FORMS OF CORROSION

1
CHAPTER 3 FORMS OF CORROSION (cont)
Chapter Outlines 3.5 Selective Leaching 3.6
Erosion Corrosion 3.7 Stress Corrosion 3.8
Hydrogen Damage
2
SELECTIVE LEACHING
3

• SELECTIVE LEACHING (Dealloying, Parting)
• Corrosion in which one constituent of an alloy is
  preferentially removed, leaving behind an altered
  (weakened) residual structure.
• Can occur in several systems.

4

• Combinations of alloys and environments subject
  to dealloying and elements preferentially removed
  

Alloy Environment Element removed
Brasses Many waters, especially under stagnant conditions Zn (dezincification)
Grey iron Aluminium bronzes Soils, many waters HCl, acids containing Chloride Fe (graphitic corrosion) Al (dealuminification)
Silicon bronzes High-temperature steam and acidic species Si (desiliconification)
Tin bronzes Copper-nickels Hot brine or steam High heat flux and low water velocity (in refinery condenser tubes) Sn (destannification) Ni (denickelification)
Copper-gold single crystals Monels Ferric chloride Hydrofluoric and other acids Cu Cu in some acids, and Ni in others
Gold alloys with copper or silver High-nickel alloys Sulfide solutions, human saliva Molten salts Cu, Ag, Cr, Fe, Mo and T
Medium- and high-carbon steels Oxidizing atmospheres, hydrogen at high temperatures C (decarburization)
Iron-chromium alloys High-temperature oxidizing atmospheres Cr, which forms a protective film
Nickel-molybdenum alloys Oxygen at high temperature Mo
5

• Dezincification
• All Cu-Zn alloys (Brasses) containing gt 15 Zn
  are susceptible . . .
• e.g. common yellow brass . . . 30 Zn 70 Cu,
  dezincifies to red copper-rich structure.
  Dezincification can be uniform...
• potable water inside
• or plug-type.... (boiler water inside,
  combustion gases outside)

Uniform dezincification of brass pipe.
Plug-type dezincification.
6
Section of one of the plugs shown before

• Overall dimensions of original material tend to
  be retained . . . residual is spongy and porous .
  . . often brittle.
• Can go unnoticed, especially if covered with
  dirt/deposit, etc.
• Uniform dezincification...
• - usually found in high brasses (highZn),
  acid environments
• Plug-type dezincification...
• - usually found in low brasses, alkaline,
  neutral or slightly acid environments.

7

• Mechanism
• (1) Zn atoms leave lattice sites . . .
• are leached into the environment selectively
• Discuss . . . w.r.t. last picture.
• (2) Generally accepted . . .
• - brass dissolves
• - Zn stays in solution
• - Cu re-deposits.
• Discuss . . . w.r.t. last picture.
• N.B. possibility for local anode-cathode couples
  .. Cu deposits accelerate attack.
• N.B. dezincified areas generally 90-95 Cu some
  Cu2O/CuO present if O2 in the environment.

8

• Prevention
• - Make environment less aggressive (e.g., reduce
  O2 content)
• - Cathodically protect
• - Use a better alloy (common cure - above not
  usually feasible)...
• - red brass (lt15 Zn) almost immune
• - Admiralty Brass. . . 70 Cu, 29 Zn, 1 Sn
• - arsenical Admiralty. . . 70 Cu, 29 Zn, 1
  Sn, 0.04 As
• (Sn and Sn-As in deposited films hinder
  redeposition of Cu)
• - For very corrosive environments likely to
  provoke dezincification, or for critical
  components, use . . .
• - cupronickels 70-90 Cu, 30-10 Ni.

9
Graphitization (misnomer . . . graphitization
is the breakdown of pearlite to ferrite C at
high temperature) Grey cast iron is the cheapest
engineering metal . . . 2-4 C, 1-3 Si. Hard,
brittle, easily cast carbon present as
microscopic flakes of matrix graphite within
microstructure.
Microstructure of grey cast iron.
100 ?m
10

• In some environments (notably mild, aqueous soils
  affecting buried pipe) the Fe leaches out slowly
  and leaves graphite matrix behind . . appears
  graphitic . . .soft . . . can be cut with a
  knife. Pores usually filled with rust. Original
  dimensions are retained.

