ASTM G102-89(2015)e1

Standard Practice for Calculation of Corrosion Rates and Related Information from Electrochemical Measurements

SIGNIFICANCE AND USE
3.1 Electrochemical corrosion rate measurements often provide results in terms of electrical current. Although the conversion of these current values into mass loss rates or penetration rates is based on Faraday's Law, the calculations can be complicated for alloys and metals with elements having multiple valence values. This practice is intended to provide guidance in calculating mass loss and penetration rates for such alloys. Some typical values of equivalent weights for a variety of metals and alloys are provided.  
3.2 Electrochemical corrosion rate measurements may provide results in terms of electrical resistance. The conversion of these results to either mass loss or penetration rates requires additional electrochemical information. Some approaches for estimating this information are given.  
3.3 Use of this practice will aid in producing more consistent corrosion rate data from electrochemical results. This will make results from different studies more comparable and minimize calculation errors that may occur in transforming electrochemical results to corrosion rate values.
SCOPE
1.1 This practice covers the providing of guidance in converting the results of electrochemical measurements to rates of uniform corrosion. Calculation methods for converting corrosion current density values to either mass loss rates or average penetration rates are given for most engineering alloys. In addition, some guidelines for converting polarization resistance values to corrosion rates are provided.  
1.2 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.

General Information

• Status: Historical
• Publication Date: 31-Oct-2015
• ICS: 77.060 - Corrosion of metals
• Technical Committee: G01 - Corrosion of Metals
• Drafting Committee: G01.11 - Electrochemical Measurements in Corrosion Testing

Standards Content (Sample)

NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
Contact ASTM International (www.astm.org) for the latest information
´1
Designation: G102 − 89 (Reapproved 2015)
Standard Practice for
Calculation of Corrosion Rates and Related Information
from Electrochemical Measurements
This standard is issued under the fixed designation G102; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
ε NOTE—Editorially corrected the legend below Eq 1 in 4.1 in November 2015.
1. Scope version of these current values into mass loss rates or penetra-
tion rates is based on Faraday’s Law, the calculations can be
1.1 This practice covers the providing of guidance in
complicated for alloys and metals with elements having
converting the results of electrochemical measurements to rates
multiple valence values. This practice is intended to provide
of uniform corrosion. Calculation methods for converting
guidance in calculating mass loss and penetration rates for such
corrosion current density values to either mass loss rates or
alloys. Some typical values of equivalent weights for a variety
average penetration rates are given for most engineering alloys.
of metals and alloys are provided.
In addition, some guidelines for converting polarization resis-
tance values to corrosion rates are provided. 3.2 Electrochemical corrosion rate measurements may pro-
vide results in terms of electrical resistance. The conversion of
1.2 The values stated in SI units are to be regarded as
these results to either mass loss or penetration rates requires
standard. No other units of measurement are included in this
additional electrochemical information. Some approaches for
standard.
estimating this information are given.
2. Referenced Documents
3.3 Use of this practice will aid in producing more consis-
2.1 ASTM Standards: tent corrosion rate data from electrochemical results. This will
make results from different studies more comparable and
D2776 Methods of Test for Corrosivity of Water in the
Absence of Heat Transfer (Electrical Methods) (With- minimize calculation errors that may occur in transforming
electrochemical results to corrosion rate values.
drawn 1991)
G1 Practice for Preparing, Cleaning, and Evaluating Corro-
4. Corrosion Current Density
sion Test Specimens
4.1 Corrosion current values may be obtained from galvanic
G5 Reference Test Method for Making Potentiodynamic
cells and polarization measurements, including Tafel extrapo-
Anodic Polarization Measurements
lations or polarization resistance measurements. (See Refer-
G59 Test Method for Conducting Potentiodynamic Polariza-
ence Test Method G5 and Practice G59 for examples.) The first
tion Resistance Measurements
step is to convert the measured or estimated current value to
3. Significance and Use current density. This is accomplished by dividing the total
current by the geometric area of the electrode exposed to the
3.1 Electrochemical corrosion rate measurements often pro-
solution. The surface roughness is generally not taken into
vide results in terms of electrical current. Although the con-
account when calculating the current density. It is assumed that
the current distributes uniformly across the area used in this
This practice is under the jurisdiction of ASTM Committee G01 on Corrosion
calculation. In the case of galvanic couples, the exposed area of
of Metalsand is the direct responsibility of Subcommittee G01.11 on Electrochemi-
the anodic specimen should be used. This calculation may be
cal Measurements in Corrosion Testing.
