ASTM G102-89(1999)

NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
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Designation: G 102 – 89 (Reapproved 1999)
Standard Practice for
Calculation of Corrosion Rates and Related Information
from Electrochemical Measurements
This standard is issued under the fixed designation G 102; 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 (e) indicates an editorial change since the last revision or reapproval.
1. Scope additional electrochemical information. Some approaches for
estimating this information are given.
1.1 This practice is intended to provide guidance in con-
3.3 Use of this practice will aid in producing more consis-
verting the results of electrochemical measurements to rates of
tent corrosion rate data from electrochemical results. This will
uniform corrosion. Calculation methods for converting corro-
make results from different studies more comparable and
sion current density values to either mass loss rates or average
minimize calculation errors that may occur in transforming
penetration rates are given for most engineering alloys. In
electrochemical results to corrosion rate values.
addition, some guidelines for converting polarization resis-
tance values to corrosion rates are provided.
4. Corrosion Current Density
2. Referenced Documents 4.1 Corrosion current values may be obtained from galvanic
cells and polarization measurements, including Tafel extrapo-
2.1 ASTM Standards:
lations or polarization resistance measurements. (See Refer-
D 2776 Test Methods for Corrosivity of Water in the Ab-
ence Test Method G 5 and Practice G 59 for examples.) The
sence of Heat Transfer (Electrical Methods)
first step is to convert the measured or estimated current value
G 1 Practice for Preparing, Cleaning, and Evaluating Cor-
to current density. This is accomplished by dividing the total
rosion Test Specimens
current by the geometric area of the electrode exposed to the
G 5 Reference Test Method for Making Potentiostatic and
solution. It is assumed that the current distributes uniformly
Potentiodynamic Anodic Polarization Measurements
across the area used in this calculation. In the case of galvanic
G 59 Practice for Conducting Potentiodynamic Polarization
couples, the exposed area of the anodic specimen should be
Resistance Measurements
used. This calculation may be expressed as follows:
3. Significance and Use
I
cor
i 5 (1)
cor
3.1 Electrochemical corrosion rate measurements often pro- A
vide results in terms of electrical current. Although the con-
where:
version of these current values into mass loss rates or penetra-
i 5 corrosion current density, μA/cm ,
cor
tion rates is based on Faraday’s Law, the calculations can be
I 5 total anodic current, μA, and
cor
complicated for alloys and metals with elements having
A 5 exposed specimen area, cm .
multiple valence values. This practice is intended to provide
Other units may be used in this calculation. In some
guidance in calculating mass loss and penetration rates for such
computerized polarization equipment, this calculation is made
alloys. Some typical values of equivalent weights for a variety
automatically after the specimen area is programmed into the
of metals and alloys are provided.
computer. A sample calculation is given in Appendix X1.
3.2 Electrochemical corrosion rate measurements may pro-
4.2 Equivalent Weight—Equivalent weight, EW, may be
vide results in terms of electrical resistance. The conversion of
thought of as the mass of metal in grams that will be oxidized
these results to either mass loss or penetration rates requires
by the passage of one Faraday (96 489 6 2 C (amp-sec)) of
electric charge.
This practice is under the jurisdiction of ASTM Committee G01 on Corrosion
NOTE 1—The value of EW is not dependent on the unit system chosen
of Metalsand is the direct responsibility of Subcommittee G01.11 on Electrochemi-
and so may be considered dimensionless.
cal Measurements in Corrosion Testing.
Current edition approved Feb. 24, 1989. Published May 1989. Originally For pure elements, the equivalent weight is given by:
e1
published as G 102– 89. Last previous edition G 102– 89 (1994) .
2 W
Discontinued—See 1990 Annual Book of ASTM Standards, Vol 03.02.
EW 5 (2)
n
Annual Book of ASTM Standards, Vol 03.02.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
G 102
Normally only elements above 1 mass percent in the alloy
where:
are included in the calculation. In cases where the actual
W 5 the atomic weight of the element, and
analysis of an alloy is not available, it is conventional to use the
n 5 the number of electrons required to oxidize an atom of
the element in the corrosion process, that is, the mid-range of the composition specification for each element,
unless a better basis is available. A sample calculation is given
valence of the element.
in Appendix X2 (1).
4.3 For alloys, the equivalent weight is more complex. It is
4.5 Valence assignments for elements that exhibit multiple
usually assumed that the process of oxidation is uniform and
valences can create uncertainty. It is best if an independent
does not occur selectively to any component of the alloy. If this
technique can be used to establish the proper valence for each
is not true, then the calculation approach will need to be
alloying element. Sometimes it is possible to analyze the
adjusted to reflect the observed mechanism. In addition, some
corrosion products and use those results to establish the proper
rationale must be adopted for assigning values of n to the
valence. Another approach is to measure or estimate the
elements in the alloy because many elements exhibit more than
electrode potential of the corroding surface. Equilibrium dia-
one valence value.
grams showing regions of stability of various phases as a
4.4 To calculate the alloy equivalent weight, the following
function of potential and pH may be created from thermody-
approach may be used. Consider a unit mass of alloy oxidized.
namic data. These diagrams are known as Potential-pH (Pour-
The electron equivalent for1gofan alloy, Q is then:
baix) diagrams and have been published by several authors (2,
3). The appropriate diagrams for the various alloying elements
nifi
Q 5 ( (3)
Wi can be consulted to estimate the stable valence of each element
at the temperature, potential, and pH of the contacting electro-
where:
lyte that existed during the test.
th
fi 5 the mass fraction of the i element in the alloy,
th
NOTE 2—Some of the older publications used inaccurate thermody-
Wi 5 the atomic weight of the i element in the alloy, and
th
namic data to construct the diagrams and consequently they are in error.
ni 5 the valence of the i element of the alloy.
