Transcript 2.5. CORROSION CONTROL

4.0 CORROSION
PREVENTION
MATERIAL
SELECTION
ALTERATION OF ENVIRONMENT
PROPER DESIGN
CATHODIC PROTECTION
ANODIC PROTECTION
COATINGS & WRAPPING
(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]
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-CrMo) (Chlorimets)
5
Dilute sulfuric acid
Lead
No
Environment
Proper material
6
Nonstaining atmospheric
exposure
Distilled water
Aluminium
Hot strong oxidizing
solution
Ultimate resistance
Titanium
7
8
9
10
Concentrated sulfuric
acid
Tin
Tantalum
Steel
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.
Dissolution rate, cm/sec
E.g : Stainless Steels
wt.% Cr
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.
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.
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.
Ternary diagram of Fe-Cr-Ni at 6500 and 10000C
AISI : American Iron and Steel Institute
Anodic polarization curves of Cr, Ni and Fe in 1 N
H2SO4 solution
Influence of Cr on corrosion resistance of iron
base alloy
Influence of Ni on corrosion resistance of iron base alloy
Influence of Cr on
iron base alloy
containing 8.39.8wt.%Ni
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).
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.

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.

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.

Influence of alloying elements on pitting
corrosion resistance of stainless steels
Influence of alloying elements on crevice
corrosion resistance of stainless steels
Influence of alloying elements on SCC
resistance of stainless steels
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 3000C
because they will loss their room temperature ductility.

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 3000C.

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.
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.

Three conditions must be present
simultaneously to produce SCC:
- a critical environment
- a susceptible alloy
- some component of tensile stress
Pure metals are more
resistance to SCC but not
immune and susceptibility
increases with strength
Tensile stress
is below yield
point
Tensile
stress
Corrosive
environment is
often specific to
the alloy system
Susceptible
material
Corrosive
environment
Stress
corrosion
cracking
Typical micro cracks formed during SCC of
sensitized AISI 304 SS
Surface morphology
Example of crack propagation during transgranular stress
corrosion cracking (TGSCC) brass
Example of crack
propagation during
intergranular stress
corrosion cracking
(IGSCC) ASTM A245
carbon steel
Fracture surface of
intergranular SCC on
carbon steel in hot nitric
solution
Fracture surface of
transgranular SCC on
austenitic stainless steel in
hot chloride solution
Fracture surface due
to intergranular SCC
Fracture surface due to
local stress has reached
its tensile strength value
on the remaining section
Electrochemical effect
pitting
Zone 1
cracking
zones
passive
Usual region for
TGSCC, mostly is
initiated by pitting
corrosion
(transgranular cracking
propagation needs
higher energy)
Zone 2
active
Usual region for IGSCC,
SCC usually occurs where
the passive film is
relatively weak



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.
Models of stress corrosion cracking






Slip step dissolution model
Discontinuous intergranular crack growth
Crack nucleation by rows of corrosion microtunnels
Absorption induced cleavage
Surface mobility (atoms migrate out of the
crack tips)
Hydrogen embrittlement→HIC
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
2. 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.

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.
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.
3. 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.





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!!!
4. 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.




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.
5. Cathodic and Anodic Protection
5.1: Cathodic Protection





Cathodic Protection (CP) was employed before the science of
electrochemistry had been developed;
CP is achieved by supplying electrons to the metal structure to
be protected. (M
Mn+ + ne) and (2H+ + 2e
H2);
Examination of equation indicates the addition of electrons to
the structure will tend to suppress metal dissolution and
increase the rate of hydrogen dissolution;
If current is considered to flow from (+) to (-), then a structured
is protected if current enters it from the electrolyte;
Conversely, accelerated corrosion occurs if current passes from
the metal to the electrolyte, this current convention has been
adopted in cathodic protection technology and is used here for
consistency;

There 2 ways to cathodically protect a structure:
(i) by an external power supply
(ii) by appropriate galvanic coupling





Figure illustrates CP by impressed current;
Here, an external dc power supply is connected to an underground tank,
the negative terminal of the power supply is connected to the tank, and
the positive terminal to an inert anode such as graphite or Duriron.
The electric leads to the tank and inert the electrode are carefully
insulated to prevent current leakage;
The anode is usually surrounded by backfill consisting of coke breeze,
gypsum or bentonite which improves electric contact between the anode
and the surrounding soil.
In figure, current passes to the metallic structure and corrosion is
suppressed;



CP by galvanic coupling to Mg is shown in Fig 6.2;
Mg is anodic with respect to steel and corrodes preferentially when
galvanic coupled; The anode in this case is called a sacrificial anode
since it is consumed during the protection of the steel structure;
CP using sacrificial anodes can also be used to protect buries pipelines;
shown Fig 6.3. the anodes are spaced along the pipe to ensure uniform
current distribution;
5.2 Anodic Protection







In contrast to CP, anodic protection (AP) is relatively new;
This technique was developed using electrode kinetics principles and is
somewhat difficult to describe without introducing advanced concepts
of electrochemical theory;
Simply, AP is based is based on the formation of a protective film on
metals by externally applied anodic currents.
This usually except for metals with active-passive transitions such as
Ni, Fe, Cr, Ti and their alloys;
If carefully controlled anodic currents are applied to these materials,
they are passivated and the rate of metal dissolution is decreased;
To anodically protect a structure, a device called a potentiostat is
required;
A potentiostat is an electronic device that maintains a metal at a
constant potential with respect to a reference electrode.



The potentiostat has three terminals, one connected to the tank,
another to an auxiliary cathode (a platinum or platinum-clad electrode)
and the third to a reference electrode (e.g. calomel cell);
In operation, the potentiostat maintains a constant potential between
the tank and the reference electrode;
The optimum potential for protection is determined by electrochemical
measurements;