Transcript Corrosion in Soils

Corrosion in Soils
Raymond F. Mignogna, MS, PE
Metallurgical Engineer
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.
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.
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.
OUTLINE
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Affected Facilities
Factors Affecting Corrosion
Soil Corrosivity
Corrosion Mechanisms
Corrosion Control Methods
Sacrificial Anodes
References
Additional Questions
Affected Facilities
• Buried Structures:
– Underground Storage Tanks
– Transmission & Distribution Pipelines
– Foundations
– Cables
• Any structure in full or partial contact with
the earth
Corrosion Damage
• Reduced Life of Structures
– I-35 Bridge Collapse
• Direct Environmental Degradation
– i.e. Oil Spills
• Cost to Domestic Economy
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(>$10 Billion/year)
• Cost In Lives and Environmental Damage
– Incalculable
Factors Affecting the
Corrosion Process
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1 - Aeration
2 - Water retention
3 - Dissolved Salt Content
4 - Soil Resistivity
5 - Soil Acidity
6 - Presence of Ionic Species
Aeration
More Air = Less Corrosion
Drier Environment Reduces
Galvanic Action
Order of Increasing Corrosion:
• Gravels
• Coarse Sands
• Fine Sands
Water Retention
More Water =
More Electrolyte =
More Corrosion
Dissolved Salt Content
More Dissolved Salt = Higher Conductivity
Higher Conductivity = Greater Corrosivity
Soil Resistivity
• Greater Resistivity = Less Current Flow
• Less Current Flow = Lower Corrosion Rate
Resistivity vs Corrosivity
Soil Resistivity,(ohm-cm) Corrosivity
0 – 500
500 - 1000
1000 – 2000
2000 – 10,000
> 10,000
Very corrosive
Corrosive
Moderately corrosive
Mildly corrosive
Negligible corrosivity
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
Ionic Species and Microbes
• Halide ions (i.e. Chloride) and Active
Bacteria Produce an Acid Environment
Active Bacteria
are fed by
Sulfate Ions (SO4-)
Sulfate Concentration,ppm
Corrosivity
>10,000
>1500 – 10,000
>150 – 1500
< 150
Severe
Corrosive
Moderate
Negligible
Corrosion Mechanism
• Galvanic Action is the primary corrosion
mechanism in soils
• Stray-current corrosion is a significant
secondary form, unique to buried
structures
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
Dissimilar Metal Corrosion in
Neutral Soils and Water
Zinc (V = -1.1)
Copper (V = -.2)
Cathode
Ion Flow
Anode
CHEMICAL REACTION
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Zn
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Cu + 2 e-
Zn +2 + 2 e-
Cu -2
Corrosion Cell on Buried Metal Surface
SOIL
Electric Current Flow
Cathode
Anode
Ionic Current Flow
Poor Aeration Region
Good Aeration Region
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
Corrosion Control
• Cathodic Protection – Applied Current
• Sacrificial Anodes
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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Power Supply
AIR
GROUND
Structure (cathode)
Anode
SACRIFICIAL ANODE
SOIL
Structure
(Steel)
Wire
Anode
(Zn or Mg)*
Ion Flow
* Zn = Zinc; Mg = Magnesium
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.
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.
SUMMARY
WHAT WE’VE DISCUSSED
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The Soil Corrosion Problem
Factors Affecting the Process
Corrosion Mechanisms
Corrosion Control Methods
Sacrificial Anodes
Current Practice
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)
QUESTIONS?
COMMENTS?
NEED MORE
INFORMATION?
Please email me at
[email protected]
or visit
www.mignogna.net