# ASTM G101-04 (Reapproved 2015)

Designation: G101 − 04 (Reapproved 2015)Standard Guide forEstimating the Atmospheric Corrosion Resistance of Low-Alloy Steels1This standard is issued under the fixed designation G101; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon (´) indicates an editorial change since the last revision or reapproval.1. Scope1.1 This guide presents two methods for estimating theatmospheric corrosion resistance of low-alloy weatheringsteels, such as those described in Specifications A242/A242M,A588/A588M, A606 Type 4, A709/A709M grades 50W, HPS70W, and 100W, A852/A852M, and A871/A871M. Onemethod gives an estimate of the long-term thickness loss of asteel at a specific site based on results of short-term tests. Theother gives an estimate of relative corrosion resistance basedon chemical composition.1.2 The values stated in SI units are to be regarded asstandard. No other units of measurement are included in thisstandard.2. Referenced Documents2.1 ASTM Standards:2A242/A242M Specification for High-Strength Low-AlloyStructural SteelA588/A588M Specification for High-Strength Low-AlloyStructural Steel, up to 50 ksi [345 MPa] Minimum YieldPoint, with Atmospheric Corrosion ResistanceA606 Specification for Steel, Sheet and Strip, High-Strength, Low-Alloy, Hot-Rolled and Cold-Rolled, withImproved Atmospheric Corrosion ResistanceA709/A709M Specification for Structural Steel for BridgesA852/A852M Specification for Quenched and TemperedLow-Alloy Structural Steel Plate with 70 ksi [485 MPa]Minimum Yield Strength to 4 in. [100 mm] Thick (With-drawn 2010)3A871/A871M Specification for High-Strength Low-AlloyStructural Steel Plate With Atmospheric Corrosion Resis-tanceG1 Practice for Preparing, Cleaning, and Evaluating Corro-sion Test SpecimensG16 Guide for Applying Statistics to Analysis of CorrosionDataG50 Practice for Conducting Atmospheric Corrosion Testson Metals3. Terminology3.1 Definitions of Terms Specific to This Standard:3.1.1 low-alloy steels—iron-carbon alloys containinggreater than 1.0 % but less than 5.0 %, by mass, total alloyingelements.3.1.1.1 Discussion—Most “low-alloy weathering steels”contain additions of both chromium and copper, and may alsocontain additions of silicon, nickel, phosphorus, or otheralloying elements which enhance atmospheric corrosion resis-tance.4. Summary of Guide4.1 In this guide, two general methods are presented forestimating the atmospheric corrosion resistance of low-alloyweathering steels. These are not alternative methods; eachmethod is intended for a specific purpose, as outlined in 5.2 and5.3.4.1.1 The first method utilizes linear regression analysis ofshort-term atmospheric corrosion data to enable prediction oflong-term performance by an extrapolation method.4.1.2 The second method utilizes predictive equations basedon the steel composition to calculate indices of atmosphericcorrosion resistance.5. Significance and Use5.1 In the past, ASTM specifications for low-alloy weath-ering steels, such as Specifications A242/A242M, A588/A588M, A606 Type 4, A709/A709M Grade 50W, HPS 70W,and 100W, A852/A852M, and A871/A871M stated that theatmospheric corrosion resistance of these steels is “approxi-mately two times that of carbon structural steel with copper.”Afootnote in the specifications stated that “two times carbonstructural steel with copper is equivalent to four times carbon1This guide is under the jurisdiction of ASTM Committee G01 on Corrosion ofMetals and is the direct responsibility of Subcommittee G01.04 on AtmosphericCorrosion.Current edition approved Nov. 1, 2015. Published December 2015. Originallyapproved in 1989. Last previous edition approved in 2010 as G101–04 (2010). DOI:10.1520/G0101-04R15.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at service@astm.org. For Annual Book of ASTMStandards volume information, refer to the standard’s Document Summary page onthe ASTM website.3The last approved version of this historical standard is referenced onwww.astm.org.Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States1structural steel without copper (Cu 0.02 maximum).” Becausesuch statements relating the corrosion resistance of weatheringsteels to that of other steels are imprecise and, moreimportantly, lack significance to the user (1 and 2)4, the presentguide was prepared to describe more meaningful methods ofestimating the atmospheric corrosion resistance of weatheringsteels.5.2 The first method of this guide is intended for use inestimating the expected long-term atmospheric corrosionlosses of specific grades of low-alloy steels in variousenvironments, utilizing existing short-term atmospheric corro-sion data for these grades of steel.5.3 The second method of this guide is intended for use inestimating the relative atmospheric corrosion resistance of aspecific heat of low-alloy steel, based on its chemical compo-sition.5.4 It is important to recognize that