expressed as follows:
Current edition approved Nov. 1, 2015. Published December 2015. Originally
approved in 1989. Last previous edition approved in 2010 as G102–89 (2010). DOI:
I
cor
10.1520/G0102-89R15E01.
i 5 (1)
cor
A
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
where:
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website.
i = corrosion current density, µA/cm ,
cor
The last approved version of this historical standard is referenced on www.ast-
I = total anodic current, µA, and
cor
m.org.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
´1
G102 − 89 (2015)
alloying element. Sometimes it is possible to analyze the
A = exposed specimen area, cm .
corrosion products and use those results to establish the proper
Other units may be used in this calculation. In some
valence. Another approach is to measure or estimate the
computerized polarization equipment, this calculation is made
electrode potential of the corroding surface. Equilibrium dia-
automatically after the specimen area is programmed into the
grams showing regions of stability of various phases as a
computer. A sample calculation is given in Appendix X1.
function of potential and pH may be created from thermody-
4.2 Equivalent Weight—Equivalent weight, EW, may be
namic data. These diagrams are known as Potential-pH (Pour-
thought of as the mass of metal in grams that will be oxidized
baix) diagrams and have been published by several authors (2,
by the passage of one Faraday (96 489 6 2 C (amp-sec)) of
3). The appropriate diagrams for the various alloying elements
electric charge.
can be consulted to estimate the stable valence of each element
at the temperature, potential, and pH of the contacting electro-
NOTE 1—The value of EW is not dependent on the unit system chosen
lyte that existed during the test.
and so may be considered dimensionless.
For pure elements, the equivalent weight is given by:
NOTE 2—Some of the older publications used inaccurate thermody-
namic data to construct the diagrams and consequently they are in error.
W
EW5 (2)
4.6 Some typical values of EW for a variety of metals and
n
alloys are given in Table 1.
where:
4.7 Calculation of Corrosion Rate—Faraday’s Law can be
W = the atomic weight of the element, and
used to calculate the corrosion rate, either in terms of penetra-
n = the number of electrons required to oxidize an atom of
tion rate (CR) or mass loss rate (MR) (4):
the element in the corrosion process, that is, the valence
of the element.
i
cor
CR5 K EW (5)
ρ
4.3 For alloys, the equivalent weight is more complex. It is
usually assumed that the process of oxidation is uniform and MR5 K i EW (6)
2 cor
does not occur selectively to any component of the alloy. If this
where:
is not true, then the calculation approach will need to be
CR is given in mm/yr, i in µA/cm ,
cor
adjusted to reflect the observed mechanism. In addition, some
rationale must be adopted for assigning values of n to the
−3
elements in the alloy because many elements exhibit more than
K = 3.27 × 10 , mm g/µA cm yr (Note 3),
one valence value.
ρ = density in g/cm , (see Practice G1 for density values
for many metals and alloys used in corrosion testing),
4.4 To calculate the alloy equivalent weight, the following
MR = g/m d, and
approach may be used. Consider a unit mass of alloy oxidized.
−3 2 2
K = 8.954 × 10 , g cm /µA m d (Note 3).
The electron equivalent for 1 g of an alloy, Q is then:
NOTE 3—EW is considered dimensionless in these calculations.
nifi
Other values for K and K for different unit systems are
1 2
Q 5 (3)
(
Wi
given in Table 2.
where:
4.8 Errors that may arise from this procedure are discussed
th
fi = the mass fraction of the i element in the alloy,
below.
th
Wi = the atomic weight of the i element in the alloy, and
4.8.1 Assignment of incorrect valence values may cause
th
ni = the valence of the i element of the alloy.
serious errors (5).
4.8.2 The calculation of penetration or mass loss from
Therefore, the alloy equivalent weight, EW, is the reciprocal
electrochemical measurements, as described in this standard,
of this quantity:
assumes that uniform corrosion is occurring. In cases where
EW5 (4) non-uniform corrosion processes are occurring, the use of these
nifi
methods may result in a substantial underestimation of the true
(
Wi
values.
Normally only elements above 1 mass percent in the alloy
4.8.3 Alloys that include large quantities of metalloids or
are included in the calculation. In cases where the actual
oxidized materials may not be able to be treated by the above
analysis of an alloy is not available, it is conventional to use the
procedure.
mid-range of the composition specification for each element,
4.8.4 Corrosion rates calculated by the method above where
unless a better basis is available. A sample calculation is given
abrasion or erosion is a significant contributor to the metal loss
in Appendix X2 (1).
process may yield significant underestimation of the metal loss
rate.
