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Escuela de Ingeniería UC

MECHANICAL ENGINEERING DEPARTMENTICM 3534 Daño de materiales… (I/2009)

M.Walczak, Dr.-Ing.3:02 PM

13

Daño de Materiales porAcción del Medio Ambiente

Lecture 13Thursday, April 21st 2009

Magdalena Walczak, Dr.-Ing.





M.Walczak, Dr.-Ing.

13

3:14 PM

FORMS OF CORROSION - LOCALIZED

Me(1)

Me(1) or (2)

CREVICE CORROSION

(ASTM G15: localized corrosion of a metal surface

at, or immediately adjacent to, an area that is

shielded from full exposure to the environment

because of close proximity between the metal and

the surface of another metal)

+ -I

AC

Anode (Ox): M→M++ e-

Cathode (Red): e- + M+→M

G<0

occurs in crevices due to differentenvironmental conditions





M.Walczak, Dr.-Ing.

13

3:14 PM


CORROSION DAMAGE

TYPOLOGY

TESTING METHODS

PROTECTION METHODS

BY ENVIRONMENTS

BY MATERIALS

ECONOMICS

FUNDAMENTALS

MECHANICALLY ASSISTED

MICROBIOLOGICALLY INDUCED

UNIFORM CORROSION

CREVICE

PITTING

FILIFORM

LOCALIZED CORROSION

INTERGRANULAR





M.Walczak, Dr.-Ing.

13

3:14 PM


Me

PITTING CORROSION

(ASTM G15: Localized corrosion of a metal surface,

confined to a point or small area, that takes the

form of cavities)

localized corrosion that results incavities or “holes”

+ -I

AC



G<0

s o u r c e : A S M H a

n b o o k s





M.Walczak, Dr.-Ing.

13

3:14 PM


ASTM G46-94 “Standard Guide for Examination and

evaluation of Pitting Corrosion”

VARIATIONS IN THE CROSS-SECTIONAL SHAPE OF PITS

STANDARD RATING CHARTS FOR PITS

PITTING FACTOR





M.Walczak, Dr.-Ing.

13

3:14 PM


screw element in a swimming hall

Material 1.4301

time to damage 12 months w w w . k

o r r o s . d e

C ASE STUDY



Escuela de Ingeniería UCMECHANICAL ENGINEERING DEPARTMENT

ICM 3534 Daño de materiales… (I/2009)M.Walczak, Dr.-Ing.

13

3:14 PM


P ASSIVITY VS. PITTING CORROSION

E f E p E corr

jcr

E

j p

① ②

③

Me

MexOy

③

Me

MexOy

O2

②

passive film is an

insulator,

oxygen evolution cannot

take place

passive film has some

electronic conductivity,

oxygen evolution takes place

with a rate depending on the

conductivity

Me

MexOy

①

Me

Cr2O3

Cr3+→ Cr6+①

local break-down ofthe passive film

further oxidation of

the metal takesplace





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3:14 PM


GROWTH OF PASSIVE FILM

most of all films grow according to either a logarithmic or inverse logarithmic rate law (phenomenology):

t B A L ln+= t DC L ln1

+=

Model 1

High field ion conduction

Me

• penetration of Mez+ through the

film due to the E-field (~ 106 V/m)

• reaction at the interface

film/electrolyte

Model 2

Place exchange

• simultaneous rotation

of M-O pairs

Model 3

Point defect on passivity

−+

++ → eV M mO M

k χ χ 2

22

( ) −−+

−++ → eV M M M

k

M χ δ

χ δ 3

++

+ → + H OO H V O

k

O 24

2

2

−++

−++ → + eO H M H MO k

)(2

22/5 χ δ

χ χ δ

χ

2

OV

M V

Metal Passive film Solution−−

++ → + eV M V mm M

k

M χ

χ 1

• passive film considered highly defective quasicrystalline structure

• movement of anion vacancies – film growth

• movement of cation vacancies – film dissolution

logarithmic inverse logarithmic





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3:14 PM


GENERAL REMARKS

1. Breakdown of passivity

2. Early stages of pit growth (metastable)

3. Late stages of pit growth (stable)

4. Repassivation phenomena

• pitting involves breakdown of the passive film

• breakdown occurs only above a certain critical potential at highly localized sites

•film breakdown is induced by certain aggressive ions (e. g., Cl-) in solution

• an induction time exists between when conditions conducive to pitting occur and when

breakdown actually takes place

• pitting potential is related to the activity of the aggressive ion ia B A E log pit −=

