John Bollinger, Farwest Corrosion Control, USA, explores the benefits of switch-mode power supply developments for cathodic protection solutions.
Understanding of corrosion and the need for cathodic protection (CP) dates back over 200 years. The effect of galvanic couples and the observed corrosion between metals can be traced back to 1792, when Giovanni Farroni first described ‘bi-metallic corrosion’ in Florence, Italy. The first application of CP to protect copper sheathing against corrosion from seawater on naval ships with the use of iron anodes can be traced back to Sir Humphry Davy in 1824. Ten years later, Michael Faraday laid the foundation of CP when he discovered the connection between corrosion weight loss and electric current.
While the early years saw iron and zinc sacrificial anodes as a primary component of CP, current rectifiers (a power supply that converts AC power to DC power) were considered state-of-the-art when Illinois Bell Telephone Company protected lead-sheathed cables with very crude impressed current rectifiers. This led the way to the practical application of impressed current systems for corrosion prevention on buried metallic pipelines.
As the oil and gas industries expanded with underground pipelines in the late 1940s and early 1950s, anodes and CP rectifiers advanced and were extensively used. While they are still key to CP solutions today, advanced electronics and an extensive selection of anode types are improving and extending the life of buried and submerged metallic structures.
Types of CP systems
There are two types of CP systems available, galvanic anode systems and impressed current systems.
- Galvanic anode CP system
- This system consists of an anode(s) of a dissimilar metal (from that of the structure to be protected). It creates a galvanic corrosion cell so that the anode (zinc, magnesium or aluminium) is the corroding element. The anodes generate their own power, much like a battery.
- Impressed current CP (ICCP) system
- This system consists of a metallic anode(s) (cast iron, graphite, platinum, or proprietary mixed metal oxide). The CP power is provided by a DC power supply, which converts AC power to DC power. The advantage of an ICCP system is that it can provide much higher DC than a galvanic system. ICCP systems are typically used on very large structures that would not be conducive to galvanic anodes.
The system design
Developing a customised solution involves much more than simply attaching a few anodes to the length of a pipeline. A properly designed anode system is installed to discharge DC to the pipeline in order to reverse the effects of corrosion on the bare metal.
A successful CP design requires attention to details such as design life, soil resistivity, coating efficiency, and more. In addition, the design must also take into consideration outside factors, such as stray CP current, AC from overhead power, casings, coatings, monitoring, maintenance, and more.
While the solution concepts have not changed much over the last 50 years, advancements in technology have greatly improved the efficiency, reliability and stability of the solutions. In the case of impressed current anodes, the use of a proper DC power supply to provide the required current is key.
The power supply or rectifier industry has evolved from crude selenium semi-conductors to the more advanced and efficient silicon diode semi-conductors that are used today.
How CP rectifiers work
A CP rectifier is essentially a transformer that initially converts the incoming AC line voltage to a lower AC voltage, and then converts the low AC voltage to the DC form required for CP. These conventional ‘transformer rectifiers’ (commonly called rectifiers) are still being manufactured and used for many installations.Simply explained, a rectifier with an adjustment mechanism to vary CP current is needed to convert AC power to the DC power, so that the structure can be protected.
The pipeline and anodes are connected to the rectifier via electrical cables, and the electrolyte (soil) completes the electrical circuit. When the rectifier is energised, protective DC is discharged from the anodes to the pipeline via the electrolyte, and then returns to the rectifier via the structure cable. All components are essential to the system, as they are designed to provide the required CP current to the pipeline that is subject to corrosion.
In most cases, a CP design is developed to work with the pipeline system for at least 20 years. It can then be replaced at the end of its effective life, thereby lengthening the expected life of the pipeline.
Conventional CP rectifiers are today’s workhorse of an impressed current CP solution. When maintained properly, they can provide DC for over 20 years. Given the sometimes remote and challenging locations where rectifiers are installed, they can provide a reliable source of DC power for the CP system.
There are several major suppliers of CP rectifiers. For the most part, they offer similar key product solutions, which employ the use of a transformer, diode stack, a disconnect breaker, and output meters contained in a protective cabinet. This design is common to the industry. Radically changing the look and operational ‘feel’ for the sake of advancement could cause confusion to an industry that is familiar and comfortable with rectifiers, how they work and their use.
Despite being durable and reliable, standard CP rectifiers do have limitations. For example, they can use more AC power than other solutions. It can also be tedious and difficult to obtain the desired DC output, and they can be a safety hazard to inexperienced workers.
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