Effect of aging process on the microstructure, corrosion resistance and mechanical properties of stainless steel AISI 204

Effect of aging process on the microstructure, corrosion resistance and mechanical properties of stainless steel AISI 204

Case Studies in Construction Materials 11 (2019) e00253 Contents lists available at ScienceDirect Case Studies in Construction Materials journal hom...

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Case Studies in Construction Materials 11 (2019) e00253

Contents lists available at ScienceDirect

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Effect of aging process on the microstructure, corrosion resistance and mechanical properties of stainless steel AISI 204 Mohammed Jacob Joseph*, Murtadha Abbas Jabbar University of Basra-College of Engineering, Mechanical Engineering Department, Iraq

A R T I C L E I N F O

A B S T R A C T

Article history: Received 5 February 2019 Received in revised form 30 March 2019 Accepted 15 May 2019

In this study, the microstructure thermostability, corrosion resistance, and mechanical properties of AISI204 stainless steel after aging were investigated. The microstructural analysis showed precipitation of M23C6 carbide along grain boundaries and its amount tends to increase with increasing aging time and temperature. Little effect of carbide precipitation on tensile strength where the maximum drop was at temperature 850  C and aging time 48 h by 5.09%. At the same aging conditions, precipitation severely affected yield strength, ductility, and the microhardness. The yield strength decreased by 46% and microhardness to 210 HV. On the other hand, the ductility increased to 21% in terms of percent reduction in area. Weight loss as a result of uniform corrosion and corrosion current density were strongly dependent on aging time and temperature. Furthermore, Passivation current density showed independent behavior as the temperature increase. © 2019 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Keywords: Stainless steel Aging Precipitation Microstructure Mechanical Corrosion

1. Introduction The AISI204 is an austenitic stainless steel belongs to the 200 series, the chromium-manganese-nitrogen series as it's called. This series developed by American and German investigators by replacement part or all the most important austenite stabilizer which is nickel by other elements, when nickel prices have been relatively high over the early 1930s [1–3]. The existence of nitrogen in stainless steel grades especially these containing manganese has several advantages such as making them more strengthen, reducing the amount of nickel for maintaining austenite structure, decreasing the tendency for precipitation because it has greater solid-solubility than carbon, and further increases in strength by cold deformation. Moreover, it improves the stress corrosion cracking (SCC) resistance for these steels [4,5]. This type of stainless steel is widely used for general applications and for pressure vessels where the high temperature and moisture are present [6]. Like other austenitic stainless steels, the AISI204 tends to be sensitized in the temperature range of (450–850  C). Sensitization phenomenon in this steel produces as an occasional event during prolonged services at high temperature, improper heat treatment, and welding process. Different factors affecting the sensitization kinetics, some of which relate to the material itself such as the chemical composition, grain size, and cold working degree. Other relate to the conditions of service that stimulate this effect. [7–9]. However, chromium-rich carbides or nitrides forms at grain boundaries during sensitization by depleting chromium from regions in the vicinity of grain boundaries of stainless steel which makes the chromium amount next to the grain boundary falls below 12% [10]. The presence of these precipitates

* Corresponding author. E-mail addresses: [email protected] (M.J. Joseph), [email protected] (M.A. Jabbar). https://doi.org/10.1016/j.cscm.2019.e00253 2214-5095/© 2019 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/ licenses/by-nc-nd/4.0/).

