The European Pipeline Research Group outlines that steel pipelines are best defended against external corrosion through a protective coating in addition to cathodic protection. The coating offers the initial—and primary—source of defense, while cathodic protection halts pitting or general corrosion where the pipe is exposed via damage to the coating. Its important to note that some degree of damage to coatings should be anticipated on all pipelines- due, for example, outside interference- so this methodical approach to rust prevention is key.
Negative impacts of coating faults
We achieve corrosion control through the use of a protective coating combined with cathodic protection, but does that mean coatings faults don’t matter? Actually, coating faults can have a number of negative impacts on pipeline integrity, including:
- If a pipeline’s protective capabilities are significantly reduced due to extensive coating deterioration, it may be unable to utilize cathodic protection.
- Some coatings lose adhesion to the substrate and shield the pipeline from the applied cathodic protection. This enables and generates the risk of pitting corrosion, with mechanisms such as Microbially influenced corrosion and near neutral stress corrosion cracking
- Coatings that lose adhesion to the surface and offer only partial cathodic protection shielding can lead to high pH stress corrosion cracking when the conditions are right.
- Cathodic protection may not protect all coating defects of different sizes to same level because the flow of cathodic protection current onto a coating defect will be controlled by the local electrical resistance of the soil – pipe interface
‘Do matter,’” says Hardy. “The style of corrosion degradation and coating failure has a significant impact on the pipeline’s corrosion risk.
Ineffective Cathodic Protection
Ineffective cathodic protection can occur when the accelerated degradation of a coating results in the total demand for full protection current not being met. However, pipe to soil potentials are commonly monitored inaccurately, causing under protection due to lack of close interval potential survey (CIPS) data and errors caused by measuring ON potentials.
A frequent reason for DCVG surveys is that they may accurately size coating flaws, allowing substantial faults to be fixed to lower the overall cathodic protection current demand. It is claimed that tiny defects are protected by the cathodic protection.
The data collected from intelligent pig inspections gives details on the size, depth and position of corrosion defects in a pipeline. The deepest pit depth is generally only seen in very small features. Additionally, the polarization of exposed pipe steel can differ based on defect size because the current distribution will be influenced by the local resistance at the soil-coating interface.
The following are common causes of loss of protection due to excess cathodic demand:
· Accelerated breakdown of thermoplastic coatings above the softening point
· Progressive failure of field applied tape
Cathodic Protection Shielding
Total Cathodic Protection Shielding
The term “total cathodic protection shielding” is used to describe a pipe that meets the following conditions:
- The coating has lost its adhesive properties,
- Water has penetrated beneath the coating,
- The pipe steel remains at or near the free corrosion potential, and
- The pH and chemistry of the water under the coating is not altered significantly by the cathodic reaction of applied cathodic protection.
Total cathodic protection shielding creates the risk of:
- Pitting and general corrosion
- Microbiologically influenced corrosion
- Near neutral pH SCC
Most corrosion that is caused by complete cathodic protection shielding is due to shoddy field application of tapes and heat shrink materials that have a polyolefin-based protective layer. However, it has also been observed on two-layer polyethylene coatings where the butyl rubber primer degrades over time and loses its adhesive properties, causing the coating to detach from the pipe. If the adhesive holding the polyolefin mill coating to the pipe steel is lost, this problem would extend to all ranges of polyolefin mill coatings.
It has been stated that thermoplastic and fusion bonded epoxy powder coatings allow some cathodic protection current to flow through the coating, thus total shielding is not expected. Microbial corrosion has been observed beneath coal tar and bitumen paints with poor adhesion, however.
Although it depends somewhat on the environmental conditions, it is generally considered that a polarised potential of –980 mV is required for the effective control of Microbially influenced corrosion. Microbial activity directly on the pipeline surface would be reduced in the high pH environment associated with partial cathodic protection shielding and high pH SCC. A significant level of cathodic protection shielding should therefore be expected with mill applied thermoplastic coatings of ~3+ mm thickness should they lose adhesion to the pipe.
Near neutral pH SCC has been associated with cathodic protection shielding by tape coatings that have lost adhesion to the pipe and with thermoplastic coatings where the cathodic protection shielding is attributed to high ground resistance where bedrock occurs at pipe depth.
Partial Cathodic Protection Shielding
Partial cathodic protection shielding occurs when the following conditions are met:
- There is a coating on the pipe that has lost its adhesive qualities or developed enough porosity to allow water to seep through,
- There is sufficient cathodic protection current flow to generate hydroxide and create an alkaline environment within or beneath the coating, and
- The potential of the pipe falls into the range –650 mV to –850 mV with respect to a copper-copper sulphate reference electrode.
Partial cathodic protection increases the risk of high pH stress corrosion cracking (SCC). High pH SCC has been most commonly found in pipelines with thermoplastic coatings, particularly those applied in the field over a wire brushed surface. In the USA, field-applied thermoplastic coatings were generally thinner than modern versions, being only 3mm thick on average.
Because of issues like soil penetration, creep under soil loads, embrittlement, and the development of porosity, these coatings didn’t last long. In the early days of gas transmission in America, thermoplastic pipe coatings were used quite often but they quickly fell out of favor because they couldn’t stand up to high pressures and temperatures.
UK high pressure gas transmission pipelines demonstrate no change in current demand over time, despite reports that some cathodic protection current is able to flow through thermoplastic and fusion bonded epoxy powder coatings as they age.
The most difficult, and expensive to manage in an integrated pipeline corrosion management strategy, are corrosion issues caused by loss of adhesion and CP shielding. These problems can only be detected with any degree of certainty after the corrosion has advanced beyond the pig’s detection threshold. Once these corrosion issues have been established, they can only be fixed by re-coating.
Open coating faults can be remedied with relatively low cost CP modifications and can be identified by intelligent pig inspection and above ground surveys. However, when the open area of bare metal on the pipeline becomes too great for effective control by CP due to an excessively large current demand, problems may occur. These sorts of issues are most common on pipelines coated in the field and on pipelines operated above the maximum recommended temperature for that particular coating.