A 200-mm (8-in.) diameter grey-iron pipe that
failed because of graphitic corrosion. The pipe
was part of a subterranean fire control system.
The external surface of the pipe was covered with
soil the internal surface was covered with
water. Severe graphitic corrosion occurred along
the bottom external surface where the pipe rested
on the soil. The small-diameter piece in the
foreground is a grey-iron pump impeller on which
the impeller vanes have disintegrated because of
graphitic corrosion.
11
(a) External surface of a grey-iron pipe
exhibiting severe graphitic corrosion.
(b) Close-up of the graphitically-corroded region
shown in (a).
(c) Micrograph of symmetrical envelopes of
graphitically-corroded iron surrounding flakes of
graphite.
20 ?m
12

• Selective Dissolution in Liquid Metals
• In liquid metal coolants (LMFBR with Na or Na-K
  coolant), austenitic alloys can lose Ni and Cr
  and revert to the ferrite phase...
• Corrosion of Inconel alloy 706 exposed to liquid
  sodium for 8,000 hours at 700?C (1290?F) hot leg
  circulating system. A porous surface layer has
  formed with a composition of ? 95 Fe, 2 Cr and
  lt 1 Ni. The majority of the weight loss
  encountered can be accounted for by this surface
  degradation. Total damage depth 45 ?m. (a) Light
  micrograph. (b) SEM of the surface of the porous
  layer.
• Alloy 706 ... 39-44 Ni, 14.5-17.5 Cr, 0.06
  C.

13

• Also in fusion-reactor environments (Li as
  coolant)....
• Light micrograph of cross-section. SEM of
  surface showing porous layer.
• Corrosion of type 316 stainless steel exposed to
  thermally convective lithium for 7488 hours at
  the maximum loop temperature of 600?C.

14

• Usually, the transport and deposition of leached
  elements is of more concern than the actual
  corrosion.
• (a) (b)
• SEM micrographs of chromium mass transfer
  deposits found at the 460?C (860?C) position in
  the cold leg of a lithium/type-316-stainless-steel
  thermal convection loop after 1700 hours. Mass
  transfer deposits are often a more serious result
  of corrosion than wall thinning. (a) Cross
  section of specimen on which chromium was
  deposited. (b) Top view of surface.

15
100 ?m
Iron crystals found in a plugged region of a
failed pump channel of a lithium processing test
loop.
16

• Selective Leaching in Molten Salts
• Molten salts are ionic conductors (like aqueous
  solutions) and can promote anodic-cathodic
  electrolytic cells . . . they can be aggressive
  to metals.
• ALSO . . . some molten salts (notably fluorides)
  are Fluxes and dissolve surface deposits that
  would otherwise be protective dealloying of Cr
  from Ni-base alloys and stainless steels can
  occur in the surface layers exposed to molten
  fluorides the vacancies in the metal lattice
  then coalesce to form subsurface voids which
  agglomerate and grow with increasing time and
  temperature.

17

• (a) (b)
• (a) microstructure of type 304L SS exposed to
  LiF-BeF2-ZrF4-ThF4- UF4 (70-23-5-1-1 mole
  respectively) for 5700 hours at 688?C.
• (b) microstructure of type 304L SS exposed to
  LiF-BeF2-ZrF4-ThF4- UF4 (70-23-5-1-1 mole
  respectively) for 5724 hours at 685?C.

18
EROSION CORROSION
19

• EROSION-CORROSION
• (Flow-Assisted or Flow-Accelerated Corrosion)
• An increase in corrosion brought about by a high
  relative velocity between the
• corrosive environment and the surface.
• Removal of the metal may be
• as corrosion product which spalls off the
  surface because of the high fluid shear and bares
  the metal beneath
• as metal ions, which are swept away by the fluid
  flow before they can deposit as corrosion
  product.
• Remember the distinction between
  erosion-corrosion and erosion
• erosion is the straightforward wearing away by
  the mechanical abrasion caused by suspended
  particles . . . e.g., sand-blasting, erosion of
  turbine blades by droplets . . .
• erosion-corrosion also involves a corrosive
  environment . . . the metal undergoes a chemical
  reaction.

20

• Erosion-corrosion produces a distinctive surface
  finish
• grooves, waves, gullies, holes, etc., all
  oriented with respect to the fluid flow pattern .
  . . scalloping...

