labs

lab 2: hardness (strength) testing

  • brinell test
    • to get approximate strength of material
    • load
      • 3000kg steel
      • 1500kg brass
      • 500kg aluminum
    • 1ksi = 1000psi
    • hardness correlation (steel only)
  • rockwell test
    • lighter 15-150kg
    • 14 different scale (B max 100,C max 150 are common) (quiz)
    • distance of indentation bw minor and major yield result
    • start with c scale first if we don’t know the material
  • the stronger the material, the less ductile
  • next week quiz
    • brinell
      • adv
        • surface finish not as critical
        • better repeatability
        • wider range of hardness
      • disadv
        • measure dia (possible human error)
        • a little slower
        • not good for thin or narrow section
    • rockwell
      • adv:
        • automatic readout
        • a little quicker
        • good for thinner or narrower sections
      • disadv
        • surface finish needs to be more smoother
        • more variation between results
        • multiple scale, but smaller range

lab 3: tensile testing

  • determine how strong and ductile is
  • majority of metal does not have yield point
  • quiz 2 - strain is measure using a strain extenemeter

lab 4: toughness testing

  • toughness is the ability of metal to absorb energy before fracturing
  • 2 measure of toughness: FATT, ECTT (more common, most important)
  • the test specimens is a 10mm bar, 55mm long or 10x10x50mm
  • the Charpy Impact Test equipment use a 300ft.lbs or 407J pendulum
  • v-notch is the most common used
    • 45 degree v-notch
    • 2mm deep
    • 0.25mm root radius

lab 5: heat treatment of steel

  • harden heat treatment (quenching)
    • the cooling rate to create martensite is called “the critical cooling rate”
    • the critical cooling rate mainly depends on chemical composition
      • carbon, chromium, molybdenum can determine how quickly a metal has to cool to form martensite
    • anything faster than critial cooling rate is not based on phase diagram
    • quenchants (order from fastest to slowest)
      • brine (soap add to water)
      • water
      • oil
      • air
    • “never cool a metal faster than required (it can crack)”
    • quenching is rarely the final heat treatment done on a metal like steel; further heat treating is required to restore ductility and toughness
  • cenmentaite: (carbon carbide) right side
  • ferrite: left side of dia
  • austenite : FCC
  • bainite stronger than pearlite

lab 6: strain hardening

lab 8: Magnetic Particle Inspection & Liquid penetrant

MPI

  • testing mediums (sorted best to lowest sensitivity)
    • wet florescent
    • contrast (black on white)
    • dry powder
  • magnetic 2 directions (x and y) to catch all cracks
Advantage Disadvantage
- portable (yoke) - if the part is not ferromagnetic, MPI does not work
- relatively cheap - parts has to be cleaned, free of oil, grease, etc.
- quick result - parts need to be demagnetized
- safer - best suited for surface flaw detection
- easy  

Liquid penetrant inspection

  • steps
    • clean parts
    • apply penetrant
    • remove excess penetrant
    • apply developer
    • inspect parts
    • clean parts
Advantages Disadvantages
- portable - cracks have to be opened to surface
- relatively cheap - slow (dwell time, cleaning)
- easy - messy
- good sensitivity  

lab 10: ultrasonic testing

  • can detect size and location of defects.
  • velocity of sound waves is dependent on
    • type of material
      • bronze: 4700m/s
      • steel: 5960m/s
      • aluminum: 6400m/s
    • temperature
      • speed of sound @ 15C at sea level 1225 kph or 340m/s
      • speed of sound @ -25C at sea level 1140 kph or 315m/s
    • density
    • pressure
  • sound is the mechanical vibration of particles in a material
  • advantage:
    • superior penetrating (up to 20ft)
    • high sensitivity
    • greater accuracy (size, shape, orientation, shap_
    • safe
    • portable
  • disadvantage
    • part geometry (difficult to inspect rough, irregular parts)
    • need couplants to provide an effectivee transfer of energy
  • application: weldments, corrosion surveys
  • methods:
    • straight beam: wall thickness, bar
    • angle beam: weld inspection

