lecture18 corrosion

Corrosion

Corrosion: a chemical or between a material and its

environment that produces a deterioration of the material and its properties

electrochemical reaction

Corrosion

Electrochemical reaction: chemical reactions in which elements are added or removed

from a chemical species and at least one of the species undergoes a change in the

number of valance electrons

Electrochemical Corrosion:

Wet (aqueous) - systems that corrode at low T with a liquid electrolyte

2

Wet (aqueous) - systems that corrode at low T with a liquid electrolyte

Dry (oxidation) - corrosion that occurs at high T (generally over 500˚C)

Aqueous:

Uniform or General

Galvanic or Two-metal

Crevice

Pitting

Intergranular

Velocity-assisted

Environment-assisted cracking

Oxidation:

High Temperature

No moisture nor dissolved electrolytes

Low Temperature

surfaces where T< 135°C

condensation of the acidic sulphur and

chlorine-containing gases

Liquid Metal Embrittlement:

- The dissolution-diffusion model

adsorption of the liquid metal on the solid metal induces dissolution and inward

diffusion

under stress these processes lead to crack nucleation and propagation

- brittle fracture theory

adsorption of the liquid metal atoms at the crack tip weakens inter-atomic bonds and

propagates the crack

Chemical Corrosion

propagates the crack

- diffusion penetration

penetration of liquid metal atoms to nucleate cracks which under stress

- ductile failure model

adsorption of the liquid metal leads to weakening of atomic bonds and nucleation of

dislocations which move under stress, pile-up and work harden the solid.

dissolution helps in the nucleation of voids which grow under stress and cause ductile

failure

Examples: Hg and aluminum alloys, Bi and copper alloys 3

Electrochemical Corrosion - Oxidation

Low Temperature


condensation

acidic sulfur


High Temperature:

no moisture nor dissolved electrolytes

e.g. direct reaction with oxygen

Y2O3 Y2O3-X

Fe Fe O

4

2 3 2 3-X

Fe Fe2O3

Oxidation – Ellingham Diagram

Low Temperature


condensation

acidic sulfur


High Temperature:

no moisture nor dissolved electrolytes

e.g. direct reaction with oxygen

Y2O3 Y2O3-X

Fe Fe O

5

2 3 2 3-X

Fe Fe2O3

4Fe + 3O2 2Fe2O3

∆G = ∆G0 + RT ln k

K = [C]c [D]d

[A]a [B]b

K= Po

Used for oxidation and reduction of ore

2

Galvanic Corrosion

Zn(s) Zn2+ + 2e-

Cu2+ + 2e- Cu(s)

Anodic

Cathodic

Zn (s) + Cu2+ Zn2+ + Cu (s)

Daniell Cell

Galvanic Cell

Two half cells

Half cell

charge balance

metal

solution of metal salt

One cell oxidizes the other

direction depends on driving force

6

An+ + ne- A

Bm+ + me- B

mA + nBm+ nB + mAn+

General Reaction

Standard Hydrogen EMF Test – 25°C

Metal sample mass increases

(gold is the cathode, +)

- +

0

Metal sample mass decreases

(iron is the anode, -)

- +

0

Used to develop EMF Series

Two outcomes:

Ag2+

(1M Ag2+) (1M H+)

H+

H+

H+

H+

Pla

tin

um

e-

e-

Go

ld

e-e-

Ag2+

Ag2+

Fe2+

(1M Fe2+) (1M H+)

H+

H+

H+

H+ H2

Pla

tin

um

e-

e-

Iro

n

e-

e-

Fe2+

Fe2+

Vmetal < 0 (relative to Pt) Vmetal > 0 (relative to Pt)

7

• EMF series • Metal with smaller

V corrodes.• Ex: Cd-Ni cell

metalo

- +

more cathodic Au

Cu

Pb

Sn

Ni

Co

+1.420 V

+0.340

- 0.126

- 0.136

- 0.250

- 0.277

metal Vmetalo

∆V = o

Standard EMF Series

Ni

1.0 M

Ni2+ solution

1.0 M

Cd2+ solution

Cd 25°C

more anodic

more cathodic

Co

Cd

Fe

Cr

Zn

Al

Mg

Na

K

- 0.277

- 0.403

- 0.440

- 0.744

- 0.763

- 1.662

- 2.262

- 2.714

- 2.924

0.153V

Data based on Table 17.1, Callister 6e.

8

Aqueous Types

Standard Calomel Electrode (SCE):

antiquated

contains mercury!

reaction between:

Hg

Hg2Cl2 (calomel)

aqueous phase: saturated KCl in H2O

referenced as 0V

9

referenced as 0V

redox potential is +0.2444 V vs. SHE at 25 °C

Standard Hydrogen Electrode (SHE):

modern method

Aqueous Types

Galvanic Corrosion:

Two metals in contact act similar to a model corrosion cell

electrically conduction path

driving force (EMF)

electrolyte (aqueous solution, air)

10

©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license.

