This document discusses corrosion control through material selection and protective coatings. It describes various metallic and nonmetallic materials suitable for different corrosive environments. Stainless steels are discussed in detail, outlining their alloying elements of chromium, nickel, and carbon and how they contribute to corrosion resistance. Stress corrosion cracking of stainless steel is also summarized, describing the three conditions required and models of cracking. The document concludes with discussing design considerations and protective coatings to control corrosion.

1. CORROSION
CONTROL
M. Awais Yaqoob
2011-ch-32
(University of Engineering and Technology, Lahore)

2. (1) MATERIAL SELECTION
(selection of proper material for a
particular corrosive service)
Metallic [metal and alloy]
Nonmetallic [rubbers (natural and synthetic),
plastics, ceramics, carbon and graphite, and
wood]

3. Metals and Alloys
No Environment Proper material
1 Nitric acid Stainless steels
2 Caustic Nickel and nickel
alloys
3 Hydrofluoric acid Monel (Ni-Cu)
4 Hot hydrochloric acid Hastelloys (Ni-Cr-
Mo) (Chlorimets)
5 Dilute sulfuric acid Lead

4. No Environment Proper material
6 Nonstaining atmospheric
exposure
Aluminium
7 Distilled water Tin
8 Hot strong oxidizing
solution
Titanium
9 Ultimate resistance Tantalum
10 Concentrated sulfuric
acid
Steel

5. E.g : Stainless Steels
Stainless steels are
iron base alloys that
contain a minimum
of approximately
11% Cr, the amount
needed to prevent
the formation of rust
in unpolluted
atmosphere.
wt.% Cr
Dissolutionrate,cm/sec

6. Alloying elements of stainless steel :
 Other than Ni, Cr and C, the following alloying elements
may also present in stainless steel: Mo, N, Si, Mn, Cu, Ti,
Nb, Ta and/or W.
 Main alloying elements (Cr, Ni and C):
1. Chromium
Minimum concentration of Cr in a
stainless steel is 12-14wt.%
Structure : BCC (ferrite forming element)
* Note that the affinity of Cr to form Cr-carbides is very
high. Chromium carbide formation along grain
boundaries may induce intergranular corrosion.

7. Binary diagram of Fe-Cr
Sigma phase
formation which is
initially formed at
grain boundaries has
to be avoided
because it will
increase hardness,
decrease ductility
and notch toughness
as well as reduce
corrosion resistance.

8. 2. Nickel
Structure: FCC (austenite forming element/stabilize
austenitic structure)
Added to produce austenitic or duplex stainless
steels. These materials possess excellent ductility,
formability and toughness as well as weld-ability.
Nickel improves mechanical properties of stainless
steels servicing at high temperatures.
Nickel increases aqueous corrosion resistance of
materials.

9. Ternary diagram of Fe-Cr-Ni at 6500
and 10000
C
AISI : American Iron and Steel Institute

10. Anodic polarization curves of Cr, Ni and Fe in 1 N
H2SO4 solution

11. Influence of Cr on corrosion resistance of iron
base alloy

12. Influence of Ni on corrosion resistance of iron base alloy

13. Influence of Cr on
iron base alloy
containing 8.3-
9.8wt.%Ni

14. 3. Carbon
Very strong austenite forming element (30x more
effective than Ni). I.e. if austenitic stainless steel
18Cr-8Ni contains ≤0.007%C, its structure will
convert to ferritic structure. However the
concentration of carbon is usually limited to ≤
0.08%C (normal stainless steels) and ≤0.03%C
(low carbon stainless steels to avoid sensitization
during welding).

15. Minor alloying elements :
 Manganese
Austenitic forming element. When necessary can be used to
substitute Ni. Concentration of Mn in stainless steel is usually
2-3%.
 Molybdenum
Ferritic forming element. Added to increase pitting corrosion
resistance of stainless steel (2-4%).
Molybdenum addition has to be followed by decreasing
chromium concentration (i.e. in 18-8SS has to be decreased
down to 16-18%) and increasing nickel concentration (i.e. has
to be increased up to 10-14%).
Improves mechanical properties of stainless steel at high
temperature. Increase aqueous corrosion resistance of material
exposed in reducing acid.

