Acom85_2 Oxidation Kinetics of Heat Resistant Alloys Part I+II

7/29/2019 Acom85_2 Oxidation Kinetics of Heat Resistant Alloys Part I+II

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acomAVESTA CORROSION MANAGMENT

Oxidation Kineticsof Heat Resistant Alloys

Part Iby Gene R. Rundell, Manager of Technical Services

Rolled Alloys, 125 West Sterns Rd.,Box 310, Temperance, Mich. 48182, USA

Alloys based on the Fe-Cr-Ni system include the aus-

tenitic stainless steels and a group of higher chromiumand nickel alloys having useful oxidation resistance attemperatures above 900 C (1650 F). These heat resis-tant alloys include AISI types 309 and 310, RA 330,Hastelloy alloy X, RA 333, and Inconel alloys 600 and601. All of these materials, in common with the austeni-tic stainlesses, have face centered cubic structures,undergo no phase change from ambient temperature tothe melting point, and are not hardenable by heat treat-ment.

Although types 309 and 310 are used largely because oftheir oxidation resistance, this property has not beendetermined by any standardized test. Indeed, a ma-terials engineer would have difficulty finding data ob-

tained by the same test for more than a few alloys. Thisproblem does not exist for the cast heat resistant alloys:the Alloy Casting Institute has developed oxidation datafor 14 cast alloys obtained via the same procedure.(These data can be found in Vol 1 of the Eighth Edition ofthe ASM Metals Handbook.)

In an effort to remedy the situation for wrought heatresistant steels, oxidation testing has been conductedat Rolled Alloys for the past three years. This project -still in progress - is the subject of this article.

Oxidation Kinetics - In the early 1920s Pilling and Bed-worth discovered that oxidation of alloy steels in-creased parabolically with exposure time. The dis-covery of what is now called "parabolic oxidation"

stimulated interest in oxidation kinetics to the extentthat books on oxidation contained at least one chapterdevoted to the subject. (Examples: see Ref 1 and 2.)

All rights reserved. Comments and correspondence can be

directed to Dr Sten Nordin, Avesta Projects AB,

P.O. Box 557,S-651 09 Karlstad, Sweden. Tel. +46 (0)54-10 27 70

Telex 66108 apab s. Telefax +46(0)54-18 82 54. No 2-1985


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Unfortunately, this interest in oxidation kinetics wasexpressed primarily by fundamental researchers -knowledge gained has generally not been used to pre-dict long time oxidation rates of commercial alloys. Forexample, the data for cast heat resistant alloys aregiven in mils/yr, and are based largely on weight lossmeasured at the end of a 100 h exposure. These data donot necessarily agree with those for long time exposurepredicted on the basis of our understanding of kinetic

behavior. A better approach: extend the exposureperiod to way beyond 100 h, and determine the amountof oxidation at frequent intervals during the extendedexposure time.

Cyclic Exposure MethodUsed to DevelopOxidation DataStatic oxidation testing at Rolled Alloys has been donewithin the guidelines established by ASTM in StandardRecommended Practice G54-77.

Tests are performed by exposing samples in still airusing an electrically heated box furnace capable ofhandling eight samples within a zone having no morethan an 8 C (14 F) thermal gradient. Test temperature isdetermined by a thermocouple placed close to thespecimens and which is independent of the furnacecontrol couple. Air is not intentionally passed into thefurnace.

Specimens consist of right circular cylinders which havebeen surface ground to a true right cylinder and finished

on 400 grit silicon carbide paper. After final grinding,they're measured to the nearest 0.0001 in. (2.5 m),cleaned and dried with acetone, and weighed to thenearest 0.1 mg. Specimens have a common diameter of11.43 mm (0.45 in.) and usually range in length fromabout 14 to 18 mm (0.55 to 0.71 in.).

Samples are placed in porcelain cups and reweighedalong with the cup prior to exposure. They're handled bymetal tongs between cleaning and exposure.

Methodology - Specimens of eight alloys are exposedsimultaneously in individual porcelain cups. At the endof 20 h the metal tray holding the cups is withdrawn andcooled in air.

Heat resistant alloys begin to spall oxide usually within

several minutes after the onset of rapid cooling. Toobtain accurate weight increase data, all of the spalledoxide must be retained. For this reason, each cup has aclose fitting porcelain cover that's added before coolingand kept on until the cup is returned to the furnace.Specimen and cup are weighed at the end of each 20 hexposure to determine weight gain per unit area. Notethat the weight gain includes that derived from bothadherent and nonadherent oxide.

After the usual exposure time of 500 h, each specimenhas been weighed 25 times. These data are used to plotan oxidation curve.

Possible sources of error in addition to weighing andmeasuring, and our comments about them, follow.

1. Weight change of porcelain cups. (Initial workexposing empty cups indicated that cup weight is notaffected by exposure to the test temperatures ofinterest.)

