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Plant and Process > 4rd Chapter > Flashcards

4rd Chapter Flashcards

1
Q

Mechanical characteristics

A 
 Strength → Tensile strength
 Stiffness → Elastic modulus (Young´s modulus)
 Toughness → Fracture resistance
 Hardness → Wear resistance
 Fatigue resistance
 Creep resistance
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2
Q

Factors for the selection of a material of construction

A

Mechanical characteristics
Corrosion resistance
Special properties such as thermal conductivity, electrical resistance, magnetic characteristics
Processing of material → forming, welding, casting
Availability in standard sizes → plates, tubes, sections
Costs

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3
Q

Tensile strength

A

Measurement of the basic strength of a material by a standard tensile test.
Maximum stress that a material can withstand while being stretched or pulled before breaking.

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4
Q

Fracture behavior (Tensile strength)

A

 some materials break sharply (without plastic deformation) → Brittle failure
 other materials are more ductile (including most metals), which means that they
experience some plastic deformation and possibly necking before fracture

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5
Q

Stiffness

A

Ability of a material to resist deformation → Bending and buckling
Stiffness is the opposite of flexibility.

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6
Q

Toughness

A

Measure of the materials resistance to crack propagation.

Connected with the materials ability to absorb energy and deform plastically without fracturing.

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7
Q

Connection with the crystal structure of metals (Toughness)

A

 Ductile materials such as aluminum, copper or steel → propagation of a crack is stopped by
local yielding at the crack tip
 Brittle materials such as glass or cast irons → structure is such that no local yielding occurs

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8
Q

Hardness

A

Measure of how resistant a material is to a permanent shape change when a compressive force is applied
It is an indication of a materials ability to resist wear.

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9
Q

Fatigue

A

Weakening caused by repeatedly applied loads due to progressive and localized structural damage
Stress values are much lower than the strength of the material (tensile strength).
Fatigue failures are likely to occur in equipment subject to cycling loading

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10
Q

Creep (Cold flow)

A

Tendency of a solid material to a gradual extension (deformation) under a steady tensile stress,
over a prolonged period of time.
It is the result of a long-term exposure to stresses that are still below the materials yield strength.

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11
Q

Effect of temperature on mechanical properties

A

Tensile strength and Young´s modulus of metals decrease at higher temperature.
Stainless steel is superior to plain carbon steels in this category.
At low temperatures (< 10 °C) metals that are generally ductile can fail in a brittle manner.
bcc lattice (body-centerd-cubic) metals are more liable to brittle failure at low temperatures than
fcc (face-centerd-cubic) or hexagonal lattice metals.
Low-temperature equipment (e.g. cryogenic plants): austenitic stainless steel (fcc) or Al-alloys

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12
Q

Young´s modulus

A

Young’s modulus is a measure of the ability of a material to withstand changes in length when under lengthwise tension or compression. Sometimes referred to as the modulus of elasticity

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13
Q

Dry oxidation

A

M + O → MO
M → M2+ + 2e
O + 2e- → O2-
where M is the metal (all metals except gold and silver) and O is oxygen
Formation of a thin layer of oxide at the metal surface
Rate of oxidation is controlled by the thickness and structure of the oxide layer.

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14
Q

Dry oxidation (Type 1)

A

Oxide occupies lower volume than metal → As oxides are usually brittle it will crack and split, exposing fresh metal to more corrosion

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15
Q

Dry oxidation (Type 2)

A

Oxide with higher volume → it will wrinkle and spring away → exposing fresh metal

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16
Q

Dry oxidation (Type 3)

A

Oxide volume matches volume of metal → formation of a thin adherent oxide layer at the surface that acts as a near total barrier to further oxidation
Aluminum → no further protection required against corrosion when used for window frames; Chromium and Nickel → Essential components of stainless steel

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17
Q

Types of Corrotion

A 
Uniform corrosion
Galvanic corrosion
Pitting corrosion
Intergranular corrosion
Effect of stress
Corrosion fatigue
Erosion-corrosion
High temperature oxidation
Hydrogen embrittlement
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18
Q

