The Corrosion MonitorTopics

Newsletter: Volume 13 - Spring 2017

Selecting the Correct Magnesium Anode

F A R W E S T C O R R O S I O N C O N T R O L C O M P A N Y

Selecting the Right Magnesium Anode

Induced AC Current Overview

Toll Free: 888-532-7937 www.farwestcorrosion.comLocations Nationwide

One of the most frequent questions we are asked is, “What size magnesium anode should I use to protect my buried structure?” As much as we wish there was a simple answer to this question, there are many considerations to make in choosing the correct anode or anode system. These considerations include:

• Soil resistivity or conductivity surrounding the structure. • The physical size of the structure to be protected. In other words, how much bare

metal must be protected? • Coating efficiency. Is the structure coated and how well is it coated? • Is the structure electrically isolated from other metallic structures? If not, can it be? • Required life of the anode system.

There are several popular magnesium anode sizes, which is expressed in anode ingot weight. These size categories include 9, 17, 32, and 48 pounds. In addition to these, there are elongated anode ingots that weigh 20, 40 and 60 pounds and, while not as widely used, these anodes can provide more protective current than their shorter cousins.

There is a common misconception in our industry that if you use a magnesium anode of double the weight, that heavier anode will produce twice the protective DC current. The fact is that anode weight does not determine cathodic protection (CP) current output. In actuality, the current output of an anode is determined by three factors:

1) Soil Resistivity - The lower the resistivity the higher the anode current output

2) Anode Diameter or Cross Sectional Area - The larger the diameter or cross sectional area of the anode, the greater the surface area. This larger surface area will have a minor effect to increase anode current output.

3) Anode Length - The length of the anode is the more controlling factor as to how much current the anode can produce. Therefore, a longer anode will produce more DC current than a shorter, even heavier anode.

To illustrate how different physical anode sizes have on anode current output and service life, refer to the chart and examples by clicking the “continue reading" link below.

In regards to anode weight, this is the critical factor for determining anode life. Very simply, the more anode mass, the longer that anode will last if all factors are equal. Therefore, a 32-pound anode will last almost twice as long as a 17-pound anode….

Introducing theModel AC-10 Current

Density Probe

Click here to continue reading "Selecting the Right Magnesium Anode"…

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Farwest Corrosion Control Company

Toll Free: 888-532-7937 www.farwestcorrosion.comLocations Nationwide

Induced AC Current OverviewInduced AC is a common problem that occurs when buried metallic structures are in a common corridor or near energized power lines. The electromagnetic fields associated with high power AC transmission lines can cause unwanted voltage to “appear” on the structure, potentially damaging the structure or creating a safety hazard to personnel.

There are two primary concerns with induced AC on metallic structures:1. Induced voltages can present a shock hazard to personnel that

physically touch the structure or metallic devices connected to the structure

2. AC is known to be the direct cause of soil side corrosion on buried and submerged structures in a similar manner as DC corrosion

Understanding the corrosion mechanisms caused by induced AC continues to be a challenge in the industry but it has been demonstrated that AC current can absolutely cause corrosion on even cathodically protected steel structures. The corrosion that can occur from induced AC is not necessarily proportional to the induced voltage. Many factors must be considered by knowing the magnitude of the induced AC current density is a key factor in evaluating the probability of induced AC corrosion.

AC corrosion is similar to DC corrosion in that the AC discharges at the coating flaws (holidays). The better the coating yields smaller areas of bare metal in contact with the earth, which results in higher current densities per unit area of the steel structure.

NACE International publication 35110 indicates what effects AC current density levels can have on the corrosion rates of buried steel structures:

• Less than 20 A/m2 = No induced AC corrosion issues• Greater than 20 A/m2 but less than 100 A/m2 = Corrosion is

unpredictable and influenced by many environmental factors• Greater than 100 A/m2 = AC corrosion is likely to occur on the structure

Reducing the AC voltage potential to 15 volts or lower, as recommended by NACE International and OSHA for personnel safety reasons, is not necessarily sufficient to mitigate AC corrosion on the structure. Therefore, it is important to determine the level of AC current density on the pipeline.

Model AC-10Introducing the

Portable AC Current Density Coupon/ProbeInduced AC (Alternating Current) on pipelines is a potential shock hazard to personnel as well as a source of corrosion damage. Current technology to detect and monitor induced AC current include the use of portable or permanent reference electrodes to measure voltage, and/or buried “current density coupons” to measure levels of AC current density.

Monitoring the current density, rather than just AC voltages, is a key factor in assessing the AC current related corrosion risk. A simple and effective method of determining current densities is to install a steel coupon, of a known surface area, in the soil adjacent to the structure. By connecting the coupon to the pipeline through a current measurement shunt or meter, the induced AC current density can be easily calculated.

Until now, only permanently installed AC current density coupons have been available. Unfortunately, the cost to install these permanent coupons can be thousands of dollars per site when considering the materials, planning, permitting, mobilization, equipment and labor.

The Farwest model AC-10 portable steel coupon/probe is the answer to this issue. Used much like a portable reference electrode and similar to a soil pin, the coupon is driven into the soil and functions like a permanent AC current density coupon. Since it’s portable, the AC-10 coupon/probe allows the CP technician a method of measuring AC current density on demand or during a normal pipeline corrosion survey.

Product Features:> Simple to use> Durable steel construction> Multiple cable connection options> Replaceable 10 cm2 conical tip for soil contact> AC current density readings are achieved within seconds

Operation:The conical tip of the AC-10 coupon/probe must be in good contact with the soil to obtain an accurate measurement. Once this is done, the current density measurement is taken via a digital multimeter (DMM) and wire connections to the coupon/probe and the pipeline. As the coupon/probe is 10 cm2 in size, the conversion from milliamps (mA) to Amperes/Meter2 is a direct numeric conversion. Example: 25.0 mA (measured) = 25 A/m2 AC current density.

Provided EquipmentThe model AC-10 coupon probe (item #04-32150) it comes complete with two cable connection points, a spare 10cm2 conical tip and a hardened 6" steel “pilot hole” drive-pin for hard soil conditions.

Priced at $149.00/each. Call or go online to place your order today.

Click here for the Model AC-10 webpage