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The oxidation resistance of nitrided mild steel
Corrosion Science.Vol. 18. pp. 555 to 571 © Pergamon Press Ltd. 1978. Printed in Great Britain
THE
OXIDATION
RESISTANCE MILD STEEL*
0010-938X/78/0601-0555 $02.00/0
OF NITRIDED
A. HENDRY Wolfson Research Group for High-Strength Materials, Crystallography Laboratory, The University of Newcastle-upon-Tyne, Newcastle-upon-Tyne 1, England Abstract--The oxidation resistance of mild steel in a simulated boiler flue gas atmosphere has been shown in laboratory tests to be significantly enhanced by a surface nitriding treatment which forms a controllable fine-grained martensitic layer without precipitation of iron nitrides. A mechanism is suggested by which improved protection results from the refinement of oxide grain size and the condensation of cation vacancies in the nitrided layer instead of at the metal/oxide interface. A parallel is drawn with the development of improved oxidation resistance in superailoys by sub-surface dispersion of incoherent stable oxide particles.
INTRODUCTION
THE MOSTwidely used alloy for furnace wall evaporator tubes in conventional fossil fuel power-generating plant is mild steel. Fireside corrosion (oxidation) of this alloy by the gaseous products of combustion can result in tube failures with a consequent costly loss of generating time. Power stations experiencing high rates of corrosion are generally those burning aggressive fuels, i.e. coals containing high levels of total chlorine, and there is also a body of evidence which indicates that a direct correlation exists between the concentration of carbon monoxide in the combustion gases adjacent to the furnace wall and the local corrosion rate. The positions of such patches of severe local corrosion are found to be relatively stable and are detected in successive N.D.T. surveys of tube-wall thickness from year to year. Attempts to obtain a low cost solution to the problem of mild steel corrosion in service have been largely concerned with coatings. These include sprayed ceramics such as silica and silicon carbide and metals based on either chromium or aluminium which, when oxidized, form protective oxide layers. The disadvantage of such coatings is their susceptibility to spalling at the coating/metal interface as a result of the difference in thermal expansion coefficients between tube metal and coating. Degradation due to mechanical disruption during handling and fabrication is also a major cause of breakdown of coatings. The inherent disadvantage of mild steel in terms of corrosion (oxidation resistance) lies in the non-protective nature of the oxide which spalls from the metal as a result of relatively minor changes in environmental conditions, thus exposing fresh metal to attack by gaseous oxidants. The coherency of the oxide layer on ferritie alloys, in the absence of alloying elements such as chromium, can only be improved by moditication of the oxide properties. The physical and chemical properties of the oxide are influenced by the mierostructure of the underlying metal substrate, and thus the oxida*Manuscript received 28 July 1977; in amended form 18 November 1977. 555
556
A. HENDRY
tion resistance of mild steel may be improved by a suitable surface treatment which leaves the microstructure and properties of the bulk material unaffected. Internal nitriding of ferritic iron alloys has been widely studied by Jack e t al. 1 and a significant increase in hardness and strength is obtained. By means of gas/metal equilibration nitrogen is introduced into the iron matrix without formation of iron nitride on the surface of the alloy. The nitrogen can then react at nitriding temperature with other solutes in the alloy to form pre-precipitation clusters, stoichiometric precipitates or nitrogen austenite depending on nitriding potential, temperature and alloy element concentration. The depth of penetration of nitrogen, and hence the thickness of the hardened layer, is dictated by Fick's Laws of diffusion and the thickness can be controlled by suitable choice of nitriding potential and temperature. A nitrided layer on the surface of a boiler tube would give increased strength to the tube and be condusive to formation of an adherent oxide scale. Nitriding produces a surface metal layer containing dispersed sites for oxide nucleation either as precipitates or fine metal grains which result in the nucleation of a fine grained and adherent oxide scale. The present work reports a novel method of achieving improved oxidation resistance of mild steel by means of a nitrided surface layer. The layer forms an integral part of the metal surface in, for example, a boiler tube and has high hardness and strength, sufficient to survive any abrasion or deformation during fabrication. Observations of the changes in microstructure of the nitrided layer and oxide during oxidation are reported and their implications discussed. A mechanism is suggested for the improvement in oxidation resistance which has wider implications in the field of oxide integrity on metal surfaces. EXPERIMENTAL
METHOD
The principles o f internal nitriding in NH~ : Hs gas mixtures are well-established and are based on the work o f Lehrer 9 whose results are summariscd in Fig. I. By a suitable choice of temperature and nitriding potential it is possible to nitrid¢ iron to form a surface layer which is completely ferritic or austenitic at the nitriding temperature without forming any of the nitrides of iron on the specimen
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=C
L e h r e r d i a g r a m f o r solubility o f n i t r o g e n in iron.
The oxidation resistance of nitrided mild steel
557
surface. The object of the present work was to form a hard and fine-grained martensitic surface layer, the depth of which can be controlled by the length of time at nitriding temperature as dictated by Fick's Laws of diffusion. The nitriding conditions were chosen to give an austenitic surface layer at nitriding temperatures which would transform to martensite on cooling to room temperature with some retained austenite. The amount of retained austenite is a function of cooling rate, and in the present case a normalizing cooling rate gave a ratio of martensite (ct') to austenite (y) of 1 : 2. The ratio o f n ' : "t is determined by X-ray diffractometry of the nitrided surface. The composition of the mild steel used in these experiments is given in Table 1, and the nitrogen concentration in the surface layer is 0.8 w/o.
TABLE I.
