereby preventing the formation of anodic regions and the associated corrosion. This is achieved by the following methods :

1. Sacrificial Anode Protection :
   In this method, a more reactive metal (like magnesium or zinc) is incorporated in the system. The reactive metal acts as the anode, sacrificing itself in place of the protected metal.
2. Impressed Current Cathodic Protection (ICCP):
   This involves applying an external current to the metal surface, usually via a power source connected to inert anodes. The current suppresses the anodic reactions that cause corrosion.

By ensuring the metal surface acts as a cathode, these methods effectively halt the corrosion process, as the metal no longer loses electrons to form ions.

The Emergence of Cathodic Disbondment

While cathodic protection is highly effective at preventing corrosion, it can inadvertently cause another issue: cathodic disbondment. This phenomenon occurs when the protective coating applied to the metal surface starts to lose its adhesion due to the effects of the cathodic protection system.

The root cause of cathodic disbondment lies in the electrochemical reactions that occur beneath the coating. When cathodic protection is employed, especially in ICCP systems, hydrogen ions may be generated at the metal-coating interface. These ions can penetrate the coating, reducing to form atomic hydrogen which, in turn, can then diffuse into the metal, leading to a buildup of pressure at the interface. Over time, this pressure can weaken the bond between the coating and the metal, causing the coating to blister, peel, or completely detach.

Real-World Implications

Cathodic disbondment poses significant risks in various industries, with some notable examples including:

1. Oil and Gas Pipelines :

Pipelines transporting oil, gas, and other hazardous materials are often buried underground, making them susceptible to corrosion. Cathodic protection is used extensively to prevent this. However, if the coating disbonds, the metal underneath becomes exposed to the environment, leading to localized corrosion. This can cause leaks, environmental contamination, and even catastrophic pipeline failures. The 2015 Santa Barbara oil spill, for instance, was partially attributed to corrosion exacerbated by disbondment of coating from the pipelines. The leak approximating 541,000 liters of crude oil resulted in hundreds of animals being coated with thick layer of oil as well as innumerable fatalities.

2. Marine Structures :

Offshore oil platforms, subsea pipelines, and ships rely heavily on cathodic protection due to their constant exposure to seawater, a highly corrosive environment. Cathodic disbondment in these structures can lead to severe corrosion, increasing maintenance costs and posing safety risks.

3. Infrastructure :

Bridges and underground storage tanks are other examples where cathodic disbondment can cause problems. The collapse of a bridge or the failure of a storage tank due to underlying corrosion can have devastating consequences, both economically and in terms of human safety. 

Mitigating Cathodic Disbondment

Addressing cathodic disbondment requires a multifaceted approach, focusing on both the coating itself and the cathodic protection system. Some key strategies include :

1. Selection of Resistant Coatings :
   Choosing coatings specifically designed to resist disbondment is crucial. These coatings are formulated to have better adhesion and to be less permeable to hydrogen ions. NSPC’s CURE-O-POXY 8268 is a specially developed phenalkamine adduct for resisting cathodic disbondment occurring in marine environments. Ballast tank linings from epoxy resin coupled with this curing agent maintain adhesion to the substrate that is constantly subjected to the highly corrosive environment.
2. Proper Application Techniques :
   Ensuring the coating is applied correctly is equally important. This involves proper surface preparation, such as abrasive blasting, to remove any contaminants that could affect adhesion. Additionally, coatings should be applied at the correct thickness and under suitable environmental conditions to ensure they cure as per the manufacturer’s recommendation.
3. Controlled Cathodic Protection :
   Adjusting the level of cathodic protection to the minimum effective current can help reduce the risk of disbondment. Over-protection, where too much current is applied, increases the likelihood of hydrogen ion formation and subsequent disbondment.
4. Regular Inspection and Maintenance :
   Routine inspection of coated surfaces is essential to detect early signs of disbondment. Techniques such as close-interval potential surveys (CIPS) and direct current voltage gradient (DCVG) surveys are commonly used in the oil and gas industry to assess the integrity of coatings and cathodic protection systems.
5. Use of Inhibitors :
   In some cases, corrosion inhibitors can be introduced to further protect the metal surface, especially in situations where disbondment has already occurred. These inhibitors work by forming a protective film over the metal surface, reducing the rate of corrosion.

Testing Methods for Cathodic Disbondment

To ensure that coatings are effective at resisting cathodic disbondment, several testing methods have been developed. These tests simulate the conditions under which disbondment can occur, allowing formulators to evaluate the performance of coatings before they are applied in the field.

1. ASTM G8 :
   This standard test method evaluates the cathodic disbondment of pipeline coatings. In this test, a coating sample is subjected to a specific cathodic potential while immersed in an electrolyte. After a set period, the coating is examined to measure the extent of disbondment.
2. ASTM G42 :
   Similar to ASTM G8, this test also evaluates the cathodic disbondment of coatings but under more aggressive conditions, such as higher temperatures or longer exposure times.
3. NACE TM0115 :
   This test method, developed by the National Association of Corrosion Engineers (NACE), is specifically designed for coatings used in the oil and gas industry. It involves applying a cathodic potential to a coated sample while immersed in a simulated soil environment. The extent of disbondment is then measured after a set period.
4. ISO 21809-3 :
   This international standard provides guidelines for the qualification testing of field-applied coatings for the corrosion protection of pipelines. It includes procedures for testing the resistance of coatings to cathodic disbondment under various conditions.
5. Electrochemical Impedance Spectroscopy (EIS) :
   EIS is a more advanced technique used to study the electrochemical properties of coatings. By applying an alternating current (AC) signal to the coated surface, the impedance (resistance) of the coating can be measured. Changes in impedance over time can indicate the onset of disbondment or other coating failures.
6. Adhesion Tests :
   Simple adhesion tests, such as the pull-off test (ASTM D4541), can also be used to evaluate a coating’s resistance to disbondment. In these tests, a metal dolly is glued to the coating surface, and a force is applied to pull it off. The force required to remove the coating is measured, providing an indication of the coating’s adhesion strength.

Conclusion

Cathodic disbondment is a complex issue that arises from the very methods designed to protect metal structures from corrosion. While cathodic protection is highly effective in preventing rust and deterioration, it can lead to the undesired coating disbondment if not carefully managed.

Understanding the mechanisms behind disbondment, recognizing the industries where it poses significant risks, and employing strategies to mitigate it are essential steps in ensuring the integrity and safety of metal infrastructure. By selecting appropriate coatings, applying them as instructed by manufacturer, controlling cathodic protection systems, and conducting thorough testing, the risks associated with cathodic disbondment can be minimized and the lifespan of these critical structures can be extended.

Further reading :

1. CURE-O-POXY 8268 – NSPC’s offering for cathodic disbondment protection.

2. The Electrochemical Society for more on corrosion.

3. Cathwell for types of corrosion.

4. Corrosionpedia for more on cathodic disbondment.

5. KTA for more on cathodic disbondment tests.

6. Marine Insight for more on ballast tanks on ships.