Erosion-corrosion of stainless alloy pump
impeller. Impeller lasted 2 years in oxidizing
conditions after switch to reducing conditions,
it lasted 3 weeks!
Erosion-corrosion of condenser tube wall.
21

• Most metals/alloys are susceptible to
  erosion-corrosion.
• Metals that rely on protective surface film for
  corrosion protection are particularly
  vulnerable, e.g. Al
• Pb
• SS
• CS.
• Attack occurs when film cannot form because of
  erosion caused by suspended particles (for
  example), or when rate of film formation is less
  than rate of dissolution and transfer to bulk
  fluid.

22

• Erosion-Corrosion found in - aqueous solutions
• - gases
• - organic liquids
• - liquid metal.
• If fluid contains suspended solids,
  erosion-corrosion may be aggravated.
• Vulnerable equipment is that subjected to
  high-velocity fluid, to rapid change in direction
  of fluid, to excessive turbulence . . .
• viz. equipment in which the contacting fluid has
  a very thin boundary layer
• - high mass transfer rates.
• Vulnerable equipment includes

- pipes (bends, elbows, tees) - valves - pumps - blowers - propellers, impellers - stirrers - stirred vessels - HX tubing (heaters, condensers) - flow-measuring orifices, venturies - turbine blades - nozzles - baffles - metal-working equipment (scrapers, cutters, grinders, mills) - spray impingement components - etc.
23

• Surface film effects
• Protective corrosion-product films important for
  resistance to erosion-corrosion.
• Hard, dense, adherent, continuous films give good
  resistance, provided that they are not brittle
  and easily removed under stress.
• Lead sulphate film protects lead against DILUTE
  H2SO4 under stagnant conditions, but not under
  rapidly moving conditions.

Erosion-corrosion of hard lead by 10 sulphuric
acid (velocity 39 ft/sec).
24

• pH affects films in erosion-corrosion of
  low-alloy steel.
• Scale generally granular Fe3O4 (non-protective).
  But at pH 6 pH 10, scale Fe(OH)2/Fe(OH)3 . . .
  hinders mass transport of oxygen and ionic
  species.

Effect of pH of distilled water on
erosion-corrosion of carbon steel at 50?C
(velocity 39 ft/sec).
25

• Dissolved O2 often increases erosion-corrosion .
  . .
• e.g. copper alloys in seawater. . . BUT . . . on
  steels, dissolved O2 will inhibit
  erosion-corrosion . . . utilized in boiler
  feedwater systems.

Effects of temperature and dissolved O2 on the
weight-loss of AISI 304 stainless steel exposed
for 800 hours in flowing water at 3.7 m/s.
26
Effect of oxygen dosing on erosion-corrosion and
potential of carbon steel in water at 150?C, pH
at 25oC 7.8.
27

• Good resistance of Ti to erosion-corrosion in
• - seawater
• - Cl- solutions
• - HNO3
• and many other environments.
• Resistance depends on formation and stability of
  TiO2 films.

28

• Chromium imparts resistance to erosion-corrosion
  to - steels
• - Cu alloys.
• Such tests have led to the marketing of a new
  alloy for condenser tubes . . CA-722 . . .
  previously IN-838 . . . with constituents . . .
  Cu-16Ni-0.4Cr.

Effect of chromium additions on seawater
impingement-corrosion resistance of copper-nickel
alloys. 36-day test with 7.5 m/s jet velocity
seawater temperature 27?C.
29

• Velocity Effects
• N.B. Turbulent flow regime for V lt Vc is
  sometimes called
• Flow-Assisted Corrosion regime.

Schematic showing the effect of flow velocity on
erosion-corrosion rate.
30

• Relationship between flow velocity, v, and
  erosion-corrosion rate, w, may be written as . .
  .
• w kva
• where k and a are constants that depend on the
  system.
• DISCUSS What happens when v 0 ?
• How do we express no dependence on velocity?
• The exponent a varies between . . .
• 0.3 (laminar flow) and
• 0.5 (turbulent flow)...
• occasionally reaching gt 1.0 for mass transfer
  and fluid shear
• effects.
• For mechanical removal of oxide films (spalling),
  the fluid shear stress at the surface is
  important, and a gt 1.0 . . . (may reach 2 - 4).