2. Contamination of cups from furnace refractories orthe metal tray. (An alloy for the tray was selected thatdevelops virtually no spall on cooling to room tempera-ture.)

3. Reaction of the specimen or its oxide with porcelain,and area of contact of specimen with the cup. (Exam-ination of both specimen and cup suggests that the cupdoes not contribute to or reduce oxidation rate.)

4. Effect of reduced surface area as the sample oxi-dizes. (A mathematical treatment has been suggestedthat would account for the effect of reduced specimendimensions on apparent weight loss per unit area. Thiseffect, however, is small for most of the alloys tested.Thus, original specimen dimensions are used through-out.)

5. Spalling of oxide in open cups upon heating.

Linear RegressionAnalysis: Its Role inData ReductionAt the end of the eight alloys' 500 h exposure, each hasbeen weighed 25 times for a total of 200 data pointsrelating weight gain to exposure time. These data areplotted to determine each alloy's general oxidationcharacteristics. Plotting is followed by a linear re-gression analysis using a programmable calculator suchas the Tl-59.

Analysis of heat resistant alloys oxidized at 980 C(1800 F) through 1150 C (2100 F) indicates that oxida-tion may be either parabolic or linear. Parabolic databecome linear when weight gain is squared. In bothcases a linear regression analysis is performed using 25pairs of data for each alloy and test temperature.

Linear regression analysis using the program we se-lected provides.:

1. Best relationship between the dependent and inde-pendent variable - a best fit curve. (Also provided arethe slope of this line and its intercept, and the calcu-lated value of weight gain for any particular exposuretime.)

2. Statistical quality of the data expressed as the coeffi-cient of determination. (The coefficient should ideallyapproach unity for good correlation. Values above 0.99are common using this approach for weight gain data.)

Oxidation Rate - Regression analysis is particularlyuseful for calculating (rather than determining graphi-cally) the oxidation rate constant. The rate constant forlinear oxidation is the slope of the line defined by:

W/A=kL t+C,

where kL is rate constant, t is oxidation exposure time,and W/A is the weight gain per unit area. In most casesoxidation is not linear during the early stages of expo-sure, thus the intercept may not pass through the origin.This results in small positive values for the constant, C.

Oxidation rate constants are fundamentally related totemperature in the Arrhenius equation. Rate constantsalso provide a way to determine weight gain at anygiven time, such as 1000 h.


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Assessing the Effect of

Oxidation on Metal LossUse of weight gain data is a simple and direct way todetermine the relationship between oxidation and time.Only one sample for each test temperature is needed.The technique does not, however, indicate the effect of

oxidation on metal loss.

In corrosion testing it is common to determine weightloss which, when converted to volume loss, indicatessurface recession. This quantity is also of great value inselecting alloys for elevated temperature service.Depth of metal loss can often be applied directly indesign because oxidation of many heat resistant alloysin air is uniform rather than intergranular.

Determining useful weight loss values from oxidationtest data is complicated by the fact that a portion of thetotal oxide is thin and tightly adherent. This adherentoxide has to be removed without changing the weight ofthe unoxidized portion of the specimen. Cathodic de-

scaling has been used for this purpose by others, and isthe method we chose.

The sample is immersed in a 40 % solution of sodiumcarbonate in sodium hydroxide heated to 550 C(1020 F). Direct current is passed through the solutionwith the specimen as the cathode for a time sufficient toremove all surface oxide (several minutes are needed).

Oxide removal by cathodic descaling usually involvessuccessive immersions, each followed by a measure-ment of weight change. In the first stage - typicallythree or four 1 min treatments - oxide removal, con-firmed by visual inspection, is rapid.

Continued exposure beyond this stage results in de-creased weight loss.

A plot of weight change vs immersion time in bothstages (Fig. 1) gives two intersecting straight lines whichcan be assumed to represent the best value for weightloss from scale removal (ASTM Standard PracticeGl-81).

Fig. 1.Weight change of heat resistant alloy samples during cathodicdescaling. Data are for RA 330 oxidized at 1095 C (2000 F),RA 333 oxidized at 980 C (1800 F), and RA 253MA oxidized at980 C (1800 F). Point of intersection of each pair of lines repre-sents weight of adherent oxide produced during oxidationtest.

Derived Data - This procedure appears to give satisfac-tory weight loss data. The ability to remove adherentsurface oxide after extended elevated temperatureexposure increases the amount of information availablefrom a single sample. In addition to finding an oxidationrate in terms of surface recession, you also obtain:

1. Total weight gain, reflecting the amount of oxygenreacting with the metal (equal to the final weight in cup

minus the starting weight in cup).2. Total weight loss, reflecting the amount of metal oxi-dized (original specimen weight minus cathodicallydescaled weight).

