Raymond F. Mignogna, MS, PE Metallurgical Engineer 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 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 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 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 Bacteria are 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

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

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

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

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

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 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 visit www.mignogna.net