31
Erosion-Corrosion in Carbon Steel and Low-Alloy
Steels N.B. these materials are used
extensively in boilers, turbines, feed-water
heaters in fossil nuclear plants. High
velocities occur in single-phase flow (water) and
two-phase flow (wet steam). Single-phase E-C
seen in H.P. feedwater heaters, SG inlets in
AGRs, feedwater pumps. Two-phase E-C more
widespread . . . steam extraction piping,
cross-over piping (HP turbine to moisture
separator), steam side of feedwater heaters.
32

• Material effects low-alloy steel . . .

Cr additions reduce E-C.
Erosion-corrosion loss as a function of time for
mild steel and 1 Cr 0.5 Mo steel in water (pH at
25?C 9.05) flowing through an orifice at 130?C.
33

• Flow dependence (single phase)...

Erosion-corrosion rate of carbon steel as a
function of flow rate of deoxygenated water
through orifice at pH 9.05 and at 149?C.
34
Mechanism... for E-C of C.S. in high temperature
de-oxygenated water... - magnetite film
dissolves reductively Fe3O4 (3n-4) H2O
2e 3Fe(OH)n(2-n) (3n-8)H - high mass
transfer rates remove soluble Fe II species -
oxide particles eroded from weakened film by
fluid shear stress - metal dissolves to try and
maintain film.
35
Mass transfer characteristics correlated by
expressions such as... Sh kRea Scb Sh
Sherwood Number Re Re
Reynolds Number Sc Sc Schmidt
Number Shear stress correlated by .
f f friction factor and at high Re, f
independent of velocity so
36

• Temperature and pH dependence for single-phase
  E-C of CS . . .

Effect of temperature on the exponent of the mass
transfer coefficient for the erosion-corrosion of
carbon steel in flowing water at various pHs.
37

• Prevention of Erosion-Corrosion
• design (avoid impingement geometries, high
  velocity, etc.)
• chemistry (e.g., in steam supply systems . . .
  for CS or low-alloy steel add O2, maintain pH gt
  9.2, use morpholine rather than NH3)
• materials (use Cr-containing steels)
• use hard, corrosion-resistant coatings.

38
CAVITATION DAMAGE Similar effect to E-C
mechanical removal of oxide film caused by
collapsing vapour bubbles.
High-speed pressure oscillations (pumps, etc.)
can create shock waves gt 60,000 psi. Surface
attack often resembles closely-spaced pitting.
39

• FRETTING CORROSION
• Similar to E-C but surface mechanical action
  provided by wear of another
• surface . . . generally intermittent,
  low-amplitude rubbing.
• Two theories . . . with same overall result . . .

40
Effects in terms of materials COMBINATIONS Frettin
g resistance of various materials
Poor Average Good
Aluminum on cast iron Aluminum on stainless steel Magnesium on cast iron Cast iron on chrome plate Laminated plastic on cast iron Bakelite on cast iron Hard tool steel on stainless Chrome plate on chrome plate Cast iron on tin plate Cast iron on cast iron with coating of shellac Cast iron on cast iron Copper on cast iron Brass on cast iron Zinc on cast iron Cast iron on silver plate Cast iron on silver plate Cast iron on amalgamated copper plate Cast iron on cast iron with rough surface Magnesium on copper plate Zirconium on zirconium Laminated plastic on gold plate Hard tool steel on tool steel Cold-rolled steel on cold- rolled steel Cast iron on cast iron with phosphate coating Cast iron on cast iron with coating of rubber cement Cast iron on cast iron with coating of tungsten sulfide Cast iron on cast iron with rubber gasket Cast iron on cast iron with Molykote lubricant Cast iron on stainless with Molykote lubricant
Source J.R. McDowell, ASTM Special Tech. Pub.
No. 144, p. 24, Philadelphia, 1952.
41

• Prevention of Fretting Corrosion
• lubricate
• avoid relative motion (add packing, etc.)
• increase relative motion to reduce attack
  severity
• select materials (e.g., choose harder component).

42
STRESS CORROSION
43

• STRESS CORROSION (Stress Corrosion Cracking -
  SCC)
• Under tensile stress, and in a suitable
  environment, some metals and alloys crack . . .
  usually, SCC noted by absence of significant
  surface attack . . . occurs in ductile
  materials.