Wet corrosion

A 
 Presence of moisture changes the situation drastically
 Electrochemical cell action is driven by the energy of oxidation that continues the corrosion
process.
 Loss of metal by corrosion
becomes much more significant.
 Formed products are deposited
loosely on the metal surface →
giving little or no protection.
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19
Q

Uniform corrosion

A

 More or less uniform loss of material by corrosion with no pitting or other forms of local attack.
 Life of the material in service can be predicted from experimentally determined corrosion rates.
 Corrosion rate depends on the pH, temperature and concentration of the corrosive fluid.
 Increase in temperature usually leads to an increased rate of corrosion

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20
Q

Galvanic corrosion

A

 Two different metals have physical or electrical contact with each other, and electrolyte is present.
Or when one metal is in contact to an electrolyte with different concentrations

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21
Q

Galvanic corrosion (Examples)

A

 Corrugated iron (Wellblech): Sheet of steel is covered with a protective zinc coating. Even if
the coating is damaged, the underlying steel is not attacked. Zinc is corroded as it is less
noble. Base metal is only corroded after the zinc has been consumed.
 Tin can (Konservendose): as tin is more noble than the underlying steel, the opposite effect
occurs. When the tin coating is broken, the underlying steel is immediately attacked.
 Statue of Liberty: corrosion between the outer copper skin and the wrought iron support
structure (found in 1980s). Extensive renovation required.
 Screw of copper in an aluminum sheet.
 Stainless steel plate bolted with carbon steel plate.

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22
Q

Preventing galvanic corrosion

A

 Electrically insulation of the two metals by using non-conductive materials between them.
 Ensure there is no contact with an electrolyte
 Water-repellent compounds like greases, paints or with a coating
 If it´s not possible to coat both metals, the coating should be applied to the more noble one.

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23
Q

Pitting corrosion

A

 Localized corrosion that leads to the formation of small holes in the surface of passivated metals.
 Pitting can be initiated by small surface defects e.g. a scratch, a local change in composition,
 Low concentrations of oxygen or high chlorine concentrations (compete as anions) can affect the
alloys ability to re-form a passivation film.

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24
Q

Preventing Pitting corrosion

A

 Pitting corrosion can be reduced by a good surface finishing.
 Alloyed steels with chromium-nickel are protected by the addition of molybdenum, which
stabilizes the passivation layer on the surface.

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25
Q

Intergranular corrosion

A

 Corrosion of material at the crystal boundaries.
 Grain boundaries in metals have different corrosion properties than the rest of the grain.
 Boundaries can become the anodic region of the corrosion cell, at which the corrosion is
concentrated.
 Intergranular corrosion is a common damage of alloys but occurs rarely with pure metals.

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26
Q

Preventing Intergranular corrosion

A

 Annealing after welding

 Use of low carbon steel

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27
Q

Effect of stress

A

Some metals: when loaded or stressed in a corrosive environment, cracks can grow steadily under
a stress intensity that is much less than the critical stress.
Examples: stainless steel in chloride solutions; brass in ammonia

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28
Q

Preventing Effect of stress corrosion

A

 Selection of materials that are not susceptible in the specific environment
 Stress relieving by annealing after fabrication and welding

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29
Q

Corrosion fatigue

A

 Caused by crack development under the simultaneous action of corrosion and cyclic stress
 Ferrous metals: fatigue endurance limit disappears → safe design is more difficult
 Example: steel in seawater for offshore structures such as oil and gas production which will be
subjected to wave action and seawater throughout their working life.

30
Q

Preventing Corrosion fatigue

A

 Reduce fatigue by minimizing vibration
 Reduce corrosion by using high-performance
alloys
 Reduce corrosion by using coatings to delay
the initiation of corrosion fatigue cracks

31
Q

Erosion-corrosion

A

 Combined action involving erosion and corrosion in the presence of a moving corrosive fluid or a
metal component moving through the corrosive fluid.
 Usually found at high flow rates around tube blockages, tube inlet ends, or in pump impellers.

32
Q

Preventing Erosion-corrosion

A 
 Control of the fluid velocity
 Use of more resilient materials
 Reduce turbulences
 Use of corrosion inhibitors
 Protection of the metal surface e.g. plastic inserts at the inlet of pipelines
33
Q

High temperature oxidation

A

 While corrosion is associated with wet conditions, oxidation can occur in dry conditions
 Carbon and low alloy steel oxidize rapidly at high temperatures (> 500 °C).