COMPOSITION OF THE MILD STEEL USED IN THE PRESENT EXPERIMENTS
C
Si
Mn
Ni
Cu
(w/o) 0.23
(w/o) 0.15
(w/o) 0.45
(w/o) 0.07
(w/o) 0.07
The experimental alloys in the form of discs 12.0 mm in diameter and 3.00 mm thick were nitrided in a vertical tube furnace at 700°C in 5NH3 : 95H, for 2 h and cooled in the flowing gas mixture after removal from the hot-zone to the cold end of the furnace. This treatment produces a martensite/ austenite surface layer approx. 100 v.m thick. The nitrided alloys were cleaned and degreased before oxidation in a Stanton-Redcroft decimilligram thermobalance equipped with a linear variable temperature controller and programmer. The oxidizing atmosphere was established by additions of oxygen or carbon monoxide to a carrier gas mixture of composition Ns + 15 v/o COs + 0.3 v/o SO~. All gases were purified and dried before use and a total gas flow rate of 300 ml rain -: (4 mm s -t) maintained during all experiments. The weight gain of nitrided specimens was determined as a function of time under the following conditions: (i) Isothermal oxidation in Ns + 15 v/o CO~ + 0.3 v/o SOs + 1.0 v/o Os at 430 and 600°C. (ii) Oxidation in Ns + 15 v/o COs + 0.3 v/o SOs + 1.0 v/o Os under thermal cycling conditions between 20(O30°C and 200-600°C. (iii) Oxidation in a cyclic oxygen potential of Ns + 15 v/o CO~ + 0.3 v/o SOs + 1.0 v/o Os and Ns + 15 v/o CO2 + 0.3 v/o SOs + 1.0 v/o CO at 430°C. The thermal cycles were of the following form; one hour at upper hold temperature, heating and cooling rates of 8°C.min -x and 1 h at lower hold temperature. The oxygen potential cycles were varied discontinuously between "oxidizing" (1.0 v/o 02) and "reducing" (1.0 v/o CO). Assuming that equilibrium is established in the gas phase, these conditions correspond to the following chemical potentials of the oxidizing species: oxidizing; Po~ = 0.01 atm., Pa~ = 10-'Satm ao = 10-sv (relative to graphite) reducing; Po2 = 10-alatm, Pss > 1.0 atm ao = 0.6 (relative to graphite). The composition of the scales was determined by X-ray diffraction after removal from the specimen surface by stripping in an iodine methanol solution. Optical metallography and scanning electron microscopy were carried out on specimens mounted in Araldite resin and polished to 1 ~m with diamond paste on a vibratory polisher. Specimens for transmission electron microscopy were prepared from 0.125 mm mild steel sheet which had been through-nitrided at 700°C in 5NHs : 95H2, and were examined in a Philips E.M.300 microscope at 100 kV.
EXPERIMENTAL
RESULTS
AND
DISCUSSION
Isothermal oxidation T h e w e i g h t g a i n o f n i t r i d e d m i l d steel s p e c i m e n s d u r i n g o x i d a t i o n a t 430 a n d 600°C in s i m u l a t e d flue gas is s h o w n in Fig. 2 a n d is c o m p a r e d w i t h results f r o m m i l d
558
A. HENDRY
steel, 2.25Cr-lMo steel and 9Cr-lMo steel oxidized under the same conditions. The curves indicate the general trend of results rather than a specific set of experimental points since considerable scatter, indicated by the error bars in Fig. 2, is observed from different tests. The order of behaviour is typical however. With the exception of mild steel all of the alloys demonstrate behaviour typical of growth of a protective oxide layer.
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Weight gain curves for oxidation of steels in N= + ]5 v/o COs + 0.3 v/o SO= + 1 v/o O, at (a) 430°C, (b) 600°C.
Thermal cycling The oxidation behaviour of nitrided mild steel compared with other alloys under thermal cycling conditions is shown in Fig. 3. The results are plotted as a function of time at the upper hold temperature and have not been normalized to take account of weight gain during the temperature cycles. Such a procedure is at best only approximate, particularly in eases where scale rupture is prevalent.
Oxygen potential cycling A comparison of the oxidation rates of nitrided mild steel and other alloys was carried out under conditions of cyclic oxygen potential varied discontinuously between
The oxidation resistance of nitrided mild steel
559
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rided " steel
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Weight gain curves for oxidation of steels in N= -t- ]5 v/o COs -t- 0.3 v/o SO= + 1 v/o O, thermally cycled between (a) 200-430°C, (b) 200-600°C.
"oxidizing" and "reducing". The results are shown in Fig. 4. The scatter of results from the chromium-containing alloys is much greater in these tests than under any other conditions. This is believed to be due to cracking and rupture of the scale. The causes of cracking are twofold. First, the outer layer of haematite on the scale is reduced to magnetite during the reducing part of the cycle, in agreement with the calculated Fe/FesOUFe=Os equilibrium in the prevailing chemical potentials in the gas phase. The consequent decrease in volume causes rupture of the underlying scale. Secondly, the decrease in oxygen potential leads to a eorrespondinginereasein sulphur potential and an increase in the degree of sulphidation at the metal/scale interface which also results in scale rupture due to the higher specific volume of iron sulphide with respect to iron oxide. This behaviour is demonstrated in Fig. 5. A sample of mild steel and of nitrided mild steel were oxidized under "oxidizing" (1.0 v/o Oz) conditions for 600 h in order to form a thick scale. Oxygen potential cycling was then commenced. During the reducing part of the cycle the rate of oxidation of the untreated steel increases sharply as a result of scale rupture and increased access of oxidizing
560
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Nifrided mild steel
I 400
[ 600
Time,
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FIG. 4. W e i g h t g a i n curves f o r o x i d a t i o n o f steels at 4 3 0 ° C in a cyclic o x i d a t i o n potential.
gases. The increased rate of weight gain is maintained during the initial part of the oxidizing cycle until the cracked scale becomes healed or blocked with fresh oxide and the rate then decreases. The process is then repeated with the onset of the next reducing cycle. Nitrided mild steel samples are not affected by oxygen potential cycling indicating that the fracture stress of the scale has not been exceeded and no loss of coherency has occurred at the oxide/metal interface. The oxidescale retains its protective nature on the metal surface.
Kinetics of oxidation of nitrided mild steel Qualitative comparison of the results presented in Figs. 2-5 shows that nitrided mild steel and 2.25Cr-lMo steel behave in a similar manner under isothermal conditions at 430°C. The improved oxidation resistance of nitrided mild steel becomes apparent at 600°C and under the more severe conditions imposed by thermal cycling
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COs + 0.3 v/o SOs + 1 v/o O, for 600 h).
The oxidation resistance of nitrided mild steel
561
and oxygen potential cycling. The specific reaction rate constants (kp) given in Table 2 for isothermal oxidation of nitrided mild steel are slightly greater than those of Flatley and Robinson a for 2.25Cr-lMo steel at 430°C and approximately one order of magnitude lower at 600°C. The values given by Flatley and Birks 4 for the rate of oxidation of pure iron in Ar + 0.5 v/o SO2 are kp = 5.13 × 10-v g2cm-%-I at 900°C and k, = 5.0 × 10-s g2cm-4s-X at 600°C which is two orders of magnitude higher than kp (600°C) for nitrided mild steel shown in Table 2.
TABLE 2.