3. Total oxide produced (1 plus 2, weight gain plus loss).

4. Weight of adherent oxide (weight change uponcathodic descaling).

5. Adherency factor, or relative amount of adherentoxide to total oxide (4 divided by 3).

6. Average weight ratio of oxygen ions to metal ions inthe oxide(s) (1 divided by 2).

Additional weight change measurements are needed to

account for both total weight gain and total weight lossof the same oxidation test specimen. One specimenexposed for 25 cycles and cathodically descaled is typi-cally weighed 36 times to the nearest 0.1 mg.

Benefits - An advantage of determining weight loss isthe ability to account for the reduced dimensions ofheat resistant alloys in service. Also, weight gain data donot permit valid comparisons between alloys when theiroxides differ.

Regarding the latter, consider the case of two Fe-Cr-Niheat resistant alloys - Ra 330 and Incoloy alloy DS -having similar nickel and chromium contents but differ-ent silicon contents. Weight gain data indicate that thealloys have similar oxidation resistance. On the other

hand, metal loss data favor the Incoloy alloy with its2.45 % Si.

Gain Loss

Alloy g/cm2

mils (m)

RA 330 (19Cr-36Ni-1.4Si) 0.009 0.91 (23)

Incoloy DS (17Cr-35Ni-2.45Si) 0.0093 0.72 (18)

Silicon may be present in the Alloy DS scale, helping toinhibit oxidation. (Data are for oxidation after 330 h at1150 C (2100 F).)

Another illustration of the same point is a comparisonbetween two nickel base heat resistant alloys, one of

which contains aluminum. Aluminum (or silicon) is pre-sent in the scale and affects the relative weight of oxy-gen ions to metal ions. Metal loss calculated from theweight loss of descaled samples eliminates this effectand is thus an effective way to express oxidation data.

However, metal loss isn't practical to use when a largenumber of samples are involved - 25 cycles of eightalloys in one 500 h run would involve 200 specimens. Onthe other hand, weight gain measurements every 20 hare not hard to perform routinely, and do provide usefuldata relating oxidation to exposure time. Both of thesecriteria are used in the data that follow.


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Observations onOxidation UsingWeight Gain DataThe alloys included in this phase of the test program arelisted in Table I. All but one are face centered cubic andcontain chromium, nickel, and iron. Several are modifiedwith strengthening elements or elements that enhance.oxidation resistance.

Four of the alloys - RA 253MA, type 309, type 310, andRA 330 - were tested for 20 cycles, each lasting 20 h at980 C (1800 F). All four exhibit parabolic oxidation (seedata for RA 253MA plotted in Fig. 2).

Parabolic oxidation rate constants are indicated fromthe slope of the weight gain squared vs time graph(Fig. 2, bottom). Figure 2 indicates that oxidation testingof one sample results in reproducible data over anextended exposure period.

Computer derived values for weight gain vs time are

shown in Table II.

To express these data in terms of metal loss, actual lossdetermined at 400 h is multiplied by the ratio of weightgain at 1000 h to that at 400 h. (We've assumed that thescaling mechanism is the same at both 400 and 1000 h.)

Metal loss data for 1000 h are given in the far rightcolumn of Table II.

Six alloys - including the same four exposed at 980 C(1800 F) - were exposed for 25 cycles at 1095 C(2000 F). A set of eight specimens was completed byincluding a lower chromium heat of 309 and a higherchromium heat of RA 330. Both weight gain and metalloss values are included in the tabulation of oxidation

data (Table III). Five hundred hour data for weight gainare the best values derived using linear regression ana-lysis data out to 500 h, and 1000 h weight gain values arecomputer extrapolations.

Five of the alloys exposed at 1095 C (2000 F) for 500 hwere included in a cyclic exposure test at 1150 C(2100 F) for 500 h. Two alloys not previously tested wereincluded to complete a set of eight specimens. Oxida-tion data are given in Table IV. Most of these materialsdisplayed unstable oxidation curves and no effort hasbeen made to extrapolate data from 500 to 1000 h. Thesingle exception is RA 333 tested to 620 h. This is theonly alloy exhibiting parabolic oxidation during cyclic

exposure at 1150 C (2100 F).

Fig. 2.At top is a plot of weight gain vs oxidation time for RA 253MAoxidized at 980 C (1800 F). This alloy exhibits parabolic oxida-tion behavior as demonstrated by the straight line plot ofweight gain squared vs time at bottom.