4.5 Valence assignments for elements that exhibit multiple
valences can create uncertainty. It is best if an independent
5. Polarization Resistance
technique can be used to establish the proper valence for each
5.1 Polarization resistance values may be approximated
from either potentiodynamic measurements near the corrosion
4 potential (see Practice G59) or stepwise potentiostatic polar-
The boldface numbers in parentheses refer to the list of references at the end of
this standard. ization using a single small potential step, ΔE, usually either
´1
G102 − 89 (2015)
TABLE 1 Equivalent Weight Values for a Variety of Metals and Alloys
NOTE 1—Alloying elements at concentrations below 1 % by mass were not included in the calculation, for example, they were considered part of the
basis metal.
NOTE 2—Mid-range values were assumed for concentrations of alloying elements.
NOTE 3—Only consistent valence groupings were used.
NOTE 4—Eq 4 was used to make these calculations.
Lowest Second Third Fourth
Elements
Common
UNS w/Constant
Variable Equivalent Variable Equivalent Element/ Equivalent Element/ Equivalent
Designation
Valence
Valence Weight Valence Weight Valence Weight Valence Weight
Aluminum Alloys:
A
AA1100 A91100 Al/3 8.99
AA2024 A92024 Al/3, Mg/2 Cu/1 9.38 Cu/2 9.32
AA2219 A92219 Al/3 Cu/1 9.51 Cu/2 9.42
AA3003 A93003 Al/3 Mn/2 9.07 Mn/4 9.03 Mn 7 8.98
AA3004 A93004 Al/3, Mg/2 Mn/2 9.09 Mn/4 9.06 Mn 7 9.00
AA5005 A95005 Al/3, Mg/2 9.01
AA5050 A95050 Al/3, Mg/2 9.03
AA5052 A95052 Al/3, Mg/2 9.05
AA5083 A95083 Al/3, Mg/2 9.09
AA5086 A95086 Al/3, Mg/2 9.09
AA5154 A95154 Al/3, Mg/2 9.08
AA5454 A95454 Al/3, Mg/2 9.06
AA5456 A95456 Al/3, Mg/2 9.11
AA6061 A96061 Al/3, Mg/2 9.01
Al/3, Mg/2,
AA6070 A96070 8.98
Si/4
AA6101 A96161 Al/3 8.99
AA7072 A97072 Al/3, Zn/2 9.06
Al/3, Zn/2,
AA7075 A97075 Cu/1 9.58 Cu/2 9.55
Mg/2
Al/3, Zn/2,
AA7079 A97079 9.37
Mg/2
Al/3, Zn/2,
AA7178 A97178 Cu/1 9.71 Cu/2 9.68
Mg/2
Copper Alloys:
CDA110 C11000 Cu/1 63.55 Cu/2 31.77
CDA220 C22000 Zn/2 Cu/1 58.07 Cu/2 31.86
CDA230 C23000 Zn/2 Cu/1 55.65 Cu/2 31.91
CDA260 C26000 Zn/2 Cu/1 49.51 Cu/2 32.04
CDA280 C28000 Zn/2 Cu/1 46.44 Cu/2 32.11
CDA444 C44300 Zn/2 Cu/1, Sn/2 50.42 Cu/1, Sn/4 50.00 Cu/2, Sn/4 32.00
CDA687 C68700 Zn/2, Al/3 Cu/1 48.03 Cu/2 30.29
CDA608 C60800 Al/3 Cu/1 47.114 Cu/2 27.76
CDA510 C51000 Cu/1, Sn/2 63.32 Cu/1, Sn/4 60.11 Cu/2, Sn/4 31.66
CDA524 C52400 Cu/1, Sn/2 63.10 Cu/1, Sn/4 57.04 Cu/2, Sn/4 31.55
CDA655 C65500 Si/4 Cu/1 50.21 Cu/2 28.51
CDA706 C70600 Ni/2 Cu/1 56.92 Cu/2 31.51
