Corrosion in Soils

Raymond F. Mignogna, MS, PEMetallurgical 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

• 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– (>$10 Billion/year)

• Cost In Lives and Environmental Damage– Incalculable

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

Aeration

More Air = Less CorrosionDrier 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 Very corrosive

500 - 1000 Corrosive

1000 – 2000 Moderately corrosive

2000 – 10,000 Mildly corrosive

> 10,000 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 Bacteriaare fed by

Sulfate Ions (SO4-)Sulfate Concentration,ppm Corrosivity

>10,000 Severe

>1500 – 10,000 Corrosive

>150 – 1500 Moderate

< 150 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

Copper (V = -.2) Zinc (V = -1.1)

Dissimilar Metal Corrosion in Neutral Soils and Water

Cathode AnodeIon Flow

CHEMICAL REACTION

• Zn Zn +2 + 2 e-

• Cu + 2 e- Cu -2

Electric Current Flow

Ionic Current Flow

AnodeCathode

SOIL

Corrosion Cell on Buried Metal Surface

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

• Impressed Current

• Requires a power supply and buried anode

• Makes structure into the cathode of an electric circuit

Anode

Cathode

Structure (cathode) Anode

Power Supply

+- i

GROUND

AIR

SOIL

Structure(Steel)

Anode(Zn or Mg)*

Wire

SACRIFICIAL ANODE

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.

SUMMARYWHAT WE’VE DISCUSSED

• 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 raymond@mignogna.org

or visitwww.mignogna.net