Materials
General Properties
Elastic behaviour – bond stretching (reversible)
Plastic behaviour – ductile, irreversible movement of
dislocations
Perfect metal: ~ TS 1 million psi
Perfect metal + dislocations TS ~ 100000 psi
Ductile v/s Brittle Fracture
Ductile – plastic deformation, starts at voids (high stress
area @ centre of metal)
Ductile – withstand heavy impacts, absorbs lot of energy
Brittle – fracture starts @ surface, fine cracks
Ductile-Brittle Transition
Ductility = f(temperature)
Y strength increases as loading rate increases (dislocations
can only move so fast). Hence transition temperature
increases
Alloys
Most materials
Minor elements in small concentrations
C + Fe produces carbon steel (higher strength), Cu is
added to aluminum to increase strength.
Allow properties depend upon phase diagram and heat
treatment
Alloys: Phase diagram and TTT
Cast Iron
C is good cathode
but improves mech
Grey
properties
White
When cast white on exterior and grey on interior.
– Problems when machining
Mg sometimes added for strength
1 –2 % Si may be added for strength
Cast Iron: Mechanical Properties
Tensile strength: 50000 psi
Fatigue limit: 20000 psi
Max working temperature: 400 – 500 oC
Brittle at low temperature
Welding: Possible but difficult
Machining: Good
Elongation: 3 to 18 %
Cast Iron: Corrosion
Similar to Fe but modified by carbon (similar PD)
Silicon oxides produce a protective film
Acids:
– Passivation in 70 % sulphuric acid
– Nitric acid – passivation is unreliable (water production
when nitric acid is reduced)
– Nitric + sulphuric (10 % to 96 %) OK
– Do not use with HCL
Cast Iron Corrosion
Neutral:
– High corrosion rates (but alloy is cheap)
– Hard water (scaling reduces corrosion rate)
– Sea water 0.004 to 0.008 ipy
Alkaline Solutions
– NaOH (30 %) OK
– NaOH (70 % if temperature less than 80 oC)
Cast Iron – Applications and Cost
Complex cast shapes
Sulphuric acid plant > 70 %
Water, seawater lines
Neutral salt solutions
Alkaline solutions
Nitrating vessels
Cheapest (5 – 30 cents per pound fabricated)
Carbon Steel - General
Brittle at temperature less than 50 oC.
Tensile strength ~ 60000 psi
Ductile (elongation ~ 30 %)
Carbon content ~ 2 %
Can be hot rolled and extruded: Suitable for chemical plant
construction on or off-site
Killed steel add Al
Carbon Steel – Corrosion in Acids
Do not use with dilute acids (no passivation)
OK for cold conc sulphuric acid (65 %)
OK with oleum
Do not use with HCl
OK for HF > 60 %
Nitric acid passivation is unreliable
Phosphoric acid will pit
OK with dry chlorine and fluorine but not bromine or
iodine
Carbon Steel – Corrosion in Neutral, Alkaline,
organics
Neutral – see cast iron
Alkaline – see cast iron
Organics
– OK for atmospheric towers (careful if chloride,
sulphide present)
– Do not use with acetic acid
Carbon Steel – Applications
Cheapest mech reliable steel
Corrosion is often accepted
Use wherever possible
At temperature greater than 500 oC, oxidation of the metal
and strength decrease.
Silicon Steel
14 – 16 % Si or higher if appreciable corrosion rates
Very brittle – thus impact resistance very low
Fresh castings – stress relieved@ 750 to 850 oC
Silicon Steel
High thermoductility (thermal shock not a problem)
Welding – difficult
– Heat to 750 oC, use oxidizing flame
Excellent acid resistance
Applications
– Considered metallic glass or stoneware
• Use: nitric, sulphuric, dil. hydrochloric acid lines
Stainless Steels
Alloys of Fe-Cr-Ni
Passivity Uniformity
Promotes formation of a phase in which
CrxCy’s are more soluble
Weld decay
– Alloy with Titanium (4 x C)
– Use low carbon stainless steel (eg 304 L)
Alloying with Mo improves corrosion resistance to non
oxidizing acids (316)
Stainless Steel Properties (304 L)
Tensile strength: 80000 psi
Good at high temperature. Has 2 x allowable stress than
mild steel @ 500 oC
Brittle at low temperature
Welding: Good – electric arc. Can be welded to mild steel
with special welding rods
Can be used to clad mild steel
Elongation: 55 % (v. ductile)
Good impact resistance
Stainless Steels – Corrosion Resistance Acids
Protection based on Cr2O3 passive film. Polish?
