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[Theoretical basis]-Corrosion of Metals and Alloys


CORROSION OF METALS AND ALLOYS

CORROSION IS A GALVANIC CELL IN WHICH THE ANODE AND CATHODE ARE ON THE SAME SURFACE. THE CORROSION CELL IS, IN EFFECT, A SHORTCIRCUITED BATTERY.

Metal dissolves

Oxidant consumed

+

_

CORROSION INVOLVES THE OXIDATION OF A MATERIAL DRIVEN BY THE REDUCTION OF AN OXIDIZING AGENT

O2 + 2H2O + 2e- → 4OH2H2O + 2e- → H2 + 2OH-

? Corrosion is the equivalent of a short-circuited battery occurring on a conductive surface ? The current is carried through the aqueous solution by the ions in the solution, e.g., Na+, Cl? Metal dissolution occurs at the anode and oxidant reduction at the cathode

THE REACTIVITY OF METALS CAN BE ORDERED ACCORDING TO THEIR STANDARD ELECTROCHEMICAL POTENTIALS
Eo values measured against an SHE reference electrode

Noble metals – hard to oxidize
Standard potentials can be adjusted to equilibrium potentials in the exposure solution of interest using the Nernst equation Ee = Eo - 2.303RT/nF.log [products]/[reactants]

Reactive metals – easy to oxidize

THE THERMODYNAMIC DRIVING FORCE FOR CORROSION IS THE DIFFERENCE IN EQUILIBRIUM POTENTIALS BETWEEN THE METAL DISSOLUTION REACTION AND THE OXIDANT REDUCTION REACTION

METAL

METAL / SOL’N INTERFACE

AQUEOUS SOLUTION

Ox + ne? ? Red Oxidant Consumed

EeOx/Red

ΔEe
M + Ox → Mn+ + Red

ECORR

EeM/Mn+

Metal Oxidized

Corrosion occurs at the corrosion potential, the unique potential at which the rates of the metal dissolution and oxidant reduction reactions are equal

M ? Mn+ + ne?

METAL

METAL / SOL’N INTERFACE

AQUEOUS SOLUTION

Ox + ne? ? Red Oxidant Consumed

EeOx/Red

Fraction of the potential used to drive metal oxidation e

M + Ox → Mn+ + Red

ECORR

Fraction of the potential required to drive oxidant reduction

E M/Mn+

Metal Oxidized

M ? Mn+ + ne?

THE OVERALL CORROSION REACTION IS THE SUM OF TWO ELECTROCHEMICAL HALF REACTIONS

CATHODE

O2 + 2H2O + 2e- → 4OH-

ANODE

Fe → Fe2+ + 2e-

Fe + O2 + 2H2O → Fe2+ + 4OH-

FOR A CHEMICAL TO ACT AS AN CORRODING OXIDANT ITS EQUILIBRIUM POTENTIAL MUST BE GREATER THAN THAT OF THE METAL

METAL

METAL / SOL’N INTERFACE

AQUEOUS SOLUTION

Oxidized state
Ox + ne? ? Red Oxidant Consumed

EeOx/Red

Reduced state

These two reactions can only couple at the corrosion potential when the cathodic and anodic currents are equal
Metal Oxidized

M + Ox → Mn+ + Red

ECORR

Oxidized state
M ? Mn+ + ne?

EeM/Mn+

Reduced state The location of the corrosion potential with respect to the two equilibrium potentials is an indication of which reaction controls the overall corrosion rate

WHICH OXIDANTS CAN CORRODE WHICH METALS ?

