Materials & Corrosion
Why worry about corrosion?
“One large chemical company spent more
than $400,000 per year for corrosion
maintenance in its sulfuric acid plants, even
though the corrosion conditions were not
considered to be particularly severe.”
Why worry about corrosion?
“A refinery employing a new process
developed a serious problem after just 16
weeks of operation; some parts showed a
corrosion loss of as much as 1/8 inch.”
Why worry about corrosion?
“The trend in the chemical process industries
toward higher temperatures and pressures
has made possible new processes or
improvements in old processes. Higher
temperatures and pressures usualy involve
more severe corrosion conditions. Many of
the present day operations would not have
been possible or economical w/o the use of
corrosion-resistant materials.”
Why Worry about Corrosion?
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Safety/Health
Environment
Operability
Profitability
Product Quality
Appearance of Facilities/Equipment
– Badly corroded and rusted equipment in a plant
would leave a poor impression on the observer
Examples of Corrosion
• Erosion-corrosion of copper water pipe
(tubing)
• Chloride Stress corrosion cracking of
stainless steels
• Chloride pitting of stainless steel
• Nitric acid attack of titanium tubing
Corrosive Environments
• Atmospheric
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Industrial
Urban
Rural
Marine
• Water
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Seawater
Freshwater
Tapwater
Treated water
Corrosive Environments
• Soil
• Industrial – commercial/institutional
Definition of corrosion
• Is the chemical or electro-chemical reaction
between a material – usually a metal – ant
it’s environment that produces a
deterioration of the material and it’s
properties.
Factors Influencing Corrosion
• Design, fabrication & materials
– Design/configuration
– Forming, welding, heat treating
– Metallurgy, composition
• Environmental factors
– Process chemistry including:
• pH
• Dissolved materials
– Salt
– Metal ions
– Gases (O2, N2, CO2, ammonia, chlorine etc.)
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Suspended matter
Temperature
Flow velocity
Micro-organisms
Types of Corrosion
2 Main Types:
1. General corrosion is the loss material over
the entire surface, at a relatively constant
rate. The metal is thinned fairly uniformly,
without appreciable localized attack.
2. Localized Attack is specific to certain
areas of the material, and can take many
forms.
Forms of Localized Attack
• Pitting is corrosion that produces sites of
localized attack that are small relative to the
overall exposed surface area. It is the most
common form of corrosion in aqueous
environments, and the major cause of
corrosion service failures in
the chemical processing
industry.
Forms of Localized Attack cont’d
• CREVICE CORROSION
is attack at narrow spaces
or gaps between two metal
surfaces, or between a
metal and non-metal. It
can occur at cracks or
seams in a metal, or under
a washer, gasket, or
deposit.
Crevice corrosion results
from a difference between
the chemistry of the bulk
environment and that in or
at the crevice.
Forms of Localized Attack cont’d
• BIOLOGICAL ATTACK is that which
caused or accelerated by organisms on the
affected surface. Fouling organic deposits
may cause crevice corrosion, or organisms
may produce chemicals that cause
corrosion.
• EROSION CORROSION is acceleration of
material loss due to the combined effects of
corrosion plus removal of material by the
moving fluid.
Forms of Localized Attack cont’d
• CAVITATION & IMPINGEMENT is
material loss due to collapse of voids or
cavities in the fluid, due to pressure changes
(cavitation), or due to impingement of
liquid droplets. This may be strictly
mechanical damage, or may
be worsened by the effects
of corrosion.
S.E.M. What is it?
Forms of Localized Attack cont’d
• Fretting is a process
combining wear and
corrosion in removal of
material from contacting
solid surfaces. It typically
involves very small
relative movements of the
components, oxidation of
the surfaces, and abrasion
by the oxidation products.
It often occurs between a
shaft and a component
fitted on the shaft.
Forms of Localized Attack cont’d
• INTERGRANULAR ATTACK is corrosion
at the boundaries of a metal grain, with little
or no attack of the grain. It results in
weakening of the metal, or separation at the
grain boundary. (The composition and
corrosion resistance of a metal grain varies
from the surface to the interior.)
Forms of localized Attack cont’d
• DEALLOYING ATTACK is preferential
removal of one constituent of an alloy in a
corrosive environment. An example is the
“dezincification” of brass, in which zinc is
leached from the brass in some aqueous
streams, leaving a weak structure of copper
and copper oxide.
Forms of Localized Attack cont’d
• GALVANIC ATTACK is attack of a metal that is
in electrical contact with a more noble metal, or a
non-metallic conductor, in a corrosive
environment. Examples are: corrosion of copper
or brass couple to steel in an aqueous
environment: or corrosion of a zinc coating on
steel. The latter is done intentionally to protect the
steel, as in roofing nails, fencing, corrugated
galvanized sheets, etc.
