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Galvanic Anodes

Intermediate Corrosion Course 2017

February 21-23, 2017 Eric Langelund – Piping & Corrosion Specialties, Inc. 1

Period 3

Overview

• Definitions and Terminology • Galvanic Anodes – An Overview

– Magnesium – Zinc

• Testing • Sample Calculations – Anode Current and Life


2017 Underground Corrosion Short Course Intermediate Corrosion Course – Period 3

Definition of Corrosion

• Practical Definition – – The Tendency of a metal to revert to its native

state – Current discharge

• Golden Rule of Corrosion, if current discharges off the metallic surface, then there will be metal loss, resulting into corrosion

• Scientific Definition – – Electrochemical degradation of metal as a result

of a reaction with its environment



IRON OXIDE BLAST FURNACE BESSEMER

PIPE MILL STEEL PIPE

PIPE CORRODING IRON OXIDE

REFINING PROCESS CORROSION PROCESS

Iron Ore is electrically charged through the molding process.

Once formed into a pipeline, and placed into the ground, the pipe will lose it’s electrical charge and try to resort back to it’s natural state.

Corrosion Process



Electro-Chemical Process of Corrosion

• The Basic Four Elements to create a galvanic corrosion cell – Anode – on the galvanic series charts, the metal that is the

most negative charged and less noble (The area were corrosion will occur)

– Cathode – on the galvanic series charts, the metal that is the most positive charged and more noble (The area that will be cathodically protected)

– Electrolyte – any substance that can conduct electricity, the surrounding environment of the anode and the cathode

– Metallic Connection – any metallic connection between the anode and the cathode that exist



Electro-Chemical Process of Corrosion

• Conventional Current Flow – Known as the flow of electrical current for design and

theory purpose – Where current will flow from the “+” to “-” direction

• Electron Flow – Contribute to ion movement – The movement of the electrons from the “-” to the “+”

direction – Opposite of Conventional Current Flow



Corrosion Cell on a Pipeline

Anode Cathode

Microscopic Corrosion Cell on the Surface of a Pipeline

Remember the golden rule of Corrosion – @ the point of current discharge, metal loss will occur (Corrosion)



Galvanic Series Chart



Cathode Anode

Positive Negative

Metallic connection

Electrolyte Conventional Current Flow

(positive)”+”

to

(negative)”-”

Corrosion Golden Rule

Any time Current discharge from a metallic surface, then metal loss will occur (Corrosion)

-.600 V -1.700 V February 21-23, 2017 Eric Langelund – Piping & Corrosion Specialties, Inc. 9


The Electrical-Chemical Process of Corrosion

• The conventional current flow path – The current will flow from the Cathode electrode

to the Anode electrode through the metallic connection

– The current will flow from the Anode to the Cathode through the electrolyte

– Returning back to the Cathode surface – Impressing current onto the surface, applying

cathodic protection




• In a nut shell, – Galvanic anodes are typically high in a negative

potential, so that there is a significant driving potential, when connected to the cathode (for example – pipeline)

– The driving potential (Open circuit potential) will decide the amount of CP currents to be applied

• Driving force




• In a nut shell, continue – The currents will travel from the Cathode to the Anode

through the lead wire (metallic Connection) – The currents will leave the anode surface, causing

corrosion, travelling to the Cathode surface (holiday areas) through the electrolyte

– A protective film is created onto the surface of the Cathode

• Also known as Cathodic Protection – The anode sacrifices, it’s self, to protect the pipelines

(Cathode) – Better known as Sacrificial Anodes




Cathode Anode

Positive Negative

Metallic connection

Electrolyte

Electron Flow

HO-

Electron Flow

(negative)”-”

to

(positive)”+” HO-

HO-

HO-

H+

H+

H+

H+

H2O

H2O

H2O

e- e-

e- e-

e-

e-

e- Mg+

Mg+

Mg+

Mg+ Mg+

-.600 V -1.700 V

H2O (water molecule)




Cathode Anode

Positive Negative

Metallic connection

Electrolyte

Electron Flow

HO-

HO-

HO-

HO-

H+

H+

H+

H+

H2O

H2O

H2O

e- e-

e- e-

e-

e-

e- Mg+

Mg+

Mg+

Mg+ Mg+

As the Cathode gains the “e-” (electrons), it approaches to a more negative state