Oxygen dissolved in water can corrode most metals

Acidic solutions can corrode Fe, Ni, etc, but not Cu, Ag, etc If Cu was galvanically coupled to Fe, the CU would act as the cathode (for oxygen reduction and the Fe would corrode

THE THERMODYNAMICS OF CORROSION PROCESSES CAN BE SUMMARIZED IN POTENTIAL (POURBAIX) DIAGRAMS

Water unstable with respect to oxygen

Stability region for water

Water unstable with respect to hydrogen

WHILE A METAL MAY CORRODE IN WATER IT MAY BECOME PROTECTED BY AN INSOLUBLE OXIDE/HYDROXIDE

Fe2+ is soluble and unlikely to form a protective oxide/hydroxide film on the metal

Fe3+ is highly insoluble and likely to form protective oxide films on the metal

FeOH+

Fe(OH)3-

Fe(OH)4-

Fe2(OH)24+

POTENTIAL – pH DIAGRAM FOR IRON SHOWING IT IS CORRODABLE IN WATER FOR pH < 9 BUT PASSIVE (i.e., PROTECTED BY AN INSOLUBLE OXIDE FILM) FOR pH > 9

ΔEe

CORROSIVE

PASSIVE

IMMUNE

COPPER IS IMMUNE TO CORROSION IN ANOXIC WATER BUT CORROSIVE IN AERATED WATER

(ΔEe)O2

CORROSIVE

PASSIVE

TITANIUM IS PASSIVE EXCEPT UNDER EXTREME ACIDIC CONDITIONS

PASSIVE

Titanium would be extremely corrosive If not protected by an oxide film

OXIDES CAN GROW BY DIFFERENT MECHANISMS AND EXHIBIT DIFFERENT PROPERTIES

Oxide vacancy injection

Dissolution of a metal ion into solution and the creaction of a cation vacancy

Anodic reaction

Cathodic reaction

SOLID STATE OXIDE GROWTH –leads to coherent protective films

FILM FORMATION BY DISSOLUTION -DEPOSITION

Metal cation dissolves, exceeds its solubility and is deposited on the corroding surface

Oxidant may have to diffuse through the pore structure of the deposit

Corrosion product deposit not coherent. Corrosion rate could depend on the porosity and transport properties of the deposit

FILM FORMATION COULD INVOLVE BOTH PROCESSES

O2

M

On-going corrosion is sustained by dissolution deposition H2O H2

Barrier layer grows by defect transport but fractures, since the oxide volume is > the metal volume

SOLID STATE GROWN FILMS ARE FEATURELESS AND VERY THIN ( 5 to 10nm)

Titanium after exposure to an NaCl solution at 150oC

Such films are chemically inert, strongly adhesive and make materials very corrosion resistant

(a)

(b)

CORROSION OF CARBON STEEL IN DEAERATED SOLUTIONS PRODUCES A CORROSION PRODUCT DEPOSIT WHOSE PROTECTIVENESS AND COMPOSITION VARY WITH SOLUTION COMPOSITION

Carbonate-dominated solutions produce a thick deposit that appears porous
SSW 20.0 kV × 2.00K SSW 15.0 μm 20.0 kV × 18.0K 1.67μm

( a )

(b)

Mixed anion solutions (CO32/HCO3-/SO42-/Cl-) produces a less crystalline and less porous deposit
15.0 μm SSW 20.0kV × 2.00K

SSW

20.0kV × 2.00K

(c)

(d)

15.0μm

Chloride-dominated solutions apparently produces no deposit
SSW 5.0kV × 2.00K SSW 15.0μ m 5.0kV × 4.00K 7.50μm

(e)

(f)

CORROSION PRODUCT DEPOSITS CAN BE HOMOGENEOUSLY DISTRIBUTED AND OF UNIFORM MORPHOLOGY

Iron carbonate on carbon steel

THEY CAN ALSO BE OF MIXED MORPHOLOGY AND UNEVENLY DISTRIBUTEDB

Iron oxides and carbonates formed on carbon steel in a solution containing carbonate, sulphate and chloride

THERE MANY DIFFERENT FORMS OF CORROSION
? ? ? ? ? ? ? ? ? ? Uniform Corrosion Galvanic Corrosion Dealloying (Selective Leaching) Intergranular Corrosion Pitting Crevice Corrosion Microbially-Influenced Corrosion Stress Corrosion Cracking Corrosion Fatigue Hydrogen-Induced Cracking

