1. CHAPTER 16: 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

2. THE COST OF CORROSION • Corrosion: • Cost:
--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

3. 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

4. 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

5. STANDARD EMF SERIES • EMF series • Metal with smaller • Ex: Cd-Ni cell
V corrodes.
• Ex: Cd-Ni cell V o o
metal
metal
metal Au Cu Pb Sn Ni Co Cd Fe Cr Zn Al Mg Na K V
+0.340 o
DV =
0.153V 5

6. CORROSION IN A GRAPEFRUIT6

7. 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 7

8. 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
Iron/Steel
Aluminum Alloys
Cadmium
Zinc
Magnesium 8

9. FORMS OF CORROSION • Stress corrosion • Uniform Attack
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. 9

10. DETERIORATIVE • Stress & Saltwater... • Heat treatment: slows
--causes cracks!
• Heat treatment: slows
crack speed in salt water!
4mm
--material:
7150-T651 Al "alloy"
(Zn,Cu,Mg,Zr) 10

12. Uniform Corrosion: Rust!
Prevention:
Paint
Plate
Sacrificial anode

13. 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”

14. 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

15. 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)

16. Causes: abrasive fluids impinging on surfaces
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

17. Selective Leaching Solutions:
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

18. 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

19. 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

20. 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 , Callister 6e. 11

21. Corrosion prevention
Sacrificial Anode
Applied Voltage

22. 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!)

25. 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. 12