-1.000 V -

+




Cathode Anode

Positive Negative

Metallic connection

Electrolyte

Electron Flow

HO-

HO-

HO-

HO-

H+

H+

H+

H+

H2O

H2O

H2O

e- e-

e- e-

e-

e-

e- Mg+

Mg+

Mg+

Mg+ Mg+

As the Anode loses the “e-” (electrons), it approaches to a more positive state

-1.200 V

-

+




Cathode Anode

Positive Negative

Metallic connection

Electrolyte

Electron Flow

HO-

HO-

HO-

HO-

H+

H+

H+

H+

H2O

H2O

H2O

e- e-

e- e-

e-

e-

e- Mg+

Mg+

Mg+

Mg+ Mg+

-1.200 V -1.000 V =

Polarization Process

The Cathode becomes more Negative

The Anode becomes more positive




Polarization Process

Cathode Anode

Positive Negative

Electrolyte

Electron Flow

HO-

HO-

HO-

HO-

H+

H+

H+

H+

H2O

H2O

H2O

e- e-

e- e-

e-

e-

e- Mg+

Mg+

Mg+

Mg+ Mg+

-1.200 V -1.000 V =

Now, what is the potential difference ?




• The water molecule will separate in the electrolyte allowing a surplus of Positive charge Hydrogen ions and Negative charge Hydroxyl ions

• The positive charged Hydrogen ions will meet the negative charged electrons on the surface of the Cathode electrode creating a Hydrogen film, also known as polarization

• The Negative charge Hydroxyl ions will meet the positive charge metal ions at the surface of the Anode electrode creating oxidation (rust); also known as Corrosion



Preventing Corrosion

• Remove the metallic connection – Insulators

• Remove the electrolyte – Coatings

• Remove the Anode and Cathode – Cathodic and Anodic polarization

• Galvanic Anodes • Impressed Systems



Corrosion Effects on Carbon Steel Pipelines



Defense against Corrosion

• Coatings are the number One defense against corrosion – But do not provide a 100% protection – Due to coating flaws known as “Holidays”, we

need a secondary supplemental application of Cathodic Protection; such as Galvanic or Impressed anode systems



Galvanic Anodes - Types

• Magnesium – Fresh Water – Soil

• Zinc – Low Resistance Soil

• < 1500 Ω CM • Brackish water

• Aluminum – Salt Water – Saline Muds



Candidates for Galvanic Anodes • Low Cathodic Protection Currents requirements

– Typically < 1 amp • Isolated fittings • Small cross areas

• No Access to any AC power sources • Limited UV Exposures • Low Soil Resistivity

– Typically < 10,000 Ω CM • Distribution in high resistance soils

– Distributed application to lower driving potentials – Anodes can be placed relatively close to the pipeline structure, along a

considerable length of the pipeline circuit • In areas where interference with other structures may occur

– High traffic areas with other foreign or self structures in the vicinity



Galvanic Anodes - Types

• The two most common for buried pipelines are Magnesium and Zinc galvanic anodes



Magnesium Anodes



Magnesium Anodes - Shapes

• Cast Ingots – Copper lead wires

• Used mainly in soil applications

– Steel straps (Marine)

• Extruded Ribbon • Caps



Varity Type of Magnesium Anodes



Magnesium Anodes - Composite

• HP Magnesium Anodes most popular use in the field application for carbon steel pipelines in the North America due to it’s high driving potentials

• HP Magnesium Anodes – 97% magnesium purity

• 3% - Aluminum and Zinc




• Normally packed in a low resistivity back fill – Provides low resistance around the anode so corrosion

will occur uniformly to extend the anode life • Attracts moisture from the soil

– Provides a homogeneous environment – Prevents passivation of the anode – Major two types of back fill

• 75% Gypsum – a hydrous form of Calcium sulfate • 20% Bentonite – (attracts moisture)

– 5% Sodium Sulfate



• Normally 10’ of #12 AWG coated copper solid wire connected to the magnesium bar




Magnesium Anodes – Applications

Drive-in anodes or known as spike anodes

Excellent for isolated risers



Magnesium Anodes – Applications

Anodes installed in water heaters

Magnesium slab Anodes on bulk head of boat



Zinc Anodes

• Normally used in Low soil resistance • Excellent ground source • Lower driving potential than Magnesium • Higher efficiency

– 90%



Zinc Anode - Composite

• 99.9% purity of Zinc • Open Circuit potential

– -1.1 Volts CSE

• Passive state with soils of high concentrates of – Oxygen Ions – Carbonates – Phosphates