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reduces the durability of the material for a certain time. Simmons [11] studied the effect of nitrides on mechanical properties of high nitrogen stainless steel and reported that these nitrides cause embrittlement. Bansod et al. [12] presented a comparative study of corrosion behavior of low nickel and 304 austenitic stainless steel after aging and they found that the low nickel low-nickel stainless steel more prone to intergranular corrosion. The present study aims to discover the microstructure evolution at different aging conditions and extent of the effect of formed precipitates during aging on mechanical properties and corrosion resistance of this stainless steel. 2. Experimental work The chemical composition of AISI204 stainless steel was obtained by means of spectrum analyzer device (model SPECTROTEST TXC25)in wt. % is shown in Table 1. Round specimens with diameter 12 mm of this stainless steel were cut into length 25 mm for microstructural observations and weight loss measurements, while for electrochemical tests and XRD analysis the specimens were cut into 1.5 mm. With regard to mechanical properties tests, specimens were prepared according to ASTM E8-E8M [13]. All Specimens were isothermally aged at [550, 650, 750, 850  C] for 24 and 48 h. For microstructural observations, specimens subjected to standard grinding and polishing processes according to ASTM E3 [14] standard. Different grades of emery papers [200, 400, 600, 800, 1000, 1200, and 2000] were used for surface grinding. The polishing process was accomplished by using of graded diamond paste up to 0.5 mm as a final polish stage. All grinding and polishing processes conducted by hand and careful cleaning between stages was done. To reveal the microstructure, the specimens were etched according to ASTM E407 standard with etching solution (45 mL HCl, 5 mL HNO3, and 50 mL H2O) [15]. The observations were done by optical microscopy type OLYPMUS (GX41) as a primary stage, then to get more information about microstructure evolution, the observations also carried out under FE-SEM (ZEISS-SUPRA 55-VP) equipped with an EDS which is located in the Pharmacy College-University of Basra. For the XRD analysis, a diffractometer with a copper target and equipped with monochromator was used which is located in Center of Nano Technology and Advanced Materials-Baghdad. The XRD patterns were recorded at 40 kV and 30 mA. The scan range was 30–100 with 0.2 step size and 10 deg./min. scan speed. Concerning weight loss measurements, four media (seawater, brackish water, soil, and wet environment) were used to evaluate corrosion resistance of samples after aging process. Experiments started with original weight measurements of specimens by means of a calibrated sensitive balance type DENVER TP-1502 with four significant digits reading. The specimens exposed to the four media for different periods (7–35days). After that, the specimens take out for cleaning, drying and thereafter reweighed. The experiments were done in duplicate to ensure the reliability of the results and the mean value was calculated. The weight loss in grams was taken as the difference in weight before and after the test, and corrosion behavior established according to the following formula [16]. wt ¼

W0  Wt S

ð1Þ

Where wt is the weight loss,  W0 the weight of the specimen before test, Wt the weight of specimen after test, and S the total surface area of specimen. The experiments were done for the first two media (i.e. seawater and brackish water) whose chemical and physical properties are shown in Table 2 through totally and static immersion in plastic beakers with 50 mL capacity. For the third medium, resistivity and pH value were also measured according to ASTM G51 and ASTM G57 respectively [17,18], where the values were 9 and 229.22 V.cm. The samples were buried at a depth of (50 cm) and all experiments conducted according to ASTM G162 [19] during the winter of 2018. Simple humidity test is adapted to perform wet environment experiments (the forth medium). The constructed corrosion test apparatus as shown in Fig. 1 consisting of an ultrasonic humidifier connected to a plastic box (chamber) by soft plastic tube. The samples placed inside the chamber on a perforated Teflon base to ensure that there is no contact between the samples and any metallic surface, in addition, to permit the condensate water droplets to go down the chamber to be

Table 1 Chemical composition of stainless steel. Element

C

Si

Mn

Cr

Cu

Ni

N

Fe

Wt. %

0.029

0.29

9.0

15.0

0.34

1.5

0.3

Bal.

Table 2 Chemical and physical properties for the used solutions. Electrolyte Solution

pH

Conductivity mS/cm

TDS mg/L

Brackish water RO water Sea water

7.46 7.4 7.5

4460 140 75200

2240 69 54060

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Fig. 1. Parts of the Simple Humidity Test Apparatus.