Types of Corrosion
Environmentally-induced cracking:
• Stress-corrosion cracking (SCC) is due to the
combined effects of a corrodent and sustained
tensile stress. SCC of AUSTENTITIC (300-series)
stainless steels by chorides is a major problem in
the chemical industry. SCC can
be caused in copper alloys in
nitrates, and in steel by caustic.
Environmentally-induced
Cracking
• CORROSION FATIGUE occurs in a
cyclically loaded part in a corrosive
environment. It occurs
at lower stress levels or
after fewer cycles that
would be the case in the
absence of the corrosive
environment.
Environmentally-induced
Cracking
• Hydrogen-induced cracking or
Embrittlement is reduction of the ductility
or toughness of a metal due to the presence
of atomic hydrogen. The hydrogen can be
present due to introduction into the molten
metal, or through absorption by the solid
metal.
Environmentally-induced
Cracking cont’d
• LIQUID METAL EMBRITTLEMENT is
brittle failure of a normally ductile metal
when coated with a thin film of liquid metal
followed by stressing in tension. Examples
of LME may occur when steel is brazed,
soldered, welded, or plated, or dip-coated
with zinc, cadmium, or tin.
Corrosion Control
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Materials selection
Design
Method of Operation
Barriers/coatings
– Paint systems
– Linings – e.g. rubber or plastic
– Cladding – metal on metal
– Galvanizing
– Plating
– Glass/ceramic – e.g. porcelain on steel
• Inhibitors
• Cathodic/Anodic Protection
Factors affecting choice of an
engineering material
Appearance
Corrosion
resistance
Strength
Materials
Selection
Availability
Fabricability
Cost
Corrosion-resistant Materials
All materials are resistant to corrosion in some
specific environments. For example, carbon steel
is resistant to many process and aqueous liquids. It
may corrode slowly or not at all. Steel is the main
material used in chemical plant equipment.
The term corrosion-resistant is used to refer to
materials that resist attack in specific or unusually
corrosive environments.
Corrosion-Resistant Materials
The following is a list of a few materials used
in various corrosive services:
• Stainless steels
• Nickel & nickel alloys
• Reactive & refractory metals such as
tantalum, titanium, zirconium, & their
alloys
• Copper/copper alloys (brasses, bronzes)
Corrosive-Resistant Materials
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Aluminum
Lead
Chromium
Plastics, such as teflon, PVC, nylon,
polypropylene, polyethylene, etc.
• Rubbers/elastomers such as nitrile (NBR or
BUNA-N) EPDM (e.g. NORDEL), Viton,
NEOPRENE, butyl, etc.
• COMPOSITES, e.g. fiber-reinforced plastic (FRP,
fiberglass)
• GLASS & CERAMICS, tile, porcelain, brick
STAINLESS STEELS
• Iron-based alloys, with at least 10.5% Chromium
• Chromium-rich oxide surface film
– “passive”
– Forms and heals in oxygen
• Additives of nickel, molybdenum, copper,
titanium, aluminum, silicon, niobium, nitrogen,
sulphur, selenium
• Processing applications, cutlery, decorative, health
and sanitary, dairy, transportation, medical
STAINLESS STEELS
FERRITIC
• Iron plus 10-25% chromium (BCC)
• Magnetic
• High strength, limited toughness
• Good ductility & fabricability
• Examples : 405,409, 429, 430, 434, 442,
monit
STAINLESS STEELS
AUSTENTITIC
• Iron plus chromium plus nickel (FCC)
• Also molybdenum, manganese, copper, nitrogen,
others
• Non-magnetic (Usually)
• Relatively low strength
• High ductility/toughness
• Susceptible to chloride attack (SCC, pitting)
• Good fabricability
• Examples: 201, 202, 205, 301, 302, 303, 304, 304L, 305, 308, 309, 310,316,
316L, 317, 321, 330, 347, 348, 384, 904L, nitronic types, Alloy 20, 20Cb-3, sanicro 28
STAINLESS STEELS
DUPLEX
• Mixed structure of BCC ferrite & FCC
austenite
• High corrosion resistance, SCC-resistant
• High strength, good toughness
• Examples: 329, 2205, 2507, 3RE60, 7-Mo
PLUS, Ferralium 255
STAINLESS STEELS
MARTENSITIC
• Martensitic structure (distorted BCC)
• Lower chromium: 10-18 %
• Higher carbon: 0.15% min
• Magnetic, hardenable
• High strength – limited toughness
• Lower corrosion resistance
• Examples: 403, 410, 414, 416, 420, 431, 440
STAINLESS STEELS
PRECIPITATION HARDENING
• Chromium-nickel grades with copper, aluminum,
titanium additions
• Hardened by ageing at moderately elevated
temperatures