lab 11: casting and forging

corrosion

  • factors that affect corrosion
    • v
    • environment (corrosiveness)
    • operating conditions
      • temp
      • fluid velocity
      • pressures
      • loading conditions
  • measuring corrosion, 2 ways to measure
    • thickness change
      • metric mmpa (mm per annum)
      • imperial mpy (mil per year) 1mil = 0.001”
    • weight change
  • group 1, general corrosion type 1
    • sometime called uniform
    • most common type
    • easiest to detect and control
    • accounts for the highest amount of metal lose on an annual basis.
  • 6 methods to control or work with general corrosion
    1. add a corrosion allowance
      • increase the wall thickness
    2. add galvanic protection
      • sacrificial anode (give the electrons for cathode)
      • impressed current: apply a current to the metal to be protected by forcing it to become cathodic
    3. material selection
      • some materials have better corrosion resistance than others; considerations:
        • cost
        • welding
    4. change the environment if possible
    5. add inhibitors
      • are chemicals that slow or stop the corrosion reaction.
    6. coatings
      • provide a barrier between the metals and its environment; 2 types
        • inert.
          • e.g. paint
        • active. (reaction)
          • e.g. galvanic steel (Zn coating)
      • considerations for selecting a coating
        • corrosion resistance
        • cost
        • ease of application
        • adherance
        • abrasion resistance
        • expand and contract with base metal
        • apprearance

lecture

  • what is an engineering material ?
    • anything used to build something
  • metals
    • metallic lustre
    • conduct thermally
    • conduct electricity
    • ductile (stretch it, gold has very good ductivity)
    • crystallized pattern of the atoms in the solid state
    • malleable (ability withstand beaten up)
    • metallic bonding
    • classification
      • ferrous (iron based)
      • non-ferrous (non-iron based)
  • non-metals
    • ceramic
      • inorgnaic
      • very hard/brittle
    • polymers
      • organic (pvc)
    • composites (carbon fiber)
  • when designing equipment what needs to be considered?
    • operating conditions
      • stresses
      • loading conditions
        • torsion
        • shear stress
      • temperatures
      • corrosive environment
      • easy fabrication
        • welding
        • machining
      • cost
      • availability of material
      • thermal expansion, weight
  • solidification
    1. nucleation (initial solidification)
    2. grain or crystal growth
  • in the solid form, metals have a crystalline structure on the atomic level
  • these geometric patterns are called space lattices
  • 95% of all metals will be one of 3 lattices (handout)
    • BCC: body center cubic
      • chromium
      • sodium
      • vanadium
      • tungstien
      • iron
    • FCC: face center cubic (more ductile, don’t change with heat)
      • gold
      • silver
      • nickle
      • aluminum
      • iron
    • HCP: hexigonal close packed
      • zinc
      • titanium
      • cobalt
  • some metals can change their lattices shape from one to another based on temperature
    • iron: BCC (low temp) -> FCC (high temp) -> BCC (low temp)
    • this ability is called allotropy
      • iron is allotropic
  • what is steel?
    • an alloy of iron and carbon
    • an alloy is a material with metallic properties containing 2 or more elements. At least one element has to be a metal.

steel making

  • most common orce is Taconite
  • pig iron is brittle due to high carbon content

    typical   Chemistry
      %C      3.5 - 4.5%
      %Mn     0.5 - 2.0%
      %Si     0.3 - 2.0%
      %S*     0.01 - 0.1%
      %P*     0.05 - 2.0%   - (*) bad
    
  • steel making steel
    • basic oxygen furnace is the most commonly used 60% world ouput

phase diagrams

  • phase
    • uniform chemical composition (usually)
    • surround by boundaries and can be identified under a microscope
    • used to show the different phases that can occur with alloys systems
    • based on chemical composition and temperature
  • AISI : American Iron and Steel Institute
    • C1010
      • first 2 digits alloy family; 10 : plain carbon steel
      • last 2 digits is the percent of carbon
  • ferrite : BCC, UTS ~ 30ksi or under 250mpa
  • pearlite : contains 2 phases (exact 0.8 % of C) much stronger than ferrite (UTS~100ksi)
    • white ferrite
    • cementile
  • %c go up, strength go up
  • %c go up, ductivity go down
  • the iron/carbon phase diagram is based on slow equilibrium cooling
  • if the cooling rate becomes too quick, then carbon doesn’t have a chance to diffuse. The lattice gets distorted into a body centered terragonal shape called Martensite (BCT)
  • Martensite is a hard (HRC 55 and up) and very brittle structure.
  • further heat treating is usually required

