corrosion in soils
DESCRIPTION
Describes the basic mechanism of soil assisted corrosion of metals and strategies to mitigate/control its effects.TRANSCRIPT
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Corrosion in Soils
Raymond F. Mignogna, MS, PEMetallurgical Engineer
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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.
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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.
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OUTLINE
• Affected Facilities• Factors Affecting Corrosion
• Soil Corrosivity • Corrosion Mechanisms • Corrosion Control Methods • Sacrificial Anodes • References• Additional Questions
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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– (>$10 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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Aeration
More Air = Less CorrosionDrier Environment Reduces
Galvanic Action
Order of Increasing Corrosion:
• Gravels
• Coarse Sands
• Fine Sands
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Water Retention
More Water = More Electrolyte = More Corrosion
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Dissolved Salt Content
More Dissolved Salt = Higher Conductivity
Higher Conductivity = Greater Corrosivity
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Soil Resistivity
• Greater Resistivity = Less Current Flow
• Less Current Flow = Lower Corrosion Rate
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Resistivity vs Corrosivity
Soil Resistivity,(ohm-cm) Corrosivity
0 – 500 Very corrosive
500 - 1000 Corrosive
1000 – 2000 Moderately corrosive
2000 – 10,000 Mildly corrosive
> 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 by
Sulfate Ions (SO4-)Sulfate Concentration,ppm Corrosivity
>10,000 Severe
>1500 – 10,000 Corrosive
>150 – 1500 Moderate
< 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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Copper (V = -.2) Zinc (V = -1.1)
Dissimilar Metal Corrosion in Neutral Soils and Water
Cathode AnodeIon Flow
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CHEMICAL REACTION
• Zn Zn +2 + 2 e-
• Cu + 2 e- Cu -2
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Electric Current Flow
Ionic Current Flow
AnodeCathode
SOIL
Corrosion Cell on Buried Metal Surface
Poor Aeration Region Good Aeration Region
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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
• Impressed Current
• Requires a power supply and buried anode
• Makes structure into the cathode of an electric circuit
Anode
Cathode
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Structure (cathode) Anode
Power Supply
+- i
GROUND
AIR
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SOIL
Structure(Steel)
Anode(Zn or Mg)*
Wire
SACRIFICIAL ANODE
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 WE’VE 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 [email protected]
or visitwww.mignogna.net