CDA715 C71500 Ni/2 Cu/1 46.69 Cu/2 30.98
CDA752 C75200 Ni/2, Zn/2 Cu/1 46.38 Cu/2 31.46
Stainless Steels:
304 S30400 Ni/2 Fe/2, Cr/3 25.12 Fe/3, Cr/3 18.99 Fe/3, Cr/6 15.72
321 S32100 Ni/2 Fe/2, Cr/3 25.13 Fe/3, Cr/3 19.08 Fe/3, Cr/6 15.78
309 S30900 Ni/2 Fe/2, Cr/3 24.62 Fe/3, Cr/3 19.24 Fe/3, Cr/6 15.33
310 S31000 Ni/2 Fe/2, Cr/3 24.44 Fe/3, Cr/3 19.73 Fe/3, Cr/6 15.36
316 S31600 Ni/2 Fe/2, Cr/3, Mo/3 25.50 Fe/2, Cr/3, Mo/4 25.33 Fe/3, Cr/6, Mo/6 19.14 Fe/3, Cr/6, Mo/6 16.111
317 S31700 Ni/2 Fe/2, Cr/3, Mo/3 25.26 Fe/2, Cr/3, Mo/4 25.03 Fe/3, Cr/3, Mo/6 19.15 Fe/3, Cr/6, Mo/6 15.82
410 S41000 Fe/2, Cr/3 25.94 Fe/3, Cr/3 18.45 Fe/3, Cr/6 16.28
430 S43000 Fe/2, Cr/3 25.30 Fe/3, Cr/3 18.38 Fe/3, Cr/6 15.58
446 S44600 Fe/2, Cr/3 24.22 Fe/3, Cr/3 18.28 Fe/3, Cr/6 14.46
Fe/2, Cr/3, Mo/3, Fe/2, Cr/3, Mo/ Fe/3, Cr/3, Mo/ Fe/3, Cr/6, Mo/6,
A
20CB3 N08020 Ni/2 23.98 23.83 18.88 15.50
Cu/1 4, Cu/1 6, Cu/2 Cu/2
Nickel Alloys:
200 N02200 NI/2 29.36 Ni/3 19.57
400 N04400 Ni/2 Cu/1 35.82 Cu/2 30.12
600 N06600 Ni/2 Fe/2, Cr/3 26.41 Fe/3, Cr/3 25.44 Fe/3, Cr/6 20.73
800 N08800 Ni/2 Fe/2, Cr/3 25.10 Fe/3, Cr/3 20.76 Fe/3, Cr/6 16.59
Fe/2, Cr/3, Mo/3, Fe/2, Cr/3, Mo/ Fe/3, Cr/3, Mo/ Fe/3, Cr/6, Mo/6,
825 N08825 Ni/2 25.52 25.32 21.70 17.10
Cu/1 4, Cu/1 6, Cu/2 Cu/2
B N10001 Ni/2 Mo/3, Fe/2 30.05 Mo/4, Fe/2 27.50 Mo/6, Fe/2 23.52 Mo/6, Fe/3 23.23
Fe/2, Cr/3, Mo/3, Fe/2, Cr/3, Mo/ Fe/2, Cr/3, Mo/ Fe/3, Cr/6, Mo/6,
B
C-22 N06022 Ni/2 26.04 25.12 23.28 17.88
W/4 4, W/4 6, W/6 W/6
Fe/2, Cr/3, Mo/3, Fe/2, Cr/3, Mo/ Fe/3, Cr/6, Mo/6,
C-276 N10276 Ni/2 27.09 Cr/3, Mo/4 25.90 23.63 19.14
W/4 6, W/6 W/6
´1
G102 − 89 (2015)
TABLE 1 Continued
Lowest Second Third Fourth
Elements
Common
UNS w/Constant
Variable Equivalent Variable Equivalent Element/ Equivalent Element/ Equivalent
Designation
Valence
Valence Weight Valence Weight Valence Weight Valence Weight
G N06007 Ni/2 (1) 25.46 (2) 22.22 (3) 22.04 (4) 17.03
Carbon Steel: Fe/2 27.92 Fe/3 18.62
(1) = Fe ⁄ 2, Cr/3, Mo/3, Cu/1, Nb/4,
(3) = Fe ⁄ 3, Cr ⁄ 3, Mo/6, Cu/2, Nb/5, Mn/2
Mn/2
(2) = Fe ⁄ 2, Cr/3, Mo/4, Cu/2, Nb/5,
(4) = Fe ⁄ 3, Cr/6, Mo/6, Cu/2, Nb/5, Mn/4
Mn/2
Other Metals:
Mg M14142 Mg/2 12.15
Mo R03600 Mo/3 31.98 Mo/4 23.98 Mo/6 15.99
Ag P07016 Ag/1 107.87 Ag/2 53.93
Ta R05210 Ta/5 36.19
Sn L13002 Sn/2 59.34 Sn/4 29.67
Ti R50400 Ti/2 23.95 Ti/3 15.97 Ti/4 11.98
Zn Z19001 Zn/2 32.68
Zr R60701 Zr/4 22.80
Pb L50045 Pb/2 103.59 Pb/4 51.80
A
Registered trademark Carpenter Technology.
B
Registered trademark Haynes International.
