CHAPTER 16: CORROSION AND DEGRADATION

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CHAPTER 16CORROSION 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?
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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
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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
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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
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STANDARD EMF SERIES
Metal with smaller V corrodes.
Ex Cd-Ni cell
EMF series
o
V
o
metal
metal
metal
Au Cu Pb Sn Ni Co Cd Fe Cr Zn Al Mg Na K
1.420 V 0.340 - 0.126 - 0.136 - 0.250 - 0.277 -
0.403 - 0.440 - 0.744 - 0.763 - 1.662 - 2.262 -
2.714 - 2.924
o
DV 0.153V
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CORROSION IN A GRAPEFRUIT
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EFFECT OF SOLUTION CONCENTRATION
Ex Cd-Ni cell with standard 1M solutions
Ex Cd-Ni cell with non-standard solutions
n e- per unit oxid/red reaction (2 here)
F Faraday's constant 96,500 C/mol.
Reduce VNi - VCd by --increasing X
--decreasing Y
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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 Magn
esium
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FORMS OF CORROSION
Stress corrosion Stress corrosion work
together at crack tips.
Uniform Attack Oxidation reduction occur
uniformly over surface.
Erosion-corrosion Break down of
passivating layer by erosion (pipe elbows).
Selective Leaching Preferred corrosion of one
element/constituent (e.g., Zn from brass (Cu-Zn)).
Pitting Downward propagation of small pits
holes.
Intergranular Corrosion along grain
boundaries, often where special phases exist.
Galvanic Dissimilar metals are physically
joined. The more anodic one corrodes.(see
Table 17.2) Zn Mg very anodic.
Crevice Between two pieces of the same metal.
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DETERIORATIVE
Stress Saltwater... --causes cracks!
Heat treatment slows crack speed in salt
water!
4mm
--material 7150-T651 Al "alloy"
(Zn,Cu,Mg,Zr)
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Uniform Corrosion Rust!

• Prevention
• Paint
• Plate
• Sacrificial anode

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Galvanic Corrosion
Causes Dissimilar metalsElectrolyteCurrent Path
Described by Galvanic Series
Solutions Choose metals close in galvanic
series Have large anode/cathode ratios Insulate
dissimilar metals Use Cathodic protection
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Pitting and Creviced Corrosion
Causes concentration gradients in electrolyte
cause some areas high in ion concentrations that
accelerate oxidation
Prevention Weld dont rivet Use non-absorbing
gaskets Polish surfaces Add drains avoid
stagnant water Adjust composition e.g., add Mo
to SS
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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 stronger carbide
formation (e.g., Ti or Nb)
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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

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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

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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
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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

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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.
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Corrosion prevention
Sacrificial Anode
Applied Voltage
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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!)
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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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