Table I: Compositions of Alloys in Cyclic Oxidation Test Program

Alloy Com osition %1

(UNS No.) C Si Mn Ni Cr W Mo Co Other

RA 253MA 0.09 1.5 0.7 10.8 20.7 - - - 0.05Ce,

(S30815) 0.18N

Type 309 0.06 0.4 1.8 13.1 23.5 - - - -

(S30900) 0.05 0.5 1.7 13.1 22.3 - - - -

Type 310 0.07 0.8 1.5 19.4 24.5 - - - -

(S31000)

RA330 0.06 1.3 1.5 35.0 18.2 - - - -

(N08330) 0.07 1.4 1.7 36.4 19.0 - - - -

RA333 0.06 1.0 1.4 45.9 24.9 2.8 2.7 2.9 -

(N06333)

Type 446 0.09 0.7 1.3 - 26.0 - - - 0.18N

(S44600)

Inconel 6012 - - - (60) (23) - - - 1.3AI

(N06601)

Incoloy 800 0.08 0.3 0.9 30.8 20.8 - - - -

(N08800)

1. Balance iron. 2. Nominal composition.


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Table II: Oxidation at 980 C (1800 F)

Allo Coefficient of Wei ht Gain, /cm2

Metal Loss

UNS No. Determination1

400h 500 h2

1000 h2

mils (m)/1000 h

RA 253MA 0.994 0.0017 0.0018 0.0026 0.31 7.9S30815

T e 309 0.997 0.0024 0.0027 0.0038 0.39 9.9

(S30900)

Type 310 0.994 0.0021 0.0023 0.0033 0.37 (9.4)

S31000

RA 330 0.984 0.0024 0.0027 0.0037 0.44 (11.2

N08330

1. From regression analysis 2 Extrapolated.

Table III: Oxidation at 1095 C (2000 F)

Allo Wei ht Gain, /cm2

Metal Loss

(UNS No.) % Ni % Cr 500 h 1000 h mils (m)/1000 h

RA 253MA 10.8 20.7 0.0038 0.0065 0.60 15(S30815)

Type 309 13.1 23.5 0.0077 0.0131 1.50 (38)

S30900 13.1 22.3 >0.03 - -

T e 310 19.4 24.5 0.0096 0.0135 1.45 37

(S31000)

RA 330 35.0 18.2 0.0098 0.0179 1.77 (45)

N08330 36.4 19.0 0.0078 0.0109 1.09 28

RA 333 45.9 24.9 0.0066 0.0091 1.01 26

(N06333)

Inconel 601 60 23 0.0053 0.0073 0.51 13

(N06601)

Table IV: Oxidation at 1150 C (2100 F)

Allo Wei ht Gain, Metal Loss

(UNS No.) % Ni % Cr g/cm2/500 h Mils (m)/500 h

RA 253MA 10.8 20.7 0.0884 10.5 (267)

(S30815)

Type 309 13.1 23.5 0.0596 7.47 (215)

(S30900)

Type 310 19.4 24.5 0.0561 7.07 (180)

(S31000)

RA 330 35.0 18.2 0.0711 9.14 (232)

(N08330) 36.4 19.0 0.0573 7.90 (201)

RA 3331 45.9 24.9 0.0100 1.23 (31)

(N06333)

Type 446 - 26.0 0.0368 4.60 (117)

(S44600)

Incoloy 800 30.8 20.8 0.1384 18.0 (457)

(N08800)1. RA 333 exhibited parabolic oxidation. Data for 1000 h (extrapolated) are:weight gain, 0.014 g/cm

2; metal loss, 1.73 mils (44 m).

Deviations fromParabolic Oxidation

Behavior NotedOxidation data - weight gain for each 20 h exposure -indicate that most of the alloys tested deviate fromparabolic oxidation behavior under conditions of cyclicexposure at 1150 C (2100 F). The shape of the oxidationcurve varies from one alloy to another.

Lower nickel alloys including RA 253MA and type 309display marked instabilities, typically followed by a re-covery period and then another instability. Inspection ofthe spalled oxide reveals that two different oxides areproduced. Initial stages of oxidation result in a fine typethat tends to dust the walls of the porcelain cup. "Break-away oxidation" is characterized by heavy flakes ofspalled oxide. In periods of recovery, the fine type again

predominates.

The presence of different oxides has been confirmed byX-ray diffraction studies of five alloys exposed at 1150 C(2100 F). In addition, breakaway oxidation of types 304and 446 stainless steels has been described in the lite-rature.


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Examples - In the case of RA 253MA (Fig. 3, top), theperiods preceding and following breakaway oxidationhave been found to be linear by regression analysis.

Type 309 (Fig. 3, bottom) displays an initial parabolicperiod of 200 h, a period of rapid oxidation that in-creases less abruptly than that of RA 253MA, and,finally, alternating short periods of rapid oxidation andrecovery.

Type 446 displays five periods of alternating increasingand decreasing oxidation rate. However, the overalleffect is slight, and linear regression analysis of all datasets indicates a close fit to linear oxidation.

RA 330 (18Cr-35Ni) exhibits alternating breakaway andrecovery in the first 200 h, followed by an extendedlinear segment. RA 333 exhibits parabolic oxidation outto 620 h.