TABLE 2 Values of Constants for Use in Faraday’s Equation Rate
where:
A
ba = slope of the anodic Tafel reaction, when plotted on base
Penetration
A
I Unit ρ Unit K Units of K
10 logarithmic paper in V/decade,
cor 1 1
Rate Unit (CR)
2 3
bc = slope of the cathodic Tafel reaction when plotted on
mpy µA/cm g/cm 0.1288 mpy g/µA cm
B 2B 3B
mm/yr A/m kg/m 327.2 mm kg/A m y
base 10 logarithmic paper in V/decade, and
B 2 3 −3
mm/yr µA/cm g/cm 3.27 × 10 mm g/µA cm y
B = Stern-Geary constant, V.
B
5.3.2 In cases where one of the reactions is purely diffusion
Mass Loss Rate
A
I Unit K Units of K
cor 2 2
Unit
controlled, the Stern-Geary constant may be calculated:
2 B 2B
g/m d A/m 0.8953 g/Ad
2 2 2 2
mg/dm d (mdd) µA/cm 0.0895 mg cm /µA dm d
b
2 2B −3 2 2
B 5 (8)
mg/dm d (mdd) A/m 8.953 × 10 mg m /A dm d
...

This document is not an ASTM standard and is intended only to provide the user of an ASTM standard an indication of what changes have been made to the previous version. Because
it may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current version
of the standard as published by ASTM is to be considered the official document.
´1
Designation: G102 − 89 (Reapproved 2010) G102 − 89 (Reapproved 2015)
Standard Practice for
Calculation of Corrosion Rates and Related Information
from Electrochemical Measurements
This standard is issued under the fixed designation G102; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
ε NOTE—Editorially corrected the legend below Eq 1 in 4.1 in November 2015.
1. Scope
1.1 This practice covers the providing of guidance in converting the results of electrochemical measurements to rates of uniform
corrosion. Calculation methods for converting corrosion current density values to either mass loss rates or average penetration rates
are given for most engineering alloys. In addition, some guidelines for converting polarization resistance values to corrosion rates
are provided.
1.2 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.
2. Referenced Documents
2.1 ASTM Standards:
D2776 Methods of Test for Corrosivity of Water in the Absence of Heat Transfer (Electrical Methods) (Withdrawn 1991)
G1 Practice for Preparing, Cleaning, and Evaluating Corrosion Test Specimens
G5 Reference Test Method for Making Potentiodynamic Anodic Polarization Measurements
G59 Test Method for Conducting Potentiodynamic Polarization Resistance Measurements
3. Significance and Use
3.1 Electrochemical corrosion rate measurements often provide results in terms of electrical current. Although the conversion
of these current values into mass loss rates or penetration rates is based on Faraday’s Law, the calculations can be complicated
for alloys and metals with elements having multiple valence values. This practice is intended to provide guidance in calculating
mass loss and penetration rates for such alloys. Some typical values of equivalent weights for a variety of metals and alloys are
provided.
3.2 Electrochemical corrosion rate measurements may provide results in terms of electrical resistance. The conversion of these
results to either mass loss or penetration rates requires additional electrochemical information. Some approaches for estimating this
information are given.
3.3 Use of this practice will aid in producing more consistent corrosion rate data from electrochemical results. This will make
results from different studies more comparable and minimize calculation errors that may occur in transforming electrochemical
results to corrosion rate values.
4. Corrosion Current Density
4.1 Corrosion current values may be obtained from galvanic cells and polarization measurements, including Tafel extrapolations
or polarization resistance measurements. (See Reference Test Method G5 and Practice G59 for examples.) The first step is to
convert the measured or estimated current value to current density. This is accomplished by dividing the total current by the
geometric area of the electrode exposed to the solution. The surface roughness is generally not taken into account when calculating
This practice is under the jurisdiction of ASTM Committee G01 on Corrosion of Metalsand is the direct responsibility of Subcommittee G01.11 on Electrochemical
Measurements in Corrosion Testing.
Current edition approved May 1, 2010Nov. 1, 2015. Published May 2010December 2015. Originally approved in 1989. Last previous edition approved in 20042010 as
ε1
G102–89(2004)G102 . –89 (2010). DOI: 10.1520/G0102-89R10.10.1520/G0102-89R15E01.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
The last approved version of this historical standard is referenced on www.astm.org.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
´1
G102 − 89 (2015)
the current density. It is assumed that the current distributes uniformly across the area used in this calculation. In the case of
galvanic couples, the exposed area of the anodic specimen should be used. This calculation may be expressed as follows:
I
cor
i 5 (1)
cor
A
where:
I = corrosion current density, μA/cm ,
cor
i = corrosion current density, μA/cm ,
cor
I = total anodic current, μA, and
cor
A = exposed specimen area, cm .