Generally poor resistance in reducing acids. Add Mo.
O.K. for sulphuric acid if oxygen present
No – No HCl
Phosphoric: Use 18/8 + Mo
OK. For oxidizing acids up to 95 %. After 95 % metal goes
transpassive
Stainless Steels – Corrosion Resistance Neutral
+ Alkaline + Organics
Neutral
– Add oxidizing agents to maintain passivity
– Chlorides – Bad Karma
Alkaline Solutions
– Not much better than mild steel
Organic Solutions
– Acetic acid – 18/8 + Mo
– Petroleum
• Same as mild steel except wider temperature range
Cost ~ $3 per pound fabricated (several times CS)
Nickel Alloys – General Properties
Major use of Ni is alloying element
BEST METAL for high conc, high temp alkaline (passive)
TS: 60000 psi
Good high and low temperature properties
Add NI to Steel changes min working temperature
– 3.5 % = - 100 oC, 9 % = -200 oC.
High thermoductility
Good machining properties
Expensive (~$4 – 7 per pound fabricated)
Acid Solutions: Corrosion based on nobility, alkaline
based on passivity. If chlorides will pit.
Used in industries where Fe will cause discoloration
Monel
70/30 Ni/Cu
Important in chemical industry
Mechanically reliable over a wide range of temperatures
– TS: 75000 psi @ room temp. 30000 psi @ 750 oC.
– Reasonable thermoductility
– Good impact resistance down to – 180 oC.
Easy to machine and gas weld
K-Monel
K-Monel: Addition of 3 % Al
K-Monel designed as a high temp, high acid alloy
TS: 80000 psi @ room temp. 30000 psi @ 800 oC.
Corrosion resistance better than Cu or Ni except in
Alkaline solutions. Nobility of Cu and Passivity of Ni (not
attacked by acid and Cu will no longer complex)
K-Monel
Acids
– Sulphuric: OK to 60 %
– HCl: OK to 10 % (hot)
– Nitric: If conc. > 5 % rapid attack
– HF: OK
– Cl, Br, I: OK
Alkaline: OK up to 50 % then cracking
Neutral: OK for NaCl
Ni-Mo Alloys
Hastelloy B: 28 % Mo, 5 % Fe, TS: 130000 psi, elongation
50 %
Hastelloy C: 15 % Mo, 17 % Cr, - excellent resistance to
acids.
10 % Si casting alloy: 8.5 % Ni, 10 % Si, 3 % Cu, TS
115000 psi.
All have outstanding corrosion resistance including
concentrated HCl (dilute HCl use monel)
Copper Alloys
Excellent thermal conductivity
Welding used deoxidized rods (0.02 – 0.08 % P) as oxygen
will segregate around the grains and cause cracking.
Ductile at low temperatures
Max Temp:
Cost approx same as stainless steel.
Easily worked.
Copper Alloys
Corrosion resistance based on nobility
Does not evolve hydrogen: Thus oxygen usually present
Do not use with oxidizing acids
OK in alkaline
Neutral: Low corrosion rate due to film production
Complexing agents…Arrggghhh
Fine for acetic, lactic and formic acids
Major Problem with Cu: Erosion-Corrosion
Also discolours
Copper Alloys: Brass + Bronze
Contain at least 50 % Cu. Developed to overcome softness,
low strength and high casting temp of Cu.
Brass = Cu-Zn: High thermoductility
– Admiralty brass 70/30 Cu/Zn. Not as corrosion resistant
as Cu and subject to dezincification (add 0.1 % As to
prevent this). Add 2 % Al for better resistance to E-C.