The occurrence and extent of instabilities are probablynot reproducible from one specimen to another andprohibit extrapolation of data to exposure times beyond500 h for cyclic oxidation at 1150 C (2100 F).

It is reasonable to assume that unstable oxidation of analloy after exposure at 1150 C (2100 F) for several hun-dred hours will be eventually reflected in cyclic tests at1095 C (2000 F).

At this time, cyclic oxidation has not been performedabove 1150 C (2100 F), and the ability to extrapolatedata beyond 500 h is uncertain. Possibly, oxidationcurves for higher temperatures such as 1205 C (2200 F)wilt indicate a breakaway oxidation mode.

Fig. 3.Examples of breakaway oxidation behavior in alloys testedunder cyclic conditions at 1150 C (2100 F). Top, RA 253MA;bottom, type 309 stainless steel.

The cyclic oxidation data presented here have beensupplemented with isothermal oxidation data at 980,1095,1150, and 1205 C (1800, 2000, 2100, and 2200 F).These data will be discussed in Part II of this article. Therelationship between weight gain and weight loss willalso be addressed in terms of the oxides that developon Fe-Cr-Ni alloys.

References1. Oxidation of Metals and Alloys, by 0. Kubaschewski

and B.E. Hopkins: 2nd Ed, Butterworth Press 1962.2. Gas Corrosion of Metals, by S. Mrowec and T.

Werber: National Bureau of Standards and NationalScience Foundation, Washington, D.C., 1978.

3. "Resistance of Iron-Nickel-Chromium Alloys to Cor-rosion in Air at 1600 to 2200 F," by A. deS. Brasunas,J.T. Gow, and O.E. Harder: Proceedings ASTM, Vol46, 1946.

Part IIThe oxidation resistance of Fe-Cr-Ni alloys as deter-mined by cyclic exposure tests in still air was reported inPart I of this two part article.

Summary - Weight gain data are convenient for deter-mining oxidation kinetics during cyclic exposure. Ex-posing a sample for twenty-five, 20 h cycles provides an

oxidation vs time relationship that may be useful forestimating longer time behavior. Extrapolation to timesas long as 1000 h, for example, is valid for temperaturesin which the oxidation kinetic mode has been estab-lished, and does not deviate for the exposure times ofinterest.

Part I also pointed out that parabolic oxidation is gen-erally observed at temperatures of 980 C (1800 F) and1095 C (2000 F) when samples are tested for 500 h.Here, the oxidation vs time relationship is given by:

(W/A)2

= kp t + C,where kp is the rate constant, t is oxidation exposure

time, and W/A is the weight gain per unit area. Devia-

tions from parabolic oxidation are common at 1150 C(2100 F) and are assumed to occur at somewhat lowertemperatures for extended exposures.

Weight loss, expressed in terms of surface recession,can be put to use in selecting alloys for elevated tem-perature service. Each oxidized sample is cathodicallytreated to remove the adherent scale and surfacerecession is then measured.

Cyclic exposure at 1150 C (2100 F) results in markedvariation within the composition range of alloys tested.Heat resistant alloys containing up to 20 % Ni exhibitoxidation resistance in direct proportion to the amountof chromium present. A similar dependency on chro-mium is observed for higher nickel alloys such as RA

330 and RA 333. Data are plotted in Fig. 1. Differentheats of a given alloy may exhibit very different oxi-dation rates depending upon the actual amounts ofchromium within the specification range. For example,oxidation rate of AISI type 309 may vary by a factor offour when chromium is increased from 22 to 23+%.


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Oxidation, mils (m) per 500 h Table II: Isothermal Oxidation Data

Fig. 1.Effect of chromium content on oxidation resistance of Fe-Cr-

Ni alloys tested under cyclic conditions at 1150 C (2100 F).Alloys on upper line contain no more than 20 % Ni; those alongthe bottom line have more than 34 % Ni.

Isothermal - In this concluding part of the article, iso-thermal (noncyclic) oxidation rate data for four alloyscontaining from 11 to 46 % Ni (see Table I) are reported.

Data were developed using the procedures describedin Part I. Four samples of each alloy were exposed fortimes ranging from 100 to 600 h to temperatures of 980,1095,1150, and 1205 C (1800, 2000, 2100, and 2200 F).Each sample for a given temperature-time combinationwas heated to the test temperature and cooled afterexposure in its porcelain cup. Total weight gain and

metal loss data were obtained, and the best relationshipbetween oxidation and time was determined by re-gression analysis.

Isothermal OxidationTests: DataDevelopment/AnalysisIsothermal oxidation rates are not only lower than cyclicoxidation rates, but do not deviate from parabolic be-

havior at 1150 and 1205 C (2100 and 2200 F).