44

• Transgranular SCC (TGSCC)

Cross section of stress-corrosion crack in
stainless steel.
45

• Intergranular SCC (IGSCC)

Intergranular stress corrosion cracking of brass.
46

• Two original classic examples of SCC
• season cracking of brass
• caustic embrittlement of CS

47

• Season Cracking
• Occurs where brass case is crimped onto bullet,
  i.e., in area of high residual stress.
• Common in warm, wet environments (e.g., tropics).
• Ammonia (from decomposition of organic matter,
  etc.) must be present.

Season cracking of German ammunition.
48

• Caustic Embrittlement
• Early steam boilers (19th and early 20th century)
  of riveted carbon steel. Both
• stationary and locomotive engines often exploded.
• Examination showed
• cracks or brittle failures around rivet holes
• areas susceptible were cold worked by riveting
  (i.e., had high residual stresses)
• whitish deposits in cracked regions were mostly
  caustic (i.e., sodium hydroxide from chemical
  treatment of boiler water)
• small leaks at rivets would concentrate NaOH and
  even dry out to solid. SCC revealed by dye
  penetrant.

Carbon steel plate from a caustic storage tank
failed by caustic embrittlement.
49

• Factors important in SCC
• environmental composition
• stress
• metal composition and microstructure
• temperature
• e.g., brasses crack in NH3, not in Cl-
• SSs crack in Cl-, not in NH3
• SSs crack in caustic, not in H2SO4, HNO3,
  CH3COOH, . . . etc.

necessary
50

• STRESS
• The greater the stress on the material, the
  quicker it will crack. (N.B. in fabricated
  components, there are usually RESIDUAL STRESSES
  from cold working, welding, surface treatment
  such as grinding or shot peening, etc., as well
  as APPLIED STRESSES from the service, such as
  hydrostatic, vapour pressure of contents, bending
  loads, etc.).

51

• Composite curves illustrating the relative
  stress-corrosion-cracking resistance
• for commercial stainless steels in boiling 42
  magnesium chloride.

DISCUSS how would you obtain such a curve and
what does it mean?
52

• The MAXIMUM stress you can apply before SCC is
  formed (c.f. MINIMUM stress to
  be applied compressively to prevent SCC) depends
  on alloy (composition and structure),
  temperature, and environment composition.
• Such THRESHOLD stresses may be between 10
  70 of the yield stress - Q.V.
• N.B. residual stresses from welding steel can be
  close to the yield point.
• N.B. corrosion products can induce large
  stresses by wedging.

53
N.B. small-radius notch tip and even
smaller-radius crack tip are STRESS RAISERS A
wedging action by corrosion products of ? 10
ksi (10,000 psi) can induce ? ? 300 ksi (300,000
psi) at the crack tip.
54

• Corrosion product wedging ? denting of S.G.
  tubes in some PWRs . . .

Boiling in crevice concentrates impurities - can
lead to acid Cl- at seawater-cooled sites.
Hour-glassing of Alloy-600 tubes led to severe
straining and cracking of tubes. Surrey PWR in
U.S. was first to replace S.Gs., because of
denting.
55

• Time to Failure

Major damage during SCC occurs in late stages as
cracks progress, cross-sectional area decreases,
stress increases until final failure occurs by
mechanical rupture.
56

• Environmental Factors
• No general pattern, SCC common in aqueous
  solutions, liquid metals also found in fused
  salts, nonaqueous inorganic liquids . . .
• N.B. Coriou (France) cracked Inconel-600 in pure
  water at ?300?C in 1959!!!