Aqueous Types

Galvanic Corrosion:

Can be detrimental, or beneficial, the difference is up to the engineer!

11

Stainless steel bolts in zinc plated steel. Zinc anodes on painted LAS.

in both cases, the zinc is less noble. It’s use determines the success of the system

Aqueous Types

Uniform Corrosion:

most common corrosion type

corrosion rate is often expressed as a distance per year

usually mils (1 mil = .001 inches = .0254 mm)

Mils penetration per year (MPY) = 534 x W / (ρAt)

W = weight loss in milligrams

ρ = density (g/cm3)

12

ρ = density (g/cm3)

A = area (in2)

t = time (hrs)

< 1 MPY is outstanding, 5-20 good, 50-200 poor

Simple Aqueous Corrosion (uniform)

Fe Fe2+ + 2e-

O2 + 4e- + 2H2O 4OH-

2Fe2+ + 4OH- 2Fe(OH)2

Fe Fe2+ + 2e-

2H+ + 2e- H2

Anodic

Cathodic

Anodic

Cathodic

Aerobic (Dissolved Oxygen)

Fe2+ + 2Cl- FeCl2(aq)

Anaerobic

Require four components: anode, cathode, conduction path (contact), and electrolyte

2Fe(OH)3 Fe2O3 + 3H2O

2Fe3+ + 6OH- 2Fe(OH)3

Fe Fe3+ + 3e-

O2 + 6e- + 3H2O 6OH-

13

Rust

O2 O2

Fe

Fe2+OH- OH-

e- e-

Fe

e- e-

H+Fe2+H+ H+H+H+H+

H2H2

H+Cl-

H+Cl-

(active corrosion) (partially passivates)


Controlling Uniform Corrosion

Uniform Corrosion:

The earliest attempts at slowing corrosion involved additions of copper to steel

Still used today

High Strength Low Alloy Steel

not made to specific chemical composition

specific mechanical properties.

carbon content between 0.05–0.25%

14

carbon content between 0.05–0.25%

retain formability and weldability

<2.0% Mn

Cu, Ni, Nb, N, V, Cr, Mo, Ti, Ca, Zr, or rare earth elements (Y, Hf)

Cu, Ti, V, and Nb are added for strengthening purposes

Cu, Si, Ni, Cr, P increase corrosion resistance

Zr, Ca, and RE’s control inclusion shape

Control-rolled, pearlite-reduced, microalloyed, acicular ferrite, dual-phase, and Weathering

Controlling Uniform Corrosion

Weathering Steel:

HSLA steel with ~0.3 wt. % Cu additions

forms adherent, solid oxide layer

don’t need to be painted

use of uncoated weathering steel

initial cost savings of >10% (don’t need to be painted)

life cycle cost savings >30% (durability)

15

The first bridge using this material was built over the New Jersey Turnpike in 1964.

Simple Aqueous Corrosion (uniform)

Fe Fe2+ + 2e-

O2 + 4e- + 2H2O 4OH-

2Fe2+ + 4OH- 2Fe(OH)2

Fe Fe2+ + 2e-

2H+ + 2e- H2

Anodic

Cathodic

Anodic

Cathodic



Anaerobic

Require four components: anode, cathode, conduction path (contact), and electrolyte


2Fe3+ + 6OH- 2Fe(OH)3

Fe Fe3+ + 3e-

O2 + 6e- + 3H2O 6OH-

16

Rust

O2 O2

Fe

Fe2+OH- OH-

e- e-

Fe

e- e-

H+Fe2+H+ H+H+H+H+

H2H2

H+Cl-

H+Cl-

(active corrosion) (partially passivates)


Stainless Steels

What is stainless steel?

- an iron alloy containing >12% chromium

Applications

- endless applications

chemical handling, marine applications, aesthetics

- one purpose: corrosion resistance

17

Petrochemical nuclear architecture consumer

“Stain-less” Steel

- each SS type has its weakness

Austenitics

- pH value < 1.0

Introduction

“Stainless” Steel

- “rustless”, “stainless”, “inox (inoxydable)”

- originally a miracle material

- chlorides

acidic chlorides (MgCl2 and BaCl2)

seawater

- galvanic

self, more noble alloys

Sensitization

- improper heat treatment

intergranular corrosion

18

New York Times: Jan 31, 1915 New York Times: Jan 22, 1922

How do Stainless Steels Work? Passivation

Fe Fe2+ + 2e-

O2 + 4e- + 2H2O 4OH-

2Fe2+ + 4OH- 2Fe(OH)2

Anodic

Cathodic


Oxidation

Begins with Cr and Fe oxidation

forms passivating layer

very rapid (ms)