Extrapolated data for oxidation after 1000 and 3000 h ateach exposure temperature are given in Table II. Notethat surface recession for all four alloys at 980 C

Weight Gain, Metal Loss,

Alloy g/cm2

mils (m)

(UNS No.) 1000 h 3000 h 1000 h 3000 h

980 C (1800 F)

RA 253MA 0.0018 0.0031 0.23 (5.8) 0.37 (9.4)

(S30815)

Type 310 0.0026 0,0044 0.30 (7.6) 0.48 (12.2)

(S31000)

RA 330 0.0033 0.0057 0.40 (10.2) 0.66 (16.8)

(N08330)

RA 333 0.0037 0.0062 0.43 (10.9) 0.71 (18.0)

(N06333)

1095 C (2000 F)

RA 253MA 0.0052 0.0134 0.37 (9.4) 0.88 (22.4)

(S30815)

Type 310 0.0091 0.0153 0.95 (24.1) 1.52 (38.6)

(S31000)

RA 330 0.0125 0.0218 1.00 (25.4) 1.69 (42.9)

(N08330)

RA 333 0.0059 0.0098 0.66 (16.8) 1.09 (27.7)

(N06333)1150 C (2100 F)

RA 253MA 0.0130 0.0359 1.17 (29.7) 3.14 (79.8)

(S30815)

Type 310 0.0140 0.0242 1.41 (35.8) 2.34 (59.4)

(S31000)

RA 330 0.0160 0.0275 1.94 (49.3) 3.39 (86.1)

(N08330)

RA 333 0.0101 0.0172 0.96 (24.4) 1.62 (41.1)

(N06333)

1205 C (2200 F)

RA 253MA 0.2179 0.0676 26 (660) 82 (2083)

(S30815)

Type 310 0.0175 0.0295 1.77 (45) 2.85 (72.4)

(S31000)

RA 330 0.0238 0.0407 2.26 (57.4) 3.85 (97.8)

(N08330)

RA 333 0.0126 0.0208 1.3 (33.3) 2.23 (56.6)

(N06333)

(1800 F) is well below 1 mil (25 m) at 3000 h. At 1095 C(2000 F) recession increases to more than 1 mil (25 m)at 3000 h for all but RA 253MA. Isothermal oxidation at1150 C (2100 F) results in increases to more than 3 mils(75 m) for 253MA and RA 330. The higher chromiumalloys AISI type 310 and RA 333 exhibit the best oxida-tion resistance with values of 2.3 and 1.5 mils (53 and

38 m) respectively. All of the alloys but 253MA, whichcontains the least chromium and nickel, retain usefuloxidation resistance at 1205 C (2200 F). Alloy 333 hasan oxidation rate of 2.2 mils (56 m) in 3000 h at thistemperature, for example.

Table I: Compositions of Alloys in Isothermal Oxidation Test Program

Alloy Composition, %1

(UNS No.) C Si Mn Ni Cr W Mo Co Other

RA 253MA 0.09 1.5 0.7 10.8 20.7 - - - 0.05Ce,

(S30815) 0.18N

Type 310 0.07 0.8 1.5 19.4 24.5 - - - -

(S31000)

RA 330 0.06 1.3 1.5 35.0 18.2 - - - -

(N08330)

RA 333 0.06 1.0 1.4 45.9 24.9 2.8 2.7 2.9 -

(N06333)

1. Balance Iron


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The addition of cerium to 253MA improves its oxidationresistance at the lower test temperatures. At 1150 C(2100 F) cerium is less beneficial, and at 1205 C (2200 F)oxidation is probably controlled more by base composi-tion than by the rare earth. Alloy 333 has useful oxida-tion resistance to at least 1205 C (2200 F). Althoughsuperior to other tested alloys at this temperature, 333exhibits slightly higher oxidation at the lowest tempera-ture, 980 C (1800 F). This behavior for highly oxidation

resistant alloys has been previously observed.(1

)Plots of the effect of oxidation temperature on metalloss are shown in Fig. 2. Both cyclic and isothermal oxi-dation data are given. Note that alloys 310 and 330 exhi-bit unusual behavior: isothermal oxidation (lower plot)does not increase as rapidly with temperature above1150 C (2100 F) as it does between 980 and 1095 C(1800 and 2000 F). Oxidation for cyclic exposure above1095 C (2000 F) increases much more rapidly with tem-perature.

Alloy 253MA, on the other hand, is less sensitive to tem-perature change than any of the alloys between 980and 1095 C (1800 and 2000 F) but is the most sensitiveof the four at temperatures above 1095 C (2000 F). Alloy

333 tested isothermally shows the same effect of tem-perature throughout the test range.

In more theoretical treatments of the effect of tempera-ture on oxidation, it is common to plot the parabolic rateconstant vs the reciprocal of absolute temperature. Theslope of such a plot is the activation energy, which willbe discussed later. The data in Fig. 2 are not meant to beused to determine activation energy, but simply to illus-trate change in oxidation rate with temperature. Theplots are useful, however, for interpolation - extrapola-tion of 50 C (90 F) is a reasonable extension beyond thetest temperature range.