57

• Environments that may cause stress corrosion of
  metals and alloys
• Material Environment
  Material Environment
• Aluminum alloys NaCl-H2O2 solutions
  Ordinary steels NaOH solutions
• NaCl solutions
  NaOH-Na2SiO2 solutions
• Seawater Ca, NH3, and
  NaNO3 Air, Water vapor
  solutions
• Copper alloys NH3 (g aq)
  Mixed acids (H2SO4-HNO3)
• Amines HCN
  solutions
• Water, Water vapor
  Acidic H2S solutions
• Gold alloys FeCl3 solutions
  Seawater
• Acetic-acid-salt solutions
  Molten Na-Pb alloys
• Inconel Caustic soda solutions
  Stainless steels Acid chloride
  solutions
• Lead Lead acetate solutions
  such as MgCl2 and BaCl2
• Magnesium alloys NaCl-K2CrO4 solutions
  NaCl-H2O2 solutions
• Rural and coastal
  Seawater
• atmospheres H2S
• Distilled water NaOH-H2S
  solutions
• Monel Fused caustic soda
  Condensing steam from

58

• Increasing temperature accelerates SCC
• Most susceptible alloys crack ? ? 100?C Mg
  alloys crack at room temperature.
• Alternate wetting and drying may aggravate SCC -
  accelerate crack growth (possibly because of
  increasing concentration of corrosive component
  as dryness is approached).

Effect of temperature on time for crack
initiation in types 316 and 347 stainless steels
in water containing 875 ppm NaCl.
59

• Some Data for Recommending Service of CS or Ni
  Alloy in Caustic
• NACE caustic soda chart super-imposed over the
  data on which it is based.

Area A Carbon steel, no stress relief necessary
stress relieve welded steam-traced lines Area
B Carbon steel stress relieve welds and
bends Area C Application of nickel alloys to
be considered in this area nickel alloy trim for
valves in areas B and C.
60

• Metallurgical Factors in IGSCC
• In austenitic SS and Ni alloys, sensitization is
  of major importance in determining susceptibility
  to IGSCC . . . depletion of grain boundaries in
  Cr because of carbide precipitation makes them
  vulnerable to attack. e.g., IGSCC of
  recirculation piping in BWRs (type 304 SS)
  induced by ? 200 ppb dissolved oxygen in the
  otherwise pure H2O coolant resulted in a major
  replacement problem. Plants using L-grade
  experienced very much less SCC.
• Al alloys (e.g., with Mg and Zn) are also
  susceptible to IGSCC because of precipitation
  within grain boundaries . . . Mg-rich
  precipitates can denude the grain boundaries of
  Mg, make them susceptible to attack in aqueous
  media.
• N.B. In grain-boundary-precipitate mechanisms for
  inducing IGSCC, very local galvanic effects
  between precipitates and matrix are important
• some precipitates are ANODIC
• some precipitates are CATHODIC.

61

• Grain boundary segregation of alloy constituents
  or impurities (without
• precipitation of separate phases) can also induce
  IGSCC.
• e.g., Mg enrichment of grain boundaries in Al
  alloys is a factor in IGSCC
• promotes local dissolution and hydrogen entry
  (maybe to form hydride, MgH)
• also . . . grain boundary enrichment of
  impurities and/or C in Fe-base alloys, Ni-base
  alloys and austenitic stainless steels can
  contribute to IGSCC
• - segregation of P, Si, S, N, B reported only
  clear link with IGSCC reported for P in
  austenitic SS in oxidizing aqueous solutions, for
  P in ferritic alloys in nitrate and caustic
  solutions.

62
Transgranular SCC Lattice structure in
metal/alloy matrix important dislocation
emergence, movement along slip planes under
stress, and similar factors that can disrupt
passivating films, will promote dissolution of
metal at highly localized and strained
areas. Irradiation-Assisted SCC (IASCC) Since ?
1987, some in-reactor components have cracked in
LWRs . . generally in core-support structures at
the top of the vessel (austenitic SS, Ni alloys).
More widespread in BWRs than PWRs . . .
radiolytic chemical species (especially oxidizing
radicals) seem to be the cause. IASCC of
Alloy-600 (Inconel) penetrations in several PWR
vessel heads have led to leaks and boric-acid
corrosion of RPV head steel (e.g., Davis Besse).
Heads replaced.
63

• Mechanism of SCC
• SCC is very complex probably no single
  mechanism, but several operating at the same
  time. Models (scientific descriptions) of
  mechanisms of two types
• dissolution
• mechanical fracture.
• Dissolution Models of Crack Propagation
• Major model is based on Film Rupture . . .
  (slip-dissolution) . . . high stresses at crack
  tip create local area of plastic deformation -
  ruptures passive films, exposed metal dissolves
  rapidly . . . some say periodic dissolution and
  re-passivation, some say crack tip always bare.