34
Q

Preventing High temperature oxidation

A

 Chromium is very effective in giving resistance to oxidation by forming a tenacious oxide film.
 Chromium alloys should be used for equipment subject to higher temperatures.

35
Q

Hydrogen embrittlement

A

 Loss of ductility due to the inclusion (and reaction) of atomic hydrogen in a metal such as steel.
 Hydrogen embrittlement resembles a material fatigue → hydrogen-induced cracking occurs
 Metals most vulnerable are high strength steels, Ti-alloys and Al-alloys.

36
Q

Preventing Hydrogen embrittlement

A

 Alloy steel (e.g. austenitic CrNi-stainless steel) or nickel and copper alloys show a greater resistance
(still susceptible) to hydrogen embrittlement, especially at temperatures above 500 °C.

37
Q

Metals

A 
− Carbon steel
− Low alloy steel
− Stainless steel
− Aluminum and alloys
− Copper and alloys
− Nickel and alloys
− Titanium and alloys
− Tantalum
− Zirconium
38
Q

Plastics

A 
− Polyvinyl chloride (PVC)
− Polyolefines (PE, PP)
− Polytetrafluorethylene (PTFE)
− Polyvinylidene fluoride (PVDF)
− Glass-fiber reinforced plastics
− Rubber
39
Q

Ceramic Materials

A 
− Glasses
− Stoneware
− Acid-resistant bricks
− Refractory materials
− Carbon
40
Q

Carbon steel (CS)

A

 Carbon steel consists mostly of iron and up to 2.06 % carbon, which is added for hardness.
 Mild steel (low carbon steel) with 0.05 - 0.30 % C is malleable and ductile.
 Mild steel is probably the most widely used material in process engineering as it is
 Cheap
 Can withstand fairly high and low temperatures (-45 to 480 °C) as the fluid is not corrosive or
reactive
 Available in a wide range of standard forms and sizes
 Easy to handle and workable (welding)
 Good tensile strength and ductility
 Carbon steel and iron are prone to corrosion (apart from certain specific environments).
 Carbon steel is susceptible to stress corrosion cracking.
 Suitable for most organic solvents (except chlorinated solvents) but traces of corrosion products
can cause discoloration.

41
Q

Low alloy steel

A

 Low allow steel shows improved mechanical properties (e.g. strength, hardness) compared to
low carbon steel as it usually contains chromium, molybdenum, nickel and other metals.
 Corrosion resistance is comparable to that of plain carbon steel.
 Materials can be exposed to mildly acidic and mildly oxidizing chemicals.

42
Q

Protective coatings

A

 A variety of paints and other organic coatings are used to protect carbon steel structures

43
Q

Stainless steel (SS)

A

 Stainless steel is the most commonly used corrosion resistant material in the chemical industry.
 Used for process equipment, especially when minimizing contamination is required.
 SS differs from carbon steel by the amount of chromium present (minimum of 10.5 wt.% Cr).
 High chromium content → stainless steel can form a passive film of chromium oxide, which prevents further surface corrosion by blocking oxygen diffusion to the steel surface
 Stainless steel is not fully corrosion-proof in low-oxygen, high salinity or at poor air circulation.
 The higher the chromium content the more resistant is the stainless steel alloy in oxidizing conditions.
 Nickels is added to increase the corrosion resistance in non-oxidizing conditions.

44
Q

According to the crystalline structure stainless steels are classified as

A

austenitic, ferritic and martensitic

45
Q

Austenitic (200 and 300 series)

A

 Represent over 70 % of total stainless steel production and are widely used in industry.
 Maximum of 0.15 % C, minimum of 16 % chromium and sufficient nickel and manganese.
 Used widely as they resist corrosion and reactivity with many acids and bases (as long as the
concentration is not very high)
 Can be used for very low and high temperature applications (-255 to 1100 °C)
 Thermal conductivity is significantly lower when compared to carbon steel.
 Austenitic stainless steel is non-magnetic in the annealed state.

