by Phil Irwin, AMETEK Solidstate Controls

In the foothills of the Andes, among the sprawling vineyards of picturesque Mendoza, Argentina, stands an innocuous equipment cabinet located by the side of an infrequently used mountain pass. The cabinet's superficial surface rust belies its importance as one of the most significant technological developments in modern day corrosion prevention. The cubicle, battered from years of an abrasive cocktail of arid Patagonian winds and coarse earth, hides the cradle of a revolutionary approach to cathodic protection. The equipment within the cabinet resulted in an entirely new range of products.

Ever since 1824, when Sir Humphry Davy first proposed the idea of attaching chunks of iron below the waterline of copper-clad British warships to prevent their rusting, cathodic protection has been one of the most commonly used techniques for the prevention of corrosion. Today, essentially two types of cathodic protection are used to protect steel and other structures from corrosion.

The first employs the use of galvanic or sacrificial anodes in various shapes and alloys with a more negative electrochemical potential than the metal (typically steel) structure they are designed to protect. The sacrificial anodes corrode over time, sparing the remainder of the structure. The anodes have only a limited life. Once they are no longer capable of protection, the structure begins to corrode.

This method is unable to provide complete protection for larger structures, such as pipelines. These types of applications require an impressed current CP system that includes a rectifier, which converts an alternating current power source to direct current that is properly calibrated to provide the required protection. Since the power source is delivered to the anode and not generated by degradation of the anode, the power supply may be recalibrated to the anode to provide additional power, when needed, as long as the anode remains functional and optimal protection to the pipeline or other structure.

Pipeline operators, however, traditionally purchased manual type rectifiers that typically required frequent site visits for troubleshooting and required significant maintenance and operational support. The pilot system installed some 20 years ago in the Piedras Coloradas oil field in the Mendoza region owned by Perez Companc incorporated state-of-the-art switch-mode power control technology along with the latest in remote control monitoring capabilities. At the time, switch-mode technology was considered somewhat complex and not fully understood. However, the application of sound fundamentals resulted in a stable, efficient, reliable and resilient solution.

That pilot system protected half a dozen well casings. The rectifier and deep ground bed system were located in the center of the field approximately equidistant to each of the producing boreholes. Now after 18 years, a recent visit to the facility revealed the corrosion protection system continuing to operate silently and maintenance free. Although peak oil production from the field is now well in the past, it continues to modestly contribute to the county's infrastructure, which is a testimony to solid application of sound engineering principles combined with new technologies.

Further Evolution of Cathodic Protection Systems

Today's advanced cathodic protection systems have evolved from the early breakthrough achieved with reliable switch-mode technology. Spurred on by their early success at Piedras Coloradas (much was learned regarding lightning protection in different terrains and the affect of altitude and geographical dynamics), an AMETEK development team launched a second generation system.

The second-generation rectifier system had many subtle refinements. Newer switch-mode-specific components had been introduced to the market that enhanced the system's product reliability and efficiency. The system also benefited from application of environmental protection strategies. In addition, a remote monitoring and control system was designed that would leverage new communications technologies. Engineering personnel could spend more time analyzing data and prioritizing corrosion prevention in a proactive mode rather than reacting to corrosion threats with heavy consequences.

The vast distances pipelines cover and the often remote location with little supporting infrastructure had to be overcome. With this in mind, a strategic alliance was formed with a third-party, low-earth orbit satellite communications provider. By integrating a data transport layer as part of the system and offering it in combination with analytical software, the power conversion solution was complete.

The system incorporated digital and analog control loops, enabling accurate constant potential cathodic protection. This complemented new polymer pipeline coatings in development that were particularly susceptible to disbondment with excessive protective potentials.

What has always been paramount in CP design has been the need for accurate, automated potential control, yet many older manual systems in the field still rely on moving coil volt meter and ammeter indications. With the incorporation of digital controls, newer systems remove the possibility of parallax errors that occur when observing a needle on a meter from differing, non-perpendicular angles, thus compromising the calibration and tolerance of the indicating device. The increased accuracy helps deliver optimized energy conversion, maximizing the effectiveness of the corrosion control device.