On December 13, 2018, the Bureau of Investigation and Enforcement (BI&E) of the Pennsylvania Public Utility Commission filed a complaint against Sunoco concerning the Dragonpipe (Mariner East pipeline system). The BI&E complaint alleges that corrosion of the steel wall of the pipeline caused a significant leak in Mariner East 1 near Morgantown in April, 2017, and that Sunoco’s failure to properly protect and inspect the pipeline contributed to the corrosion.
Most (but not all) of the BI&E criticisms are directed at Sunoco’s failure to implement cathodic protection effectively. Shoddy practices left the pipeline unprotected, and corrosion ensued. To understand exactly what the BI&E is talking about, I had to do some background reading on cathodic protection. Here is a summary of what I have learned about the subject. No doubt some readers know more about this than I do, so please add your clarifying comments.
In a second blog post, I will get into the specifics of the BI&E complaint. Here, I will just provide an overview of how cathodic protection works and what can go wrong.
What is “cathodic protection”? Cathodic protection is a method of keeping steel from corroding. By the nature of its manufacture, steel inevitably has microscopic spots which are electrically more negative or more positive than neighboring spots. If there is moisture present, a tiny local current will be created between these positive and negative spots. The current and the moisture (in the presence of oxygen) will cause a chemical reaction resulting in corrosion. (In the case of steel, this means the creation of ferrous hydroxide, which is rust.) If nothing is done to prevent it, the corrosion will gradually eat away the steel. In steel pipelines, it is obviously important to prevent this from happening.
This corrosion can be prevented by electrically connecting the pipeline to another piece of buried material, referred to as an “anode”. This buried material, usually metal, is maintained at a more negative electrical potential than the steel pipeline. A voltage is applied that is big enough to make the whole pipeline positive with respect to the anode, which causes a current to flow from the anode, through the soil, to the pipeline (which, in this circuit, is a “cathode”). In this situation, the tiny micro-currents will no longer take place along the pipeline’s surface. Instead, they will take place on the surface of the anode, which is slowly corroded away.
In this way, the anode is slowly “sacrificed”, preventing corrosion of the pipeline. As long as the current from the anode is sufficient to prevent the local micro-currents on the surface of the pipeline, corrosion will not occur. That’s the basis of “cathodic protection”.
What can go wrong? Cathodic protection can fail if any of the following steps is skipped:
- The equipment has to be working correctly. Is it powered up? Are the connections good?
- The anode has to be buried correctly. Is it surrounded by conductive backfill so that the voltage is conveyed to the surrounding soil?
- The soil has to conduct electricity adequately. Is the current from the anode reaching the pipe?
- The voltage has to be low enough. The anode must be kept at a certain negative voltage, compared to the pipeline. Under typical conditions, this is -850 mV (minus -0.850 volts). If the voltage is closer to zero, the protective effect is lost and corrosion can occur.
- But the voltage can’t be permitted to be too low either. If the voltage is too far from zero (more negative than -850 mV), coating problems can be triggered, which can actually promote corrosion. This problem is called “cathodic shielding”.
- Precautions must be taken to avoid electrical interference from other pipes, overhead wires, and other sources. These can affect both the cathodic protection and the testing procedures.
- Testing must be done on a regular schedule (yearly, for many of the tests).
- Careful records of the testing (including the initial baseline conditions) must be kept.
- Test results must be compared with previous values, to make sure there haven’t been changes indicating a problem with any of the items above.
- There must be well-documented procedures for investigating potential problems and correcting them as necessary.
- There must be follow-through to make sure the procedures are followed.
As the PUC complaint makes clear, Sunoco has frequently failed to perform several of these steps, and that resulted in the corrosion at Morgantown (and perhaps elsewhere). That is the subject of my next blog post.