Bronze = Cu-Sn (5-10 % Sn)
– Phosphor bronze (phosphorous) increases hardness as
does addition of 5 % Zn (Gunmetal Bronze).
– Aluminum-Bronze (93 % Cu, 7 % Al) good high temp
properties
Aluminum
Corrosion resistance is based on “passivity.”
Poor mech properties (10000 – 20000 psi TS)
– Ad 3 % Mg (TS 30000 psi). But reduces the corrosion
resistance.
Light weight, high thermoductility
Fusion welding (tungsten metal arc + inert gas)
Cost ~$1.50 per pound fabrication
Corrosion resistance is better than indicated by P-D
Can be coupled with stainless steel
Aluminum – Corrosion Resistance
Nitric
Sulphuric
HCL – No – No – No.
Aluminum – Corrosion
Neutral
– Do not use if chloride or sulphur compounds present
– Do not use in high temp water
– OK for H2O2 up to 90 %
Alkaline Solutions
– Rapid attack
Organics
– Fair corrosion resistance with organic acids (low temp)
– Does not discolour
Lead
Used to be standby in chemical plants with difficult
corrosion problems. Now Monel, Al, polymer etc.
Uses diffusion barrier approach for protection
TS: 20000 psi
Low creep stress 100’s psi
Add Cu. Ag, Al to harden
Max Working temp is only 200 oC.
Melts easily on the stove top (327 oC)
Welding use pure Pb wire, no flux
Cost $500 per ton, high scrap value
Softest and most easily worked
Fine grains, susceptible to intergranular attack.
Lead – Corrosion Resistance
Not shown is formation of salts.
Sulphuric acid forms lead sulphate
Nitric acid – steady attack
Neutral: Very good.
Alkaline: DNUse
Titanium
Strong like Arnold, with excellent corrosion resistance
based on passivity. TS: 44000 to 100000 psi
Very light weight (4.5 g/cm3 v/s 7.9 g/cm3 for CS)
Expensive. Often used as lining
Thermoductility is OK – use in thin sections
Can be passivated
by hydrogen
evolution reaction Add Pt or Pd
and make
passivation
easier
Corrosion resistance
Good for boiling 5 % sulphuric acid
Great for all concentrations of nitric acid
Outstanding resistance to seawater, brines and other
neutral chlorides.
Better resistance to SCC than stainless steel
Tantalum
Corrosion resistance of GLASS!!!
– OK for all solutions except HF
Excellent thermal properties
Ten times as expensive as Ti
Zirconium
Similar mechanical properties to C-S but almost
transparent to neutrons.
Corrosion rate is based upon passivity
Good for all concentrations of HCL at temperature less
than bpt.
No good with HF
OK with medium concs of sulphuric and nitric acid up to
200 oC.
Cost $50 per pound fabricated
Non-Metals
In many cases superior corrosion resistance but inferior
mechanical and thermal properties
Silicates: Stoneware, glass, acid resistant bricks, enamel
– Corrosion resistant in all except HF, hot conc
phosphoric acid, alkali > 10 %.
– Use in acidic environments and under tension (brittle)
– Stoneware adsorption columns, acid resitant brick
lining, glass reaction vessel, HX, enamelled C-S
reactors.
Non-Metals
Carbon (graphite)
– Acid resistant linings
– HX
– Good for acids and alkalis at all concentrations up to
the bpt.
– Graphite bonded with resin has max working temp of
around 250 oC.
– Brittle, but can be used in thick sections because of
high thermal conductivity
Non-Metals
Plastics
– In general low working temperature, low strength and
low thermal conductivity.
– DNU with oxidizing acids, aromatics and chlorinated
hydrocarbons (swelling + dissolution)
– Uses: Pipes/Valves/Fittings/Linings
– Construction: Hot gas or solvent welding
Non-Metals
Thermosetting Phenolics, Epoxyresins
– Laminated structures/tanks etc.
– Expensive but can compete with stainless steel
– Max working temp ~ 200 oC
Vulcanized rubber
– Linings, cured in situ
– Excellent abrasion resistance (mining)
• Max working temp ~ 100 oC
High Temperatures
Creep
High Temperature Oxidation
Temperature below
which oxidation
is negligible