64
periodic rupture
Schematic representation of crack propagation by
the film rupture model.
65

• Mechanical Fracture Models of Crack Propagation
• Corrosion Tunnel

• Corrosion tunnel models.
• Schematic of tunnel model showing the initiation
  of a crack by the formation of corrosion tunnels
  at slip steps and ductile deformation and
  fracture of the remaining ligaments.
• (b) Schematic diagram of the tunnel mechanism
  of SSC and flat slot formation.

66

• Adsorption of impurities at the crack tip
  promotes the nucleation of dislocations
• lead to shear-like fracture (seemingly brittle).
• Tarnish Rupture
• Cracks propagate by alternate film growth and
  (brittle) film fracture, followed by rapid film
  formation over exposed metal.
• Film-Induced Cleavage
• thin film forms
• brittle crack initiates in layer
• crack moves from film into matrix
• crack continues through ductile matrix until it
  blunts and stops
• process repeats.
• Adsorption-Induced Brittle Fracture
• Species adsorbing at crack tip alter inter-atomic
  bond strengths, lower stress required for
  fracture propagation should be continuous.
• Hydrogen Embrittlement
• Cathodic processes involving hydrogen-ion
  reduction can inject H into matrix . . . this can
  embrittle metal, promote cracking . . . most
  likely in ferritic steels but also possible in
  Ni-base, Ti and Al alloys (contributes to SCC of
  carbon steel feeders at Point Lepreau ?).

67

• Prevention of SCC
• 1. Lowering the stress below the threshold value
  if one exists. This may be done by annealing in
  the case of residual stresses, thickening the
  section, or reducing the load. Plain carbon
  steels may be stress-relief annealed at 590 to
  650?C, and the austenitic stainless steels are
  frequently stress-relieved at temperatures
  ranging from 820 to 930?C.
• 2. Eliminating the critical environmental species
  by, for example, de-gasification,
  demineralization, or distillation.
• 3. Changing the alloy is one possible recourse if
  neither the environment nor stress can be
  changed. For example, it is common practice to
  use Inconel (raising the nickel content) when
  type 304 stainless steel is not satisfactory.
  Although carbon steel is less resistant to
  general corrosion, it is more resistant to
  stress-corrosion cracking than are the stainless
  steels. Thus, under conditions which tend to
  produce stress-corrosion cracking, carbon steels
  are often found to be more satisfactory than the
  stainless steels. For example, heat exchangers
  used in contact with seawater or brackish waters
  are often constructed of ordinary mild steel.

68

• 4. Applying cathodic protection to the structure
  with an external power supply or consumable
  anodes. Cathodic protection should only be used
  to protect installations where it is positively
  known that stress-corrosion cracking is the cause
  of fracture, since hydrogen embrittlement effects
  are accelerated by impressed cathodic currents.
• 5. Adding inhibitors to the system if feasible.
  Phosphates and other inorganic and organic
  corrosion inhibitors have been used successfully
  to reduce stress-corrosion cracking effects in
  mildly corrosive media. As in all inhibitor
  applications, sufficient inhibitor should be
  added to prevent the possibility of localized
  corrosion and pitting.
• 6. Coatings are sometimes used, and they depend
  on keeping the environment away from the metal -
  for example, coating vessels and pipes that are
  covered with insulation. In general, however,
  this procedure may be risky for bare metal.
• 7. Shot-peening (also known as shot-blasting)
  produces residual compressive stresses in the
  surface of the metal. Very substantial
  improvement in resistance to stress corrosion
  found as a result of peening with glass beads.
  Type 410 stainless was exposed to 3 NaCl at room
  temperature type 304 to 42 MgCI2 at 150?C and
  aluminum alloy 7075-T6 to a water solution of
  K2Cr2O7-CrO3-NaCl at room temperature.

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• Corrosion Fatigue The fatigue fracture of a
  metal aggravated by a corrosive environment or
  the stress corrosion cracking of a metal
  aggravated by cyclic stress.
• N.B. Fatigue fracture usually occurs at stresses
  below the yield point but after many cyclic
  applications of the stress.
• Typical S-N curves

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• Fatigue-fractured material often shows most of
  the fracture face shiny metallic, with the final
  area to fracture (mechanically by brittle
  fracture of a reduced cross-section) having a
  rough crystalline appearance . . .
• If corrosion-fatigue occurs, the shiny-metallic
  area might be covered with corrosion products
  BUT normal fatigue fractures may also develop
  corrosion products - depends on environment,
  stress pattern, etc.