the role of carbon and alloy content of steels

  • when alloying elements are added to a base metal, one of three things can happen (or combination of the three)

solid solution

  • SSS: gold alloy with silver
  • ISS: carbon added to iron is interstitial
  • interstice is a “open space” in lattice

elements can seperate into their own phases

intermetallic compounds can form

  • hard brittle
  • e.g. Fe_3C - iron cardide (cementite)
  • tungsten carbide

organizations and societies

steel designations

  • there are many different designations and technical societies
  • e.g. ASTM
    • A: ferrous metals
    • B: non ferrous
    • C: calibration and testing
  • AISI: American Iron and Steel Institute
    • 4-5 digits
    • first 2 alloy group
    • last 2/3 %C/100
    • 10xx plain carbon steel
    • 41xx chromium/molybdenum
    • 86xx low chrome/nickel/moly
  • API: American Petroleum Institute
    • API uses ASTM/AISI grades and they use their own designations
      • e.g. API 5CT (casing/tubing) grade J-55 min 0.2%ys max 0.2ys min uts 55ksi 80 ksi 75ksi (380mpa) (550mpa) (520mpa) grade N-80 80ksi 110ksi 100ksi (550mpa) (760mpa) (690mpa)

cast iron

  • on the iron carbon phase dia CI’s have bw 2-6.5% carbon
  • most commercial CI’s have between 2-4% carbon and 1-3% silicon
  • ci’s cannot be formed by forging or rolling; they have to be shaped by molding
  • weldability tends to be poor
  • 4 types (groups): white, malleable, gray, nodular (ductile ci)

stainless steel

  • developed for their corrosion resistance
  • in order for a steel to become stainless chromium is required
  • a minimum of 11% Cr is needed to form a protective chromium oxide layer () 11-30%
  • this layer self heals. If the oxide layer is damaged or removed it will reform as long as oxygen is present
  • 5 main groups of stainless steels (based on the metallurgy and heat treatment)

ferritic (AISI 400 series)

  • 12-27% Cr
  • low carbon (usually 0.12% max)
  • can not strengthen by heat treating
  • magnetic
  • poor weldability
  • corrosion resistance bw martensitic and austenitic
  • applications:
    • cook ware
    • exhaust systems
    • high temp application

martensitic (AISI 400 series)

  • 12-17% Cr
  • % carbon up to 1.2%
    • there are low carbon types as well.
  • design for higher strength and hardness
  • strengthened by heat treating
  • magnetic
  • some grades are weldable
  • applications:
    • high quality blade steel e.g. 440C
    • low carbon grades are used for values, chokes, flange (e.g 410)
    • high strength fasteners

austenitic (AISI 200 and 300 series)

  • most widely used type of stainless* (about 50%)
  • 16-25% Cr and 7-20%Ni (FCC) (300 series), ‘rarely use marganesee (200 series)’
  • Ni is an austenite stabilizer, so these types are FCC (very good toughness)
  • non-magnetic
  • cannot strengthened by heat treating
  • easiest type of stainless to weld (account for about 90% of all stainless welding)
  • very good low temperature toughness (due to nickel content)
  • good or high temp application
  • applications:
    • chemical and food processing
    • fuel lines
    • valves, chokes, piping
    • refrigerator cars

duplex (trade name e.g. ferralium 255))

  • two phased microstructure (ferrite and austenite)
  • 20-25% Cr
  • 4-7% Ni
  • 2-4 Mo
  • better strength and corrosion resistance to chlorides than austenitic stainless.
  • good toughness and weldability
  • applications:
    • heat exchanger tubing
    • offshore drilling equipment
    • desalination plants

precipitation hardened (sometimes named after %Cr/%Ni content) (ASTM 600 series)

