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FORM OF CORROSION - Coggle Diagram

- - - - Al base alloy is
        susceptibl to IGC?
        
        Phase present at GB is anodic to Al
        
        depleted zone of Cu adjacent to GB
  - - - Sensitisation of SS during welding is known as Weld
      - Decay Weld Decay results in IGC within the heat
      - affected zone (HAZ)
  - - - Found in high strength aluminium alloy
        
        alloy that have been extruded or worked heavily
        
        microstructure of elongated grains
        corrosion product build up along GB exert pressure between grain
        
        end result is a lifitng or leafing effect
      - PREVENTION
        
        Removal of atmospheric pollutants such as H2S and SO2
        
        Selection of resistant but lower strength Al alloys (6xxx series)
        
        Heat treatment.
- - - - Dealloying is the removal of one element from the alloy to the electrolyte.
      - exposed surface on metal may be attacked
      - The cause of dealloying is a “galvanic effect”
  - - - is the removal of zinc in chloride water
        from brass(Cu Zn alloy)
        
        leave behind
        
        porous,
        
        weak layer of Cu and
        
        copper oxide corrosion product
        
        occur in slow moving or stagnant solution
      - MECHANISM
        
        The more active alloying element, zinc, selectively dissolves or “leaches” out of the brass leaving behind a porous, weak copper structure.
        
        Both Zn and Cu dissolve, and the more noble Cu than redeposits as a porous layer.
        
        Both mechanisms occur simultaneously i.e. dissolution of Zn and redeposition of copper.
      - PREVENTION
        
        Use cupronickles
        
        select low zinc red brass - immune to dezincification
        
        addition of P, As or Sb in single phase
    - - occur in the gray cast iron which has continuous graphite network in it microstructure(graphite flakes)
      - graphite = cathode
        
        accelerate anodic dissolution of nearby iron
        
        leave behind the graphite network
        
        it will maintain structural shape but loss mechanical strength
      - WHERE IT IS OBSERVED?
        
        in buried iron cast that exposure to soil
        (old pipe cast iron fitting, pump housing)
        
        ductile and malleable cast iron
        do not suffer from GR
        
        because do not have continuous
        graphite network
      - PREVENTION
        
        Use more resistant material such as other grades of cast
        iron.
        
        Coating
        
        Cathodic protection either by ICCP or sacrificial anodes
- - - - due to relative motion of a corrosive fluid and a metal surface
      - Corrosion is accelerated by impact of solid particles. These particles may
        
        remove metal, or
        
        remove oxide
        
        allow metal corrode faster
    - - burrs left at cut tube ends can upset smooth
      - Erosion corrosion occurs in valves, heat exchanger
      - tubes, pumps and at elbows and tees in pipelines and
      - any structural features that change the flow direction
      - Erosion corrosion of a brass tube in flowing seawater
    - - velocities
        
        velocity can increases turbulence which in turn results in erosion corrosion
      - water flow
        
        causing localized turbulence and high flow
    - - 1.reduce the fluid velocity and promote laminar flow; increased pipe diameters are useful.
      - Rough surfaces are generally undesirable.
      - Designs creating turbulence flow restrictions and obstructions are undesirable.
      - Abrupt changes in flow direction should be avoided.
  - - - Cavitation corrosion is a form of erosion corrosion and is caused by formation and collapse of bubbles of vapour
      - Vapour bubbles form because of pressure changes
        (which falls 0 across surfaces exposed to high velocity liquid flow)
    - - When the pressure increases again the collapse
      - vapour bubbles creates an intense shockwave that
      - removes metal or oxide from the metal surface
    - - Careful design to minimise pressure drops across the metal surface
      - Optimum material selection
      - Environmental modifications:
        
        Abrasive particles in fluids can be removed (filtering or settling)
        
        Deaeration (removal of air)
      - Cathodic protection
      - Use protective coating (rubber can absorb shock waves)
  - - - Fretting corrosion is a damage that occurs at the asperities of contact metal surfaces.
        
        It occurs at the interface of two highly stressed surfaces in the presence of repeated relative surface motion
      - The most common type of fretting is caused by vibration
    - - VIBRATION
        
        The protective film on the
        metal surfaces is removed
        
        Austenitic stainless steels, Ti and Al alloys (all have adherent protective films) are the most susceptible to fretting corrosion because they are relatively soft materials
        
        by the rubbing action and exposes fresh, active metal to the corrosive action of the atmosphere
        
        Where the surfaces are not
        designed to move against each other)
    - - Careful design: design the parts to exhibit less or relative motion
      - Use of lubrication on contacting surfaces
      - Use insulation material between surfaces
      - Reduce load between surfaces
      - Select a more resistant material
- - - - CONDITION FOR SCC?
        
        specific environment
        
        susceptible alloy
        
        tensile stress(residual or applied)
      - HOW IT HAPPEN?
        
        IGSCC
        
        crack may propagate along GB
        
        TGSCC
        
        May run individual grain
      - MECHANISM OF SCC?
        
        ANODIC DISSOLUTION
        
        stress
        
        cause failure of passive film
        
        passivation rate
        
        FILM INDUCED CLEAVAGE
        
        tip of crack
        
        covered by brittle film
        
        crack grow, when the surface film
        reform at crack tip surface
        
        Hydrogen embrittlement
        
        source of hydrogen
        
        welding
        
        electroplating
        
        corrosion
        
        contact with gaseous hydrogen
        
        WHAT EFFECT OF H2?
        
        hydrogen cause crack growth,
        ultimately brittle fracture
        
        produced by cathodic reaction
        
        H2 atom weaken inter atomic bond at crack tip
      - METHOD FOR TESTING OF SCC
        
        SLOW STRAIN RATE TEST(SSRT)
        
        Tensile test to constant strain rate
        
        immersed in susceptible solution until failure
        
        FRACTURE MECHANICS
        
        test at pre-cracked specimen to constant load
        
        measure crack growth rate, with the specific environment
        
        function of stress intensity factor
      - PREVENTION
        
        Remove stress (often difficult, especially for residual stresses)
        
        Avoid the necessary environment
        
        Apply electrochemical protection where possible
        
        Use a different material
    - - HYDROGEN INDUCED CRACKING(HIC)
        
        THEORIES OF MECHANISM?
        
        pressure theory
        
        surface energy theory
        
        decohesion theory
        
        PREVENTION
        
        minimise hydrogen content
        
        reduce internal stress
        
        heat treatment
        
        inhibitors
    - - PREVENTION?
        
        Cathodic protection
        
        Inhibitors
        
        Select a more resistant material
      - Stress are cyclic
  - - - residual
        
        thermal stress(SCC)
    - - shock
        
        impact damage
        
        vibration(fretting corrosion)
    - - fault condition
        
        SCC corrosion fatigue
        
        fretting corrosion
  - - - Due to its relatively hydrogen small and size, affect properties is able to such as penetrate into metals strength and ductility.
    - - H2 presence in steel reduces the tensile ductility and toughness and causes premature failure under static loads
      - Type of HE
        
        Internal HE
        
        Environmental HE
    - - THEORIES OF MECHANISM?
        
        pressure theory
        
        surface energy theory
        
        decohesion theory