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Standard Expressions for Corrosion Rate

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easy calculation with minimum opportunity for error. ready ... The corrosion rates of usefully resistant materials generally range between 1 and 200 mpy. ... – PowerPoint PPT presentation

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Title: Standard Expressions for Corrosion Rate


1
CHAPTER 2
  • Standard Expressions for Corrosion Rate

2
  • 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.

3
where W weight loss, mg D density of
specimen, g/cm3 A area of specimen, sq in. T
exposure time, hr
4
  • Conversion from other units to obtain mils per
    year is

5
  • Some equivalent metric penetration rates are
  • The corrosion rates of usefully resistant
    materials generally range between 1 and 200 mpy.

6
  • 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.

7
  • 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

8
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.

9
  • 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.

10
  • The atomic weight a and n values for alloys can
    be calculated from weighted averages of their
    components.

11
  • 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

12
Table 2 - 1 Comparison of Corrosion -
rateExpressions for Engineering applications
13
Table 2 - 1 Comparison of Corrosion -
rateExpressions for Engineering applications
(continue)
14
Table 2 - 1 Comparison of Corrosion -
rateExpressions for Engineering applications
(continue)
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