Corrosion in Soils

1
Corrosion in Soils

• Raymond F. Mignogna, MS, PE
• Metallurgical Engineer

2
ECONOMICS OF CORROSION

• In the United States alone, the cost of corrosion
  to the economy has been variously estimated at
  between 10 and 15 billion dollars annually.
• Worldwide, that figure balloons to over 45
  billion dollars.
• Corrosion of metals in soils represents a
  substantial portion of that cost.

3
THE SOIL CORROSION PROBLEM

• Whenever metals are in contact with soils, the
  potential for corrosion of one or more of them
  exists. In many cases, the corrosion can be
  severe, leading to catastrophic failure of
  structures or components. This presentation will
  describe the 6 factors that lead to corrosion of
  metals in soils, outline the basic mechanism of
  soil corrosion and select which strategy
  engineers should use to mitigate or avoid metal
  corrosion when designing facilities or equipment
  that will be in contact with soils.

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ISSUES RELEVANT TO SOIL CORROSION

• 1 There are 6 factors that affect the corrosion
  of metals in contact with soils.
• 2 The relative corrosivity of soils can be
  described as a function of level of aeration,
  water retention, dissolved salt content, soil
  resistivity, acidity, and presence of ionic
  species.
• 3 The process of galvanic action when metals
  are in contact with soils.
• 4 The two primary soil corrosion mitigation
  strategies used in modern engineering practice.
• 5 Two metals are most commonly used as
  sacrificial anodes in soil corrosion protection.

5
OUTLINE

• Affected Facilities
• Factors Affecting Corrosion
• Soil Corrosivity
• Corrosion Mechanisms
• Corrosion Control Methods
• Sacrificial Anodes
• References
• Additional Questions

6
Affected Facilities

• Buried Structures
• Underground Storage Tanks
• Transmission Distribution Pipelines
• Foundations
• Cables
• Any structure in full or partial contact with the
  earth

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Corrosion Damage

• Reduced Life of Structures
• I-35 Bridge Collapse
• Direct Environmental Degradation
• i.e. Oil Spills
• Cost to Domestic Economy
• (gt10 Billion/year)
• Cost In Lives and Environmental Damage
• Incalculable

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Factors Affecting the Corrosion Process

• 1 - Aeration
• 2 - Water retention
• 3 - Dissolved Salt Content
• 4 - Soil Resistivity
• 5 - Soil Acidity
• 6 - Presence of Ionic Species

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AerationMore Air Less CorrosionDrier
Environment ReducesGalvanic Action

• Order of Increasing Corrosion
• Gravels
• Coarse Sands
• Fine Sands

10
Water RetentionMore Water
More Electrolyte
More Corrosion
11
Dissolved Salt Content

• More Dissolved Salt Higher Conductivity
• Higher Conductivity Greater Corrosivity

12
Soil Resistivity

• Greater Resistivity Less Current Flow
• Less Current Flow Lower Corrosion Rate

13
Resistivity vs Corrosivity

• Soil Resistivity,(ohm-cm) Corrosivity
• 0 500 Very corrosive
• 500 - 1000 Corrosive
• 1000 2000 Moderately corrosive
• 2000 10,000 Mildly corrosive
• gt 10,000 Negligible
  corrosivity

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Soil Acidity

• Steels greater corrosion in acid soils
• -- passive in neutral/alkaline
  soils
• Aluminum passive in neutral soils
• -- greater corrosion in
  strong acid
• or alkaline soils

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Ionic Species and Microbes

• Halide ions (i.e. Chloride) and Active Bacteria
  Produce an Acid Environment

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Active Bacteriaare fed bySulfate Ions (SO4-)

• Sulfate Concentration,ppm Corrosivity
• gt10,000
  Severe
• gt1500 10,000
  Corrosive
• gt150 1500
  Moderate
• lt 150
  Negligible

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Corrosion Mechanism

• Galvanic Action is the primary corrosion
  mechanism in soils
• Stray-current corrosion is a significant
  secondary form, unique to buried structures

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Galvanic Corrosion

• Dissimilar materials are in contact
• Two different metals or alloys
• Same nominal alloy in different environments
• Copper alloy valves/steel piping
• Result is accelerated steel corrosion
• Steel alloy in soil having a conductivity gradient

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Dissimilar Metal Corrosion in Neutral Soils and
Water
Zinc (V -1.1)
Copper (V -.2)
Ion Flow
Cathode
Anode
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CHEMICAL REACTION

• Zn Zn 2 2 e-
• Cu 2 e- Cu -2

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Corrosion Cell on Buried Metal Surface
SOIL
Electric Current Flow
Cathode
Anode
Ionic Current Flow
Good Aeration Region
Poor Aeration Region
22
Stray-Current Corrosion

• External Induced Electrical Current
• Independent of environmental factors
• Currents follow paths other than their intended
  circuits due to
• Poor electrical connections
• Poor insulation

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Corrosion Control

• Cathodic Protection Applied Current
• Sacrificial Anodes

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Impressed Current Protection
Anode
Cathode

• Impressed Current
• Requires a power supply and buried anode
• Makes structure into the cathode of an electric
  circuit

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i
-

Power Supply
AIR
GROUND
Anode
Structure (cathode)
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SACRIFICIAL ANODE
SOIL
Structure (Steel)
Wire
Anode (Zn or Mg)
Ion Flow
Zn Zinc Mg Magnesium
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ANODE PLACEMENT

• Remote Anodes 50-100 yards or more from
  structure. Uniform current flow.
• Close Anodes within a few yards. Higher
  current to localized region.
• Linear Anodes ribbon/wire. Used primarily for
  pipelines.

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Modern Practice

• Cathodic Protection used in conjunction with
  coatings on structures.
• Provides a reduction of power and equipment costs
  to 5/10 of cost of cathodic protection alone.
• Generally results in complete protection.

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SUMMARYWHAT WEVE DISCUSSED

• The Soil Corrosion Problem
• Factors Affecting the Process
• Corrosion Mechanisms
• Corrosion Control Methods
• Sacrificial Anodes
• Current Practice

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REFERENCES

• 1 Corrosion Understanding the Basics J.R.
  Davis, ed., ASM (2000)
• 2 Handbook of Corrosion Engineering Pierre R.
  Roberge, McGraw-Hill (1999)
• 3 Practical Handbook of Corrosion Control in
  Soils Sam Bradford, CASTI (2001)

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QUESTIONS? COMMENTS? NEED MORE
INFORMATION? Please email me at
raymond_at_mignogna.org or visit www.mignogna.net