It is V&A’s mission to preserve the integrity of infrastructure by designing systems that prevent corrosion-induced failure. When determining the most efficient way to do so, V&A engineers and customers are regularly faced with a design choice: whether to install a cathodic protection (CP) system or a corrosion monitoring system. The correct answer depends on factors such as electrolyte corrosivity, structure material, structure coating, and budget, to cite a few. This article introduces the differences between both systems and the general considerations that are considered when recommending either.
Different, but related…
Although CP systems and corrosion monitoring systems are treated as two different remedies, they represent a similar approach to the problem taken to different extents. A more comprehensive discussion of the science behind a CP system is available in our blog, CP systems utilize either a galvanic anode (GACP) or ICCP (Impressed Current Cathodic Protection) to apply a DC current to affected structures. This current lowers the electrical potential of the structure with respect to the electrolyte, which is the driving force of electrochemical corrosion, which is the driving force of electrochemical corrosion.
Figure 1: Dielectric insulating flange, used to stop electrical flow between two segments of pipe.
The first step in designing a CP system is to determine which section of the structure will be protected by it. This section needs to be electrically continuous for the protective current to reach it in its entirety. Bonding flanges are used to provide a low-resistance path between two electrically isolated segments of pipe that would not be able to be protected by the same CP system otherwise. Likewise, the section needs to be isolated from other (foreign) structures to prevent them from draining the protective current away. For pipelines, one can use a dielectrically insulating flange (see Figure 1) to prevent electrical flow between two segments.
To easily test the effectiveness of components such as anodes, bonding flanges, and insulating flanges, test stations (TS) are installed along the structure. These usually consist of a circuit board in an accessible location with wire leads connected to relevant parts of the structure. Additional test stations can be built to obtain other relevant information, such as structure-to-soil potentials (monitoring TS) or the effect of nearby foreign structures (foreign pipeline TS).
The term “corrosion monitoring system” refers to the combination of components that is necessary to make the structure “CP-ready”, but without the actual CP current and anodes. For instance, a corrosion monitoring system on a pipeline could include any insulating and bonding flanges, their associated test stations, and additional monitoring test stations typically every 1,000- feet to measure potentials. A corrosion monitoring system does not provide cathodic protection.
What advantages are there to installing a corrosion monitoring system?
Corrosion monitoring systems are a prophylactic approach to corrosion problems. They are recommended when the electrolyte corrosivity is not high enough to justify a full CP system requirement. The usual V&A recommendation when installing corrosion monitoring systems is to perform on-site testing at least once a year. The testing consists of making sure that all components are working properly and that the structure-to-electrolyte potential remains at a healthy level. If it is determined that the structure is at risk of corroding, an upgrade from corrosion monitoring to a CP system is more convenient and less expensive than installing a CP system from scratch, since some of the required components are already installed.