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• N.B. In normal fatigue, the frequency of the
  stress cycles is not important. (can do
  accelerated fatigue tests at high frequency - the
  total number of cycles determines fatigue).
• BUT in corrosion fatigue, low-cycle stresses are
  more damaging than high-frequency stresses.
• Environment is important.
• e.g., in seawater
• Al bronzes and type 300 series SS lose
  20-30 of normal fatigue resistance
• high-Cr alloys lose 60-70 resistance.
• N.B. Cyclic loads mean lower allowable stresses,
  this must be designed into components if there
  is also a corrosive environment, the allowable
  stresses are EVEN LOWER.

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• Prevention of Corrosion Fatigue
• change design so as to reduce stress and/or
  cycling.
• reduce stress by heat treatment (for residual
  stress), shot peening (to change surface residual
  stresses to COMPRESSIVE).
• use corrosion inhibitor with care!
• use coatings . . . electrodeposited
• Zn
• Cr
• Ni
• Cu
• and
• nitrided layers (heating of steels in contact
  with N-containing material e.g., NH3, NaCN,
  etc.).

73
HYDROGEN DAMAGE
74

• HYDROGEN EFFECTS
• Hydrogen can degrade metals by
• hydrogen blistering
• hydrogen embrittlement
• decarburization
• hydrogen attack.

75

• Blistering
• Hydrogen enters the lattice of a metal, diffuses
  to voids, creates high internal stresses ?
  blisters . . .
• Blistering may occur during exposure to
• hydrocarbons
• electroplating solutions
• chemical process streams
• pickling solutions
• H-containing contaminants during welding
• general corrosive environments.

Cross section of a carbon steel plate removed
from a petroleum process stream showing a large
hydrogen blister. Exposure time 2 years.
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• Embrittlement
• Similar to blistering . . . hydrogen enters metal
  lattice . . .BUT . . .interaction with metal
  lattice different. High-strength (and more
  brittle) steels are susceptible.
• H-embrittlement different from SCC in nature of
  cracks . . . stress-corrosion cracks usually
  propagate anodically

77

• Hydride-forming metals are susceptible to H-
  embrittlement . . .e.g., Zr-alloy pressure tubes
  (in CANDUs) and fuel sheathing (in all water-
  cooled reactors) pick up hydrogen (or deuterium
  in heavy water ) by general corrosion. The
  hydrogen (D) migrates through the metal lattice
  to cool regions and to regions of high tensile
  stress - can precipitate as a separate phase -
  zirconium hydride.
• These hydrides are themselves brittle, and crack,
  and the crack can propagate through the material,
  with more hydride progressively precipitating at
  the crack tip.

N.B. Enough hydride can precipitate to form a
hydride blister . . . c.f.
hydrogen blister.
78

• N.B. The mechanism of hydrogen uptake by metals
  must involve
• ATOMIC HYDROGEN - molecular hydrogen cannot
  diffuse through metal lattices.
• BUT . . . remember that molecular hydrogen may
  absorb and dissociate on metal surfaces.

Schematic illustration showing the mechanism of
hydrogen blistering.
79

• Decarburization and Hydrogen Attack
• High temperature process - C or carbide in steels
  can react with gaseous hydrogen . . .
• C 2H2 ? CH4
• Note that the reaction can occur with atomic H in
  the metal lattice . . .
• C 4H ? CH4
• May crack the steel from high internal pressure.
• May cause loss of strength as C disappears.

80

• Prevention of Blistering
• use steels with few or no voids
• use coatings
• use inhibitors
• remove impurities that can promote hydrogen
  evolution . . . S2- (particularly bad), As,
  CN-, etc.
• use different materials (Ni-base alloys have low
  diffusion rates for hydrogen).

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• Prevention of Embrittlement
• reduce corrosion rate (inhibitors, coatings,
  etc.)
• change electroplating process to minimize H
  effects (voltage, current density, bath
  composition, etc.)
• bake material to remove H
• minimize residual stresses
• use less susceptible material
• maintain clean conditions during welding.