Environmentally Induced Corrosion

UNIFORM CORROSION

Carbon steel pipe failure due to external corrosion

Internal corrosion of a carbon steel pipe

ALLOYING CAN SIGNIFICANTLY SUPPRESS UNIFORM CORROSION
For example, addition of small amounts of Pd to titanium significantly reduces the corrosion rate even in boiling concentrated acid

HOWEVER, CORROSION IS COMMONLY NOT UNIFORMLY DISTRIBUTED

Some sites are heavily corroded and others untouched

PITTING IS A COMMON FORM OF LOCALIZED CORROSION

PITTING REQUIRES THAT THE ANODE AND CATHODE BE SPATIALLY SEPARATED
For example on a carbon steel surface under a water droplet, when an oxygen concentration difference is established

Absorption of atmospheric O2 produces a high oxygen concentration in the periphery of the droplet

Soluble Fe2+ diffuses to the high O2 region where it reacts with the O2 to produce an insoluble Fe3+ deposit (Fe2O3)

Anodic iron dissolution site with a low O2 concentration

passive oxide

metal/alloy

Aqueous?solution? containing?aggressive? anions?(e.g.?Cl‐ )?and? an?oxidant?(e.g.? dissolved?O2)

Localized pitting starts when the passive film is breached

passive oxide

metal/alloy O2 M MO

Aqueous?solution? containing?aggressive? anions?(e.g.?Cl‐ )?and? an?oxidant?(e.g.? dissolved?O2)

This breakdown site could be repaired if the re-growth of the passive film is rapid

Cl- utilizes surface defects to destabilize the passive oxide

The incipient pit acidifies before it can re-grow the passive oxide Dissolved metal is re-deposited at the pit mouth, when less acidic conditions are encountered. This protects the pit from neutralization by the non-acidic bulk of solution

pH = 8

pH = 3 to 4

Repassivation (i.e., reformation of the insulating oxide is prevented since the oxide solubility is significantly increased In the acidic conditions in the pit

PITTING CAN ADOPT MANY DIFFERENT MORPHOLOGIES DEPENDING ON THE MATERIAL PROPERTIES AND THE EXPOSURE CONDITIONS

CREVICE CORROSION IS A MORE INSIDIOUS FORM OF LOCALIZED CORROSION

OH‐

O2

Neutral?aqueous? solution

H+

Passive Oxide

Mn+ M

For pitting, acidity created within the crevice relatively easily lost by diffusion out of the pit. This makes it difficult to establish the low pH that prevents repassivation

OH‐

O2 H+

Crevice former
H+ Acidification

Neutral?aqueous? solution

Passive oxide Oxide passive

Mn+

M
e‐

In a crevice diffusive loss of acidity is severely inhibited, and acidic conditions easier to establish and maintain

CREVICE CORROSION OF TITANIUM DOES NOT OCCUR BELOW A CRITICAL TEMPERATURE
? For a sufficiently high temperature, film fracture allows slow metal dissolution. ? Hydrolysis of the dissolved metal cations causes acid formation. ? For non-creviced areas this is insignificant since the acidity is rapidly neutralized ? Within the crevice, the large meatl surface area to water volume ratio and hindered diffusion allow acidification ? This leads to dissolution of the oxide film and the initiation of crevice corrosion

Proposed Crevice Corrosion Mechanism for Ti
? Crevice corrosion is a form of localized corrosion occurring in a crevice between metal/nonmetal or metal/metal. It develops in four stages.

