CP Design Center

Introduction

Magnesium anodes are the primary sacrificial anode used for underground cathodic protection. There is a great deal of technical information and many publications detailing the application of magnesium anodes. This review is intended to provide summary information and brief guidelines in the selection of anodes.

Magnesium Alloy

Two alloys are commonly used for cathodic protection applications. These alloys are generically known as High-Potential Magnesium and H-1 alloy. High potential alloys are produced from primary magnesium refined from saltwater. This alloy provides the maximum output with open circuit voltage exceeding -1.70 volts relative to copper sulfate. H-1 alloy also known as AZ-63 is produced from secondary magnesium sources through recycling facilities. This alloy provides a much lower driving voltage in the range of -1.40 volts relative to copper sulfate. The selection of alloy requires consideration of current requirements, soil resistivity, and costs. In most cases where current demand requires the use of multiple anodes, high potential alloy is the most economical choice.

Standard chemistry requirements for the two alloys are:

High Potential Anode Chemistry
Per ASTM B843 Industry Standard for high potential magnesium anodes
Aluminum 0.01% max
Manganese 0.50 - 1.3%
Copper 0.02% max
Silicon 0.05% max
Iron 0.03% max
Nickel 0.001% max
Others, each 0.05% max
Magnesium Remainder
Element
H-1 Magnesium Alloy Chemistry
Content %
Grade A
Grade B*
Grade C
Aluminum 5.3-6.7 5.3-6.7 5.3-6.7
Zinc 2.5-3.5 2.5-3.5 2.5-3.5
Manganese .15 Min. .15 Min. .15 Min.
Silicon .10 Max. .20 Max. .30 Max.
Copper .02 Max. .05 Max. .10 Max
Nickel .002 Max .003 Max .003 Max
Iron .003 Max. .003 Max. .003 Max.
Other Impurities .20 Max.
.20 Max.
.30 Max.
Magnesium Remainder Remainder Remainder

* The H-1 alloy supplied is Grade A .



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