4.6 Some typical values of EW for a variety of metals and
Therefore, the alloy equivalent weight, EW, is the reciprocal
of this quantity: alloys are given in Table 1.
EW 5 (4)
nifi
The boldface numbers in parentheses refer to the list of references at the end of
(
Wi this standard.
TABLE 1 Equivalent Weight Values for a Variety of Metals and Alloys
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
AA6070 A96070 Al/3, Mg/2, 8.98
Si/4
AA6101 A96161 Al/3 8.99
AA7072 A97072 Al/3, Zn/2 9.06
AA7075 A97075 Al/3, Zn/2, Cu/1 9.58 Cu/2 9.55
Mg/2
AA7079 A97079 Al/3, Zn/2, 9.37
Mg/2
AA7178 A97178 Al/3, Zn/2, 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
G 102
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
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
A
20CB3 N08020 Ni/2 Fe/2, Cr/3, Mo/3, 23.98 Fe/2, Cr/3, Mo/ 23.83 Fe/3, Cr/3, Mo/ 18.88 Fe/3, Cr/6, Mo/6, 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
825 N08825 Ni/2 Fe/2, Cr/3, Mo/3, 25.52 Fe/2, Cr/3, Mo/ 25.32 Fe/3, Cr/3, Mo/ 21.70 Fe/3, Cr/6, Mo/6, 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
B
C-22 N06022 Ni/2 Fe/2, Cr/3, Mo/3, 26.04 Fe/2, Cr/3, Mo/ 25.12 Fe/2, Cr/3, Mo/ 23.28 Fe/3, Cr/6, Mo/6, 17.88
W/4 4, W/4 6, W/6 W/6
C-276 N10276 Ni/2 Fe/2, Cr/3, Mo/3, 27.09 Cr/3, Mo/4 25.90 Fe/2, Cr/3, Mo/ 23.63 Fe/3, Cr/6, Mo/6, 19.14
W/4 6, W/6 W/6
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) 5 Fe/2, Cr/3, Mo/3, Cu/1, Nb/4, (3) 5 Fe/3, Cr/3, Mo/6, Cu/2, Nb/5, Mn/2
Mn/2
(2) 5 Fe/2, Cr/3, Mo/4, Cu/2, Nb/5, (4) 5 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.
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.
4.7 Calculation of Corrosion Rate—Faraday’s Law can be
−3
used to calculate the corrosion rate, either in terms of penetra-
K 5 3.27 3 10 , mm g/μA cm yr (Note 3),
tion rate (CR) or mass loss rate (MR) (4): r5 density in g/cm , (see Practice G 1 for density values
for many metals and alloys used in corrosion test-
i
cor
CR 5 K EW (5)
ing),
r
MR 5 g/m d, and
MR 5 K i EW (6)
−3 2 2
2 cor
K 5 8.954 3 10 ,gcm /μA m d (Note 3).
where:
NOTE 3—EW is considered dimensionless in these calculations.
CR is given in mm/yr, i in μA/cm ,
cor
G 102
Other values for K and K for different unit systems are 5.3.1 Calculate Stern-Geary constants from known Tafel
1 2
given in Table 2. slopes where both cathodic and anodic reactions are activation
4.8 Errors that may arise from this procedure are discussed controlled, that is, there are distinct linear regions near the
below. corrosion potential on an E log i plot:
4.8.1 Assignment of incorrect valence values may cause
ba bc
B 5 (7)
serious errors (5).
2.303 ~ba 1 bc!
4.8.2 The calculation of penetration or mass loss from
where:
electrochemical measurements, as described in this standard,
ba 5 slope of the anodic Tafel reaction, when plotted on
assumes that uniform corrosion is occurring. In cases where
base 10 logarithmic paper in V/decade,
non-uniform corrosion processes are occurring, the use of these
bc 5 slope of the cathodic Tafel reaction when plotted on
methods may result in a substantial underestimation of the true
base 10 logarithmic paper in V/decade, and
values.
B 5 Stern-Geary constant, V.
4.8.3 Alloys that include large quantities of metalloids or
5.3.2 In cases where one of the reactions is purely diffusion
oxidized materials may not be able to be treated by the above
controlled, the Stern-Geary constant may be calculated:
procedure.
4.8.4 Corrosion rates calculated by the method above where
b
B 5 (8)
abrasion or erosion is a significant contributor to the metal loss
2.303
process may yield significant underestimation of the metal loss
where:
rate.
b 5 the activation controlled Tafel slope in V/decade.
5. Polarization Resistance
5.3.3 It should be noted in this case that the corrosion
5.1 Polarization resistance values may be approximated current density will be equal to the diffusion limited current
from either potentiodynamic measurements near the corrosion density. A sample calculation is given in Appendix X4.
potential (see Practice G 59) or stepwise potentiostatic polar-
5.3.4 Cases where both activation and diffusion effects are
ization using a single small potential step, DE, usually either 10
similar in magnitude are known as mixed control. The reaction
mV or − 10 mV, (see Test Method D 2776). Values of 65 and
under mixed control will have an apparently larger b value than
620 mV are also commonly used. In this case, the specimen
predicted for an activation control, and a plot of E versus log
current, DI, is measured after steady state occurs, and DE/DI is
I will tend to curve to an asymptote parallel to the potential
calculated. Potentiodynamic measurements yield curves of I
axis. The es
...