• Good formability (form then harden)
• Susceptible to heat damage
• Good corrosion resistance
• Examples: 15-5 PH, 17-4 PH, 17-7 PH, PH 14-4
Mo, Custom 450, custom 455
OTHER ALLOYS
COPPER & IT’S ALLOYS
Copper
• 99+% copper
• Water service, marine, heat exchangers,
architectural
• Not heat treatable
• Resists biofouling
COPPER & IT’S ALLOYS
BRASSES
• 5-45% Zinc
• Dealloying above 15% zinc
• Tin added to stop dealloying (admiralty and
naval brasses)
COPPER & IT’S ALLOYS
BRONZES
• Phosphour bronze – 10% tin
• Silicon bronze – 1-3% Si
• Aluminum bronze – 5-10% Al
COPPER & IT’S ALLOYS
CUPRO-NICKELS
• 90/10 Cu/Ni, 80/20, 70/30
• Resistant to seawater, erosion, pitting
ALUMINUM & IT’S ALLOYS
• Protected by barrier oxide film
• Corrodes at low and high pH
• Resistant to nitric acid (oxidizing)
1xxx – 99+% Al
2xxx
• alloyed with copper
• Strong, heat-treatable
• Lower corrosion resistance
ALUMINUM & IT’S ALLOYS
4xxx – alloyed with Si
5xxx – Al-Mg-Mn, Al-Mg-Cr, Al-Mg-Mn-Cr
NICKEL & IT’S ALLOYS
NICKEL
• Alloy 200, commercially pure
• Plating/cladding
• Resistant to caustic
MONEL
• Alloy 400, approx. 30% copper
• Very good fabricability
NICKEL & IT’S ALLOYS
NICKEL-MOLY
• Hastelloy B, B-2
• Resistant to HCI
NICKEL-CHROMIUM
• Inconel or Alloy 600
(77% Ni-15%Cr-bal Fe)
NICKEL & IT’S ALLOYS
Ni-Fe-Cr
• Incoloy or Alloy 800 (21Cr-32Ni-bal Fe)
• Resistant to chlorides
Ni-Fe-Cr-Mo
• Includes Alloys 20 & 20Cb3, Incoloy 825,
Hastelloy F & G
• Increased resistance to sulphuric, phosphoric, and
organic acids and to SCC and chloride pitting
NICKEL & IT’S ALLOYS
Ni-Cr-Mo
• Hastelloy C, C276, C-22
• Inconel or Alloy 625
• Resistant to hot acid mixtures
NON-METALLICS
PLASTICS
Thermoplastics
• Polyethylene, PP, ABS, PVC, CPVC,
PVDC
• Fluorocarbons – PTFE or teflon, FEP, PFA,
CTFE etc.
• Nylon
• Acetals, acrylics, polycarbonates, styrenes
Plastics Cont’d
Thermosetting resins
• Usually reinforced with glass
• Epoxy, phenolics, polyester, furanes
Rubber & Elastomers
• Synthetic & natural rubbers
Carbon & graphite
Glass
Ceramics & Refractories
Concrete
Wood
METALS
MATERIAL
Advantages
Disadvantage
Carbon Steel
Low cost, readily available, resists
abrasion, standard fabrication,
resists alkali
Poor resistance to acids & strong
alkali, often causes discolouration
and contamination
Stainless steel
Resists most acids, reduces
discolouration, available with a
variety of alloys, abrasion less than
mild steel
Not resistant to chlorides, more
expensive, fabrication more
difficult, alloy materials may have
catalytic effects
Monel-Nickel
Little discoloration, contamination,
resistant to chlorides
Not resistant to oxidizing
environments, expensive
Hasteloy
Improved over Monel-Nickel
More expensive than Monel-Nickel
Other exotic
metals
Improves specific properties
Can be very high cost
Non-metals
Material Advantages
Disadvantage
Glass
Useful in laboratory and batch system,
low diffusion at walls
Fragile, not resistant to high
alkali, poor heat transfer,
poor abrasion resistance
Plastics
Good at low temperature, large variety to
select from with various characteristics,
easy to fabricate, seldom discolours,
minor catalytic effects possible
Poor at high temperature,
low strength, not resistant to
high alkali conditions, low
heat transfer, low cost
Ceramics
Withstands high temperatures, variety of
formulations available, modest cost
Poor abrasion properties,
high diffusion at walls (in
particular hydrogen), low
heat transfer, may encourage
catalytic reactions
From "Analysis, Synthesis, and
Design of Chemical Processes"
Tensile Strength of Steel
1400
1200
1000
800
carbon steel
stainless steel
3-D Column 3
600
400
200
0
ambient
400 C
550C
Data in Text
In pages 168-174 of the text you
can find a table recommending
specific materials for specific
chemicals, and descriptions of
some common alloys
The Methanol Project
• Process streams in the methanol section are
not corrosive
• Sulphur can cause problems in earlier
stages, but that is not our problem
• Long term storage (days) of methanol in
carbon steel can cause side reactions that
generate impurities which interfere with
proper operation of fuel cells
Workshop
• What material should we use for the
majority of the equipment in the methanol
reaction and purification section?
• What equipment requires other material?
• What would be some good choices for that
material?