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Pitting and Crevice Corrosion
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Pi#ng Corrosion and
Crevice Corrosion (Corrosion Engineering)
Pi#ng Corrosion Lecture Outline • Defini:on • Examples • Observa:on • Mechanism • Effect of Environment • Effect of Alloy Composi:on • Pi#ng Corrosion – Other Alloys • Impacts
Types of Corrosion
• Uniform • Galvanic corrosion • Pi#ng corrosion • Crevice corrosion • Intergranular corrosion • Selec:ve leaching • Erosion corrosion and fre#ng • Environmentally induced cracking • Hydrogen damage
Pi#ng Corrosion Defini:on • Highly localized aNack occurring at a rapid penetra:on rate due to breakdown of a passive metal.
• The local sites of breakdown are oQen associated with microscopic defects in the metal or alloy.
• ASM defini:on: – Corrosion of a metal surface confined to a point or small area that takes the form of cavi:es
Example of Pi#ng Corrosion
• Requirements for pi#ng corrosion: – Alloy composi:on (stainless steel, nickel alloys, aluminum alloys, :tanium alloys, copper alloys…)
– Passive film – Solu:on composi:on (Cl-‐, Br-‐, I-‐, F-‐)
– Surface heterogenei:es
Observa:on of Pi#ng Corrosion
• Visual observa:on: – Density – Diameter/Area – Depth
• Pit vs. general corrosion
Observa:on of Pi#ng Corrosion -‐ 2
• Electrochemistry: – Change in environment affects the current response
– Higher passive current
– Significant current increase before transpassivity
Log(i co
rr)
MΔSΦ-‐ΦSHE
Addi:on of chloride
Pi#ng corrosion
Passive
Transpassive
Pi#ng Poten:al and Significant Values Log(i co
rr)
MΔSΦ-‐ΦSHE Erep
Transpassive
• Current response as a func:on of the poten:al applied
Epit
Metastable pi#ng
Ac:ve pi#ng
• Determine occurrence of pi#ng knowing the corrosion poten:al in the environment of interest
Log(i co
rr)
MΔSΦ-‐ΦSHE Erep
Transpassive
Epit
1.
2. 3.
4.
Use of Pi#ng Poten:al
Mechanism – Pit Chemistry • The anodic reac:on (release
e-‐) and cathodic reac:on (consume e-‐) are separated.
• Cathodic reac:on on passive film:
O2+2H2O+e-‐è4OH-‐ • Anodic reac:on in pit: FeèFe2++2e-‐ • Hydrolysis of metal ion: Fe2++2Cl-‐+2H2OèFe(OH)2+2HCl • Hydrolysis acidifies the pit
solu:on
Mechanism – Ini:a:on 1 • Mul:ple mechanisms suggested: Chloride adsorp:on. (a)
FeOOH èFe3+ + 3OH-‐
(b) FeOOH + Cl-‐ è FeOCl + OH-‐
FeOCl + H2O è Fe3+ + Cl-‐ + 2OH-‐
(c) Fe è Fe2+ + 2e-‐
Mechanism – Ini:a:on 2 • Structural defects (grain boundaries or disloca:on pile ups). • Weaken passive film. • Preferen:al passive film dissolu:on exposing the bare
material and ini:a:ng pi#ng corrosion.
Passive film Passive film
Passive film Passive film
Mechanism – Ini:a:on 3 • Chemical heterogenei:es. Such as Manganese Sulfide (MnS) in Stainless Steels.
Passive film Passive film MnS
Passive film Passive film
– Weak passive film – Suggested mechanism:
• MnS dissolve preferen:ally
• Bare surface exposed • Pi#ng corrosion ini:ates
– OR • MnS/Matrix galvanic cell • Matrix dissolve and undercut MnS
• Bare surface exposed • Pi#ng corrosion ini:ates
Effects of Environment – Chloride • Pi#ng corrosion is highly dependent on the chloride concentra:on – Pi#ng poten:al increases – Repassiva:on poten:al stays constant
Epit
Erep
Poten:
al
Pi#ng Poten
:al
Effect of Environment – Temperature 1 • The pi#ng poten:al decreases as the temperature increases.
• Materials resistant to pi#ng corrosion at low temperature may become suscep:ble at high temperature
Effect of Environment – Temperature 2
• Cri:cal pi#ng temperature CPT: temperature above which a significant current density is measured when a fixed poten:al is applied
Time
Tem
pera
ture
Current
density
CPT
100 µA/cm2
Effect of Alloy Composi:on 1
• Pi#ng Resistance Equivalent Number (PREN) empirical equa:on to rank stainless steels
PREN= Cr+3.3(Mo+0.5W)+16N
Effect of Alloy Composi:on 2 • Alloy composi:on impact microstructure and defects.
