Standard Expressions for Corrosion Rate

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CHAPTER 2

• Standard Expressions for Corrosion Rate

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• A good corrosion rate expression should involve
• familiar units
• easy calculation with minimum opportunity for
  error
• ready conversion to life in years
• penetration
• whole number without cumbersome decimals.

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where W weight loss, mg D density of
specimen, g/cm3 A area of specimen, sq in. T
exposure time, hr
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• Conversion from other units to obtain mils per
  year is

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• Some equivalent metric penetration rates are
• The corrosion rates of usefully resistant
  materials generally range between 1 and 200 mpy.

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• where W is the weight loss in milligrams, D is
  the density in g/cm3, A is the area in cm2, and T
  is the time exposure in hours.

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• Corrosion rates determined by linear
  polarization and other electrochemical techniques
  are expressed in terms of current density. These
  expressions can be converted into penetration
  rates by the following expression based on
  Faradays law

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Corrosion penetration rate

• where
• a is the atomic weight of the metal
• i is the current density in ?A/cm2
• n is the number of electrons lost (valence
  change)
• D is the density in g/cm3
• K is a constant depending on the penetration rate
  desired.

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• For mpy, ?m/yr, and mm/yr, the values of K are
  0.129, 3.27, and 0.00327, respectively. The
  parameter n is determined by analysis of the
  solution or by measuring the potential and pH and
  referring to appropriate thermodynamic data.

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• The atomic weight a and n values for alloys can
  be calculated from weighted averages of their
  components.

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• Consider iron corroding in air - free acid at
  an electrochemical corrosion rate of 1 ?A/cm2. It
  dissolves as ferrous ions (Fe2), and thus, n
  2. The corrosion rate in mpy is obtained by
  substitution

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Table 2 - 1 Comparison of Corrosion -
rateExpressions for Engineering applications
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Table 2 - 1 Comparison of Corrosion -
rateExpressions for Engineering applications
(continue)
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Table 2 - 1 Comparison of Corrosion -
rateExpressions for Engineering applications
(continue)