Transcript Slide 1

CHAPTER 3
FORMS OF CORROSION
(cont’)
Chapter Outlines
3.5
3.6
3.7
3.8
Selective Leaching
Erosion Corrosion
Stress Corrosion
Hydrogen Damage
SELECTIVE LEACHING
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.
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
Dezincification
All Cu-Zn alloys (Brasses) containing > 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
Uniform dezincification of
brass pipe.
-
or plug-type.... (boiler water inside, combustion gases outside)
Plug-type
dezincification.
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 (high[Zn]), acid environments;
Plug-type dezincification...
- usually found in low brasses, alkaline, neutral or slightly acid environments.
Prevention
- Make environment less aggressive (e.g., reduce O2 content);
- Cathodically protect;
- Use a better alloy (common cure - above not usually feasible)...
- “red” brass (<15% 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.
“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
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.
(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
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 < 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.
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.
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.
100 m
Iron crystals found in a plugged region of a failed pump channel
of a lithium processing test loop.
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 Nibase 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.
(a)
(b)
(a) microstructure of type 304L SS exposed to LiF-BeF2-ZrF4-ThF4- UF4 (7023-5-1-1 mole % respectively) for 5700 hours at 688C.
(b) microstructure of type 304L SS exposed to LiF-BeF2-ZrF4-ThF4- UF4 (7023-5-1-1 mole % respectively) for 5724 hours at 685C.
EROSION CORROSION
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.
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.
Erosion-corrosion of condenser
tube wall.
Impeller lasted ~ 2 years in oxidizing
conditions;
after switch to reducing conditions, it lasted
~ 3 weeks!
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.
- 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:
Erosion-Corrosion found in:
- 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.
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).
pH affects films in erosion-corrosion of low-alloy steel.
Effect of pH of distilled water
on erosion-corrosion of carbon
steel at 50C (velocity 39 ft/sec).
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.
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.
Effect of oxygen dosing on
erosion-corrosion and potential of
carbon steel in water at 150C,
pH at 25oC= 7.8.
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.
Chromium imparts resistance to erosion-corrosion to: - steels;
- Cu alloys.
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.
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.
Velocity Effects
Schematic showing the
effect of flow velocity on
erosion-corrosion rate.
N.B. Turbulent flow regime for V < Vc is sometimes called
Flow-Assisted Corrosion regime.
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 > 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 > 1.0 . . . (may reach 2 - 4).
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.
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.
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.
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.
Mass transfer characteristics correlated by expressions such as...
Sh
= kRea Scb
Re
=
Sc
=
dv


D
Sh = Sherwood Number =
Re = Reynolds Number
Sc = Schmidt Number
Shear stress correlated by ….

v 2
= f
and at high Re, f independent of velocity so
f = friction factor
  v2
kd
D
Temperature and pH dependence for single-phase E-C of CS . . .
Effect of temperature on the
exponent of the mass transfer
coefficient for the erosioncorrosion of carbon steel in
flowing water at various pHs.
Prevention of Erosion-Corrosion
•
design (avoid impingement geometries, high velocity, etc.);
•
chemistry (e.g., in steam supply systems . . . for CS or lowalloy steel add O2, maintain pH > 9.2, use morpholine rather
than NH3);
•
materials (use Cr-containing steels);
•
use hard, corrosion-resistant coatings.
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 > 60,000
psi. Surface attack often resembles closely-spaced pitting.
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 . . .
Effects in terms of materials COMBINATIONS
Fretting resistance of various materials
Poor
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
Average
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
Good
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.
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).
STRESS CORROSION
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.
“Transgranular” SCC (“TGSCC”)
Cross section of stress-corrosion
crack in stainless steel.
“Intergranular” SCC (“IGSCC”)
Intergranular stress corrosion
cracking of brass.
Two original classic examples of SCC:
• “season cracking” of brass;
• “caustic embrittlement” of CS;
“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.
“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.
Factors important in SCC:
•
•
•
•
e.g.,
environmental composition;
stress;
metal composition and microstructure;
temperature;
}
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
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.).
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?
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”.
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.
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.
Time to Failure
Major damage during SCC
occurs in late stages as
cracks progress, crosssectional area decreases,
stress increases until final
failure occurs by mechanical
rupture.
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!!!
Environments that may cause stress corrosion of metals and alloys
Material
Aluminum alloys
Environment
NaCl-H2O2 solutions
NaCl solutions
Seawater
Air, Water vapor
Copper alloys
NH3 (g & aq)
Amines
Water, Water vapor
Gold alloys
FeCl3 solutions
Acetic-acid-salt solutions
Inconel
Caustic soda solutions
Lead
Lead acetate solutions
Magnesium alloys NaCl-K2CrO4 solutions
Rural and coastal
atmospheres
Distilled water
Monel
Fused caustic soda
Hydrofluoric acid
Hydrofluosilicic acid
Nickel
Fused caustic soda
Material
Ordinary steels
Stainless steels
Titanium alloys
Environment
NaOH solutions
NaOH-Na2SiO2 solutions
Ca, NH3, and NaNO3
solutions
Mixed acids (H2SO4-HNO3)
HCN solutions
Acidic H2S solutions
Seawater
Molten Na-Pb alloys
Acid chloride solutions
such as MgCl2 and BaCl2
NaCl-H2O2 solutions
Seawater
H2S
NaOH-H2S solutions
Condensing steam from
chloride waters
Red fuming HNO3, N2O4,
seawater, methanol-HCI
Increasing temperature accelerates SCC:
Effect of temperature on
time for crack initiation
in types 316 and 347
stainless steels in water
containing 875 ppm NaCl.
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).
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.
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.
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.
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.
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 repassivation, some say crack tip always bare.
periodic rupture
Schematic representation
of crack propagation by the film
rupture model.
Mechanical Fracture Models of Crack Propagation
Corrosion Tunnel;
Corrosion tunnel models.
(a) 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.
•
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 Nibase, Ti and Al alloys (contributes to SCC of carbon steel feeders at Point Lepreau …?).
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.
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.
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:
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.
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 highfrequency 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.
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.).
HYDROGEN DAMAGE
HYDROGEN EFFECTS
Hydrogen can degrade metals by:
•
hydrogen blistering;
•
hydrogen embrittlement;
•
decarburization;
•
hydrogen attack.
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.
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;
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.
N.B. The mechanism of hydrogen uptake by metals must involve
ATOMIC HYDROGEN - molecular hydrogen cannot diffuse through metal
lattices.
Schematic illustration showing the
mechanism of hydrogen blistering.
BUT . . . remember that molecular hydrogen may absorb and dissociate on
metal surfaces.
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.
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).
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.