46
Q

Austenitic (200 and 300 series) Examples

A

− 304 SS (X5CrNi18-10, code 1.4301): most versatile and generally used stainless steel.
Applied for a wide range of home and commercial applications such as kitchen benches,
chemical containers, heat exchangers, etc.
− 316 SS (X5CrNiMo17-12-2, code 1.4401): called marine grade stainless. Used primarily
due to its increased resistance to corrosion in chloride environments.

47
Q

Super austenitic stainless steel

A

− Super austenitic stainless steel (high nickel stainless steel) contains 29-30 % nickel, about
20 % chromium and more than 6 % molybdenum.
 Great resistance to chloride pitting and crevice corrosion (Spaltkorrosion).
 High nickel content ensures better resistance to stress-corrosion cracking when
compared to the 300 series.
 Higher alloy content makes them more expensive.

48
Q

Ferritic

A

 Stainless steel with a bcc structure and 10.5 - 27 % Cr, very little Nickel but Al, Ti, Mo
 Generally lower corrosion resistance than austenitic stainless steel but show better
engineering properties
 Usually less expensive

49
Q

Martensitic

A

 Martensitic structure that contains 12 - 14 % Cr, less than 2 % Ni, 0.2 - 1 % Mo
 Martensitic stainless steel is very strong and tough as well as highly machinable.
 Can be hardened by heat treatment but not as corrosion-resistant as the other two SS classes.

50
Q

Duplex and super-duplex stainless steel

A

 Mixed microstructure of austenitic and ferritic phases.
 Chromium content of around 20 % (duplex) and 25 % (super-duplex).
 Corrosion resistance and the strength is improved compared to austenitic stainless steel.
 Super-duplex steel was developed for aggressive off-shore conditions.

51
Q

Aluminum and Alloys

A

 Pure aluminum has a lack of mechanical strength but is more corrosion resistant than its alloys.
 Good corrosion resistance due to passivation (Al2O3 layer on surface).
 Attacked by mineral acids and bases.
 Good choice for very low temperatures (e.g. refrigerant systems).
The main aluminum alloys are the so-called Duralumin (Dural) which contain copper (about 4 %),
manganese (0.5 - 1 %) and magnesium (0.5 - 1.5 %).

52
Q

Copper and Alloys

A

 Traditionally used in the food industry (e.g. brewing & whisky distillation)
 The material is soft, easily workable and is often used for tubes and small-bore pipes.
 Copper and copper alloys are attacked by oxidizing acids like sulfuric or nitric acid.
 They are resistant to caustic alkalis (except ammonia) and to many organic acids and salts.
 Copper alloys are a good material to use for heat transfer equipment.

53
Q

Brass (Copper and Alloys)

A

 Alloy of copper and zinc with a comparable corrosion resistance than pure copper but with
improved mechanical properties.
 Strength and corrosion resistance can be improved by the addition of aluminum.
 Tin has similar effects and is used especially in seawater applications (naval brasses).

54
Q

Bronze (Copper and Alloys)

A

 Alloy of copper and tin (other so-called bronzes are Al-bronzes and Si-bronzes).
 Bronze resists corrosion (also from seawater) and metal fatigue more than steel.
 The main use (also for brass) in the chemical industry is for valves, fittings and heat
exchanger tubes and sheets.

55
Q

Nickel and Alloys

A

 Show good mechanical properties and can be worked easily.
 Best class of metals for high temperature applications (up to 1100 °C) and reactive chemicals.
 Example: handling caustic alkalis at higher temperatures.
 Nickel and nickel alloys are not prone to corrosion cracking like stainless steel.

56
Q

MONEL (Nickel and Alloys)

A

 Group of nickel-copper alloys that are primarily composed of nickel (up to 63 %) with up to 34
% copper and small amounts of iron and other elements.
 After stainless steel probably the most used alloy for chemical plants.
 Monel resists saltwater, sulfuric and hydrochloric acid, and caustics such as sodium hydroxide.
 Monel is more expensive than stainless steel.

57
Q

INCONEL and INCOLOY (Nickel and Alloys)

A

 Group of nickel, chromium and iron-based superalloys.
 Highly oxidation and corrosion resistant materials that are designed for extreme environments
 Acid resistance even at high temperatures

58
Q

HASTELLOY (Nickel and Alloys)

A

 Highly corrosion resistant metal alloys (“superalloys” or “high-performance-alloys”) based on
nickel with up to 30 % chromium and up to 29 % molybdenum and other elements.
 Good to use with high concentrations of oxidizing chemicals such as peroxides, nitric acid or
sulfuric acid.