PARABOLIC REACTION RATE CONSTANTS (kp) FOR NITRIDED MILD ffI"EEL
Temp. (*C) 430 600 20(0-430 200-600
tu
Total time (h)
(11)
520 1300 788 858
520 1300 390 410
Parabolic rate constant kp (gScm-%-t) 2.76 2.47 5.0 1.95
X I0-xt × 10-to × 10-12 x 10-*o
No experimental values of k, during thermal cycling or oxygen potential cycling have been published, and only qualitative comparison of rates is therefore possible in Figs. 3 arid 4. In order to determine the oxidation kinetics of a specimen in which the nitrided layer is completely oxidized, and to examine the effect on subsequent oxidation of the underlying un-nitrided metal, a nitrided mild steel specimen with a nitrided layer thickness of 87.5 ~tm was oxidized isothermally at 600°C in N~ + 15 v/o CO~ + 0.3 v/o SO2 + I v/o O~ for 2100 h. The results are shown in Fig. 6 for a single specimen. From a number of experimental observations it can be assumed that under the conditions used 1 g.cm -2 of weight gain is approximately equivalent to 4 ~m of metal loss as oxide and sulphide. Oxidation of the entire 175 ~m nitrided layer (2 x 87.5 ~m layers) will therefore lead to aweight gain ot'43.75 g.cm -2. Inspection of Fig. 6 shows that this weight gain occurs at 1600 h and corresponds to the breakdown of parabolic oxidation kinetics and a transition (1500-1600 11)to a linear regime. bfter oxidation of the nitrided layer is complete, the rate of oxidation in the linear region of the curve is much lower than that of conventional mild steel. A weight gain of 22.5 g.cm -2 was recorded in the final 500 h of the present test--the linear region of Fig. 6---wjaereas the same weight gain was recorded in 40 h with conventional mild steel, Fig. 2(b). The values of Flatley and Birks 4 for linear rate constants during the initial stages of oxidation of iron in Ar + 0.5 v/o SO2 which, on extrapolation to 600°C, give kL = 6.6 × 10-6 g.em-2s -1 can be compared with the present value of kz = 1.2 × 10-6 g.cm-2s -1 for the final linear stage of Fig. 6. Indeed the value of Flatley and Birks parabolic rate constant for oxidation of iron at 600°C in Ar + 0.5 v/o SOz (k v = 5.0 × 10-6 g.2cm~s-1) gives a weight gain in 500 h which is an order of magnitude higher than that obtained in the final 500 h of the present experiment.
562
A. HENDRY
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F1G. 6. Weight gain curve for long-term oxidation of nitrided mild steel at 600°C in N2 + 15 v/o COs + 0.3 v[o SOs + 1 v/o 09. The mechanism of oxidation of nitrided mild steel The ability of an oxide layer to accommodate stresses during growth and hence remain attached coherently to the underlying metal surface, is primarily dependent on grain size and porosity within the layer. Grain size is dictated by the nucleation and growth processes by which the initial layer of oxide is formed on the metal surface, in particular by the number and distribution of nucleation sites for oxide formation. The extremely fine microstructure of a nitrided layer compared to that of the unnitrided bulk material (Fig. 7) should lead to a larger number of suitable sites for oxide nucleation and a consequently finer grain size in the scale than would be the case in the absence of the nitrided surface. These factors combined with the high degree of residual stress associated with the martensite transformation which results in an increased compressive stress on the oxide, will lead to improved adhesion of the scale to the metal surface. The microstructure of the nitrided layer on mild steel has been examined by transmission electron microscopy in order to determine the microstructural changes which occur as a result of exposure to elevated temperatures during nitriding and oxidation. Through-nitrided mild steel foils were sealed in an evacuated silica capsule and aged at 430°C for times up to 500 h. The microstructure of the as-nitrided a l l ~ consists of packets of twinned martensite needles surrounded by retained austenite (Fig. 8). The martensite packets are typically 1 Izm in width containing individual twin laths approximately 15 nm wide, and surrounded by austenitic grains containing a high density of dislocations adjacent to the martensite packets. On ageing at 430°C the structure transforms slowly and 7'-Fe4N is precipitated from the metastable austenite phase while the martensite plates coarsen. After 500 h at 430°C the structure is still relatively fine and the coarse lath martensite is surrounded by ferrite containing a fine, platelike dispersion of %,'-Fe4N of ca. 25 nm din (Fig. 9). The microhardness has correspondingly decreased from 600 to 400 VMN after 500 h, which is still significantly high for mild steel. The nitrided layer therefore retains a high hardness and finely divided microstructure after 500 h at 430°C and consequently will maintain
The oxidationresistanceof nitrided mild steel
563
good coherency with its oxide scale as no gross microstructural changes have occurred. The microstructure of the oxide scale on mild steel and nitrided mild steel are morphologically similar but show a distinct difference in mechanical behaviour. The oxide on nitrided specimens is strongly adherent whereas that on mild steel is spalled and laminated. The chemical composition of the scale as determined by S.E.M. is similar for both alloys and no significant variation in the relative proportions of oxide and sulphide have been detected. Figure 10 illustrates the difference in oxide adhesion between nitrided and un-nitrided mild steel. A magnetite/sulphide layer is present at the metal/scale interface on both specimens, but mild steel scales also conrain layers of sulphide within the scale thickness indicative of exfoliation during growth. These observations indicate that the same oxidation mechanism operates in both cases; the scale grows by outward diffusion of iron cations through the scale and inward diffusion of oxygen anions is negligible, although inward diffusion of sulphur anions occurs. This is in accord with the accepted mechanism of oxidation of iron in sulphurous gases) The structure of the oxide layer on a nitrided specimen oxidized at 600°C for 850 h is shown in Fig. 11 where the fine grain size of the innermost layer of oxide and sulphide compared to the larger grain size of the outer areas of the scale can be clearly seen. Since the scale grows by outward diffusion of iron cations the layer of oxide immediately adjacent to the metal surface continues in that position as the metal is consumed. Therefore in the situation where all of the nitrided layer is oxidized the essential protective element, the five grain size oxide layer, will still be present and continue to confer improved oxidation resistance on the alloy over that of conventional mild steel. This is confirmed by the final section of the test shown in Fig. 6. The fine grain size of the oxide layer formed on nitrided specimens is better able to accommodate growth stresses than a coarser grain size oxide. It has been shown by Douglass 6 that the mechanical strength of oxides increases with decreasing grain size. This is attributed to grain boundary creep processes which are sufficiently rapid at oxidizing temperature to relieve the internal stresses in the oxide by a HerringNabarro creep mechanism. Recent investigations of the mechanical properties of oxides grown in steam in service in power stations Manning and Metcalfe7 show that scales containing fine grain size oxide have higher fracture stresses and strains than other M,O4-type oxides of coarser grain size. Since oxidizing/reducing cycles have no effect on nitrided mild steel it cart be concluded that the fracture strain of the oxide is not exceeded by the volume decrease on reduction during the reducing half-cycle. The effect of grain size and creep processes on oxide fracture has also been reviewed by Stringer.' Regardless of the absolute strength of the oxide layer, the ultimate integrity of the oxide on the metal surface is governed by its weakest point, which is the scale/metal interface. Vacancy condensation at the interface to form voids is a primary cause of exfoliation during oxidation of mild steel. Vacancies arise as a result of oxide growth by outward diffusion of iron cations which is coupled with the inward migration of cation vacancies. A build up of vacancies occurs in the oxide adjacent to the metal/ oxide interface and a driving force exists for the condensation of vacancies on a suitable heterogeneous site. The interface is the most likely site for vacancy condensation, consequent void formation and subsequent exfoliation.