As indicated in Fig. 2, cyclic exposure increases oxida-

tion for all alloys at all test temperatures. The differenceis most pronounced at temperatures above 1095 C(2000 F). Of the four alloys, 333 is the most oxidationresistant under cyclic exposure at 1150 C (2100 F).

Oxides and Their Effecton Weight Gainand Metal LossThe primary reason for gathering total weight gain(adherent plus spalled oxide) data is to develop infor-mation regarding oxidation kinetics, defined as the rela-tionship among oxidation, time, and temperature. Metalloss values express oxidation in terms of surface reces-sion and are obtained by converting weight loss tovolume loss.

These data may also be used to calculate the relation-ship between total weight gain and metal loss. The ratioof the relative amount of oxygen absorbed to theamount of metal oxidized should agree with the weightrelationships of oxygen to metal in the oxides present(Table III). This supposition has been tested by oxidation

testing of pure metals using the same procedures andequipment. Certain pure metals form only one oxide inair. Assuming that these oxides do not deviate from stoi-chiometry and form only at the surface, then experi-mental values should agree with the relative atomicweights of the oxide.

Table III: Oxygen to Metal Weight Ratios

Cu2O - Copper oxidizes in air at ambient pressure toCu2O. The atomic weight ratio of oxygen to copper is16.00/2(63.54), or 0.1259. Six samples of electrolytictough pitch copper (99.9 % Cu) were prepared for oxi-dation testing as described in Part I and exposed at tem-peratures of 900 C (1652 F) or 1000 C (1832 F) for

various times ranging from 20 to 120 min. The ratiosobtained from these tests are close to the "ideal":

0.1271, 0.1276, 0.1259, 0.1277, 0.1261, and 0.1274.

NiO - Nickel forms NiO when oxidized in air above 300 C(570 F). The atomic weight ratio of oxygen to nickel is0.273. The ratios obtained by oxidizing commerciallypure nickel (99.6 % Ni) at 850 C (1560 F) and 1150 C(1920 F) for times ranging from 2 to 142 h are: 0.271,0.260, 0.279, and 0.271. NiO, unlike Cu2O, is tightlyadherent, and its accurate removal is more difficult.Nevertheless, the average oxygen to metal ratio agreeswith the stoichiometric ratio within 1 %.

Alloys' Oxides - The ratio of oxygen to metal, obtainedfrom total weight gain and metal loss test data, for eachalloy oxidation specimen has also been determined andfound to correlate well with our knowledge of alloyoxidation.

Iron-chromium-nickel alloys derive their useful oxida-tion resistance from the formation of a protective oxide,

Cr2O3. The occurrence of breakaway oxidation is inter-

preted in terms of changes to this oxide. In addition, themodification of the oxide by Fe2O3 and NiO is a majorcontributor to the excellent oxidation resistance of aus-tenitic Fe-Cr-Ni alloys.(

2)

Assuming that Cr2O3 forms on the four alloys testedhere, the ratio of weight gain to metal loss is 0.462. (Thismay be reduced slightly by substitution of iron for chro-

mium in the oxide M2O3.)The ratio of weight gain to metal loss is given in Table IVfor isothermal test samples. At test temperatures of1095 C (2000 F) and above the ratios for 310, 330, and333 are close to that observed for oxygen to chromiumin Cr2O3 (0.462): nine of ten fall in the 0.45 to 0.50 range.At 980 C (1800 F) the ratios for these three alloys arelower, falling between 0.38 and 0.41. The reason for thisis not known.

Three of the alloys in Table IV exhibit ratios well above0.46 at 1095 C (2000 F) or 1150 C (2100 F). All three aremodified with silicon which forms SiO2 (ratio of 1.14) inFe-Cr alloys and, apparently, also in Fe-Cr-Ni alloys.Inconel alloy 601 (23Cr-60Ni-1.3AI) has a ratio of 0.63,reflecting the presence of AI2O3 (ratio of 0.89).

The weight gain to metal loss ratio is not suggested as away to identify oxides. X-ray diffraction is a far bettertechnique. The relationship is, however, useful for esti-mating metal loss from weight gain. Because, as shown

Oxide Ratio Occurrence

Cr2O3 0.462 Forms a protective oxide on Fe-Cr,Fe-Cr-Ni, and Ni-Cr alloys.

Fe2O3 0.430 May form a series of solid solutions with

Cr2O3 on alloys containing sufficient iron.

Fe3O4 0.382 Less protective, forming on Fe-Cr alloys ofmarginal oxidation resistance.

May dissolve some Cr3O4.

SiO2 1.14 Forms a thin layer on heat resistant Fe-Cr-

Ni alloys containing silicon. Beneficial.