Other units may be used in this calculation. In some computerized polarization equipment, this calculation is made automatically
after the specimen area is programmed into the computer. A sample calculation is given in Appendix X1.
4.2 Equivalent Weight—Equivalent weight, EW, may be thought of as the mass of metal in grams that will be oxidized by the
passage of one Faraday (96 489 6 2 C (amp-sec)) of electric charge.
NOTE 1—The value of EW is not dependent on the unit system chosen and so may be considered dimensionless.
For pure elements, the equivalent weight is given by:
W
EW 5 (2)
n
where:
W = the atomic weight of the element, and
n = the number of electrons required to oxidize an atom of the element in the corrosion process, that is, the valence of the
element.
4.3 For alloys, the equivalent weight is more complex. It is usually assumed that the process of oxidation is uniform and does
not occur selectively to any component of the alloy. If this is not true, then the calculation approach will need to be adjusted to
reflect the observed mechanism. In addition, some rationale must be adopted for assigning values of n to the elements in the alloy
because many elements exhibit more than one valence value.
4.4 To calculate the alloy equivalent weight, the following approach may be used. Consider a unit mass of alloy oxidized. The
electron equivalent for 1 g of an alloy, Q is then:
nifi
Q 5 (3)
(
Wi
where:
th
fi = the mass fraction of the i element in the alloy,
th
Wi = the atomic weight of the i element in the alloy, and
th
ni = the valence of the i element of the alloy.
Therefore, the alloy equivalent weight, EW, is the reciprocal of this quantity:
EW 5 (4)
nifi
(
Wi
Normally only elements above 1 mass percent in the alloy are included in the calculation. In cases where the actual analysis of
an alloy is not available, it is conventional to use the mid-range of the composition specification for each element, unless a better
basis is available. A sample calculation is given in Appendix X2 (1).
4.5 Valence assignments for elements that exhibit multiple valences can create uncertainty. It is best if an independent technique
can be used to establish the proper valence for each alloying element. Sometimes it is possible to analyze the corrosion products
and use those results to establish the proper valence. Another approach is to measure or estimate the electrode potential of the
corroding surface. Equilibrium diagrams showing regions of stability of various phases as a function of potential and pH may be
created from thermodynamic data. These diagrams are known as Potential-pH (Pourbaix) diagrams and have been published by
several authors (2, 3). The appropriate diagrams for the various alloying elements can be consulted to estimate the stable valence
of each element at the temperature, potential, and pH of the contacting electrolyte that existed during the test.
NOTE 2—Some of the older publications used inaccurate thermodynamic data to construct the diagrams and consequently they are in error.
4.6 Some typical values of EW for a variety of metals and alloys are given in Table 1.
The boldface numbers in parentheses refer to the list of references at the end of this standard.
´1