the methods presentedhere are based on calculations made from test data for flat,boldly exposed steel specimens. Atmospheric corrosion ratescan be much higher when the weathering steel remains wet forprolonged periods of time, or is heavily contaminated with saltor other corrosive chemicals. Therefore, caution must beexercised in the application of these methods for prediction oflong-term performance of actual structures.6. Procedure6.1 Atmospheric corrosion data for the methods presentedhere should be collected in accordance with Practice G50.Specimen preparation, cleaning, and evaluation should con-form to Practice G1.6.2 Linear Regression Extrapolation Method:6.2.1 This method essentially involves the extrapolation oflogarithmic plots of corrosion losses versus time. Such plots ofatmospheric corrosion data generally fit well to straight lines,and can be represented by equations in slope-intercept form,(3-5):logC 5 logA1Blogt (1)where:C = corrosion loss,t = time, andA and B = constants. A is the corrosion loss at t = 1, and Bis the slope of a log C versus log + plot.C may be expressed as mass loss per unit area, or as acalculated thickness loss or penetration based on mass loss.6.2.2 The method is best implemented by linear regressionanalysis, using the method of least squares detailed in GuideG16. At least three data points are required. Once the constantsof the equation are determined by the linear regressionanalysis, the projected corrosion loss can be calculated for anygiven time. A sample calculation is shown in Appendix X1.NOTE 1—Eq 1 can also be written as follows:C 5 AtB(2)Differentiation of Eq 2 with respect to time gives the corrosion rate (R)at any given time:R 5 ABt~B21!(3)Also, the time to a given corrosion loss can be calculated as follows:t 5 ~C/A!1/B(4)6.2.3 Examples of projected atmospheric corrosion lossesover a period of fifty years for low-alloy weathering steels invarious environments are presented in Appendix X1.NOTE 2—It has been reported (6 and 7) that for some environments, useof log-log linear regression extrapolations may result in predictions whichare somewhat lower or somewhat higher than actual losses. Specifically, inenvironments of very low corrosivity, the log-log predictions may behigher than actual losses (6), whereas in environments of very highcorrosivity the opposite may be true (7). For these cases, use of numericaloptimization or composite modeling methods (7 and 8) may provide moreaccurate predictions. Nevertheless, the simpler log-log linear regressionmethod described above provides adequate estimates for most purposes.6.3 Predictive Methods Based on Steel Composition—Twoapproaches are provided for prediction of relative corrosionresistance from composition. The first is based on the data ofLarrabee and Coburn (6.3.1). Its advantage is that it iscomparatively simple to apply. This approach is suitable whenthe alloying elements are limited to Cu, Ni, Cr, Si, and P, andin amounts within the range of the original data. Corrosionindices by either of the two approaches can be easily deter-mined by use of the tool provided on the ASTM website athttp://www.astm.org/COMMIT/G01_G101Calculator.xls.6.3.1 Predictive Method Based on the Data of Larabee andCoburn—Equations for predicting corrosion loss of low-alloysteels after 15.5 years of exposure to various atmospheres,based on the chemical composition of the steel, were publishedby Legault and Leckie (9). The equations are based onextensive data published by Larrabee and Coburn (10).6.3.1.1 For use in this guide, the Legault-Leckie equationfor an industrial atmosphere (Kearny, NJ) was modified toallow calculation of an atmospheric corrosion resistance indexbased on chemical composition. The modification consisted ofdeletion of the constant and changing the signs of all the termsin the equation. The modified equation for calculation of theatmospheric corrosion resistance index (I) is given below. Thehigher the index, the more corrosion resistant is the steel.I 5 26.01 ~%Cu!13.88 ~%Ni!11.20 ~%Cr!11.49 ~%Si!117.28 ~%P! 2 7.29 ~%Cu!~%Ni!29.10 ~%Ni!~%P! 2 33.39 ~%Cu!2NOTE 3—Similar indices can be calculated for the Legault-Leckieequations for marine and semi-rural atmospheres. However, it has beenfound that the ranking of the indices of various steel compositions is thesame for all these equations. Therefore, only one equation is required torank the relative corrosion resistance of different steels.6.3.1.2 The predictive equation should be used only for steelcompositions within the range of the original test materials inthe Larrabee-Coburn data set (7). These limits are as follows:Cu 0.51 % maxNi 1.1 % maxCr 1.3 % maxSi 0.64 % maxP 0.12 % max4The boldface numbers in parentheses refer to a list of references at the end ofthis standard.G101 − 04 (2015)26.3.1.3 Examples of averages and ranges of atmosphericcorrosion resistance indices calculated by the