Al2O3 0.89 Forms a protective oxide on Fe-Cr-Ni and

Cr-Ni alloys containing aluminum.


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Fig. 2.Effect of temperature on oxidation of heat resistant alloys.Alloy 253MA exhibits one type of behavior; 310 and 330, asecond type; and 333, a third. These data indicate that cyclicexposure increases oxidation for all alloys at all test tempera-tures.

Table IV: Weight Gain to Metal Loss Ratios

Test Temperature, C (F)

980 1095 1150 1205

Alloy (UNS No.) (1800) (2000) (2100) (2200)

RA 253MA (S30815) 0.35 0.64 0.50 0.41

Type 310 (S31000) 0.41 0.46 0.46 0.45

RA 330 (N08330) 0.38 0.59 0.49 0.49

RA 333 (N06333) 0.40 0.46 0.50 0.48

Incoloy DS - - 0.65 -

Inconel 601 (N06601) - - 0.63 -


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above, silicon and aluminum change the relationshipbetween these criteria, weight loss is the preferred wayof expressing oxidation rate. (A comparison of weightgain and metal loss was made in Part I.)

Oxide Adherency - We've assumed that the sum ofweight gain and metal loss is equal to the total amountof oxide. The amount of adherent oxide is the differencein weight after cathodic descaling. The ratio of adherent

to total oxide (in percent) is a useful measure of the oxide's"adherency", and is a more satisfactory descrip-tion, than "free-flaking", "adherent", or "semiadherent".

Adherency values have been determined for each oxi-dation test specimen (both cyclic and isothermal tests).Values range from 2 to 100 % adherent oxide. Cyclicexposure at 1150 C (2100 F) results in the lowest values(greatest spalling). Isothermal exposure at 980 C(1800 F) results in the highest values.

The cerium in 253MA improves oxide adherency. After500 h of cyclic exposure at 1095 C (2000 F) this alloyretains 82 % of its total oxide compared with values of40 to 50 % for unmodified Fe-Cr-Ni alloys.

It is not surprising that good oxide adhesion and oxida-tion resistance go hand in hand. The relationship isshown in Fig. 3 for 32 cyclic tests of ten alloys at threetemperatures.

Activation - This final note relates to the Arrheniusequation, a relationship commonly referenced in oxida-tion studies. It relates reaction rate (oxidation rate in thiscase) to temperature:

k = Ae-(Q/RT)

where k is the rate constant, A is a constant, R is the uni-versal gas constant (1.987 cal/K/mol [8.33 J/K/mol]),T is the absolute temperature (K), and Q is the activa-tion energy in cal/mol or J/mol.

Activation energy cannot be determined if the kineticmode changes with temperature; for example, fromparabolic to linear. In addition, it cannot be determinedif the scaling mechanism changes over the range of testtemperatures.

Several values for activation energy have been deter-mined with these limitations in mind (Table V). Thedeviations from parabolic oxidation during cyclic expo-sure at 1150 C (2100 F) favor the use of isothermal oxi-dation which is parabolic for three of four alloys.

Since activation energy is the slope of the straight lineplot of rate constant vs temperature, it tells us how fastthe rate constant is changing with temperature. Alloys

having a similar oxidation resistance at a given tem-perature, however, may have quite different activationenergies.

Fig. 3.Relationship between adherency (ratio of adherent to totaloxide) and oxidation resistance. Conditions that favor ad-herency also tend to promote oxidation resistance. Ten alloysand three test temperatures are represented.

Table V: Activation Energy for Isothermal OxidationBased on Parabolic Weight Gain

Alloy Temperature Range, Activation Energy

(UNS No.) C (F) kcal/mol (kJ/mol)

RA 253MA 980-1095 (1800-2000) 31 (130)

(S30815) 1095-1205 (2000-2200) 146 (612)

Type 310 980-1095 (1800-2000) 78 (327)2

(S31000)

RA 330 980-1150 (1800-2100) 68 (285)

(N08330)

RA 333 980-1205 (1800-2200) 39 (163)

(N06333)

1. Rate constant based on linear oxidation.2. Activation energy in cyclic oxidation is 88 kcal/mol (369 kJ/mol).

References1. "Resistance of Iron-Nickel-Chromium Alloys to Cor-

rosion in Air at 1600 to 2200 F," by A. deS. Brasunas,J.T. Gow, and O.E. Harder: Proceedings ASTM, Vol46, 1946.

2. Gas Corrosion of Metals, by S. Mrowec andT. Werber: National Bureau of Standards and Na-tional Science Foundation, Washington, D.C., 1978.

The articles were originally published in the April (Part I)and May (Part II) issues of Metal Progress Magazine1985. They are reprinted here with the kind permissionof the Author.


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Acom85_2 Oxidation Kinetics of Heat Resistant Alloys Part I+II

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