  • are hardened and strengthened to the highest levels of all stainless steels
  • usually named after the Cr/Ni content e.g. 17-4 (17%Ni, 4%Cr) (martensitic microstructure)
  • magnetic / weldable
  • good toughness
  • applications:
    • aerospace components
    • petroleum equipment e.g. valve stems
    • high strength fasteners

review midterm

  • List the three components of a charge loaded into a blast furnace to make pig iron and briefly explain what each component is for?
    • iron ore: the source of element iron after chemical reduction in the blast furnace.
    • lime stone: an fluxing agent that remove impurities and forms a slag over the molten metallic iron. Slag not only contains most of the impurities but also acts as an oxidation shield.
    • coke (solid product of destructive distillation of coal.) Reducing agent (carbon monoxide) to remove oxygen from the ore.
  • What are the 2 stages in the transition of a liquid to a solid state?
    • nucleation
    • gain or crystal growth
  • What is the most common type of furnace used in steel making?
    • basic oxygen furnace
  • What is the name of the device that measures strain on the UTM?
    • strain extensometer
  • What does the term fully killed steel mean?
    • completely remove oxygen
  • Name the two phases in Pearlite. What is the carbon content of Pearlite?
    • white ferrite & cementile
  • What is the crystalline structure of Martensite?
    • body center tetragonal (BCT)
  • What 2 elements are known for deoxidizing the melt during steel making?
    • manganese, silicon, or aluminum
  • Describe a Substitution Solid Solution.
    • formed when atoms of a solute metal substitute for atoms of a solvent metal
  • Carbon in steel is substitutional? True or False
    • carbon in steel is substitutional
  • What is the crystalline form for austenite and what is the maximum solubility of Carbon?
    • FCC; max 2% C
  • What marks are common with fatigue failures?
    • beach marks are centered around a common point, fatigue crack origin
  • What is Allotropy?
    • metal’s ability to change lattices shape from one to another based temperature
  • What are the A3 and A1 lines called on the iron carbon phase diagram?
    • A1: lower critical temperature (LCT)
    • A3: upper critical temperature (UCT)
  • What is the difference between Yield Point and 0.2% offset Yield Strength.
    • yield point is the stress at witch the material exhibits an increase in strain without a corresponding increase in stress.
    • 0.2% offset yield strength is the stress at which the material exhibits 0.002mm/mm of permanent (plastic) deformation.
  • A material shows with a diameter of 15.9 mm shows a strain of 0.0004 mm/mm at 105 Kilo-newton’s. What is the modulus of elasticity?
  • Modulus of Elasticity is a measure of what property?
    • measures the stiffness and the ability to bend of material
  • The charpy impact machine generates how much foot-lbs of energy?
    • 300 ft.lbs or 407J
  • What are the dimensions of a standard chary impact test sample?
    • 10mm x 10mm x 55mm
    • 45 V-notch, 2mm deep, 0.25mm root radius
  • What types of indenters are used for Brinell, Rockwell B and C scales? What are the loads?
    • Brinell’s indenter: 10mm diameter tungsten carbide ball
      • loads: 500kg, 1500kg, 3000kg
    • Rockwell’s B scale indenter: 1/16” diameter tungsten carbide ball
      • loads: 10kg/100kg
    • Rockwell’s C scale indenter: 120 cone-shaped diamond (Brale diamond)
      • loads: 10kg/150kg
  • Describe fatigue. What is the endurance limit and fatigue strength?
    • fatigue are failures associated with fluctuating or cyclic stress
    • fatigue strength is the cyclic stress level that a material fails at; the higher stress level, the fewer number of cycles that will be required to cause the material to fail.
    • endurance limit is a limiting stress below which a load may be applied an infinite number of times without causing failure
  • What are the 3 stages of creep?
    • Primary: creep occurring at a decreasing rate (cold working)
    • Secondary: exhibit minimum creep rate, constant rate
    • Tertiary: exhibit creep at an accelerating rate, followed by fracture.
  • What is ECTT and FATT?
    • Fracture Appearance Transition Temperature (FATT) is the temperature when the fracture surface is 50% ductile and 50% brittle
    • Energy Criteria Transition Temperature (ECTT) is the temperature where the fracture requires some specific amount energy (20 joules or 15 ft-lbs)
  • What is the microstructure of hypoeutectic steels?
    • 0.008 to 0.8% C
    • contain pearlite and ferrite
  • A bar 10mm X 8mm is subjected to a force of 15,000 Newton’s. What is the stress on the bar?
  • What is the approximate carbon content of an AISI 1010 steel?
    • 0.10% (last 2 digit)
  • What are the four groups of cast irons? Which type has flaked shape graphite? Which type has no freed-up graphite?
    • white cast iron (carbides)
    • malleable cast iron (freed up graphite)
    • gray cast iron (flaked shape graphite) (most common)
    • nodular cast iron (rounded graphite)
  • Which type of cast iron is used for pump housings or sewer covers?
    • gray cast iron
  • What is the minimum carbon content of cast irons on the iron carbon phase diagram?
    • minimum 2%
  • Which association is responsible for the prevention of corrosion?
    • NACE (National Organization of Corrosion Engineers
  • Which association is responsible for the design of pressure vessels?
    • ASME (American Society of Mechanical Engineers)
  • What is a common type of steel used for pressure vessels?
    • fully killed astm A516 Cr70
  • Which group of stainless steel is used for high quality blade steels?
    • martensitic AISI 400 series e.g 440C
  • Which group of stainless steels are non-magnetic
    • austenitic
  • Which group of stainless steels are considered the easiest to weld?
    • austenitic
  • How does chromium make stainless steel stainless?
    • a minimum of 11% Cr is needed to form a protective chromium oxide layer () 11-30%
    • this layer self heals. If the oxide layer is self heals or removed it will reform as long as oxygen is present
  • What is the minimum amount of chromium needed to make stainless steel stainless?
    • minimum 11% Cr
  • What were the 3 stages of heat treatment for the cold working lab?
    • recovery, recrystalization, grain growth
  • Does the hardness continue to go up from cold working?
    • No, there is a limit
  • Directional properties resulting from cold working is called?
    • anisotropy
  • What happens to the ductility of a cold worked metal?
    • ductivity decreases when the material is cold worked
  • What is sensitization?