564
A. HENDRY
On nitrided alloys the vacancy condensation mechanism is apparently modified as a result of the coherency of the metal/oxide interface such that voids are not formed at the interface but appear at some depth into the metal (Fig. 12). The exact mechanism by which this occurs cannot be unequivocally demonstrated by the techniques presently available, but is believed to result from stress relaxation in the surface layer at the oxidizing temperature giving rise to triple point cracks. These cracks immediately below the surface of the metal can then act as vacancy sinks, thus eliminating vacancy condensation at the oxide/metal interface. Vacancy condensation and voids in the sub-surface metal layer could have a deleterious effect on long term adherence of the oxide as the interface moves inward through this region. It appears from Fig. 6 however that in prolonged tests the voids or cracks are absorbed into the oxide layer without initiating breakaway of the scale from the nitrided metal layer. When the nitrided layer is completely oxidized the vacancy condensation mechanism reverts to the oxide/metal interface (Fig. 13) which in turn leads to increased access of oxidizing gases to the metal via pores or cracks and an increase in the rate of oxidation (Fig. 6). However the fine grain oxide layer initially nucleated on the nitrided surface is still located adjacent to the metal and its superior mechanical properties relative to the coarse grain oxide normally present on mild steel result in continued ability to accommodate growth stresses and a rate of oxidation lower than that of conventional mild steel.
Analogy with the oxidation of superalloys The proposed mechanism of oxidation resistance outlined above is not unique to nitrided mild steel. Nucleation of fine-grain oxide layers and a consequent improvement in oxidation resistance was first observed in superalloy materials containing mechanically produced dispersions of rare earth oxides within the surface of the alloy. 9 In these systems improved resistance to gaseous oxidation is a result of improved adhesion of oxide layers on the metal surface and is associated with a refned oxide grain size. It is postulated that the rare earth oxide particles within the sub-surface layer of metal act as nucleation sites for oxide formation and that their size and dispersion result in a very fine grained oxide structure. Similar results have been obtained from alloys containing small quantities of alloy elements which form oxides with high stability. These alloy elements are internally oxidized in service producing a fine dispersion of oxides which act as nuclei for formation of the outer oxide layer. The resultant oxide is fine grained and highly adherent. Irving, Stringer and Whittlea° have demonstrated this principle on Co20 ~oCr alloys containing Nb or Ta, Jones and Stringer u on Co25 ~o Cr-Si alloys, and Golightly, Stott and Wood t2 on Fe27 ~oCr-Al alloys. CONCLUSIONS The oxidation resistance of mild steelunder conditions similar to those experienced in service as furnace wall evaporator tubes in coal-fired boilers has been shown to be significantlyincreased by a surface nitriding treatment which avoids formation of iron nitridcs.Nitrided mild steelhas a rate of oxidation similar to that of medium chromium ferritic steels,
1
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Optical micrograph of a nitrided surface layer on mild steel. 5NH3 : 95H2, 700°C.
F
FIG. 8. FIG. 9.
Transmission electron micrograph of the nitrided surface layer on mild steel. 5NH3 : 95H~, 700°C. Transmission electron micrograph of the nitrided surface layer on mild steel after ageing for 500 h ht vacuo.
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FIc. 10. Scanning electron micrographs and corresponding sulphur distribution of scales in steels oxidized in N= + 15 v/o CO"- + 0.3 v/o SO.. + I v/o O~ at 430°C for 500 h (a) untreated mild steel; (b) nitrided mild steel.
Mei'a I Inner oxide/sulphide layer
Oufer oxide layer
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FIG. 11.
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Optical micrograph of oxide microstructure on nitrided mild steel oxidized in N~ + 15 v/o CO.~ + 0.3 v/o SO~ + I v/o O~. at 600°C for 850 h.
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FIG. 12. Optical micrograph of sub-surface voids in.the nitrided layer of an oxidized specimen. FIG. ]3. Optical micro~'aph of the oxide/metal interface of a nitrided mild steel sample in which the nitrided metal layer has been completely oxidized by exposure for ZlO0 h at 600°C.
~uI
The oxidation resistance of nitrided mild steel
571
A mechanism involving the refinement of oxide grain size and modification of the condensation process for cation vacancies is postulated to explain the increased resistance to oxide exfoliation on nitrided mild steel compared to the untreated alloy. A similar mechanism has been advanced for the oxidation resistance of superalloys containing dispersions of stable oxide particles. Oxidation of both nitrided and unnitrided mild steel proceeds by outward diffusion of iron cations through the inner oxide layer. In principle, any treatment which results in the formation of fine grain oxide on the surface of an alloy should lead to some improvement in oxidation resistance of the alloy. The particular advantage of nitriding is that the process can be controlled to give the depth and morphology of surface metal layer required, and can be carried out under conditions of time and temperature such that the mierostructure and properties of the bulk material are unaffected. Acknowledgements--The oxidation experiments described in this paper were carried out in the labora-
tories of the CEGB Midlands Region Scientific Services Department, and the paper is published by permission of the Central Electricity Generating Board (Midlands Region). The author also acknowledges the assistance given by Professor K. H. Jack, Crystallography Laboratory, University of Newcastle-upon-Tynein providing nitriding and electron microscopyfacilities, and by Dr. D. J. Lees, CEGB (Midland Region) Scientific Services Department.
REFERENCES 1. K. H. JACK,Heat Treatment '73, p. 39. The Metals Sot., London (1974). 2. F. LEHRER,Z. Elektrochem. 36, 383 (1930). 3. T. FLATLEYand M. T. ROmNSON,CEGB Report No. 5SD/MID/Rl3/75. 4. T. FLATLEYand N. BmKS,J.LS.I. 209, 523 (1971). 5. A. RAHMEL,Corros. Sci. 13, 125 (1973). 6. D. L. DOUGLASS,Oxidation of Metals and Alloys, p. 137. A.S.M.E. (1971). 7. M. J. MANNINGand E. METCALFE,C.E.R.L. Report No. RD/L/N229/74. 8. J. STRINGER,Corros. Sci. 10, 513 (1970). 9. J. STRINGER,B. A. WILt.COXand R. J. JAFFEE,Oxid. Metals 5, 11 (1972). 10. G. N. IRVING,J. STRINGERand D. P. WmTTLe, Corros. Sci. 15, 337 (1975). ll.D. E. JONESand J. STRINGER,Oxid. Metals 9, 409 (1975). 12. F. A. GOLIGHTLY,F. H. STOTTand G. C. WOOD,16th Corrosion Science Symposium, Swansca (1975).
