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Source: 2002 Study by CC Technologies

• Direct cost of metallic corrosion in US: $276 billion/year or 3.1% of GNP

• Direct costs include use of more expensive material, labor, equipment, lost revenue, etc.

• Indirect costs such as lost productivity estimated to be equal to direct costs

• Largest sector: utilities, in particular, drinking water and sewer systems: $36B/year

• Comparisons:

Health care $2.3 trillion

Defense budget $533 billion

Cost of Metallic Corrosion in US

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Corrosion can have catastrophic consequences:

• Boilers and other pressure vessels

• Submarines and ships

• Pipelines

• Nuclear power plants and waste containers

• Bridges

• Aging aircraft Aloha Airlines incident, 1988

Corrosion and Safety

San Bruno, CA pipeline rupture

I-35 bridge collapse

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Conservation of Resources

The world’s natural resources are limited.

Corrosion is simply extractive metallurgy in reverse. Metal returns to its natural state Energy is wasted

D D

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“ ”

Electrochemical Nature of Corrosion

CorrosionCorrosion

Corrosion results from the oxidation of the metal to either (i) a soluble metal ion or complex, or (ii) an insoluble oxide, hydroxide or other salt.

Reactions of most metals with oxygen is thermodynamically favored – kinetics dictate significance of corrosion.

O2 + 4H+ + 4e- ↔ 2H2O E0 = 1.23 V

M+n + ne- ↔ M Eo << 1.23 V

Metal

ions

e-Anodic zone

M → Mn+ + ne-

Cathodic zoneO2 + 4H+

+ 4e- → 2H2O

CorrosionCorrosion

Oxidizing agents are either oxygen, water or hydrogen ions

M + n/4O2 + n/2H2O → Mn+ + nOH-

M + n/4O2 + nH+ → Mn+ + n/2H2O

M + nH+ → Mn+ + n/2H2

M + nH2O → Mn+ + n/2H2 + nOH-

Charge balance must be maintained.

CorrosionCorrosion

Rate of corrosion = jcorr (A/cm2)

Mass loss/change = ∆w (kg/m2-s) = jcorrM/nF

Active corrosion

Passivation Region

Breakdown (oxide conversion)

Typical slow scan i-E curve for an active/passive metal (e.g., stainless steel)

CorrosionCorrosion

Pitting and crevice corrosion are the two major forms of localized corrosion.

Localized metal dissolution occurs leading to the formation of cavities within a passivated area. Presence of halides or other salts facilitates the breakdown of passivating oxide layers and leads to enhanced rates of localized corrosion.

CorrosionCorrosion

CorrosionCorrosion

CorrosionCorrosion

Thermodynamics

∆Grxn = - nFEocell

Eocell = Eo

C – EoA >0

CorrosionCorrosion

Kinetics

Ecorr = Eeq at Ecorr -IC = IA = Icorr

Ecorr depends on the dynamics of the two reactions.

IC and IA at Ecorr are called the partial exchange currents.

IA M → Mn+ + ne-

E

Ecorr

2H2O + 2e- → H2 + 2OH-

Icorr α Jcorr α ∆wIC

Log I = Log Io + αAnFη2.3 RTLog I

E

CorrosionCorrosion

Procedures for Corrosion Protection

Cathodic protection = potential of a metal is shifted more negatively where rate of corrosion is less. Lower IA but increase IC.

Hydrogen embrittlement – absorb H2 in grain boundaries and changes metal structure.

Add sacrificial metal (Al, Zn, Mg)

Log I

E

Anodic protection = shift potential of metal into a range where passivation occurs. Can be performed prior to use – formation of passivating oxide film.

CorrosionCorrosion

Modification of the medium

Removal of O2 from the system.

Addition of oxygen scavengers (N2H4).

Add cheap base to remove H+ (lime).

Add complexing agents to remove aggressive ions.

Inhibitors to act via adsorption and block reaction sites on metal.

Paints and coatings.

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Consider a metal electrode immersed in a solution:

Electrochemical Potential

• Potential drop across metal electrode / electrolyte interface from charge separation.

• Potential drop across reference electrode / electrolyte.

• No potential gradient or concentration gradient in solution.

• Voltage difference between RE sample is measure of potential drop at sample/electrolyte interface.

metal/solution

potential drop

E

measured

voltage or

potential

from charge separation

at interface

V

metal in

solution

It is impossible to measure the absolute value of any half-cell electrode potential; only a cell potential can be measured using a second electrode (reference electrode).

RE:

reference

electrode

RE/solution

potential drop

We often refer to the potential drop as a potential, especially when it is measured vs a reference electrode.

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Electrolytic Migration Migration is the cause of failure in some microelectronic applications

+ -

H2O

+ -

Al (Cl-)

Ag

Ag Ag+ + e-

Ag+ + e- Ag

substrate

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This charge separation at the interface, called the Electrical Double Layer, was suggested by Helmholtz to have a structure similar to a capacitor.

Structure of the Interface

Shrier, Corrosion

One plate of the capacitor is thought to be the excess electrons at the metal surface (inner Helmholtz plane, IHP) and the other is a plane of excess positively charged ions in solution adjacent to the surface (outer Helmholtz plane, OHP). Note that the charge could be the other way around. The electric field varies linearly and is extremely large, on the order of 107 V/cm. This is the reason why electrons can easily cross the interface during electrochemical reactions.

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There are two possible structures for the Helmholtz double layer, which are related to the waters of solvation associated with ions in solution.

Structure of the Interface

Note: arrows around cations should point in: +-

Small cations are so tightly bound to their solvent sheath that contact adsorption is not possible. The larger the cation, the greater the tendency to adsorb directly on the surface. Anions tend to adsorb directly on the surface. Cations are attracted to surfaces with negative charge and anions to surfaces with positive charge. Adsorbed solvent dipoles tend to be oriented like those in the ion solvation sheath.

Shrier, Corrosion

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The simplified Helmholtz double layer has limited applicability, and is unable to explain certain experimental observations (i.e. electrocapillary measurements on Hg).

Structure of the Interface

Gouy and Chapman later suggested that the thermal energy of the ions would result in a diffuse layer with the concentration of excess counter ion at a maximum close to the electrode surface and gradually decreasing with distance into the electrolyte (on the order of nm).

Shrier, Corrosion

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The Gouy-Chapman diffuse layer model also doesn’t match well with all experimental data. Stern then synthesized the Helmholtz and Gouy-Chapman models with the concept of two regions of charge separation, with the interphase region as a whole remaining electrically neutral: In concentrated solutions, most of the charge is in the Helmholtz double layer, and the diffuse region can be neglected. Conversely, in dilute solutions, the diffuse region dominates.

Structure of the Interface

• Region from electrode to OHP where

potential varies linearly.

• Region from OHP into the solution where potential decays exponentially.

Shrier, Corrosion

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Contact adsorption of anions and large cations is possible because the hydration sheath does not cover the entire surface of the ion and adsorption can occur with relatively minor disturbance of the sheath.

Structure of the Interface

Contact adsorption is chemical in nature; negatively-charged ions may even adsorb on a negatively-charged electrode. A triple layer structure may result:

Bockris and Reddy, Modern Electrochemistry