Applied Electrochemistry
This presentation is the property of its rightful owner.
Sponsored Links
1 / 27

Applied Electrochemistry PowerPoint PPT Presentation


  • 111 Views
  • Uploaded on
  • Presentation posted in: General

Applied Electrochemistry. Dept. Chem. & Chem. Eng. Lecture 14 Metal Electrochemistry. Dept. Chem. & Chem. Eng. 1. Electrodeposition. 2. Corrosion. 3. Industrial electrolytic process. Outline. Definition.

Download Presentation

Applied Electrochemistry

An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -

Presentation Transcript


Applied electrochemistry

Applied Electrochemistry

Dept. Chem. & Chem. Eng.


Applied electrochemistry

Lecture 14

Metal Electrochemistry

Dept. Chem. & Chem. Eng.


Applied electrochemistry

1

Electrodeposition

2

Corrosion

3

Industrial electrolytic process

Outline


Applied electrochemistry

Definition

Corrosion is the deterioration of materials by chemical interaction with their environment.  The term corrosion is sometimes also applied to the degradation of plastics, concrete and wood, but generally refers to metals.


Applied electrochemistry

Corrosion:

--the destructive electrochemical attack of a material.

--Al Capone's ship, Sapona, off the coast of Bimini.

Cost:

--4 to 5% of the Gross National Product (GNP)*

--this amounts to just over $400 billion/yr**

Photos courtesy L.M. Maestas, Sandia National Labs. Used with permission.

* H.H. Uhlig and W.R. Revie, Corrosion and Corrosion Control: An Introduction to Corrosion Science and Engineering, 3rd ed., John Wiley and Sons, Inc., 1985.

**Economic Report of the President (1998).


Applied electrochemistry

The Rusting Mechanism (Peel)

  • 4Fe + 6H2O + 3O2 4Fe(OH)3

    • gives ferric hydroxide

  •  2Fe(OH)3 Fe2O3 3H2O

    • gives iron oxide (rust) and water

Basic “rusting” or corrosion requirements

1. The metal is oxidized at the anode of an electrolytic cell

2. Some ions are reduced at the cathode

3. There is a potential or voltage difference between the anode and cathode

4. An electrolyte (fluid) must be present

5. The electrical path must be completed


Applied electrochemistry

Corrosion of zinc in acid

• Two reactions are necessary:

-- oxidation reaction: Zn → Zn2+ + 2e-

-- reduction reaction: 2H+ + 2e → H2(gas)

Adapted from Fig. 17.1, Callister 6e. (Fig. 17.1 is from M.G. Fontana, Corrosion Engineering, 3rd ed., McGraw-Hill Book Company, 1986.)

• Other reduction reactions:

-- in an acid solution

-- in a neutral or base solution


Applied electrochemistry

-

+

Cd

Ni

25°C

1.0 M

1.0 M

2

+

2+

Cd

solution

Ni

solution

Standard EMF

• Metal with smaller

V corrodes.

• Ex: Cd-Ni cell

• EMF series

o

V

o

metal

metal

metal

Au

Cu

Pb

Sn

Ni

Co

Cd

Fe

Cr

Zn

Al

Mg

Na

K

+1.420 V

+0.340

- 0.126

- 0.136

- 0.250

- 0.277

- 0.403

- 0.440

- 0.744

- 0.763

- 1.662

- 2.262

- 2.714

- 2.924

o

DV =

0.153V

Data based on Table 17.1, Callister 6e.

5


Applied electrochemistry

Thermodynamic Driving Force

  • Like all chemical reactions – Thermodynamics

  • What is the driving force for the reaction? (otherwise stated as what is the electrochemical potential for the reaction)

    • Dissimilar metals

    • Different cold work states

    • Different grain sizes

    • Difference in local chemistry

    • Difference in the availability of species for a reaction (concentration cells)

    • Differential aeration cells


Applied electrochemistry

For:

1

1


Applied electrochemistry

  • Introduce: The total electropotential is

    • G = -nFE

Where:

F = Faraday’s constant (total charge on Avogadro’s number of electrons)

n = the number of electrons transferred

E = The electrode potential


Applied electrochemistry

Nernst Equation:

The Basic equation which describes ALL corrosion reactions

For the Given Example:

Note: pH = -log10[H+]


Applied electrochemistry

Pourbaix Diagram

Source: www.corrosionsource.com


Applied electrochemistry

EFFECT OF SOLUTION CONCENTRATION

• Ex: Cd-Ni cell with

standard 1M solutions

• Ex: Cd-Ni cell with

non-standard solutions

n = #e-

per unit

oxid/red

reaction

(=2 here)

F =

Faraday's

constant

=96,500

C/mol.