THE
OXIDATION
RESISTANCE MILD STEEL*
0010-938X/78/0601-0555 $02.00/0
OF NITRIDED
A. HENDRY Wolfson Research Group for High-Strength Materials, Crystallography Laboratory, The University of Newcastle-upon-Tyne, Newcastle-upon-Tyne 1, England Abstract--The oxidation resistance of mild steel in a simulated boiler flue gas atmosphere has been shown in laboratory tests to be significantly enhanced by a surface nitriding treatment which forms a controllable fine-grained martensitic layer without precipitation of iron nitrides. A mechanism is suggested by which improved protection results from the refinement of oxide grain size and the condensation of cation vacancies in the nitrided layer instead of at the metal/oxide interface. A parallel is drawn with the development of improved oxidation resistance in superailoys by sub-surface dispersion of incoherent stable oxide particles.
INTRODUCTION
THE MOSTwidely used alloy for furnace wall evaporator tubes in conventional fossil fuel power-generating plant is mild steel. Fireside corrosion (oxidation) of this alloy by the gaseous products of combustion can result in tube failures with a consequent costly loss of generating time. Power stations experiencing high rates of corrosion are generally those burning aggressive fuels, i.e. coals containing high levels of total chlorine, and there is also a body of evidence which indicates that a direct correlation exists between the concentration of carbon monoxide in the combustion gases adjacent to the furnace wall and the local corrosion rate. The positions of such patches of severe local corrosion are found to be relatively stable and are detected in successive N.D.T. surveys of tube-wall thickness from year to year. Attempts to obtain a low cost solution to the problem of mild steel corrosion in service have been largely concerned with coatings. These include sprayed ceramics such as silica and silicon carbide and metals based on either chromium or aluminium which, when oxidized, form protective oxide layers. The disadvantage of such coatings is their susceptibility to spalling at the coating/metal interface as a result of the difference in thermal expansion coefficients between tube metal and coating. Degradation due to mechanical disruption during handling and fabrication is also a major cause of breakdown of coatings. The inherent disadvantage of mild steel in terms of corrosion (oxidation resistance) lies in the non-protective nature of the oxide which spalls from the metal as a result of relatively minor changes in environmental conditions, thus exposing fresh metal to attack by gaseous oxidants. The coherency of the oxide layer on ferritie alloys, in the absence of alloying elements such as chromium, can only be improved by moditication of the oxide properties. The physical and chemical properties of the oxide are influenced by the mierostructure of the underlying metal substrate, and thus the oxida*Manuscript received 28 July 1977; in amended form 18 November 1977. 555
556
A. HENDRY
tion resistance of mild steel may be improved by a suitable surface treatment which leaves the microstructure and properties of the bulk material unaffected. Internal nitriding of ferritic iron alloys has been widely studied by Jack e t al. 1 and a significant increase in hardness and strength is obtained. By means of gas/metal equilibration nitrogen is introduced into the iron matrix without formation of iron nitride on the surface of the alloy. The nitrogen can then react at nitriding temperature with other solutes in the alloy to form pre-precipitation clusters, stoichiometric precipitates or nitrogen austenite depending on nitriding potential, temperature and alloy element concentration. The depth of penetration of nitrogen, and hence the thickness of the hardened layer, is dictated by Fick's Laws of diffusion and the thickness can be controlled by suitable choice of nitriding potential and temperature. A nitrided layer on the surface of a boiler tube would give increased strength to the tube and be condusive to formation of an adherent oxide scale. Nitriding produces a surface metal layer containing dispersed sites for oxide nucleation either as precipitates or fine metal grains which result in the nucleation of a fine grained and adherent oxide scale. The present work reports a novel method of achieving improved oxidation resistance of mild steel by means of a nitrided surface layer. The layer forms an integral part of the metal surface in, for example, a boiler tube and has high hardness and strength, sufficient to survive any abrasion or deformation during fabrication. Observations of the changes in microstructure of the nitrided layer and oxide during oxidation are reported and their implications discussed. A mechanism is suggested for the improvement in oxidation resistance which has wider implications in the field of oxide integrity on metal surfaces. EXPERIMENTAL
METHOD
The principles o f internal nitriding in NH~ : Hs gas mixtures are well-established and are based on the work o f Lehrer 9 whose results are summariscd in Fig. I. By a suitable choice of temperature and nitriding potential it is possible to nitrid¢ iron to form a surface layer which is completely ferritic or austenitic at the nitriding temperature without forming any of the nitrides of iron on the specimen
60
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r a =N- ferrite / ) , =N-austenite
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300 FIG. 1.
400
500 Temperature,
600
700
=C
L e h r e r d i a g r a m f o r solubility o f n i t r o g e n in iron.
The oxidation resistance of nitrided mild steel
557
surface. The object of the present work was to form a hard and fine-grained martensitic surface layer, the depth of which can be controlled by the length of time at nitriding temperature as dictated by Fick's Laws of diffusion. The nitriding conditions were chosen to give an austenitic surface layer at nitriding temperatures which would transform to martensite on cooling to room temperature with some retained austenite. The amount of retained austenite is a function of cooling rate, and in the present case a normalizing cooling rate gave a ratio of martensite (ct') to austenite (y) of 1 : 2. The ratio o f n ' : "t is determined by X-ray diffractometry of the nitrided surface. The composition of the mild steel used in these experiments is given in Table 1, and the nitrogen concentration in the surface layer is 0.8 w/o.
TABLE I.
COMPOSITION OF THE MILD STEEL USED IN THE PRESENT EXPERIMENTS
C
Si
Mn
Ni
Cu
(w/o) 0.23
(w/o) 0.15
(w/o) 0.45
(w/o) 0.07
(w/o) 0.07
The experimental alloys in the form of discs 12.0 mm in diameter and 3.00 mm thick were nitrided in a vertical tube furnace at 700°C in 5NH3 : 95H, for 2 h and cooled in the flowing gas mixture after removal from the hot-zone to the cold end of the furnace. This treatment produces a martensite/ austenite surface layer approx. 100 v.m thick. The nitrided alloys were cleaned and degreased before oxidation in a Stanton-Redcroft decimilligram thermobalance equipped with a linear variable temperature controller and programmer. The oxidizing atmosphere was established by additions of oxygen or carbon monoxide to a carrier gas mixture of composition Ns + 15 v/o COs + 0.3 v/o SO~. All gases were purified and dried before use and a total gas flow rate of 300 ml rain -: (4 mm s -t) maintained during all experiments. The weight gain of nitrided specimens was determined as a function of time under the following conditions: (i) Isothermal oxidation in Ns + 15 v/o CO~ + 0.3 v/o SOs + 1.0 v/o Os at 430 and 600°C. (ii) Oxidation in Ns + 15 v/o COs + 0.3 v/o SOs + 1.0 v/o Os under thermal cycling conditions between 20(O30°C and 200-600°C. (iii) Oxidation in a cyclic oxygen potential of Ns + 15 v/o CO~ + 0.3 v/o SOs + 1.0 v/o Os and Ns + 15 v/o CO2 + 0.3 v/o SOs + 1.0 v/o CO at 430°C. The thermal cycles were of the following form; one hour at upper hold temperature, heating and cooling rates of 8°C.min -x and 1 h at lower hold temperature. The oxygen potential cycles were varied discontinuously between "oxidizing" (1.0 v/o 02) and "reducing" (1.0 v/o CO). Assuming that equilibrium is established in the gas phase, these conditions correspond to the following chemical potentials of the oxidizing species: oxidizing; Po~ = 0.01 atm., Pa~ = 10-'Satm ao = 10-sv (relative to graphite) reducing; Po2 = 10-alatm, Pss > 1.0 atm ao = 0.6 (relative to graphite). The composition of the scales was determined by X-ray diffraction after removal from the specimen surface by stripping in an iodine methanol solution. Optical metallography and scanning electron microscopy were carried out on specimens mounted in Araldite resin and polished to 1 ~m with diamond paste on a vibratory polisher. Specimens for transmission electron microscopy were prepared from 0.125 mm mild steel sheet which had been through-nitrided at 700°C in 5NHs : 95H2, and were examined in a Philips E.M.300 microscope at 100 kV.