Larrabee-Coburnmethod for 72 heats of each of two weathering steels are shownin Table X2.1.6.3.2 Predictive Method Based on the Data of Townsend—Equations for predicting the corrosion loss of low alloy steelsbased on a statistical analysis of the effects of chemicalcomposition on the corrosion losses of hundreds of steelsexposed at three industrial locations were published byTownsend (11).6.3.2.1 In this method, the coefficients A and B, as definedfor Eq 1, are calculated as linear combinations of the effects ofeach alloying element, according to Eq 5 and 6.A 5 ao1Σaixi(5)B 5 bo1Σbixi(6)where:A and B = constants in the exponential corrosion loss func-tion as defined for Eq 1,aoand bo= constants for three industrial locations as givenin Table 1,aiand bi= constants for each alloying element as given inTable 1 for three industrial locations, andxi= compositions of the individual alloyingelements.The A and B values calculated from Eq 4 and 5 can be usedto compute corrosion losses, corrosion rates, and times to agiven loss at any of the three sites by use of Eq 2-4,respectively.6.3.2.2 For purposes of calculating a corrosion index fromthe Townsend data, the following procedure shall be followed.(1) For each of the three test sites,Aand B values for pure,unalloyed iron at are calculated by use of the regressionconstants given in Table 1, and Eq 5 and 6.(2) The times for pure iron to reach a 254-µm loss at thethree sites are then calculated by use of Eq 4.(3) For a given low alloy steel, A and B values at each siteare calculated from the regression constants and coefficients inTable 1, and Eq 5 and 6.(4) The losses of the low alloy steel at each site arecalculated from Eq 1 at the times required for pure iron to lose254 µm at the same site as determined in (1).(5) The respective differences between the 254-µm loss forpure iron and the calculated loss for the low alloy steel at eachsite as determined in (4) are averaged to give a corrosion index.(6) Examples of corrosion indices calculated by theTownsend method are shown in Table 2 for pure iron and avariety of low-alloy steel compositions. The upper limit of thecomposition ranges of each element in the Townsend data arealso given in Table 2.6.3.3 The minimum acceptable atmospheric corrosion indexshould be a matter of negotiation between the buyer and theseller.7. Report7.1 When reporting estimates of atmospheric corrosionresistance, the method of calculation should always be speci-fied.Also, in the Linear Regression Extrapolation Method (6.2)of this guide, the data used should be referenced with respectto type of specimens, condition and location of exposure, andduration of exposure.8. Keywords8.1 atmospheric corrosion resistance; compositional effects;corrosion indices; high-strength; industrial environments; low-alloy steel; marine environments; rural environments; weath-ering steelsTABLE 1 Constants and Coefficients for Calculating the Rate Constants A and B from CompositionA (µm) B (T in months)n 275 227 248 275 227 248site Bethlehem, PA Columbus, OH Pittsburgh, PA Bethlehem, PA Columbus, OH Pittsburgh, PAConstant 15.157 16.143 14.862 0.511 0.539 0.604Carbon 6.310A3.350 –0.102 –0.103 –0.046ManganeseA–2.170 –2.370 –0.097 –0.019 0.042Phosphorus –1.770 –10.250 –5.120 –0.592 –0.333 –0.546Sulfur –27.200 –15.970A2.408 0.908 1.004Silicon 6.50 2.96 1.38 –0.20 –0.16 –0.13Nickel 1.970 –1.380 1.180 –0.080 –0.029 –0.088ChromiumA2.560 2.370 –0.103 –0.095 –0.174CopperA0.990 –1.970 –0.072 –0.067 –0.068AluminumA1.580 5.520AA–0.087VanadiumA6.110AA–0.193ACobalt 1.580 –1.770 –2.560 –0.063 –0.053 0.044Arsenic 3.150 –6.110 –7.690 –0.157A0.097MolybdenumAA–2.960 –0.078 –0.038ATin –3.740 –7.490 –9.860 –0.151 –0.038ATungstenA–5.520A–0.148AAACoefficient has greater than 50 % probability of chance occurrence.G101 − 04 (2015)3APPENDIXES(Nonmandatory Information)X1. PROJECTED ATMOSPHERIC CORROSION PENETRATIONS FOR WEATHERING STEELSX1.1 Projected atmospheric corrosion losses in fifty yearsfor flat, boldly exposed specimens of Specifications A588/A588M and A242/A242M Type 1 weathering steels in rural,industrial, and marine environments are shown in Figs. X1.1-X1.3. (The “loss” shown in the figures is the average thicknessloss per surface, calculated from the mass loss per unit area.The uniformity of the thickness loss varies with the type ofenvironment.) These figures were developed from data (13) forspecimens exposed for time periods up to 8 or 16 years invarious countries. The specific exposure locations are given inTables X1.1-X1.3, and the compositions of the steels are givenin Table X1.4. In this test program, specimens were exposed infour orientations: 30° to the horizontal facing north and facingsouth, and vertical facing north and facing south. (The backsurface of each specimen was protected with a durable paintsystem.) For the lines plotted in Figs. X1.1-X1.3, data for thetest orientations showing th