removed periodically. A fixed level of temperature and relative humidity (T = 26.12  C, RH = 94.42%) was maintained inside the chamber and digital hygrometer was used for measuring them through a demountable hole in one of the chamber sides. A drinking water (RO water) whose chemical and physical properties are shown in Table 2 was used to fill the humidifier tank. Regarding electrochemical tests, corrosion cell with three electrodes, the working electrode at which the AISI204 samples were installed with 1 cm2 exposed area, Ag/AgCl as a reference electrode, and platinized Titanium as an auxiliary electrode was used. The tests started by open circuit potential OCP recording with time up to 600 s after one-hour immersion period in 1 M NaCl solution. Then, initial and final potential limited to define the path which the scan will take. After that, polarization test carried out with standard scan rate 0.1667 mV/s for all experiments. Next, Tafel lines constructed by means of M Lab software to extract Ecorr and Icorr. Whereas the passive current Ipass extracted directly from the curve. Polarization test provides quantitative information about corrosion resistance of metal in a certain environment, where the parameters such as the corrosion potential Ecorr and corrosion current Icorr are used to identify the active degradation ability of metal while the passivation current Ipass is used to evaluate the stability of the passive film. Mechanical properties tests were carried out both for aged and non-aged specimens. The tests performed in triplicate to ensure the reliability of the results. Axial tensile test conducted at room temperature by means of testing equipment type INSTRON 600DX at a constant strain rate of 15 s1.Vickers microhardness test of the matrix of specimens, those used for optical microscopy examination conducted according to the ASTM E92 [20] at room temperature with a load 10 g and dwell 10 s. A modern microhardness tester model vhs-30b was used for this purpose and the mean value of three readings of each specimen was taken. 3. Results and discussion 3.1. Microstructure analysis The optical and FE-SEM micrographs of stainless steel AISI204 at annealed conditions (material as received) and at different aging conditions are illustrated in Figs. 2 and 3 respectively. Initially, the microstructure is a single and homogeneous austenite phase without any apparent precipitates. Annealing twins (AT) also characterized the microstructure which are pointed by blue arrows in (Fig. 2a). The appearance of such these twins indicates that the material was deformed and then annealed. For thermally aged specimen, it is clear that the evolution of the microstructure has occurred and resulted in small precipitates formed on preferential sites at the grain boundaries. The amount of these precipitates coarsened and agglomerated with increase the aging time and aging temperature. Fig. 4 shows the XRD analysis of specimens at different aging conditions. From this figure, it is obvious that this analysis in close agreement with microstructural observation where the as-received material exhibited only the austenite peaks. Moreover, patterns of aged specimens showed new peaks correspond to the precipitated phase. These peaks increase in their intensities as aging time and temperature increase indicative of an increase in the amount of precipitated phase. With regard to EDS analysis, a proportion of the precipitated phase bounded by a yellow frame as illustrate in Fig. 5 to ensure that only this region exposing to the spectrum. Two region have been selected for each sample and the average value was taken. Fig. 6 show the EDS spectrum of precipitated phase. It can be seen from this spectrum the elements these have high peaks are Fe, Cr, Mn, and C. Based upon this analysis the chemical composition of the precipitated phase is belong to M23C6 carbide phase. Iron occupies the largest proportion for the metallic part of carbide followed by Cr and then Mn. As it is noted, the solute element in particular nitrogen disappears in this phase, although its weight percent, more than the C element in AISI204 stainless steel so it is possible to say that the C element has a higher affinity than the N element. All the aged samples showed carbides precipitation along their grain boundaries and no nitrides precipitation had observed. The average concentration of Fe, Cr, Mn, and C in M23C6 as a function of aging temperature and for the two aging times is shown in Fig. 7. It can be seen the elements, notably, Cr and C, they increase in their concentrations remarkably when the aging temperature increases from (550–650  C), then show a fixed level while the Mn concentration remained constant with

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Fig. 2. Optical Micrographs of AISI204 stainless steel, (a) as-received, Aged at (b) 550  C for 24 h. (c) 650  C for 24 h. (d) 750  C for 24 h. (e) 850  C for 24 h. (f) 550  C for 48 h. (g) 650  C for 48 h. (h) 750  C for 48 h. (i) 850  C for 48 h.

aging temperature change. Moreover, the Fe concentration decreases when aging temperature reaches 650  C and then continues at this value till the aging temperature of 850  C. Furthermore, the effect of increasing aging time is not clear on the carbide concentration because all curves show the same trend and approximately compatible. 3.2. Corrosion resistance evaluation 3.2.1. Weight loss measurements Weight loss plots of specimens those aged for 24 and 48 h then exposed to the four media are shown in Figs. 8 and 9 respectively. It can be seen from these figures that the mass per area increases with increasing aging time and temperature as a result of uniform corrosion effect instead of pitting corrosion that observed in the as-received material. Despite the release of general corrosion in all sensitized samples, but it is the end effect for sensitization. As it known, aging process caused a chemical segregation at grain boundaries. This chemical segregation (i.e. Precipitation of M23C6 carbide along grain boundaries) makes the whole grain cathode with respect to its boundary and once exposing to corrosive environment, localized corrosion such intergranular corrosion activates. In the case of sensitized AISI204 stainless steel, the effect of intergranular corrosion was severe and reflected as uniform corrosion. It can also see that the most effective media on aged specimens was mainly the wet environment because of the significant effect of oxygen, then the soil, after that seawater, and lastly the brackish water. Sharp decline showed by aged specimen at period (21 day) in soil medium. This belongs to receding the amount of rainfall (i.e. the amount of moisture in soil) during that period. Furthermore, the as-received material did not show any reaction with this medium.