Flanar flaws - disavantage of radiaction

  • it would pickup thin crack that is horizontal with the source

non-ferrous metals

  • non-iron based
  • there are non-ferrous alloys that do contain small amounts of iron.
  • the metal covered
    • aluminum
    • copper
    • tin/lead alloys
    • nickel
    • cobalt
    • zinc
    • cemented carbides

aluminum

  • next to iron based alloys, Al is the second most widely used metal on the planet
  • comes from the ore bauxite
  • like iron it is very easily alloyed
  • very versitile/economical
  • density 2.7 g/cm3 (1/3 the density of iron)
  • FCC lattice
  • non-magnetic
  • corrosion resistance come from a oxide layer
  • some grades can be strengthened by heat treating, others have to be cold worked
  • some grades are weldable
  • Al has about 60% the electrical conductivity of copper
  • high thermal conductivity
  • some grades have a high weight to strength ratio e.g 7075 - uts 80ksi or 540mpa
    • designation AA/ANSI, 4 digit system
      • .xxxx wrought (forging)
      • .xxx.x casting

copper and copper alloys

  • ranks 3rd behind steel and al production
  • most Cu is found in sulphides but small amounts can be found in its native state
  • known for excellent thermal and electrical conductivity (second behind silver)
  • denser than iron (8.9g/cm3)
  • FCC structure
  • very good ductility; cold work easily
  • due to high thermal conductivity it can be difficult to weld; high preheats required
  • soldering (<450C) and brazing (>450C - used bronze) is used to join copper (don’t melt the material, not strong as welding)
  • above 80% is used in its commercially pure form, the remainder is used for alloying
  • the two common alloys brass (yellow) and bronze (reddish, for long time found 5000years ago)

brass

  • alloy of copper and zinc (Zn)
  • depend on alloy content there can be a wide range of properties
    • easy to work
    • easy to machine
    • good corrosion resistance
  • cold work brass + ammonia = bad (season cracking can occur)

bronze

  • traditionally an alloy of Cu and Sn (Tin)
  • other metals / elements will also make bronze
    • nickel
    • chromium
    • silicon
    • phosphorus
    • aluminum
  • tin bronze has good strength and good fatigue strength
  • low coefficient of friction (good for bearing surfaces)

tinc and lead

  • tin + lead = babbitt
  • babbitt is a bearing material

nickle

  • Ni is a very important engineering material for modern industry.
  • Ni uses (2011)
    • stainless steel 65%
    • other alloys (superalloys) - 22%
    • electroplating - 8%
    • other - 5%
  • pure Ni = uts 65 ksi or 450mpa
  • density 8.9g/cm3 iron 7.8g/cm3
  • fcc
  • magnetic up to 360C
  • can dramatically improve toughness for low temp applications