Zinc Anodes - Composite

• Normally packed in a low resistivity back fill – Provides low resistance around the anode so corrosion will

occur uniformly to extend the anode life • Attracts moisture from the soil

– Provides a homogeneous environment – Prevents passivation of the anode

• Sulfate ion’s in the Gypsum back fill – Major two types of back fill

• 50% Gypsum – a hydrous form of Calcium sulfate • 50% Bentonite – (attracts moisture)



Zinc Anodes - Composite

Typically Type I is used for sea water applications



Zinc Anodes - Shapes • Cast

– Bars • Normally used with back fill material for soil applications

– Bracelets • Normally used in underwater applications

– Slabs – Buttons

• Normally used for exposed carbon steel bolts

• Extruded Ribbon – Normally used in AC mitigation

• Pencils and Rods – Normally used in isolated fittings



Zinc Anodes – Applications

Type 1 Cast Zinc Anodes – used on sea water ships – Maritime ship hulls



Zinc Anodes – Applications

Type II - Extruded Zinc Ribbon anode

Mainly used for AC interference mitigation



Anode Testing • Due to high amount of bad anodes received over seas,

testing has become more important than ever • Testing recommendation practice

– Third Party – ASTM G97 (Laboratory Evaluation of Magnesium Test Specimen for

Underground Applications) – Test pencils taken from a random selected batch

• Tested for a period of 14 days – During the test and at the conclusion of the test

• Open circuit potentials measured • Weight loss measured • Material Composite of the anode and the back fill material

– Two Key Factors • Open Circuit Potentials • Anode Efficiency



Anode Testing

• Magnesium – Open Circuit Potentials

• Norm – -1.7 in reference of a Calomel electrode or – -1.774 in reference of a copper-copper sulfate electrode

– Anode Efficiency • Norm

– 50% – 500 amps per Hour (amp/hr)



Anode Testing –Meaning of Results

• Open Circuit Potentials – Decides the output of the anode

• Less Open Circuit Potentials (-1.774 CSE), Less protective currents

• Anode Efficiency – Decides the life expectancy

• Less Eff. % (50%)



Cost of Testing

• Normally Expensive – In the range of $600.00 to $1,200.00 – Depending on the amount data requested

• In Comparison to possible failures of Anodes in the field, very in-expensive – Some anodes found with a life expectancy of 5 years or

less (based field trials and laboratory testing) – Rework frequency cost can exceed in the millions.



Anode Life Expectancy Formula – Zinc and Magnesium

Anode Life = 0.116 x anode weight (lbs) x eff. x Utilization Factor

Design Current (amperes)

Magnesium Anode Formula -

Zinc Anode Formula -

Anode Life = 0.0424 x anode weight (lbs) x eff. x Utilization Factor

Design Current (amperes)



Anode Life Expectancy

Anode Life = 0.116 x anode weight (lbs) x eff. x Utilization Factor Design Current (amperes)

Theoretical ampere-hour per pound of the anode material Given by manufacture.





Anode weight – Expressed in pounds of the magnesium bar not the entire package, for example, 17lb anode is the weight of the mag. bar, therefore, 17 would be the correct number to enter.





Efficiency percentage - Expressed in a decimal number, for an example, 50% would be expressed as .50





Utilization Factor – It represents the percentage of the anode being used before failure, for an example, magnesium anodes is 85% utilization, so it will be represented in the formula as .85





Design Current (amperes) – Express as the anode output with the surrounding environment, placed in the formula as a decimal format, for an example, 100 milliamps would be written as .100 amps




8.3 yrs Anode Life = 0.116 x 17(lbs) x .50 x .850 .100 amps

Sample calculation – Magnesium anode with an output of 100 milliamps

What if, efficiencies drop below 50%?


Life expectancy of anode has dropped

The efficiency is now at 30%





Sample calculation

What if, we could apply a small resistor into the circuit of the anode, and reduce the anode output?