HCP Cr2O3 layer

kinetic inhibitor

tenacious

allows cold work, welding, etc

19

(fully passivates)

Fe-Cr(OH)2-3

O2 O2

Fe2+

OH-OH-

e- e-

Cr-rich

base metal

Cr2O3

Fe-Cr

Transpassive Corrosion

Fe Fe2+ + 2e-

2H+ + 2e- H2

Anodic

Cathodic


Low pH Corrosion

Breakdown

increased potential (e.g. very low pH)

attacks passivating layer

accelerated in pitted area

pitting

20

Fe-Cr(OH)2-3

Fe2+

e- e-

(partially passivates)

Fe-Cr

Cr-rich

base metal

Cr2O3

H+ H+ H+

H2H2

Fe2+

Cr2+

Crevice Corrosion

1) Oxygen is consumed in crevice by slow passive corrosion

2) Passive corrosion continues, and pH falls by Cr3+ hydrolysis

O2 + 4e- + 2H2O 4OH- (Cathodic)

Cr Cr3+ + 3e- (Anodic)

Cr3+ + 3H2O Cr(OH)3 + H+

H+ + Cl- HCl(aq)

localized decrease in pH (<2)

3) Passive film breaks down in acid and rapid active corrosion starts

4) Active corrosion causes even stronger acidification and stabilizes the crevice corrosion

Breakdown of the Passivating Layer

4) Active corrosion causes even stronger acidification and stabilizes the crevice corrosion

21

e-

OH-

O2

Cl-

M+H+

Breakdown

Surface Roughness

Local environment

pH

Film disruption

chloride ions

Underlying microstructure

Breakdown of the Passivating Layer

Steel

Cl- Cl-

Cl-

γ α’

H+H+H+

Flow

Electrolyte

Underlying microstructure

inclusions

slip bands

surface roughness

phase interfaces

grain boundaries

Stress gradients

22

Steel

©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson

Learning™ is a trademark used herein under license.

Chromium Additions to Steel

- Beginning in 18201

- added to medium steel (0.10-0.30% C)

improved hardenability

- 1913, stainless steel “invented”

Harry Brearley

Brown Firth Laboratories

Fe-12.8% Cr-0.24% C

(martensitic)

Carbide Formation

Fe-Cr Phase Diagram (Calphad)Fe-Cr-0.1C Phase DiagramCarbide Formation

23

Fe-Cr Phase Diagram (Calphad)Fe-Cr-0.1C Phase DiagramCarbide Formation

- too brittle for practical use

- solubility limit (<0.02%C)

- Cr23C6 precipitates

1000hr

400°C

600°C

800°C

1000°C

0.1 hr 1 hr 10 hr 100 hr

Cr23C6

Air cool

Chromium Additions - Carbide

Temper-Brittleness

brittle carbide on grain boundary

limitation with early Fe-Cr alloys

Carbon Content

Lower C, less propensity to carbide formation

move the nose

“stainless iron” c. 1920

24

Aitchison, L. Engineering Steels: An exposition of the properties…. D. Van Nostrand Company. New York. 1921.

1000hr

400°C

600°C

800°C

1000°C

0.1 hr 1 hr 10 hr 100 hr

Air cool

Quench

Cr23C6

0.30%0.15%0.05%

Sourmail, T. Stainless Steel. University of Cambridge. 2010.

Nickel and Chromium Additions

Fe-Ni

Austenite stabilizer

demotes martensite formation

tough, solid solution

1889 – first nickel additions (for strength)1

Austenitic Stainless Steel

25


1914 – austenitic stainless steel invented

<1% C, <20% Ni and 15-40% Cr

Friedrich Krupp AG

Eduard Maurer and Benno Strauss

Provide ductility, toughness, workability

Fe-Ni Phase diagram (Calphad)

1Riley

Not so Fast


- invented in 1914

Friedrich Krupp AG

Eduard Maurer and Benno Strauss

- limited use throughout the war effort

high temperature applications (e.g. valves)

“Krupp Krankheit” (Krupp’s Disease)

“that peculiar property of nickel chromium steels known as temper-brittleness”

26

“that peculiar property of nickel chromium steels known as temper-brittleness”

- intergranular disintegration

without apparent external cause

- decay is always observed from the surface

corrosion process

- only in isolated cases

traced to faulty heat treatment

Aitchison, L. Engineering Steels: An exposition of the properties…. D. Van Nostrand Company. New York. 1921.