• Alloy composi:on changes the pH of the pit solu:on. – Room temperature pH of concentrated salt solu:ons
Salt 1N 3N Saturated
NiCl2 3.0 2.7 2.7
FeCl2 2.1 0.8 0.2
CrCl3 1.1 -‐0.3 -‐1.4
Pi#ng Corrosion – Other Alloys • Nickel alloys are highly resistant to pi#ng corrosion
• Aluminum alloys: light alloys used for aeronau:c and automo:ve. Pi#ng ini:ates at microstructures (e.g. Cu-‐rich phases) linked to micro-‐galvanic cells
• Copper: annealed or half-‐hard tubes in cold tap water. Other possible as a func:on of pH, material condi:on and temperatures
• Other materials: Titanium, Zinc, Tin, Cadmium, Zirconium, Magnesium
Impact • High localized stress
leading to fracture (failed axle)
• Leaks with low amount
of materials damage
• Explosion if under pressure
Lecture Review • Require passive alloy and chloride solu:on • Pi#ng poten:al:
Ecorr<Epit : no corrosion Ecorr>Epit : corrosion • anode in the pit and cathode outside • Solu:on in the pit contains chloride and low pH • Compe::on:
Crea.on>Diffusion: corrosion Crea.on<Diffusion: no corrosion
• Chloride, temperature and alloy composi:on greatly affect pi#ng corrosion
• Nickel alloys, aluminum alloys, copper and others may suffer pi#ng corrosion
Small damages result in big failures
Crevice Corrosion
Crevice Corrosion Lecture Outline • Defini:on • Examples • Observa:on • Mechanism • Effect of Environment • Effect of Alloy Composi:on • Pi#ng Corrosion – Other Alloys • Impacts
Crevice Corrosion Defini:on
• Breakdown of passivity on a metal or alloy at a :ght crevice site due to the development of an aggressive crevice solu:on.
• ASM Defini:on: – Localized corrosion at or immediately adjacent to an area that is shielded from full exposure to the environment due to close proximity between the metal and the surface of another material
Example of Crevice Corrosion • Requirements for crevice corrosion: – Creviced system combined to exposed area
– Alloy composi:on (stainless steel, nickel alloys, aluminum alloys, :tanium alloys, copper alloys…)
– Passive film – Solu:on composi:on (Cl-‐, Br-‐, I-‐, F-‐)
Observa:on • Visual:
– Expose creviced specimens and perform visual assessment of crevice corrosion
– Amount of “feet” having corroded
– Extent of corrosion
Observa:on 2 • Electrochemically:
– Crevice poten:al to iden:fy when crevice will ini:ate
– Can crevice corrosion or pi#ng corrosion ini:ate?
MΔSΦ-‐ΦSHE
Passive
Log(i co
rr)
Epit Ecrev
Transpassive
Mechanisms
• Three models suggested: – Acidifica:on: similar to pi#ng corrosion
– IR drop: poten:al drop due to solu:on resistance
– Stabiliza:on of metastable pi#ng
Mechanism – Acidifica:on 1 1. Deple:on of oxygen
in crevice 2. Separa:on of the
anode and the cathode
3. Chloride diffusion and hydrolysis of metal ion
4. Aggressive environment depassivate crevice
M+
M
O2 OH-‐ M+
M
O2 OH-‐
M+
M e-‐
M(OH)a + aH+
Cl-‐
O2 OH-‐
M+
M e-‐
M(OH)a + aH+
Cl-‐
O2 OH-‐
Mechanism – Acidifica:on 2
Reduc:on Cathodic
Chloride diffusion
Hydrolysis
Oxida:on Anodic
Mechanism – IR Drop 1. Current is flowing
through the crevice from inside to the mouth
2. Due to the solu:on resistance, the poten:al drops:
E=IR 3. As the poten:al
decreases, it reaches the ac:ve peak
Mechanism – Stabilization of metastable pitting
• Metastable pits die because the pit solu:on diffuse out in “infinite” bulk solu:on
• Crevice ini:ate because the metastable pit solu:on diffuse out in a small crevice solu:on
Neutral pH Low Cl-‐
Acidic pH High Cl-‐
Crevice Former Proper:es effect • Porous crevice former: Solu:on diffuse out
• Small crevice length: Solu:on diffuse out
• Wide crevice gap: Lower corrosion rate ANack near the mouth
Pi#ng vs. Crevice Corrosion : Epit in 1 M NaCl : Ecrev in 0.5 M NaCl : Ecrev in 1 M NaCl (fine) : Ecor in 1 M NaCl
• Can pi#ng corrosion occur?
• Can crevice corrosion occur?
• Which one is more likely?
Effect of Environment
• Chloride • Temperature/pH
• What is the likeliness of crevice corrosion when you increase the chloride content?
• What is the likeliness of crevice corrosion when you decrease the pH?
• How do the cri:cal temperatures compare?
Examples of Crevice
• Flanges • Rivets • Rocks • ?
Lecture Review • Requirements: crevice, passivity, chloride • Observa:on: visual or electrochemical • Crevice poten:al above which crevice corrosion occurs • Mechanisms: acidifica:on, IR drop or metastable pit • Compe::on:
Crea:on (dissolu:on, hydrolysis) vs. Diffusion • Small crevice former: less crevice corrosion • Loose crevice former: less crevice corrosion • Ecrev < Epit : if possible, crevice corrosion ALWAYS occur before pi#ng corrosion
• High chloride • High temperature BAD • Low pH