EXPERIMENTAL
RESULTS
AND
DISCUSSION
Isothermal oxidation T h e w e i g h t g a i n o f n i t r i d e d m i l d steel s p e c i m e n s d u r i n g o x i d a t i o n a t 430 a n d 600°C in s i m u l a t e d flue gas is s h o w n in Fig. 2 a n d is c o m p a r e d w i t h results f r o m m i l d
558
A. HENDRY
steel, 2.25Cr-lMo steel and 9Cr-lMo steel oxidized under the same conditions. The curves indicate the general trend of results rather than a specific set of experimental points since considerable scatter, indicated by the error bars in Fig. 2, is observed from different tests. The order of behaviour is typical however. With the exception of mild steel all of the alloys demonstrate behaviour typical of growth of a protective oxide layer.
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Weight gain curves for oxidation of steels in N= + ]5 v/o COs + 0.3 v/o SO= + 1 v/o O, at (a) 430°C, (b) 600°C.
Thermal cycling The oxidation behaviour of nitrided mild steel compared with other alloys under thermal cycling conditions is shown in Fig. 3. The results are plotted as a function of time at the upper hold temperature and have not been normalized to take account of weight gain during the temperature cycles. Such a procedure is at best only approximate, particularly in eases where scale rupture is prevalent.
Oxygen potential cycling A comparison of the oxidation rates of nitrided mild steel and other alloys was carried out under conditions of cyclic oxygen potential varied discontinuously between
The oxidation resistance of nitrided mild steel
559
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rided " steel
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Weight gain curves for oxidation of steels in N= -t- ]5 v/o COs -t- 0.3 v/o SO= + 1 v/o O, thermally cycled between (a) 200-430°C, (b) 200-600°C.
"oxidizing" and "reducing". The results are shown in Fig. 4. The scatter of results from the chromium-containing alloys is much greater in these tests than under any other conditions. This is believed to be due to cracking and rupture of the scale. The causes of cracking are twofold. First, the outer layer of haematite on the scale is reduced to magnetite during the reducing part of the cycle, in agreement with the calculated Fe/FesOUFe=Os equilibrium in the prevailing chemical potentials in the gas phase. The consequent decrease in volume causes rupture of the underlying scale. Secondly, the decrease in oxygen potential leads to a eorrespondinginereasein sulphur potential and an increase in the degree of sulphidation at the metal/scale interface which also results in scale rupture due to the higher specific volume of iron sulphide with respect to iron oxide. This behaviour is demonstrated in Fig. 5. A sample of mild steel and of nitrided mild steel were oxidized under "oxidizing" (1.0 v/o Oz) conditions for 600 h in order to form a thick scale. Oxygen potential cycling was then commenced. During the reducing part of the cycle the rate of oxidation of the untreated steel increases sharply as a result of scale rupture and increased access of oxidizing
560
A. I4_JENDRV 8.0
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Nifrided mild steel
I 400
[ 600
Time,
I 800
h
FIG. 4. W e i g h t g a i n curves f o r o x i d a t i o n o f steels at 4 3 0 ° C in a cyclic o x i d a t i o n potential.
gases. The increased rate of weight gain is maintained during the initial part of the oxidizing cycle until the cracked scale becomes healed or blocked with fresh oxide and the rate then decreases. The process is then repeated with the onset of the next reducing cycle. Nitrided mild steel samples are not affected by oxygen potential cycling indicating that the fracture stress of the scale has not been exceeded and no loss of coherency has occurred at the oxide/metal interface. The oxidescale retains its protective nature on the metal surface.
Kinetics of oxidation of nitrided mild steel Qualitative comparison of the results presented in Figs. 2-5 shows that nitrided mild steel and 2.25Cr-lMo steel behave in a similar manner under isothermal conditions at 430°C. The improved oxidation resistance of nitrided mild steel becomes apparent at 600°C and under the more severe conditions imposed by thermal cycling
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F]G. 5. Weight gain curves for mild stee] at 430°C in a cyclic oxidation potential (a) n i t r i d c d m i l d steel; (b) untreated mild steel (specimens pre-oxidizcd in Ns + ] 5 v / o
COs + 0.3 v/o SOs + 1 v/o O, for 600 h).
The oxidation resistance of nitrided mild steel
561
and oxygen potential cycling. The specific reaction rate constants (kp) given in Table 2 for isothermal oxidation of nitrided mild steel are slightly greater than those of Flatley and Robinson a for 2.25Cr-lMo steel at 430°C and approximately one order of magnitude lower at 600°C. The values given by Flatley and Birks 4 for the rate of oxidation of pure iron in Ar + 0.5 v/o SO2 are kp = 5.13 × 10-v g2cm-%-I at 900°C and k, = 5.0 × 10-s g2cm-4s-X at 600°C which is two orders of magnitude higher than kp (600°C) for nitrided mild steel shown in Table 2.
TABLE 2.