G102 − 89 (2015)
TABLE 1 Equivalent Weight Values for a Variety of Metals and Alloys
NOTE 1—Alloying elements at concentrations below 1 % by mass were not included in the calculation, for example, they were considered part of the
basis metal.
NOTE 2—Mid-range values were assumed for concentrations of alloying elements.
NOTE 3—Only consistent valence groupings were used.
NOTE 4—Eq 4 was used to make these calculations.
Lowest Second Third Fourth
Elements
Common
UNS w/Constant
Variable Equivalent Variable Equivalent Element/ Equivalent Element/ Equivalent
Designation
Valence
Valence Weight Valence Weight Valence Weight Valence Weight
Aluminum Alloys:
A
AA1100 A91100 Al/3 8.99
AA2024 A92024 Al/3, Mg/2 Cu/1 9.38 Cu/2 9.32
AA2219 A92219 Al/3 Cu/1 9.51 Cu/2 9.42
AA3003 A93003 Al/3 Mn/2 9.07 Mn/4 9.03 Mn 7 8.98
AA3004 A93004 Al/3, Mg/2 Mn/2 9.09 Mn/4 9.06 Mn 7 9.00
AA5005 A95005 Al/3, Mg/2 9.01
AA5050 A95050 Al/3, Mg/2 9.03
AA5052 A95052 Al/3, Mg/2 9.05
AA5083 A95083 Al/3, Mg/2 9.09
AA5086 A95086 Al/3, Mg/2 9.09
AA5154 A95154 Al/3, Mg/2 9.08
AA5454 A95454 Al/3, Mg/2 9.06
AA5456 A95456 Al/3, Mg/2 9.11
AA6061 A96061 Al/3, Mg/2 9.01
Al/3, Mg/2,
AA6070 A96070 8.98
Si/4
AA6101 A96161 Al/3 8.99
AA7072 A97072 Al/3, Zn/2 9.06
Al/3, Zn/2,
AA7075 A97075 Cu/1 9.58 Cu/2 9.55
Mg/2
Al/3, Zn/2,
AA7079 A97079 9.37
Mg/2
Al/3, Zn/2,
AA7178 A97178 Cu/1 9.71 Cu/2 9.68
Mg/2
Copper Alloys:
CDA110 C11000 Cu/1 63.55 Cu/2 31.77
CDA220 C22000 Zn/2 Cu/1 58.07 Cu/2 31.86
CDA230 C23000 Zn/2 Cu/1 55.65 Cu/2 31.91
CDA260 C26000 Zn/2 Cu/1 49.51 Cu/2 32.04
CDA280 C28000 Zn/2 Cu/1 46.44 Cu/2 32.11
CDA444 C44300 Zn/2 Cu/1, Sn/2 50.42 Cu/1, Sn/4 50.00 Cu/2, Sn/4 32.00
CDA687 C68700 Zn/2, Al/3 Cu/1 48.03 Cu/2 30.29
CDA608 C60800 Al/3 Cu/1 47.114 Cu/2 27.76
CDA510 C51000 Cu/1, Sn/2 63.32 Cu/1, Sn/4 60.11 Cu/2, Sn/4 31.66
CDA524 C52400 Cu/1, Sn/2 63.10 Cu/1, Sn/4 57.04 Cu/2, Sn/4 31.55
CDA655 C65500 Si/4 Cu/1 50.21 Cu/2 28.51
CDA706 C70600 Ni/2 Cu/1 56.92 Cu/2 31.51
CDA715 C71500 Ni/2 Cu/1 46.69 Cu/2 30.98
CDA752 C75200 Ni/2, Zn/2 Cu/1 46.38 Cu/2 31.46
Stainless Steels:
304 S30400 Ni/2 Fe/2, Cr/3 25.12 Fe/3, Cr/3 18.99 Fe/3, Cr/6 15.72
321 S32100 Ni/2 Fe/2, Cr/3 25.13 Fe/3, Cr/3 19.08 Fe/3, Cr/6 15.78
309 S30900 Ni/2 Fe/2, Cr/3 24.62 Fe/3, Cr/3 19.24 Fe/3, Cr/6 15.33
310 S31000 Ni/2 Fe/2, Cr/3 24.44 Fe/3, Cr/3 19.73 Fe/3, Cr/6 15.36
316 S31600 Ni/2 Fe/2, Cr/3, Mo/3 25.50 Fe/2, Cr/3, Mo/4 25.33 Fe/3, Cr/6, Mo/6 19.14 Fe/3, Cr/6, Mo/6 16.111
317 S31700 Ni/2 Fe/2, Cr/3, Mo/3 25.26 Fe/2, Cr/3, Mo/4 25.03 Fe/3, Cr/3, Mo/6 19.15 Fe/3, Cr/6, Mo/6 15.82
410 S41000 Fe/2, Cr/3 25.94 Fe/3, Cr/3 18.45 Fe/3, Cr/6 16.28
430 S43000 Fe/2, Cr/3 25.30 Fe/3, Cr/3 18.38 Fe/3, Cr/6 15.58
446 S44600 Fe/2, Cr/3 24.22 Fe/3, Cr/3 18.28 Fe/3, Cr/6 14.46
Fe/2, Cr/3, Mo/3, Fe/2, Cr/3, Mo/ Fe/3, Cr/3, Mo/ Fe/3, Cr/6, Mo/6,
A
20CB3 N08020 Ni/2 23.98 23.83 18.88 15.50
Cu/1 4, Cu/1 6, Cu/2 Cu/2
Nickel Alloys:
200 N02200 NI/2 29.36 Ni/3 19.57
400 N04400 Ni/2 Cu/1 35.82 Cu/2 30.12
600 N06600 Ni/2 Fe/2, Cr/3 26.41 Fe/3, Cr/3 25.44 Fe/3, Cr/6 20.73
800 N08800 Ni/2 Fe/2, Cr/3 25.10 Fe/3, Cr/3 20.76 Fe/3, Cr/6 16.59
Fe/2, Cr/3, Mo/3, Fe/2, Cr/3, Mo/ Fe/3, Cr/3, Mo/ Fe/3, Cr/6, Mo/6,
825 N08825 Ni/2 25.52 25.32 21.70 17.10
Cu/1 4, Cu/1 6, Cu/2 Cu/2