STAGES IN PITTING





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3:14 PM


BREAKDOWN OF PASSIVITY




4. Repassivation phenomenaTheories on the passivi ty breakdown and pit in itiation

1. The competitive adsorption theory

2. Complex ion formation theory3. Ion penetration theory

4. The chemical-mechanical model

5. The Cation Vacancy Condensation Mechanism

• O2- and Cl- anions adsorb onto metal surfaces in a competitive

manner. Whereas the adsorption of O2- results in passivity,

adsorption of Cl- does not. The pits develop on the spots where O2-

adsorbed on the metal surface is replaced by Cl- ions.

• According to this model Epit is the minimum electrode potential at

which Cl- ions become capable of replacing the passivating oxygen.





13

3:14 PM










5. The cation vacancy condensation mechanism

• A small number of Cl- ions jointly

adsorb around a cation in the filmsurface to form a complex ion MeCln-;

Once formed, this complex will readily

dissolve into solution, leading to local

thinning of the film.

• The higher electric field at this point

then causes a rapid transfer of another

cation to the surface where it again

complexes with the Cl- ions.

• This “auto-accelerated” dissolution

will eventually result in film breakdown P .

M a r c u s “

C o r r o s i o n M e c h a n i s m s i n T h e o r y a n d P

r a c t i c e ”





13

3:14 PM











• Cl- incorporates into the film and

migrates through the passive film orthrough pores or defects in the film to

the metal/film interface, resulting in

film breakdown.

P .

M a r c

u s “ C o r r o s i o n M e c h a n i s m s i n T h e o r y a n d P r a c t i c e ”





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3:14 PM











• The adsorption of Cl- on the film

greatly reduces the surface tension ofthe film, which in turn increases the

pressure P. When P exceeds some

critical value, breakdown occurs

P .

M a r c u s “ C o r r o s i o n M e c h a n i s m s i n T h e o r y a n d P

r a c t i c e ”





13

3:14 PM











• Based on the point defect model, film breakdown is attributed to the accumulation of cation

vacancies at the metal/film interface to form a cation vacancy condensate. When thecondensate grows to a critical size, mechanical instability occurs -> breakdown

−+

++ → eV M mO M

k χ

χ 2

2

2

( ) −−+

−++ → eV M M M

k

M χ δ χ δ 3

++

+ → + H OO H V O

k

O 24

2

2

−++

−++ → + eO H M H MO k )(

2 22/

5 χ δ χ

χ δ

χ

2

OV

M V

Metal Passive film Solution

−−++ → + eV M V m

m M

k

M χ χ 1





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3:14 PM











P .

M a r c u s “ C o r r o s i o

n M e c h a n i s m s i n T h e o r y a n d P r a c t i c e ”





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3:14 PM


METASTABLE PIT GROWTH





F





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3:14 PM


METASTABLE PIT GROWTH





Metastable pitting occurs at potentials lower than Epit

Pit initiation is followed by rapid repassivationCurrent transient characterized by i peak and t life - life time

i peak

t lifet

i

The occurrence of metastable pitting below the E pit

indicates a potential risk,

especially when crevice conditions cannot be completely excluded

With rising applied potential the

current transient gets higher

current and longer life time

F





13

3:14 PM


STABLE PIT GROWTH





• Pitting corrosion occurs by the breakdown of

passive film (detrimental influence of Cl- )

• Highly restricted mixing of the pit electrolyte

and the bulk solution is highly restricted

• Build up of a concentration differences (pH, O2,

Cl-)

• Large cathode area(external surface) and small

anode area(pit).

• External surface is generally passivated due to

high O2 and high pH or cathodically protected.

• Within the pit:1) Rapid metal dissolution(anodic dissolution).

2) Increase in [Cl-]

3) Hydrolysis reaction by cation leads to acidification of

solution

Rapid dissolution occurs in the pit, while oxygen reduction

takes place on the adjacent metal surfaces


n b o o k s

F





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3:14 PM


STABLE PIT GROWTH





CORROSION CURRENTS IN PITTING

j(3)corr

j(1)corr

j(2)corr


n b o o k s

F





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3:14 PM


REPASSIVATION





Pitting corrosion is stopped when pit internal and

external solutions are mixed; pits are repassivated.

E > E pit : pits nucleation occurs

E prot < E < E pit : new pits can not be formed, but t he existing pits may propagate

E < E prot : the metal remains passive.