PARABOLIC REACTION RATE CONSTANTS (kp) FOR NITRIDED MILD ffI"EEL
Temp. (*C) 430 600 20(0-430 200-600
tu
Total time (h)
(11)
520 1300 788 858
520 1300 390 410
Parabolic rate constant kp (gScm-%-t) 2.76 2.47 5.0 1.95
X I0-xt × 10-to × 10-12 x 10-*o
No experimental values of k, during thermal cycling or oxygen potential cycling have been published, and only qualitative comparison of rates is therefore possible in Figs. 3 arid 4. In order to determine the oxidation kinetics of a specimen in which the nitrided layer is completely oxidized, and to examine the effect on subsequent oxidation of the underlying un-nitrided metal, a nitrided mild steel specimen with a nitrided layer thickness of 87.5 ~tm was oxidized isothermally at 600°C in N~ + 15 v/o CO~ + 0.3 v/o SO2 + I v/o O~ for 2100 h. The results are shown in Fig. 6 for a single specimen. From a number of experimental observations it can be assumed that under the conditions used 1 g.cm -2 of weight gain is approximately equivalent to 4 ~m of metal loss as oxide and sulphide. Oxidation of the entire 175 ~m nitrided layer (2 x 87.5 ~m layers) will therefore lead to aweight gain ot'43.75 g.cm -2. Inspection of Fig. 6 shows that this weight gain occurs at 1600 h and corresponds to the breakdown of parabolic oxidation kinetics and a transition (1500-1600 11)to a linear regime. bfter oxidation of the nitrided layer is complete, the rate of oxidation in the linear region of the curve is much lower than that of conventional mild steel. A weight gain of 22.5 g.cm -2 was recorded in the final 500 h of the present test--the linear region of Fig. 6---wjaereas the same weight gain was recorded in 40 h with conventional mild steel, Fig. 2(b). The values of Flatley and Birks 4 for linear rate constants during the initial stages of oxidation of iron in Ar + 0.5 v/o SO2 which, on extrapolation to 600°C, give kL = 6.6 × 10-6 g.em-2s -1 can be compared with the present value of kz = 1.2 × 10-6 g.cm-2s -1 for the final linear stage of Fig. 6. Indeed the value of Flatley and Birks parabolic rate constant for oxidation of iron at 600°C in Ar + 0.5 v/o SOz (k v = 5.0 × 10-6 g.2cm~s-1) gives a weight gain in 500 h which is an order of magnitude higher than that obtained in the final 500 h of the present experiment.
562
A. HENDRY
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F1G. 6. Weight gain curve for long-term oxidation of nitrided mild steel at 600°C in N2 + 15 v/o COs + 0.3 v[o SOs + 1 v/o 09. The mechanism of oxidation of nitrided mild steel The ability of an oxide layer to accommodate stresses during growth and hence remain attached coherently to the underlying metal surface, is primarily dependent on grain size and porosity within the layer. Grain size is dictated by the nucleation and growth processes by which the initial layer of oxide is formed on the metal surface, in particular by the number and distribution of nucleation sites for oxide formation. The extremely fine microstructure of a nitrided layer compared to that of the unnitrided bulk material (Fig. 7) should lead to a larger number of suitable sites for oxide nucleation and a consequently finer grain size in the scale than would be the case in the absence of the nitrided surface. These factors combined with the high degree of residual stress associated with the martensite transformation which results in an increased compressive stress on the oxide, will lead to improved adhesion of the scale to the metal surface. The microstructure of the nitrided layer on mild steel has been examined by transmission electron microscopy in order to determine the microstructural changes which occur as a result of exposure to elevated temperatures during nitriding and oxidation. Through-nitrided mild steel foils were sealed in an evacuated silica capsule and aged at 430°C for times up to 500 h. The microstructure of the as-nitrided a l l ~ consists of packets of twinned martensite needles surrounded by retained austenite (Fig. 8). The martensite packets are typically 1 Izm in width containing individual twin laths approximately 15 nm wide, and surrounded by austenitic grains containing a high density of dislocations adjacent to the martensite packets. On ageing at 430°C the structure transforms slowly and 7'-Fe4N is precipitated from the metastable austenite phase while the martensite plates coarsen. After 500 h at 430°C the structure is still relatively fine and the coarse lath martensite is surrounded by ferrite containing a fine, platelike dispersion of %,'-Fe4N of ca. 25 nm din (Fig. 9). The microhardness has correspondingly decreased from 600 to 400 VMN after 500 h, which is still significantly high for mild steel. The nitrided layer therefore retains a high hardness and finely divided microstructure after 500 h at 430°C and consequently will maintain
The oxidationresistanceof nitrided mild steel
563
good coherency with its oxide scale as no gross microstructural changes have occurred. The microstructure of the oxide scale on mild steel and nitrided mild steel are morphologically similar but show a distinct difference in mechanical behaviour. The oxide on nitrided specimens is strongly adherent whereas that on mild steel is spalled and laminated. The chemical composition of the scale as determined by S.E.M. is similar for both alloys and no significant variation in the relative proportions of oxide and sulphide have been detected. Figure 10 illustrates the difference in oxide adhesion between nitrided and un-nitrided mild steel. A magnetite/sulphide layer is present at the metal/scale interface on both specimens, but mild steel scales also conrain layers of sulphide within the scale thickness indicative of exfoliation during growth. These observations indicate that the same oxidation mechanism operates in both cases; the scale grows by outward diffusion of iron cations through the scale and inward diffusion of oxygen anions is negligible, although inward diffusion of sulphur anions occurs. This is in accord with the accepted mechanism of oxidation of iron in sulphurous gases) The structure of the oxide layer on a nitrided specimen oxidized at 600°C for 850 h is shown in Fig. 11 where the fine grain size of the innermost layer of oxide and sulphide compared to the larger grain size of the outer areas of the scale can be clearly seen. Since the scale grows by outward diffusion of iron cations the layer of oxide immediately adjacent to the metal surface continues in that position as the metal is consumed. Therefore in the situation where all of the nitrided layer is oxidized the essential protective element, the five grain size oxide layer, will still be present and continue to confer improved oxidation resistance on the alloy over that of conventional mild steel. This is confirmed by the final section of the test shown in Fig. 6. The fine grain size of the oxide layer formed on nitrided specimens is better able to accommodate growth stresses than a coarser grain size oxide. It has been shown by Douglass 6 that the mechanical strength of oxides increases with decreasing grain size. This is attributed to grain boundary creep processes which are sufficiently rapid at oxidizing temperature to relieve the internal stresses in the oxide by a HerringNabarro creep mechanism. Recent investigations of the mechanical properties of oxides grown in steam in service in power stations Manning and Metcalfe7 show that scales containing fine grain size oxide have higher fracture stresses and strains than other M,O4-type oxides of coarser grain size. Since oxidizing/reducing cycles have no effect on nitrided mild steel it cart be concluded that the fracture strain of the oxide is not exceeded by the volume decrease on reduction during the reducing half-cycle. The effect of grain size and creep processes on oxide fracture has also been reviewed by Stringer.' Regardless of the absolute strength of the oxide layer, the ultimate integrity of the oxide on the metal surface is governed by its weakest point, which is the scale/metal interface. Vacancy condensation at the interface to form voids is a primary cause of exfoliation during oxidation of mild steel. Vacancies arise as a result of oxide growth by outward diffusion of iron cations which is coupled with the inward migration of cation vacancies. A build up of vacancies occurs in the oxide adjacent to the metal/ oxide interface and a driving force exists for the condensation of vacancies on a suitable heterogeneous site. The interface is the most likely site for vacancy condensation, consequent void formation and subsequent exfoliation.