16.  Tungsten
Is added to increase the strength and toughness of
martensitic stainless steel.
 Nitrogen (up to 0.25%)
Stabilize austenitic structure. Increases strength and corrosion
resistance. Increases weld ability of duplex SS.
 Titanium, Niobium and Tantalum
To stabilize stainless steel by reducing susceptibility of the
material to intergranular corrosion. Ti addition > 5x%C.
Ta+Nb addition > 10x%C.

17.  Copper
Is added to increase corrosion resistance of stainless steel
exposed in environment containing sulfuric acid.
 Silicon
Reduce susceptibility of SS to pitting and crevice corrosion as
well as SCC.

18. Influence of alloying elements on pitting
corrosion resistance of stainless steels

19. Influence of alloying elements on crevice
corrosion resistance of stainless steels

20. Influence of alloying elements on SCC
resistance of stainless steels

21. Five basic types of stainless steels :
 Austenitic - Susceptible to SCC. Can be hardened by only
by cold working. Good toughness and formability, easily to
be welded and high corrosion resistance. Nonmagnetic
except after excess cold working due to martensitic
formation.
 Martensitic - Application: when high mechanical strength
and wear resistance combined with some degree of corrosion
resistance are required. Typical application include steam
turbine blades, valves body and seats, bolts and screws,
springs, knives, surgical instruments, and chemical
engineering equipment.
 Ferritic - Higher resistance to SCC than austenitic SS. Tend
to be notch sensitive and are susceptible to embrittlement
during welding. Not recommended for service above 3000
C
because they will loss their room temperature ductility.

22.  Duplex (austenitic + ferritic) – has enhanced resistance to
SCC with corrosion resistance performance similar to AISI
316 SS. Has higher tensile strengths than the austenitic
type, are slightly less easy to form and have weld ability
similar to the austenitic stainless steel. Can be considered as
combining many of the best features of both the austenitic
and ferritic types. Suffer a loss impact strength if held for
extended periods at high temperatures above 3000
C.
 Precipitation hardening - Have the highest strength but
require proper heat-treatment to develop the correct
combination of strength and corrosion resistance. To be
used for specialized application where high strength
together with good corrosion resistance is required.

31. Stress Corrosion Cracking of Stainless Steel
 Stress corrosion cracking (SCC) is defined as crack
nucleation and propagation in stainless steel caused by
synergistic action of tensile stress, either constant or slightly
changing with time, together with crack tip chemical
reactions or other environment-induced crack tip effect.
 SCC failure is a brittle failure at relatively low constant
tensile stress of an alloy exposed in a specific corrosive
environment.
 However the final fracture because of overload of
remaining load-bearing section is no longer SCC.

32.  Three conditions must be present
simultaneously to produce SCC:
- a critical environment
- a susceptible alloy
- some component of tensile stress

33. Tensile
stress
Corrosive
environment
Susceptible
material
Stress
corrosion
cracking
Tensile stress
is below yield
point
Corrosive
environment is
often specific to
the alloy system
Pure metals are more
resistance to SCC but not
immune and susceptibility
increases with strength

34. Typical micro cracks formed during SCC of
sensitized AISI 304 SS
Surface morphology

35. Example of crack propagation during transgranular stress
corrosion cracking (TGSCC) brass

36. Example of crack
propagation during
intergranular stress
corrosion cracking
(IGSCC) ASTM A245
carbon steel

37. Fracture surface of
transgranular SCC on
austenitic stainless steel in
hot chloride solution
Fracture surface of
intergranular SCC on
carbon steel in hot nitric
solution

38. Fracture surface due
to intergranular SCC
Fracture surface due to
local stress has reached
its tensile strength value
on the remaining section

39. Electrochemical effect
pitting
passive
active
cracking
zones
Usual region for
TGSCC, mostly is
initiated by pitting
corrosion
(transgranular cracking
propagation needs
higher energy)
Usual region for IGSCC,
SCC usually occurs where
the passive film is
relatively weak
Zone 1
Zone 2

40.  Note that non-susceptible alloy-environment combinations,
will not crack the alloy even if held in one of the potential
zones.
 Temperature and solution composition (including pH,
dissolved oxidizers, aggressive ions and inhibitors or
passivators) can modify the anodic polarization behavior to
permit SCC.
 Susceptibility to SCC cannot be predicted solely from the
anodic polarization curve.