REPASSIVATION POTENTIAL = PROTECTION POTENTIAL

E prot E pit

j

FORMS OF CORROSION





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3:14 PM


IgNoble prize in physics, 2005

John Mainstone and the late Thomas Parnell(http://improbable.com)

Pitch drop experimentsince 1927

FORMS OF CORROSION





13

3:14 PM


EVALUATION OF PITTING

1) Number and depth of pits in a suitable standard solution (ASTM G46-94)

2) Pitting potential (= film breakdown potential, critical pitting potential )

3) Critical pitting temperature4) Minimum [Cl-] causing pitting

The least noble potential where pitting

corrosion will initiate and propagate.

Pits are initiated by short polarization over

E pit and then set back to a given E x,

current measured. If E x= E pit the measured

current is constant

The different curves in the diagrams representcurrent transients (i(t )) for different E x

PITTING POTENTIAL ( E pit)source: ASTM F746-87 “Standard test method for

pitting or crevice corrosion of metallic surgical implant materials”

FORMS OF CORROSION L





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3:14 PM




2) Film breakdown potential or pitting potential


the lowest temperature on the test surface

at which stable propagating pitting occurs

(under specified testing conditions)

CPT is determined by measuring anodic

current with increasing the solution

temperature . At CPT current density

increases abruptly.

source: ASTM G150-99 “Standard test method for

electrochemical critical pitting temperature testing

of stainless steels”

CRITICAL PITTING TEMPERATURE (CPT)

FORMS OF CORROSION L





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3:14 PM




2) Film breakdown potential or pitting potential


Talbot & Talbot “Corrosion

Science and Technology”

AISI 304 steel in 0.05 M sulfuric acid solutions

FORMS OF CORROSION L CASE STUDY





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3:14 PM

FORMS OF CORROSION - LOCALIZED C ASE STUDY

April, 1992

Guadalajara, Mexico

Fatalities: 206 (500 injured)

galvanized steel pipeline occluding steel gasoline pipeline - leakage of gasoline into the main sewer

FORMS OF CORROSION LOCALIZED





13

3:14 PM


F ACTS ABOUT PITTING CORROSION

• Pits initiate above a certain potential called pitting potential (=

breakdown p., rupture p., critical pitting p.)

• Metastable and stable pit growth

• A passive surface is essential for pitting to take place

• Breakdown of the passive film at highly localized sites

• Mediation of halide ions (Cl, F, I, Br) in the passivity breakdown

• Strong dependence on temperature (critical pitting temperature)

• No pitting below protection (repassivation) potential

Me






13

3:14 PM


CORROSION DAMAGE

TYPOLOGY

TESTING METHODS

PROTECTION METHODS

BY ENVIRONMENTS

BY MATERIALS

ECONOMICS

FUNDAMENTALS

MECHANICALLY ASSISTED

MICROBIOLOGICALLY INDUCED

UNIFORM CORROSION

CREVICE

PITTING

FILIFORM

LOCALIZED CORROSION

INTERGRANULAR






13

3:14 PM


Me

FILIFORM CORROSION(EDGE CREEP CORROSION)

(ASTM G15: Corrosion that occurs under some

coatings in the form of randomly distributed

threadlike filaments)

localized corrosion that results incavities or “holes”

+ -I

AC



G<0






13

3:14 PM


Filiform corrosion on cast aluminum alloy automotive wheels that

were lacquered with a clear coat to prevent dulling of the aluminum.

A S M H

a n d b o o k

w w w . c

e b e l c o r . o r g

Filiform corrosion on organically coated aluminum alloy (laboratory sample)






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3:14 PM


cathode (alkaline)anode (acid) corrosion products

FILIFORM CORROSION OF AL

0.5 mm

Increasing potential

map of Ecorr (x,y)

300 mV

cathode

Cl- e-

anodeMaterial: AA2024-T3

Polymer coating: 50 µm Epoxy

FFC initiation: NaCl drop in defect

a d a p t e d f r o m A

S M

H a n d b o o k

EXAMPLE (courtesy prof. Grundmeier)




Escuela de Ingeniería UC ICM 3534 Daño de materiales… (I/2009)

13

3:14 PM


Me

F ACTS ABOUT FILIFORM CORROSION

• occurs at coated metallic surface

• initiation damage sites

• formation of filament by movement of the “acid head”

• driving force: differential areation between the acid head and

alkaline tail

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