• Reduce VNi - VCd by

--increasing X

--decreasing Y

7


Applied electrochemistry

H 2H+ +2e-

0.1

for H2

io

ANODIC

2

0

E

on Fe

A

0.0

A

2H+ +2e- H2

-0.1

CATHODIC

-0.2

POTENTIAL VOLTS (SHE)

-0.3

Fe Fe+2 +2e-

ANODIC

0

io Fe

E

-0.4

B

B

-0.5

Fe+2 +2e- Fe

CATHODIC

-0.6

-0.7

-10

-9

-8

-7

-6

-5

-4

-3

-2

10

10

10

10

10

10

10

10

10

CURRENT DENSITY, A/cm²

Kinetics Describes Rate of Reaction Evan’s Diagram


Applied electrochemistry

Aa

= Area Inside Crevice (Anodic)

Ac

= Area Outside Crevice (Cathodic)

Aa << Ac

M M+ +e-

Aa

A

io

i

c

+

o

H+

H

Aa

io

E

O2

Ac

ia

E

O2

O2 /OH +

½ O2 + H2O + 2e- 2OH-

E

M/M+

icorrin very

aggresive environment

Crevice

Effect

No Crevice

PLUS Crevice

Effect

Effect

Log

i

Area Effects


Applied electrochemistry

Effect of Oxidizer Concentration (e.g., Oxygen) on the Electrochemical Behavior of an Active - Passive Metal

Increasing Oxidant

Concentration

M M+

Log i

[Fontanna and Greene, Corrosion Engineering, McGraw-Hill, 1967]


Applied electrochemistry

Passivity

Passivity is defined as corrosion resistance due to formation of thin surface films under oxidizing conditions with high anodic polariza tion.

For example Fe is passivated in concentrated nitric acid.


Applied electrochemistry

Effect of Temperature and Dissolved O2

35

LEGEND

30

FRESH WATER @ 50°F

FRESH WATER @ 90°F

FRESH WATER @ 120°F

25

VELOCITY = 2.5 FPS

pH=7, R=100M-0HM

20

pH=7, R=2500M-0HM

CORROSION RATE (MPY))

15

10

5

0

2

3

4

5

6

7

0

1

8

9

10

11

DISSOLVED O2 (PPM)

0

0.7

1.4

2.1

2.8

3.5

4.2

4.9

5.6

6.3

7.0

7.7

DISSOLVED O2 (Mg/H2O)


Applied electrochemistry

Forms of corrosion

• Stress corrosion

Stress & corrosion

work together

at crack tips.

• Uniform Attack

Oxidation & reduction

occur uniformly over

surface.

• Erosion-corrosion

Break down of passivating

layer by erosion (pipe

elbows).

• Selective Leaching

Preferred corrosion of

one element/constituent

(e.g., Zn from brass (Cu-Zn)).

• Pitting

Downward propagation

of small pits & holes.

Fig. 17.8, Callister 6e.

(Fig. 17.8 from M.G.

Fontana, Corrosion

Engineering, 3rd ed.,

McGraw-Hill Book

Company, 1986.)

• Intergranular

Corrosion along

grain boundaries,

often where special

phases exist.

• Galvanic

Dissimilar metals are

physically joined. The

more anodic one

corrodes.(see Table

17.2) Zn & Mg

very anodic.

• Crevice Between two

pieces of the same metal.

Fig. 17.6, Callister 6e. (Fig. 17.6 is courtesy LaQue Center for Corrosion Technology, Inc.)

Fig. 17.9, Callister 6e.


Applied electrochemistry

Types of Aqueous Corrosion Cells

  • a. General Corrosion

  • b. Localized Corrosion

    • Pitting

    • Crevice Corrosion

    • Under-deposit Corrosion

    • MIC

  • c. Tuberculation

  • d. Galvanic Corrosion


Applied electrochemistry

Factors Affecting Corrosion

  • Material properties

  • Metallurgical factors

  • Passivity

  • Environment

  • Metallurgical factors

  • Chemical segregation

  • Presence of multiple phases

  • Inclusions

  • Cold Work

  • Non-uniform stresses

  • Passivity

  • Example with steel in nitric acid…dilute solutions will cause rapid attack, strong solutions have little visible effect.

  • Surface film can be formed

  • Some types of steel may do this with rust

  • Aluminum does this

  • Need to watch passive film, but can be used for simple protection


Applied electrochemistry

Controlling corrosion

• Self-protecting metals!

--Metal ions combine with O2

to form a thin, adhering oxide layer that slows corrosion.

• Reduce T (slows kinetics of oxidation and reduction)

• Add inhibitors

--Slow oxidation/reduction reactions by removing reactants

(e.g., remove O2 gas by reacting it w/an inhibitor).

--Slow oxidation reaction by attaching species to

the surface (e.g., paint it!).

• Cathodic (or sacrificial) protection

--Attach a more anodic material to the one to be protected.

Adapted from Figs. 17.13(a), 17.14 Callister 6e. (Fig. 17.13(a) is from M.G. Fontana, Corrosion Engineering, 3rd ed., McGraw-Hill Book Co., 1986.)


Applied electrochemistry

Introduction

Corrosion Processes

Corrosion of Metals

Passivity of Metals

Atmospheric Corrosion of Steels


  • Login