Microcell Theory1

local galvanic couple

(Cr-Fe)23C6 cathode (noble)

matrix anode

weakness: self healing, >12% Cr

Stress Theory2

local stresses around carbide

imperfect passivation

weakness: magnitude

Sensitization Corrosion Theories

18Cr-8Ni

(Cr-Fe)23C6

<12Cr

weakness: magnitude

Carbide Corrosion Theory3

carbides preferentially corrode

weakness: noble

Chromium Depletion Theory4,5,6,7

Cr below passivating

generally applicable

27

18%

12%

8%

Cr

1Stickler, 1963 2Lula, 1954 3Shvartz, 1959 4Strauss et al, 1930 5Bain,1933 6Hillert, 1969 7Tedmon, 1971

~70% Cr

Chromium Depletion Theory

18% Cr

12% Cr

0.1% C0.08% C

20nm 0nm 20nm

5.68% C

28

18%

12%

8%

Cr

18% Cr

12% Cr

0.3% C

0.08% C

20nm 0nm 20nm

Chromium Depletion Theory II

18Cr-8Ni(Cr-Fe)23C6<12Cr

Microcell Theory1

local galvanic couple

(Cr-Fe)23C6 cathode (noble)

matrix anode

weakness

self healing

resistance above 12% Cr

Stress Theory2


Vacancy Theory3,4,5,6

sintering

preferential diffusion

weaknesses

rate

29

18%

12%

8%

Cr


imperfect passivation

weakness

magnitude

Chromium Depletion Theory3,4,5,6

Cr below passivating

generally applicable

1Stickler and Vinckier, 1963 2Lula et al, 1954. 3Strauss et al, 1930 4Bain et al,1933 5Hillert et al, 1969 6Tedmon et al, 1971

Goal/Hypothesis

Provide explanation for “Krupp Krankheit”

Method

Relate problem to heat treatment

carbide formation

Increasing Carbon Content

0.04% C, 9.30% Ni, 18.3% Cr

0.06% C, 9.22% Ni, 18.0% Cr

strain

UTSσy

1930 – Strauss, Schottky, and Hinnüber

0.06% C, 9.22% Ni, 18.0% Cr

0.12% C, 8.40% Ni, 18.6% Cr

Summary

1) > potential w/> C = carbides

2) > carbide precip at > T, but < corrosion

3) depletion in immediate region, at higher t the C had more time to diffuse

4) > hardness - martensite being formed? (magnetism detected)

5) corrosion still seen in austenitic nickel-containing alloys (no magnetism)

7) 25%Cr, 20% Ni, 0.15% C has only weak tendency to intergranular corrosion

σy

30

Strauss, B., H. Schottky, and J. Hinnüber. Zeitschrift für anorganische und allgemeine Chemie. Vol 188. No. 1. pp 309-324. 1930.

Shvartz and Kristal

C control for sensitization

Cr control for self-healing

Baumel

GB diffusion for sensitization

volume diffusion for self-healing

Strawström and Hillert

sensitization and self-healing

Modeling efforts

sensitization and self-healing

uniform grain boundary film

alloy-carbon-M23C6 local equilibrium

Tedmon, Vermilyea, and Rosolowski

sensitization only

non uniform carbide film

free energy of carbide formation

31

Strawstrom, C., and M. Hillert. An Improved Depleted-Zone Theory of Intergranular Corrosion of 18-8 Stainless Steel. Journal of the Iron and

Steel Institute. Vol 207. pp 77-85. 1969.

20nm 0nm 20nm20nm 0nm 20nm

Analytical Techniques

Early Studies

Corrosion Testing

(Moneypenny-)Strauss Test: H2SO4-CuSO41

Huey Test: boiling HNO3

optical microscopy

showed pits/trenches

low resolution

no chemical informationAitchinson, 1921

32

Models

all proposed before proof of depletion

Strauss, Schottky, and Hinnüber, 1930

Bain , Aborn and Rutherford,1933

Strawström and Hillert, 1969

Tedmon, Vermilyea, and Rosolowski, 1971

1Strauss et al, 1930 2Mahl, 1940 2Schaefer and Harker, 1942

Bain, 1933

First Confirmation

Joshi and Stein, 1972

Auger electron spectroscopy

fracture surfaces

sulfur, silicon, nitrogen, and phosphorus at GB

confirmed Cr-depletion

Subsequent Study

AES1,2

Atom Probe13

Proof of Chromium Depletion

Atom Probe13

STEM/EDS3,4,5,6,7,8,9,10,11,12

increasing resolution

1977 >50nm

1983-1984 50nm

1983-1986 25nm

1989 5nm

Model Validation

main chromium depletion effect

33

Hopkinson, B.E. and K.G. Carroll. Metallurgy: Chromium Distribution around Grain BoundaryHall, 1984 – 50nm probe sizeOrtner, 1989 – 5nm probe size

lecture18 corrosion

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