Corrosion

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CORROSION AND DEGRADATION
ISSUES TO ADDRESS...
• Why does corrosion occur?
• What metals are most likely to corrode?
• How do temperature and environment affect
corrosion rate?
• How do we suppress corrosion?
1
THE COST OF CORROSION
• 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**
2
CORROSION OF ZINC IN ACID
• Two reactions are necessary:
-- oxidation reaction:
-- reduction reaction:
• Other reduction reactions:
-- in an acid
solution
-- in a neutral or base solution
3
•
STANDARD HYDROGEN (EMF)
TEST
Two outcomes:
--Metal sample mass
--Metal sample mass
--Metal is the anode (-)
--Metal is the cathode (+)
(relative to Pt)
(relative to Pt)
Standard Electrode Potential
4
STANDARD EMF SERIES
• EMF series
metal
Au
Cu
Pb
Sn
Ni
Co
Cd
Fe
Cr
Zn
Al
Mg
Na
K
o
Vmetal
+1.420 V
+0.340
- 0.126
- 0.136
- 0.250
o
ΔV =
- 0.277
0.153V
- 0.403
- 0.440
- 0.744
- 0.763
- 1.662
- 2.262
- 2.714
- 2.924
• Metal with smaller
o
Vmetal corrodes.
• Ex: Cd-Ni cell
5
CORROSION IN A GRAPEFRUIT
6
EFFECT OF SOLUTION
CONCENTRATION
• Ex: Cd-Ni cell with
standard 1M
solutions
• Ex: Cd-Ni cell with
non-standard
solutions
• Reduce VNi - VCd by
--increasing X
--decreasing Y
n = #eper unit
oxid/red
reaction
(=2 here)
F=
Faraday's
constant
=96,500
C/mol.
7
GALVANIC SERIES
• Ranks the reactivity of metals/alloys in seawater
Platinum
Gold
Graphite
Titanium
Silver
316 Stainless Steel
Nickel (passive)
Copper
Nickel (active)
Tin
Lead
316 Stainless Steel
Iron/Steel
Aluminum Alloys
Cadmium
Zinc
Magnesium
8
FORMS OF CORROSION
• Stress corrosion
• Uniform Attack
Stress & corrosion
work together
at crack tips.
Oxidation & reduction
occur uniformly over
surface.
• Erosion-corrosion
Break down of passivating
layer by erosion (pipe
elbows).
• Selective Leaching
• Pitting
Preferred corrosion of
one element/constituent
(e.g., Zn from brass (Cu-Zn)).
Downward propagation
of small pits & holes.
• Intergranular
Corrosion along
grain boundaries,
often where special
phases exist.
• Galvanic
• Crevice Between two
Dissimilar metals are
pieces of the same metal.
physically joined. The
more anodic one
corrodes.(see Table
17.2) Zn & Mg
very anodic.
9
DETERIORATIVE
• Stress & Saltwater...
--causes cracks!
• Heat treatment: slows
crack speed in salt water!
--material:
4μm
7150-T651 Al "alloy"
(Zn,Cu,Mg,Zr)
10
Uniform Corrosion: Rust!
Prevention:
•
Paint
•
Plate
•
Sacrificial anode
Galvanic Corrosion
Causes:
Dissimilar metals
Electrolyte
Current Path
Described by Galvanic Series
Solutions:
Choose metals close in galvanic
series
Have large anode/cathode ratios
Insulate dissimilar metals
Use “Cathodic protection”
Pitting and Creviced
Corrosion
Causes:
concentration gradients in
electrolyte cause some areas high in ion
concentrations that accelerate oxidation
Prevention:
Weld – don’t rivet
Use non-absorbing gaskets
Polish surfaces
Add drains – avoid stagnant water
Adjust composition; e.g., add Mo to SS
Intergranular Corrosion
Occurs in specific alloys – precipitation of corrosive
specimens along grain boundaries and in particular
environments
e.g. : Chromium carbide forming in SS, leaving adjacent areas depleted in Cr
Solutions:
High temp heat treat to redissolve carbides
Lower carbon content (in SS) to minimize carbide
formation
Alloy with a material that has
formation (e.g., Ti or Nb)
stronger carbide
Erosion Corrosion
Causes: abrasive fluids impinging on
surfaces
Commonly found in piping, propellers,
turbine blades, valves and pumps
Solutions:
• Change design to minimize or eliminate fluid
turbulence and impingement effects.
• Use other materials that resist erosion
• Remove particulates from fluids
Selective Leaching
• Occurs in alloys in which one
element is preferentially removed –
e.g., in Brass, Zinc is electrically
active and is removed, leaving
behind porous Copper
• Occurs in other metals, such as Al,
Fe, Co, Cr
Solutions:
• Use protective coating to protect surfaces
• Use alternative materials
Stress Corrosion
Aka: stress corrosion cracking
Cracks grow along grain
boundaries as a result of residual or
applied stress or trapped gas or
solid corrosion products
e.g., brasses are sensitive to
ammonia
Stress levels may be very low
Solutions:
Reduce stress levels
Heat treatment
Atmosphere control
Hydrogen Embrittlement
• Metals loose strength when Hydrogen is
absorbed through surface, especially along
grain boundaries and dislocations
• Often occurs as a result of decorative
plating
• High strength steels particularly susceptible
• Can be removed by “baking” the alloy
CONTROLLING CORROSION
• Self-protecting metals!
--Metal ions combine with O
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 Fig.
17.14,
Callister
6e.
11
Corrosion prevention
Sacrificial Anode
Applied Voltage
Surface coatings & Passivation
Some materials, such as
Aluminum or Stainless
Steel, form oxide
barrier coatings that
prevent oxidation at
active surface – this is
called “passivation”
Surface can be coated with protective layers:
painted, anodized, plated (Caution!!! Cracks in
plating or paint can lead to crevice corrosion!)
SUMMARY
• Corrosion occurs due to:
--the natural tendency of metals to give up electrons.
--electrons are given up by an oxidation reaction.
--these electrons then are part of a reduction reaction.
• Metals with a more negative Standard Electrode
Potential are more likely to corrode relative to
other metals.
• The Galvanic Series ranks the reactivity of metals in
seawater.
• Increasing T speeds up oxidation/reduction reactions.
• Corrosion may be controlled by:
-- using metals which form
a protective oxide layer
-- reducing T
-- adding inhibitors
-- painting
--using cathodic protection.
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