AC corrosion: Integrity aspects and cost-effective mitigation measures
Electromagnetic interference from above ground high voltage transmission lines causes an AC voltage to be induced in paralleled buried pipelines. Besides being a safety concern to pipeline operators, the induced AC voltage poses a risk of causing AC corrosion of the pipeline steel, despite the application of cathodic protection. AC corrosion is known to be developing at small coating defects and can reach corrosion rates in excess of 10 mm/year.
Industry standards such as EN 15280:2013, ISO 18086:2015 or NACE SP12424-2018 specify a range of criteria related to the AC and DC current density to mitigate AC corrosion. This is because both of these have been found to be highly influential parameters on AC corrosion.
A lot of effort is put into reducing the interference level (AC voltage) by installation of extensive grounds, mitigation wires and DC de-coupler systems. This allows for a certain reduction of the pipeline AC voltage, which is the driving force for AC corrosion. A reduction in the AC current density in the coating defects follows directly from Ohms law.
The relation between the interference level (AC voltage) and the resulting current density discharge at a coating defect is defined as the spread resistance. While soil resistivity is often (mistakenly) considered a static parameter in cathodic protection design, this spread resistance is extremely dependent on the cathodic protection level and local soil conditions. The spread resistance can vary up to several decades depending on cathodic protection levels due to the electrochemical reactions occurring on the cathodically protected steel surface. The cathodic protection may either produce conducting ions at the very vicinity of the coating defect, or it may produce highly resistive layers. This means that the effect of cathodic protection level on the spread resistance, and therefore the AC current density, is far more significant than what might be achieved by means of grounding installations. AC mitigation can be a simple matter of reducing the level of cathodic protection.
It seems counter-intuitive to reduce the cathodic protection level in order to protect a buried pipeline against AC corrosion. But based on exhaustive field observations as well as profound research results, it is the most efficient mitigation measure to implement. All experience has proven that coating defects can be safely protected against AC corrosion, even at AC voltage levels far beyond the 15 V safety threshold, entirely by carefully controlling the level of cathodic protection.
To verify that the pipeline integrity is not compromised while maintaining this balance between AC interference and cathodic protection level, a remote monitoring system that continuously measures the corrosion rate as well as all relevant electrical fingerprints is applied. The measured parameters include:
- Corrosion rate (ER)
- On potential
- Off potential
- IR-free potential
- DC current density
- AC voltage
- AC current density
- Spread resistance
MetriCorr’s presentation will focus on risk assessment, mitigation and continuous monitoring of cathodic protection and AC corrosion. In addition, several field case studies will be presented to illustrate the most cost-effective method of AC corrosion mitigation.