Stage 1

Aerated NaCl Solution
Crevice Former O2 (Cathodic reaction) O2 OHO2

? Corrosion occurs OHCrevice normally both inside Ti3+/4+ and outside the crevice leading to oxide film thickening.

e-

Oxide

Ti3+/4+

Ti

e(Anodic reaction)

Mechanism of Crevice Corrosion – cont’d
? Stage 2 ― crevice initiation Aerated NaCl Solution
Cl- O2 Crevice Former H2O Ti(OH) + xH+ O2 depletion x 3+/4+ Ti Oxide OHO2

In the crevice ? O2 becomes depleted;
transport restricted by the small gap. ? Low pH

? High [Cl-]
? Breakdown of the passive film, initiation of crevice corrosion

Ti
Anode

eCathode

Mechanism of Crevice Corrosion – cont’d
? Stage 3 ― crevice propagation Aerated NaCl Solution
O2 Crevice Former H2O Ti(OH) + xH+ x H2 Ti3+/4+ OHOxide Habs O2

In the crevice
? pH decreases further ? Extensive propagation of crevice corrosion

Anode

Ti

e-

eCathode

? Stage 4― all O2 consumed → crevice repassivation

A SMALL AMOUNT OF IRON IMPURITY CHANGES THE GRAIN STRUCTURE OF TITANIUM
C Ti-2 Pure Ti 0.010 * O 0.127 * N 0.010 * Ni 0.019 * Fe 0.107 * Ti Bal. 99.99

Pure Ti α- phase h.c.p

Ti-2 β- phase b.c.c

100 ?m 40 ?m

> 100 ?m

(20 ~ 40 ?m)
42

Microstructure of the Three Ti-2 Materials
Material 1 (Fe = 0.042 wt%)
3 mm

Material 2 (Fe = 0.078 wt%)
50 ?m

Material 3 (Fe = 0.12 wt%)
100 ?m

Grain size decreased with the increasing Fe content ? Grain growth is unrestricted, leading to large grains. ? β-phase or intermetallic particles may be present in materials 2 and 3.

Metallographic Cross Sections
Material 1
200 ?m

? General attack.

300 ?m

Material 2
2 1

? Severe and deep intergranular attack. (?) ? Shallow and general attack. Minor amount of intergranular attack.

200 ?m

Material 3

Maximum penetration depth by crevice corrosion for two commercial specimens of titanium (95oC in 0.27 M NaCl)
Maximum penetration depth, dmax (μm)
1800 1500 1200 900 600 300 0 0 20 40 60 80 100

40 ?m

0.078 wt% 0.107 wt%

Duration of experiment (days)

40 ?m

? Comparison of dmax for the two specimens shows that Ti-2 with higher Fe content (0.107%) accumulates less crevice corrosion damage than that with Fe content of 0.078%.
45

Crevice corrosion for different times at 95oC (0.27 M NaCl)

H+
TiO2

H2 + 2eTiO2

Ti

- 4e-

TixFe

- 4e-

Ti

?TixFe intermetallic particles act as proton reduction catalysts and
enhance crevice corrosion resistance by forcing crevice corroding sites to repassivate
46

CREVICE CORROSION OF TITANIUM CAN BE AVOIDED BY ALLOYING WITH SMALL AMOUNTS OF PALLADIUM

Commercially pure titanium

Titanium containing 0.07 to 0.16 wt% Pd

Pd ennobles the titanium surface, since the Ti is preferentially dissolved, leaving behind an unreactive Pd-enriched surface

GALVANIC CORROSION INVOLVES THE SHORT CIRCUIT COUPLING OF TWO DISSIMILAR METALS OR ALLOYS

STRESS CORROSION CRACKING REQUIRES THAT THREE CONDITIONS BE MET SIMULTANEOUSLY

STRESS CORROSION CRACKING
? ? ? ? Requires tensile stresses. Normally ductile materials can be brittle Occurs for stresses below the materials yield stress Stresses can be externally applied. But residual stresses in the material (e.g., from fabrication, treatment or welding processes) often cause failure. Cracks initiate and propagate at a low rate (10-9 to 10-6 m/sec) (< 1 to 2 years for a 5 cm wall thickness !!) Can initiate and crack with little outside evidence of corrosion and no warning before catastrophic failure Other forms of corrosion, such as pitting or crevice corrosion can transition to SCC

?