B N10001 Ni/2 Mo/3, Fe/2 30.05 Mo/4, Fe/2 27.50 Mo/6, Fe/2 23.52 Mo/6, Fe/3 23.23
Fe/2, Cr/3, Mo/3, Fe/2, Cr/3, Mo/ Fe/2, Cr/3, Mo/ Fe/3, Cr/6, Mo/6,
B
C-22 N06022 Ni/2 26.04 25.12 23.28 17.88
W/4 4, W/4 6, W/6 W/6
Fe/2, Cr/3, Mo/3, Fe/2, Cr/3, Mo/ Fe/3, Cr/6, Mo/6,
C-276 N10276 Ni/2 27.09 Cr/3, Mo/4 25.90 23.63 19.14
W/4 6, W/6 W/6
´1
G102 − 89 (2015)
TABLE 1 Continued
Lowest Second Third Fourth
Elements
Common
UNS w/Constant
Variable Equivalent Variable Equivalent Element/ Equivalent Element/ Equivalent
Designation
Valence
Valence Weight Valence Weight Valence Weight Valence Weight
G N06007 Ni/2 (1) 25.46 (2) 22.22 (3) 22.04 (4) 17.03
Carbon Steel: Fe/2 27.92 Fe/3 18.62
(1) = Fe ⁄ 2, Cr/3, Mo/3, Cu/1, Nb/4,
(3) = Fe ⁄ 3, Cr/3, Mo/6, Cu/2, Nb/5, Mn/2
Mn/2
(2) = Fe ⁄ 2, Cr/3, Mo/4, Cu/2, Nb/5,
(4) = Fe ⁄ 3, Cr/6, Mo/6, Cu/2, Nb/5, Mn/4
Mn/2
Other Metals:
Mg M14142 Mg/2 12.15
Mo R03600 Mo/3 31.98 Mo/4 23.98 Mo/6 15.99
Ag P07016 Ag/1 107.87 Ag/2 53.93
Ta R05210 Ta/5 36.19
Sn L13002 Sn/2 59.34 Sn/4 29.67
Ti R50400 Ti/2 23.95 Ti/3 15.97 Ti/4 11.98
Zn Z19001 Zn/2 32.68
Zr R60701 Zr/4 22.80
Pb L50045 Pb/2 103.59 Pb/4 51.80
A
Registered trademark Carpenter Technology.
B
Registered trademark Haynes International.
4.7 Calculation of Corrosion Rate—Faraday’s Law can be used to calculate the corrosion rate, either in terms of penetration
rate (CR) or mass loss rate (MR) (4):
i
cor
CR 5 K EW (5)
ρ
MR 5 K i EW (6)
2 cor
where:
CR is given in mm/yr, i in μA/cm ,
cor
−3
K = 3.27 × 10 , mm g/μA cm yr (Note 3),
ρ = density in g/cm , (see Practice G1 for density values for many metals and alloys used in corrosion testing),
MR = g/m d, and
−3 2 2
K = 8.954 × 10 , g cm /μA m d (Note 3).
NOTE 3—EW is considered dimensionless in these calculations.
Other values for K and K for different unit systems are given in Table 2.
1 2
4.8 Errors that may arise from this procedure are discussed below.
4.8.1 Assignment of incorrect valence values may cause serious errors (5).
4.8.2 The calculation of penetration or mass loss from electrochemical measurements, as described in this standard, assumes
that uniform corrosion is occurring. In cases where non-uniform corrosion processes are occurring, the use of these methods may
result in a substantial underestimation of the true values.
4.8.3 Alloys that include large quantities of metalloids or oxidized materials may not be able to be treated by the above
procedure.
4.8.4 Corrosion rates calculated by the method above where abrasion or erosion is a significant contributor to the metal loss
process may yield significant underestimation of the metal loss rate.
TABLE 2 Values of Constants for Use in Faraday’s Equation Rate
A
Penetration
A
I Unit ρ Unit K Units of K
cor 1 1
Rate Unit (CR)
2 3
mpy μA/cm g/cm 0.1288 mpy g/μA cm
B 2B 3B
mm/yr A/m kg/m 327.2 mm kg/A m y
B 2 3 −3
mm/yr μA/cm g/cm 3.27 × 10 mm g/μA cm y
B
Mass Loss Rate
A
I Unit K Units of K
cor 2 2
Unit
2 B 2B
g/m d A/m 0.8953 g/Ad
2 2 2 2
mg/dm d (mdd) μA/cm 0.0895 mg cm /μA dm d
2 2B −3 2 2
mg/dm d (mdd) A/m 8.953 × 10 mg m /A dm d
A
EW is assumed to be dimensionless.
B
SI unit.
´1
G102 − 89 (2015)
5. Polarization Resistance
5.1 Polarization resistance values may be approximated from either potentiodynamic measurements near the co
...