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

4. 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

7. 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

8. 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

9. Aeration More Air = Less Corrosion Drier Environment Reduces Galvanic Action
Order of Increasing Corrosion:
Gravels
Coarse Sands
Fine Sands

10. Water Retention More 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 – Very corrosive
Corrosive
1000 – Moderately corrosive
2000 – 10, Mildly corrosive
> 10, Negligible corrosivity

14. 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

15. Ionic Species and Microbes
Halide ions (i.e. Chloride) and Active Bacteria Produce an Acid Environment

16. Active Bacteria are fed by Sulfate Ions (SO4-)
Sulfate Concentration,ppm Corrosivity
>10, Severe
>1500 – 10, Corrosive
>150 – Moderate
< Negligible

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

18. 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

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

20. CHEMICAL REACTION
Zn Zn e-
Cu + 2 e Cu -2

21. 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

23. Corrosion Control Cathodic Protection – Applied Current
Sacrificial Anodes

24. Impressed Current Protection
Anode
Cathode
Impressed Current
Requires a power supply and buried anode
Makes structure into the cathode of an electric circuit

25. i - +
Power Supply
AIR
GROUND
Anode
Structure (cathode)

26. SACRIFICIAL ANODE SOIL Structure (Steel) Wire Anode (Zn or Mg)*
Ion Flow
* Zn = Zinc; Mg = Magnesium

27. ANODE PLACEMENT
Remote Anodes – 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.

28. 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.

29. SUMMARY WHAT WE’VE DISCUSSED
The Soil Corrosion Problem
Factors Affecting the Process
Corrosion Mechanisms
Corrosion Control Methods
Sacrificial Anodes
Current Practice

30. 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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