?

?

STRESS CORROSION CRACKING CAN PROPAGATE EITHER INTERGRANULARLY OR TRANSGRANULARLY DEPENDING ON MATERIALS PROPERTIES AND THE MECHANISM OF CRACK PROPAGATION

Intergranular SCC

Transgranular SCC

INTERGRANULAR CORROSION DUE TO COMPOSITIONAL DIFFERENCES BETWEEN GRAINS AND GRAIN BOUNDARIES CAN LEAD TO STRESS CORROSION CRACKING

304 Stainless steel before IGC

304 stainless steel after IGC

Crack

Grain boundary is Cr-depleted due to Cr carbide precipitation during inappropriate heat treatment (i.e., sensitization during welding

THE FILM RUPTURE MECHANISM OF STRESS CORROSION CRACKING

Thin passive film formed by the solid state process

THE TARNISH RUPTURE MECHANISM OF STRESS CORROSION CRACKING

Similar to the film rupture mechanism, except the film more likely to be formed by metal dissolution – deposition. This mechanism is thought to apply to copper and brasses

THE ANODIC DISSOLUTION MECHANISM OF STRESS CORROSION CRACKING

? Cracking can be hydrogen assisted if hydrogen is absorbed during the corrosion process. ?Hydrogen absorption leads to local embrittlement which enhances crack propagation

HYDROGEN INDUCED CRACKING ? Metals such as Titanium and Zirconium are generally passive and extremely unreactive ? However, if the passive film is degraded (e.g., stress fractured) water reduction can lead to hydrogen absorption ( Ti + H2O → Ti4+ + 4Hads) ? This hydrogen can precipitate as hydrides (TiHx) leading to brittle fracture once the hydrogen content of the material is above a critical level

HYDROGEN ABSORPTION LEADING TO HYDRIDE CRACKING OF ZIRCONIUM

Generally flaws in the oxide and galvanic coupling to a readily corrodable metal (e.g., carbon steel) are required for hydrogen absorption to occur at lower temperatures

ONCE CRACKING PROPAGATION IS UNDERWAY HYDROGEN CAN DIFFUSE UP STRESS GRADIENTS TO THE CRACK TIP ALLOWING CRACKING TO CONTINUE

HOW DOES HYDROGEN GET INTO THE METAL AND ACCUMULATE TO THE LEVEL NEEDED TO INITIATE AND PROPAGATE CRACKING ?

Critical stress level below which cracking will not occur

Critical hydrogen level below which cracking will not occur

EASY PATHWAY Incomplete passivation of intermetallic Precipitates at grain boundaries leaves open absorption “windows”
E > -0.6 V

DIFFICULT PATHWAY Chemical transformations in the oxide induced by galvanic coupling lead to the oxide becoming permeable to hydrogen
E < -0.6 V H+ H2

H+

H2 TiIV TiIII TiO2 Ti Habs Habs

Habs

TixM

Absorption Efficiency High

Absorption Efficiency Low

HYDROGEN TRANSPORT TO DEEP LOCATIONS 105 TIMES FASTER THROUGH GRAIN BOUNDARY β-PHASE THAN THROUGH α-PHASE GRAINS

Body centred cubic β-phase Atomic packing factor = 0.68

Hexagonal close packed α-phase Atomic packing factor = 0.72

A sites B sites A sites

MICROBIALLY-INFLUENCED CORROSION

BIOFILMS HAVE COMPLEX GEOMETRIES AND CHEMISTRY

METAL SURFACE

THE BIOFILM ESTABLISHES AN OXYGEN CONCENTRATION CELL IN A SIMILAR MANNER TO THAT FOR CORROSION UNDER A WATER DROPLET

? Localized corrosion occurs beneath the biofilm ? Corrosion rates can be extremely high


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