41. Models of stress corrosion cracking
 Slip step dissolution model
 Discontinuous intergranular crack growth
 Crack nucleation by rows of corrosion micro-
tunnels
 Absorption induced cleavage
 Surface mobility (atoms migrate out of the
crack tips)
 Hydrogen embrittlement HIC→

42. Control/prevention :
 Reduce applied stress level
 Remove residual tensile stress (internal stress)
 Lowering oxidizing agent and/or critical
species from the environment
 Add inhibitor
 Use more resistant alloys
 Cathodic protection

43. Alteration of Environment
 Typical changes in medium are :
 Lowering temperature – but there are cases where
increasing T decreases attack. E.g hot, fresh or salt water
is raised to boiling T and result in decreasing O2
solubility with T.
 Decreasing velocity – exception ; metals & alloys that
passivate (e.g stainless steel) generally have better
resistance to flowing mediums than stagnant. Avoid very
high velocity because of erosion-corrosion effects.

44.  Removing oxygen or oxidizers – e.g boiler feedwater
was deaerated by passing it thru a large mass of scrap
steel. Modern practice – vacuum treatment, inert gas
sparging, or thru the use of oxygen scavengers. However,
not recommended for active-passive metals or alloys.
These materials require oxidizers to form protective oxide
films.
 Changing concentration – higher concentration of
acid has higher amount of active species (H ions).
However, for materials that exhibit passivity, effect is
normally negligible.

45. Environment factors affecting
corrosion design :
 Dust particles and man-made pollution – CO, NO,
methane, etc.
 Temperature – high T & high humidity accelerates
corrosion.
 Rainfall – excess washes corrosive materials and
debris but scarce may leave water droplets.
 Proximity to sea
 Air pollution – NaCl, SO2, sulfurous acid, etc.
 Humidity – cause condensation.

46. Design Do’s & Don’ts
 Wall thickness – allowance to accommodate for corrosion
effect.
 Avoid excessive mechanical stresses and stress
concentrations in components exposed to corrosive
mediums. Esp when using materials susceptible to SCC.
 Avoid galvanic contact / electrical contact between dissimilar
metals to prevent galvanic corrosion.
 Avoid sharp bends in piping systems when high velocities
and/or solid in suspension are involved – erosion corrosion.
 Avoid crevices – e.g weld rather than rivet tanks and other
containers, proper trimming of gasket, etc.

47.  Avoid sharp corners – paint tends to be thinner at sharp
corners and often starts to fail.
 Provide for easy drainage (esp tanks) – avoid remaining
liquids collect at bottom. E.g steel is resistant against
concentrated sulfuric acid. But if remaining liquid is
exposed to air, acid tend to absorb moisture, resulting in
dilution and rapid attack occurs.
 Avoid hot spots during heat transfer operations – localized
heating and high corrosion rates. Hot spots also tend to
produce stresses – SCC failures.
 Design to exclude air – except for active-passive metals and
alloys coz they require O2 for protective films.
 Most general rule : AVOID HETEROGENEITY!!!

48. Protective Coatings / Wrapping
 Provide barrier between metal and environment.
 Coatings may act as sacrificial anode or release substance
that inhibit corrosive attack on substrate.
 Metal coatings :
 Noble – silver, copper, nickel, Cr, Sn, Pb on steel.
Should be free of pores/discontinuity coz creates
small anode-large cathode leading to rapid attack
at the damaged areas.
 Sacrificial – Zn, Al, Cd on steel. Exposed substrate
will be cathodic & will be protected.
 Application – hot dipping, flame spraying, cladding,
electroplating, vapor deposition, etc.

49.  Surface modification – to structure or composition by use
of directed energy or particle beams. E.g ion implantation
and laser processing.
 Inorganic coating : cement coatings, glass coatings, ceramic
coatings, chemical conversion coatings.
 Chemical conversion – anodizing, phosphatizing, oxide
coating, chromate.
 Organic coating : paints, lacquers, varnishes. Coating liquid
generally consists of solvent, resin and pigment. The resin
provides chemical and corrosion resistance, and pigments
may also have corrosion inhibition functions.