59
Q

Titanium and Alloys

A

 One of the most expensive materials you can select.
 Lightweight and excellent chemical resistance as they are safe to use for almost all organics, acids
and bases, except hydrofluoric acid or concentrated sulfuric acid.
 Fields of applications: Heat exchangers (for both shell and tube and plate) for sea water
environments.

60
Q

Plastics as materials of construction

A

 Plastics are increasingly used as corrosion resistant materials.
 Mechanical strength and maximum operating temperature of polymers are low when compared
to metals.
 In contrast to metals: Plastics are flammable.
 Reinforced plastics (with glass or carbon fibers) can achieve a similar strength than low carbon
steel and can be used for pressure vessels and pressure pipelines
 Unlike metals: polymers can absorb solvents, which causes a swelling and softening

61
Q

Polyvinyl chloride (PVC)

A

 PVC is a commonly used thermoplastic material in chemical plants.
 It has a good resistance to water, seawater, gasoline, oil, most inorganic acids (except strong
sulfuric and nitric acid) and inorganic salt solutions.
 Due to swelling it is inappropriate when processing many organic solvents.
 The maximum operating temperature is limited to 60 °C.

62
Q

Polyolefines (PP and PE)

A

 Polypropylene (PP) has a comparable resistance like PVC but offers the advantages of shape
retention up to a temperature of about 120 °C (resistant to boiling water) and a good toughness
also at low temperatures (-50 °C).
 PP is stronger than polyethylene and is used for pipelines, tanks, plates or column internals.
 Polyethylene (PE) is available in different densities and is a relatively cheap, tough, flexible plastics.

63
Q

Polytetrafluoroethylene (PTFE)

A

 Commonly known under the tradename Teflon®.
 Most stable plastics as it is resistant to all chemicals, except molten alkalis and fluorine.
 PTFE can be used within a wide temperature range of -200 to 260 °C.
 Used as coating material to realize non-stick properties to surfaces such as filter plates.
 Disadvantages are high costs, a low mechanical strength and PTFE is difficult to process (sticking
together is nearly impossible and welding is extremely difficult)

64
Q

Polyvinylidene fluoride (PVDF)

A

 Similar properties like PTFE but processing is easier.
 Shows a good resistance to inorganic acids and alkalis, and organic solvents.
 The maximum operating temperature is 140 °C.
 In biomedical applications PVDF is used for example as material for filters and membranes.

65
Q

Corrosion charts

A

 Corrosion charts can be used as guidelines for the preliminary screening of materials that are
likely to be suitable.

66
Q

Contamination

A

 Textile processing: stainless steel or aluminum are often used in place of carbon steel, which
would be suitable with the exception that even slight rusting will mark the textiles.
 Processing acetylene: pure metals and alloys containing gold, silver, copper, mercury are
unsuitable in order to prevent the formation of explosive acetylides
 Brass (copper-zinc alloy) heat-exchanger: trace quantities of mercury in a process stream
lead to the formation of a mercury-copper amalgam and can cause serious equipment
failures.
 Consider even unexpected sources of contamination e.g. the failure of a mercury-in-steelthermometer

67
Q

From Cheap to Expensive

A 
Carbon steel
Low allow steel
Stainless steel
Aluminum and Alloys
Copper and Alloys
Brass
Bronze
Nickel and Alloys
MonelTM
InconelTM
IncoloyTM
HastelloyTM
Titanium and Alloys
68
Q

Surface treatment

A

 The lower the surface roughness, the better, since corrosive material and bacteria are less likely
to stick to a very smooth surface.

69
Q

Narrow down materials of construction based on

A

(1) Consider the inlet and outlet temperatures and concentrations of all components in the streams
going through the considered piece of equipment.
(2) Create a list of potentially safe and practical materials for each piece of equipment.
(3) Calculate the cost of that piece of equipment made with those materials → narrow it down to
the final decision.

70
Q

Cladded materials

A

 Cladding is when one material is layered on top of another as a protective layer

Plant and Process (5 decks)

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