CORROSION:-

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CORROSION-

• TYPES
• EFFECTS
• PREVENTION
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  PRESENTED BY
• 
  GAUTAM AHUJA
• 
  1207535

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What is Corrosion?

• Corrosion is the deterioration of materials by
  chemical interaction with their environment.  The
  term corrosion is sometimes also applied to the
  degradation of plastics, concrete and wood, but
  generally refers to metals.

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TYPES OF CORROSION-

• Uniform corrosion.
• Galvanic corrosion.
• Pitting Crevice corrosion.
• Stress corrosion cracking.
• Erosion corrosion.
• Microbiologically induced corrosion.

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UNIFORM CORROSION-

• Uniform (or general) corrosion refers to the
  relatively uniform reduction of thickness over
  the surface of a corroding material. It is
  relatively easy to measure, predict and design
  against this type of corrosion damage. While
  uniform corrosion may represent only a small
  fraction of industrial corrosion failures, the
  total tonnage wasted is generally regarded as the
  highest of all forms.

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GALVANIC CORROSION-

• Galvanic corrosion is an electrochemical process
  in which one metal corrodes preferentially when
  in electrical contact with a different type of
  metal and both metals are immersed in an
  electrolyte. When two or more different sorts of
  metal come into contact in the presence of an
  electrolyte a galvanic couple is set up as
  different metals have different electrode
  potentials. The electrolyte provides a means for
  ion migration whereby metallic ions can move from
  the anode to the cathode. This leads to the
  anodic metal corroding more quickly than it
  otherwise would the corrosion of the cathodic
  metal is retarded even to the point of stopping.
  The presence of electrolyte and a conducting path
  between the metals may cause corrosion where
  otherwise neither metal alone would have corroded.

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PITTING CREVICE CORROSION-

• Pitting Corrosion is the localized corrosion of a
  metal surface confined to a point or small area,
  that takes the form of cavities. Pitting is one
  of the most damaging forms of corrosion. Pitting
  corrosion forms on passive metals and alloys like
  stainless steel.The resulting pits can become
  wide and shallow or narrow and deep which can
  rapidly perforate the wall thickness of a metal.

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STRESS CORROSION CRACKING-

• It is the cracking induced from the combined
  influence of tensile stress and a corrosive
  environment. The impact of SCC on a material
  usually falls between dry cracking and the
  fatigue threshold of that material. Usually, most
  of the surface remains unattacked, but with fine
  cracks penetrating into the material.

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EROSION CORROSION-

• Erosion corrosion is an acceleration in the rate
  of corrosion attack in metal due to the relative
  motion of a corrosive fluid and a metal surface.
  The increased turbulence caused by pitting on the
  internal surfaces of a tube can result in rapidly
  increasing erosion rates and eventually a leak.
  Erosion corrosion can also be aggravated by
  faulty workmanship.

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MICROBIAL CORROSION-

• Microbial corrosion or biological corrosion, is
  the deterioration of metals as a result of the
  metabolic activity of microorganisms.There are
  about a dozen of bacteria known to cause
  microbial corrosion of carbon steels, stainless
  steels, aluminum alloys and copper alloys in
  waters and soils with pH 49 and temperature
  10oC50oC. These bacteria can be broadly
  classified as aerobic anaerobic. Sulphate
  reducing bacteria (SRB) is anaerobic and is
  responsible for most instances of accelerated
  corrosion damages to ships and offshore steel
  structures. Iron and manganese oxidizing bacteria
  are aerobic and are frequently associated with
  accelerated pitting attacks on stainless steels
  at welds.

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EFFECTS OF CORROSION-

• Reduced Strength
• Downtime of equipment
• Escape of fluids
• Lost surface properties
• Reduced value of goods
• Economic losses

• The consequences of corrosion are many and
  varied and the effects of these on the safe,
  reliable and efficient operation of equipment or
  structures are often more serious than the simple
  loss of a mass of metal.  Failures of various
  kinds and the need for expensive replacements may
  occur even though the amount of metal destroyed
  is quite small. 

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CORROSION PREVENTION-

• Electrically insulate the two metals from each
  other. Unless they are in electrical contact,
  there can be no galvanic couple set up.
• Coating the two materials or if it is not
  possible to coat both, the coating shall be
  applied to the more noble, the material with
  higher potential.
• Another way is to keep the metals dry and/or
  shielded from ionic compounds (salts, acids,
  bases), for example by painting or encasing the
  protected metal in plastic or epoxy, and allowing
  them to dry.
• Proper selection of materials with known
  resistance to the service environment .
• Control pH, chloride concentration and
  temperature
• Cathodic protection and/or Anodic Protection.
• Regular mechanical cleaning if possible.
• Chemical treatment with biocides to control the
  population of bacteria.
• Complete drainage and dry-storage.

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