cobalt

  • is between Fe and Ni on the periodic table
  • 8.8g/cm3
  • allotropic: HCP-FCC-HCP
  • magnetic
  • largest use is for superalloys

super alloys

  • developed for high temperatures up to 1100C or 2000F
  • many uses
    • aerospace
    • petrochemical
    • offshore marine apps
  • 3 groups of superalloys

nickel based

  • most common of the 3 groups
  • known for good corrosion resistance
  • some have excellent low temp impact strength e.g. 718 inconel

cobalt based

  • have the best creep resistance
  • some have excellent wear resistance e.g. stellite #6

iron based

  • high end austenitic stainless steel with better strength and corrosion resistance.

zinc

  • soft material with a low melting temp (420C)
  • HCP
  • good atmospheric corrosion resistance
  • most common use:
    • sacrificial anode (galvanized steel)

cemented carbides

  • considered a ceramic
  • part of a group of materials that are hard with good wear resistance
  • e.g. tungsten carbide
    • parts are made in molds at high temperatures and pressures
    • the tc particles are held together by a metal binded (cobalt or nickel)

thermal expansion

  • material subjected to changing temperatures can undergo changes in dimensions.
  • the amount of dimensional change is dependent on
    • type of material
    • changing temperatures.
  • formula:
    • = coefficient of thermal expansion
    • = change in length
    • L = original length
    • = temp change
  • unit: (mm/mm C) or (in/in F)
  • steel:
  • aluminum:
  • Cu:
  • if expansion is restricted or not permitted then stresses will build up.
    • formula:
      • : stress in mpa or psi
      • : coefficient of T.E.
      • E: modulus of elasticity
      • : change in temp
  • sample question
    1. what is the change in length in a steel bridge 300m long temp change -30 to 30 C
    2. if the bridge is not allowed to expand what stress will be build up
    3. imperial units
      • A bar 1/2”x1/2”x24” long changes in temp from 60F to 120F
        • what is the
        • not allowed to expand
        • we can calculate the force acting on the bar
    4. what change of length does a 30 foot boom on a crane go through from 0F to 80F? ,
    5. a 1.5m magnesium rod is snuggly placed between 2 fixtures at 20C. what stress will be created if the rod is not allowed to expand and the temperature goes up to 45C? ,

shear stress

  • shear stresses act parallel to the stress surface
  • : occurs in bolts, shear pins, lap welds
  • simplest type of happens in bolts
    • = force causing shear / area resisting shear = F/A
    • single shear: 2-lap welds
    • double shear: 3-lap welds
  • sample questions
    • find maximum stress for 1/2” thick plate with two semicircular notches, F=25000lbf, b= 0.5”, h=6”, r=1”
    • calculate the following for 10mm thick plate, hole dia d=17mm, width of the plate w=130mm, F=20kN
      • max stress @ the hole
      • max stress on the full width of the plate
    • 10mm dia bolt fastens 2 plates together. if the allowable shear stress what load can be applied
    • a clevis pin is subjected to 50,000lbf the dia of the pin is 2”. what is the \sigma_s
    • two aluminum sheets are joined by 6 spot welds. Each spot weld is 6.5mm in diameter. Determine the shear stress
    • calculate the shear stress on the 7/8 diameter bolt, F = 10,500lbf
    • a diameter bolt can take a maximum shear stress of 50,000psi. what is the maximum load that the bolt can take?

torsion

  • caused by twisting or rotating
  • imperial
      • T: torque in lb
      • N: RPM
      • HP: horse power
  • metric
      • T: N.m
      • 1 HP = 746 watts
      • outside diameter
      • inside diameter
      • r: distance to center of shaft
  • angle of twist of shaft
      • radians
      • L = length
      • G = modulus of shear
      • G_steel = 83 Gpa (imperial)
  • ex: sample torque questions
    • determine the max in a shaft transmitting 300kw @ 300 rpm
      • solid 100mm dia (don’t use inside dia in solving)
        • KW = TN/9540 => T = 300kw*9540/300rpm = 9540N.m
      • hollow shaft ,
    • design a solid shaft to transmit 50kw @ 1130rpm; the allowable shear stress = 70mPa
      • find T
      • find diameter
    • determine the maximum torsional stress on a shaft that is transmitting 750KW at 250 RPM if
      • Find T
      • the shaft is solid and 125mm in dia
      • the shaft is hollow. ,

corrosion

generic electrochemical cell

  1. anode - only one reaction can occur at the anode; anode loses electrons (oxidation)
  2. cathode - 4 possible reactions; accepts electrons (reduction)
  3. electrolyte
  4. pathway for electron travel

