How Do You Select an Appropriate MMO Anode for a Cathodic Protection System?

How Do You Select an Appropriate MMO Anode for a Cathodic Protection System?

The choice between impressed and sacrificial cathodic protection depends many factors and may be just personal preference. There are, however, situations where one or the other provides the correct choice. The advantages and disadvantages of each type of CP system are described in below table.

Advantages and Disadvantages of Impressed Current and Sacrificial Anode CP Systems.

Impressed Current	Sacrificial Anodes
Advantages
Variable control of current and potential	Self contained
Can be automated	Can be self adjusting
Light weight and fewer anodes	Polarity of connections always correct
Varied anode geometry	Needs no supervision
Long life with inert anodes	Simple to install
Disadvantages
Complex installation and maintenance	Expensive method of generating electricity
Requires external power source	No variable control
Anodes require dielectric shields	Anodes add weight
Anodes may be damaged	Anodes have finite life
Probability of stray current corrosion	Small lead resistance reduces current

How do you select an appropriate MMO anode for a cathodic protection system?
Anodes, for both impressed current and sacrificial anodes, are selected according to their size and chemical composition. This determines the current output and design life. Specifications for impressed current anodes are provided in Table 1 and for sacrificial anodes in Tables 2 and 3.
    
Table 1 Impressed Current Anodes

Anode Material	Recommended Current density A/m2	Maximum Voltage, V	Consumption Rate, g/A-yr	Comments
Scrap Steel	Varies	-	200 - 9,000	Difficult life rediction
Graphite	10	-	30 - 450	Very brittle
Silicon-ChromiumCast Iron	10 - 100	-	90 - 250	Very brittle
Lead-Silver	250 - 500	-	30 - 90	Heavy, Poor mechanical properties
Lead-Platinum	100	-	2 - 60
Magnetite	10 - 500	-	40	Very Brittle
Platinized Titanium	250 - 700	9	0.01	5 μm thick Pt film provides 10 year life
Platinized Tantalum	500 - 1000	100	0.01	5 μm thick Pt film provides 10 year life
Platinized Columbium	500 - 1000	100	0.01	5 μm thick Pt film provides 10 year life
Lithium-Ferrite Ceramic	15 - 2000	9.7	1-2	Lightweight and ough

Table2  Sacrificial Anode Types and Use.   

Anode	Preferred Use	Approx. Potential Volts ref. Ag/AgCl
Magnesium, High Potential	Soils with resistance > 2000 Ω-cm	-1.75
Magnesium, Standard	Soils with resistance < 2000 Ω-cm, and in aqueous environments with controllers if necessary	-1.50
Zinc, Hi-Amp	Seawater, brackish water, saline mud. Temps < 60o C	-1.05
Zinc, Hi-Purity	Underground, fresh water, and saline environments > 60o C	-1.05
Galvalum I	Submerged seawater, max. temp 25 oC	-1.05
Galvalum II	Saline mud	-1.04
Galvalum III	Seawater, brackish water, saline mud	-1.10
Reynode		-1.05
Al-Sn-In Alloy		-1.05

Table3  Sacrificial Anode Properties.

Property	Anode Material Type
	Magnesium	Zinc	Galvalum 1	Galvalum II	Galvalum III
Density, kg/m3	1940	7130	2700	2700	2700
Electrochem Equiv, g/coulomb	0.126E-3	0.339E-3	0.093E-3	0.093E-3	0.093E-3
Theoretical Ah/Kg	2,205	819	2,987	2,987	2,987
Current Efficiency %	0.55	0.95	0.95	0.57	0.85
Actual Ah/Kg	1,212	780	2,830	1,698	2,535
Actual Kg / Amp / Year	7.95	11.25	3.10	5.16	3.46
Potential V, ref. Ag/AgCl	-1.75	-1.05	-1.05	-1.04	-1.10