Life expectancy of anode has increased (doubled)

The current output is reduced to half as much by adding resistance



Installation of Anodes • Never –

– Pick up the anode by the wire connections – Place the anode above the pipeline – Place the anode flat horizontally,

• Position with the wire end higher then the rest of the anode body – Pour water directly on pre-packaged anodes before back

filling, could cause voids around anode, loss of anode output

– Drop anode into excavation site – Place anode over voids, could cause fracture onto the

magnesium bar, loss of efficiency



Installation of Anodes

• Do’s – – Place the anode perpendicular from the pipeline

• Minimum distance of 2 to 3 feet • If possible, stretch to the complete length of the wire

with out applying tension to the point of connection • Place as deep as the bottom of the pipeline

– Recommend to place the anode at least one foot below the pipeline, if possible



Installation of Anodes

• Do’s – – Tamp the dirt firmly around the anode – If desire to kick start anode

• Apply water, only after several feet of back fill has been achieved and tamped

• If water applied to anode directly could result in voids and drop in anode effiecicy



Installation of Anodes • Can’s –

– Attach anode to a gathering wire to save pavement cost • Also known as “Banking anodes”

– Banking Anodes – • Use Sundae theory to calculate anode output due to spacing's

– Anode currents may fight against each other and this will cause a loss of the anode current output, resulting in the CP design falling short

• Recommend of using a minimum of # 8 AWG copper for the gathering wire

– Reduce circuit resistance • Follow the normal installation



Anode Installation - Types

Banking Anodes (use of a gathering wire)

Single Installation or Direct connection



Anode Installation – Sunde Theory Sunde Theory Chart Single Anode Current

Reduction Factors (C)

# of Anodes inConcentrated Bed Anode spacing

5 Feet 10 Feet 15 Feet 20 Feet

2 1.84 1.92 1.95 1.96

3 2.45 2.70 2.79 2.85

4 3.04 3.45 3.62 3.71

5 3.59 4.19 4.43 4.56

6 4.12 4.90 5.22 5.41

7 4.65 5.60 6.00 6.22

8 5.15 6.28 6.77 7.04

9 5.67 6.96 7.54 7.87

10 6.16 7.64 8.38 8.68

Design calculation used to compensate the current reduction based on spacing of the anodes







2 1.84 1.92 1.95 1.96

3 2.45 2.70 2.79 2.85

4 3.04 3.45 3.62 3.71

5 3.59 4.19 4.43 4.56

6 4.12 4.90 5.22 5.41

7 4.65 5.60 6.00 6.22

8 5.15 6.28 6.77 7.04

9 5.67 6.96 7.54 7.87

10 6.16 7.64 8.38 8.68

Sample Calculation -

CP Design – circuit needs 125 milliamps

What is the amount of anodes needed for the circuit at a spacing of 10 feet?

Based on soil resistivity – 5000 Ω CM



Anode Installation – Anode Output Calculation

120,000 • f • y

ρ = I m

120,000 • 1 • 1.21 5,000 ohms cm = 29.04 mA

120,000 is based on DA Tefankjian of magnesium anodes for good coating.

Soil Resistivity Magnesium Calculation

Sample Calculation







2 1.84 1.92 1.95 1.96

3 2.45 2.70 2.79 2.85

4 3.04 3.45 3.62 3.71

5 3.59 4.19 4.43 4.56

6 4.12 4.90 5.22 5.41

7 4.65 5.60 6.00 6.22

8 5.15 6.28 6.77 7.04

9 5.67 6.96 7.54 7.87

10 6.16 7.64 8.38 8.68

Sample Calculation -

CP Design – circuit needs 125 milliamps

What is the amount of anodes needed for the circuit at a spacing of 10 feet?

Based on soil resistivity – 5000 Ω CM

**Anode output 29 milliamps**



Anode Installation Design – Sunde Theory

• Number of anodes = It

• Total number of anodes needed = 125 milliamps divided by 29 milliamps • 4.3 anodes, round off to the number, “4” anodes

• Sunde theory reduction factor – 3.45 • 3.45 x 29 milliamps = 100 milliamps

– Based on Sunde theory, design will fall short of 25 milliamps – Another anode needs to be added to achieve the 125 milliamp

design

Ia



Anodes - Effective in Interference Remediation (AC and DC)

• Interference problems, – galvanic anodes can be used for mitigating the current

safely from the pipelines to prevent corrosion • Zinc anodes are excellent ground source for helping mitigating AC

currents – Zinc Ribbon – Zinc Grounding Cells – Zinc Voltage Gradient mats

• Magnesium anodes are some times used for mitigating DC interference currents, in place of a interference bond



In Conclusion

• If soil resistivity is < 1500 Ω CM, maybe best to use Zinc Anodes

• Test Anodes – Open Circuit Potentials – Efficiency

• Designing – Life Expectancy Calculation



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