Zinc/Platinum cell in non-aerated hydrochloric acid

  • anodic reaction: (Zinc)
    • losing electron
  • cathodic reaction:
    • picking up electron
  • other possible cathodic reaction
    • aerated acidic environment
    • basic (caustic) or neutral environment
    • metal plating
      • (metal)
      • (number of electrons)
      • (plated on cathode)
  • corrosion definition: deterioation of a material due to a reaction with its environment; non metals can corrode as well
  • corrosion accounts for billions of dollars in lost material
  • cost of corrosion can fall into 2 categories
    1. direct
      • replacement costs (value, labor, paint, etc.)
    2. indirect
      • down time
      • loss of production
      • loss of a customer
  • 2 main types of corrosion
    1. dry
      • elevated temps
    2. wet
      • a liquid phase has to be present
  • 8 types of wet corrosion
    • group 1: general (uniform) form 1
    • group 2: localized:
      • galvanic corrosion - 2
        • sometimes called 2-metal corrosion
        • 3 requirements
          • electrochemically different metals
          • electrically connect
          • must be exposed to an electrolyte
        • area effect of galvanic corrosion
          • large anode + small cathode = low corrosion rate @ anode
          • small anode + large cathode = high corrosion rate @ anode
        • prevention
          • avoid dissimilar metals if possible
          • if dissimilar metals are used, insulate them from each other
          • use a 3rd metal to act as a sacrificial anode
          • inhibitors
      • crevice - 3
        • sometimes called gasket corrosion
        • occurs in gaps or cracks where an electrolyte can get into
        • prevention
          • eliminate any crevices or cracks (welding or coating)
          • non-absorbant gasket or insulating material
      • pitting - 4
        • very similar to crevice corrosion but can occur on open surfaces -2 steps
          • initiation
          • propogation
        • occurs on metals like stainless steel where the oxide layer gets damaged
        • once initiation begins a crevice is tarted and corrosion takes off
        • prevention
          • smooth surfaces have better have better pitting resistance than rough surfaces
          • inhibitors
          • coating
      • intergranular - 5
        • this form of corrosion occurs at the grain boundaries.
        • gain boundaries become anodic due to a different chemical composition.
        • when this occurs the metal will fall apart
        • cold worked Al + chlorides will undergo this type of corrosion and its called exfoliation
      • dealloying (selective leaching) - 6
        • occurs in alloys where one element becomes anodic and is leached out.
        • eg. Brass => cooper (cathode) & Zn (anode) (reddish - copper, yellowish - brass)
    • group 3: cracking
      • stress corrosion cracking (SCC) - 7
        • 3 requirements
          • Susceptible material
          • Corrosive environment
          • Tensile stress (can be applied or residual)
        • Many materials are prone to SCC - Al, Ti, Cu, Austenitic Stainless
          • E.g
          • Brass + ammonia = not good
          • Austenitic stainless steel + chlorides = not good
        • One prevention is to minimize the tensile stress
      • hydrogen induced cracking - 8
        • Caused by the interaction of atomic hydrogen and a metal
        • Atomic is small enough migrate through steel if it get strapped in a void in the steel H_2 gas forms damaged the steel as the pressure builds.
        • With pressure vessel plate or any metals used for valves (petroleum) fully killed steel is required.
    • erosion is inluded
      • cavitation
      • flashing
  • note: corrosion is a chemical reaction
  • factors that affect corrosion
    • material properties
      • chemical composition
      • metallurgical factors
        • cold working increases, corrosion increases
        • residual stress (welding)
        • inclusions
        • multiple phases

final review questions

  • What are methods commonly used to heat treat steel and what result do they produce?
    • full anneal (above UCT)
      • slow-furnace cooled, max time for carbon diffusion
      • produce the softest, most ductile steel
    • normalizing (above UCT)
      • still air cooled, minimum time for carbon diffusion
      • produce a smaller ferrite and fine pearlite structure
    • hardening (quenching) (above UCT)
      • water/brine quenched, no time for carbon diffusion
      • produce max hard steel by forming martensite (BCT)
    • tempering (below UCT)
      • no crystal structure change takes place
      • restore some ductility and toughness to the steel
    • stress-relief anneal
      • heat, hold and cool slow
      • remove locked-in localized stresses
  • What metal can exfoliate?
    • Aluminum
  • What are the 2 most common radiographic sources used in industrial radiography?
    • Cobalt 60, Iridium 192
  • In radiography the denser the material being examined the lighter the image will be.