564
A. HENDRY
On nitrided alloys the vacancy condensation mechanism is apparently modified as a result of the coherency of the metal/oxide interface such that voids are not formed at the interface but appear at some depth into the metal (Fig. 12). The exact mechanism by which this occurs cannot be unequivocally demonstrated by the techniques presently available, but is believed to result from stress relaxation in the surface layer at the oxidizing temperature giving rise to triple point cracks. These cracks immediately below the surface of the metal can then act as vacancy sinks, thus eliminating vacancy condensation at the oxide/metal interface. Vacancy condensation and voids in the sub-surface metal layer could have a deleterious effect on long term adherence of the oxide as the interface moves inward through this region. It appears from Fig. 6 however that in prolonged tests the voids or cracks are absorbed into the oxide layer without initiating breakaway of the scale from the nitrided metal layer. When the nitrided layer is completely oxidized the vacancy condensation mechanism reverts to the oxide/metal interface (Fig. 13) which in turn leads to increased access of oxidizing gases to the metal via pores or cracks and an increase in the rate of oxidation (Fig. 6). However the fine grain oxide layer initially nucleated on the nitrided surface is still located adjacent to the metal and its superior mechanical properties relative to the coarse grain oxide normally present on mild steel result in continued ability to accommodate growth stresses and a rate of oxidation lower than that of conventional mild steel.
Analogy with the oxidation of superalloys The proposed mechanism of oxidation resistance outlined above is not unique to nitrided mild steel. Nucleation of fine-grain oxide layers and a consequent improvement in oxidation resistance was first observed in superalloy materials containing mechanically produced dispersions of rare earth oxides within the surface of the alloy. 9 In these systems improved resistance to gaseous oxidation is a result of improved adhesion of oxide layers on the metal surface and is associated with a refned oxide grain size. It is postulated that the rare earth oxide particles within the sub-surface layer of metal act as nucleation sites for oxide formation and that their size and dispersion result in a very fine grained oxide structure. Similar results have been obtained from alloys containing small quantities of alloy elements which form oxides with high stability. These alloy elements are internally oxidized in service producing a fine dispersion of oxides which act as nuclei for formation of the outer oxide layer. The resultant oxide is fine grained and highly adherent. Irving, Stringer and Whittlea° have demonstrated this principle on Co20 ~oCr alloys containing Nb or Ta, Jones and Stringer u on Co25 ~o Cr-Si alloys, and Golightly, Stott and Wood t2 on Fe27 ~oCr-Al alloys. CONCLUSIONS The oxidation resistance of mild steelunder conditions similar to those experienced in service as furnace wall evaporator tubes in coal-fired boilers has been shown to be significantlyincreased by a surface nitriding treatment which avoids formation of iron nitridcs.Nitrided mild steelhas a rate of oxidation similar to that of medium chromium ferritic steels,
1
FIG. 7.
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Optical micrograph of a nitrided surface layer on mild steel. 5NH3 : 95H2, 700°C.
F
FIG. 8. FIG. 9.
Transmission electron micrograph of the nitrided surface layer on mild steel. 5NH3 : 95H~, 700°C. Transmission electron micrograph of the nitrided surface layer on mild steel after ageing for 500 h ht vacuo.
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FIc. 10. Scanning electron micrographs and corresponding sulphur distribution of scales in steels oxidized in N= + 15 v/o CO"- + 0.3 v/o SO.. + I v/o O~ at 430°C for 500 h (a) untreated mild steel; (b) nitrided mild steel.
Mei'a I Inner oxide/sulphide layer
Oufer oxide layer
80#m i
FIG. 11.
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Optical micrograph of oxide microstructure on nitrided mild steel oxidized in N~ + 15 v/o CO.~ + 0.3 v/o SO~ + I v/o O~. at 600°C for 850 h.
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FIG. 12. Optical micrograph of sub-surface voids in.the nitrided layer of an oxidized specimen. FIG. ]3. Optical micro~'aph of the oxide/metal interface of a nitrided mild steel sample in which the nitrided metal layer has been completely oxidized by exposure for ZlO0 h at 600°C.
~uI
The oxidation resistance of nitrided mild steel
571
A mechanism involving the refinement of oxide grain size and modification of the condensation process for cation vacancies is postulated to explain the increased resistance to oxide exfoliation on nitrided mild steel compared to the untreated alloy. A similar mechanism has been advanced for the oxidation resistance of superalloys containing dispersions of stable oxide particles. Oxidation of both nitrided and unnitrided mild steel proceeds by outward diffusion of iron cations through the inner oxide layer. In principle, any treatment which results in the formation of fine grain oxide on the surface of an alloy should lead to some improvement in oxidation resistance of the alloy. The particular advantage of nitriding is that the process can be controlled to give the depth and morphology of surface metal layer required, and can be carried out under conditions of time and temperature such that the mierostructure and properties of the bulk material are unaffected. Acknowledgements--The oxidation experiments described in this paper were carried out in the labora-
tories of the CEGB Midlands Region Scientific Services Department, and the paper is published by permission of the Central Electricity Generating Board (Midlands Region). The author also acknowledges the assistance given by Professor K. H. Jack, Crystallography Laboratory, University of Newcastle-upon-Tynein providing nitriding and electron microscopyfacilities, and by Dr. D. J. Lees, CEGB (Midland Region) Scientific Services Department.
REFERENCES 1. K. H. JACK,Heat Treatment '73, p. 39. The Metals Sot., London (1974). 2. F. LEHRER,Z. Elektrochem. 36, 383 (1930). 3. T. FLATLEYand M. T. ROmNSON,CEGB Report No. 5SD/MID/Rl3/75. 4. T. FLATLEYand N. BmKS,J.LS.I. 209, 523 (1971). 5. A. RAHMEL,Corros. Sci. 13, 125 (1973). 6. D. L. DOUGLASS,Oxidation of Metals and Alloys, p. 137. A.S.M.E. (1971). 7. M. J. MANNINGand E. METCALFE,C.E.R.L. Report No. RD/L/N229/74. 8. J. STRINGER,Corros. Sci. 10, 513 (1970). 9. J. STRINGER,B. A. WILt.COXand R. J. JAFFEE,Oxid. Metals 5, 11 (1972). 10. G. N. IRVING,J. STRINGERand D. P. WmTTLe, Corros. Sci. 15, 337 (1975). ll.D. E. JONESand J. STRINGER,Oxid. Metals 9, 409 (1975). 12. F. A. GOLIGHTLY,F. H. STOTTand G. C. WOOD,16th Corrosion Science Symposium, Swansca (1975).