CORROSION Section 2: Basic Corrosion Measurements

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CORROSIONSection 2 Basic Corrosion Measurements

• By
• D.H. Lister W.G. Cook
• Department of Chemical Engineering
• University of New Brunswick

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• If a piece of metal
• corrodes to this
• How would we know how much corrosion had
  occurred?
• Clearly, we need before and after measurements
  but of what?

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• Size?
• Length, breadth and thickness before
• What about after?
• Volume before, volume after .. by liquid
  displacement .
• Sensitive enough?
• Mass?
• Weigh before, weigh after (if piece small enough
  for reasonable balance note that measuring
  weight losses to a fraction of a mg in a few
  grams is the norm in corrosion testing).
• To be of any use, a weight loss measurement has
  to be scaled to the size of the specimen
  corrosion is a surface related phenomenon.

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• Because corrosion is a surface phenomenon, weight
  loss is related to surface area (SA)
• For example
• a metal coupon having a total surface area of
  10 cm2 loses 1 mg when exposed to a particular
  environment for a month
• which should be a characteristic amount for that
  metal in that environment in a month.

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• Corrosion engineers traditionally work in units
  of dm2, thus
• BUT even though mdms give reasonable numbers
  for most corrosion situations, the month is a bad
  unit Why?
• So are often used . mdd

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• If we know the metal density, we can convert the
  weight loss to penetration
• for ? in g/cm3
• Which isnt a very practical unit.

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• However, we note that 1 cm 104 mm, thus
• Another common unit for corrosion penetraion (in
  the US) is the mil or milli-inch per year
  (0.001 in/year).

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• NOTE these rates are averaged over the exposure
  time, and are quoted as if corrosion were
  constant with time it often is not
• measure rate 1 measure rate 2

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• Often, our bit of metal ..
• after exposure, looks like this
• It is covered with scale.
• How would we assess corrosion under these
  conditions?

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• Scaled (e.g., oxidized) specimens often
  experience a weight gain thus, the
  high-temperature oxidation of iron proceeds
• 4Fe 3O2 ? 2Fe2O3
• (4?56)g (4?56 6?16)g
• If we know scale composition (e.g., type of
  oxide) and weight gain, we can calculate the Fe
  in the oxide and, therefore, the Fe corroded.

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• BUT we can rarely be sure that the scale
  accounts exactly for the corrosion some might
  fall off (spall) some might have been deposited
  from the environment
• NOW WHAT?

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• Consider the scaling process
• Weight before Wo g
• Surface area A dm2
• Exposure time ? days
• Find condition weight after W1 g

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• We can only be sure that the remaining metal
  provides an unequivocal reference, so
• Descale
• Descaled weight W2 g

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• (divided by ? for rate mdd)
• We also have a measure of the weight of scale
• If we assume the scale is composed of Fe2O3, we
  can calculate the iron in scale as

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• The difference between iron in scale and total
  amount corroded is either
• Iron released to environment OR
• Iron deposited from environment

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• How do we descale?
• Chemically or electrochemically usually in weak
  acids.
• BUT such descaling can also dissolve some of
  the remaining metal, so how do we account for
  this?
• employ an inhibitor
• use a blank

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• A blank is a bare specimen put into the
  descaling solution along with the specimen
• during the descaling, if the blank corrodes by
• we apply this blank correction to the specimen
  measurement
• DRAWBACKS? DISCUSS
• EXAMPLE.

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• Note such corrosion measurements and units are
  useful for general or uniform corrosion but they
  dont work for localized corrosion (e.g. pitting
  or cracking). A component can fail by stress
  corrosion cracking (SCC) with no detectable
  weight loss

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• For these situations, we use concepts such as
  crack propagation rate or pit propagation rate.
• For example
• percent through-wall for tubes, pipes,
  vessels, etc.
• crack depth or pit depth usually in mil or
  mm
• propagation rate
  etc.

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Examples of Corrosion

• General corrosion .
• General corrosion of the reactor vessel head of
  the Davis-Besse PWR. (note this exterior general
  corrosion was indicative of a much more serious
  local corrosion problem due to a leak in the
  pressure boundary)

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• Classic crevice corrosion. In this case caused by
  a carbon rubber exhaust flapper clamped onto a
  stainless exhaust flange plate. Notice that the
  worst corrosion is where the clamp pressed the
  rubber the tightest.

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• Stress Corrosion
• Stress corrosion of stainless steel type 304
  autoclave. (Mallinckrodt Chemical Works)

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• How would you measure (estimate) the corrosion
  of plant components?
• For example
• pressure vessels or pressure tubes
• feeder pipes
• steam generator tubes
• feedwater pipes and heaters
• etc.
• DISCUSS

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• Methods involve
• Non-Destructive Evaluation (NDE) or
• Non-Destructive Testing (NDT)
• and include
• Ultrasonic Testing (UT)
• Eddy Current
• Acoustic Emission
• etc.

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• Some useful conversion factors