  • What is the function of a penetrameter in radiography?
    • it uses to measure the radiographic quality based on definition (shapeness of the image) and contrast
  • What are the advantages and limitations the different methods NDE we looked at?
  advantage disadvantage
RT - can be portable (isotope camera) - can be extremely dangerous
  - permanent record (film) - expensive startup cost
  - good visual detail from radiographs - can’t tell how deep a defect is
UT - superior penetrating (up to 20ft) - part geometry (difficult to inspect rough, irregular parts)
  - high sensitivity - need couplants to provide an effectivee transfer of energy
  - greater accuracy (size, shape, orientation)  
  - safe  
  - portable  
MPI - portable (yoke) - if the part is not ferromagnetic, MPI does not work
  - relatively cheap - parts has to be cleaned, free of oil, grease, etc.
  - quick result - parts need to be demagnetized
  - safer - best suited for surface flaw detection
  - easy  
LPI - portable - cracks have to be opened to surface
  - relatively cheap - slow (dwell time, cleaning)
  - easy - messy
  - good sensitivity  
  • What is UT used for?
    • used for detect size and location of defects (cracks, etc.)
  • What does the x and y axis on a typical UT machine represent?
    • x: time/distance, y: amplitude
  • In UT what is a couplent used for?
    • couplants used to provide an effective transfer of energy bw the transducers and test pieces
  • What is copper alloyed with to make brass?
    • zinc + cooper = brass
  • What is the crystalline structure of aluminum? What series is used for pop cans?
    • FCC lattice
    • Al series 3004 for pop can
  • Why were superalloys developed?
    • for the need of good materials can retain strength under long exposure to high heat environment, and good low temp ductivity
    • for creep resistance
  • Duplex stainless steels are a combination of what types of stainless steels?
    • autenistic and ferritic stainless steels
  • Which stainless steel is used for high grade blades steel?
    • martensitic AISI 400 series e.g 440C
  • What is the crystal structure of nickel?
    • FCC
  • What is the main use of nickel?
    • used for create stainless steels
  • What are the components of a corrosion cell?
    1. anode - only one reaction can occur at the anode; anode loses electrons (oxidation)
    2. cathode - 4 possible reactions; accepts electrons (reduction)
    3. electrolyte
    4. pathway for electron travel
  • What is the anodic reaction when steel is exposed to aerated dilute H2SO4? What are the cathodic reactions?
    • anodic reaction:
    • cathodic reaction: H2 goes off
  • What is meant by corrosion allowance?
    • an added additional thickness compensate for general corrosion
  • What is galvanic corrosion? What conditions are required for galvanic corrosion to occur?
    • corrosion damage induced when two dissimilar metals are coupled in a corrosive electrolyte
    • 3 requirements
      • electrochemically different metals
      • electrically connect
      • must be exposed to an electrolyte
  • What happens when a zinc block is fastened to a steel structure that is exposed to aerated fresh water?
    • galvanic corrosion happen
  • What is meant by the term crevice corrosion? What conditions must be present for this type of attack to occur?
    • sarifical anode
  • What is de-alloying?
    • occurs in alloys where one element becomes anodic and is leached out.
    • brass: zinc out, cooper left over
  • What is erosion corrosion?
    • induced by the velocity of a fluid and metal surface
  • What is cavitation?
    • caused by fluid bubble collapsing and pitting on metal surface
  • What is intergranular corrosion?
    • occur at grain boundaries which become anodic, and then removed
  • What is stress corrosion cracking?
    • metal failure due to combined effects of corrosion (metal + corrosive env) and tensile stress (external or residual)
  • What is used in domestic water heaters to cathodically protect them?
    • magniseum strip