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Fig. 3. FE-SEM Micrographs of AISI204 stainless steel, (a) as-received, Aged at (b) 550  C for 24 h. (c) 650  C for 24 h. (d) 750  C for 24 h. (e) 850  C for 24 h. (f) 550  C for 48 h. (g) 650  C for 48 h. (h) 750  C for 48 h. (i) 850  C for 48 h.

3.2.2. Electrochemical tests Fig. 10 shows OCP results after one-hour immersion period in 1 M NaCl solution. It is obvious from this figure that the aging process tends to shift the potential to more negative values. Initially, the as-received sample starting at 315 mV and overtime gradually decreases to 360 mV at time 600 s. These values of starting and ending potentials change substantially for aged samples. In the case (aged at 550  C for 24 h.), the potential starts at 401 mV and then decreases with slow rate till reaches 434 mV after 600 s. Decreasing of potential continues for sample (aged at 650  C for 24 h.) with rate more than that observed in the previous case although the starting potential was less (i.e. 382 mV). Moreover, noticeable variation in the potential of the samples those aged at (750 and 850  C for 24 h.), they exhibited (459 and 474 mV) at time of 600 s respectively. Fig. 11 shows the polarization curve obtained for one of the specimens used in this test. The polarization curves of all samples showed similar trend in their behavior and the one thing observed is the change of the three parameters (i.e. Icorr,

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Fig. 4. XRD Patterns for the Specimen (a) As-received, (b) Aged at 550  C for 24 h. (c) Aged at 750  C for 24 h. (d) Aged at 550  C for 48 h. (e) Aged at 750  C for 48 h.

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Fig. 5. Micrograph Represents Selection Method of Precipitated Phase particle for EDS Analysis (sample aged at 550  C for 24 h.).

Fig. 6. EDS Spectrum of the Precipitated Phase of sample Aged at 550  C for 24 h.

Ecorr, and Ipass) as shown in Figs. 12 and 13. To estimate the effect of aging process on corrosion resistance, the extracted corrosion potential Ecorr and corrosion current density Icorr are plotted as a function of the aging temperature as shown in Figs. 14 and 15. From Fig. 14 which show the variation of corrosion potential with aging temperature, it can be seen that the highest value of potential exhibited by the as-received material which is 368 mV due to the absence of carbides. Additionally, it is obvious that the aging process promotes the active dissolution of samples in the saline solution due to the presence of carbides. For samples aged for 24 h, the potential decreased to 444 mV at the temperature of 550  C, then shifting rapidly to 484 mV at 650  C. Next, a slight increase to 456 mV occurred at 750  C, thereafter returned to decrease again to 450 mV at 850  C. The samples those aged for 48 h exhibited reaction (i.e. went to more negative values) with

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Fig. 7. Evolution of Carbide Concentratinon (average value) With Aging Time and Temperature.

Fig. 8. Weight loss curves of samples those aged for 24 h then exposed to the four medium, (a) seawater, (b) brackish water, (c) Soil, (d) Wet Environment.

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Fig. 9. Weight loss curves of samples those aged for 48 h then exposed to the four medium, (a) seawater, (b) brackish water (c) Soil, (d) Wet Environment.

Fig. 10. Open Circuit Potential Variation of AISI204 Stainless Steel for Different Aging Conditions.

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Fig. 11. Extraction of Corrosion Parameters from Polarization Curve for the Sample That Aged at 550  C for 48 h.

Fig. 12. Polarization Behavior of the Isothermally Aged Samples at 24 h.

saline solution higher than those aged for 24 h expect at 650  C. Despite this difference in corrosion potential at 650  C, however, a linear behavior of corrosion current density with aging temperature is observed. The corrosion current is an important parameter to determine corrosion resistance more than the corrosion potential because of it is directly proportional to corrosion rate according to Faraday's law. So, the corrosion current density takes into account only to estimate the corrosion behavior of samples at different aging conditions without having to address the corrosion rate. Fig. 15 shows the corrosion current density variation with aging temperature at two aging times. It can be seen from this figure the lowest value of corrosion current showed by the as-received material (i.e. the material free of any precipitates) and then increased substantially for the aged samples. Same trend and almost linear behavior of curves during the aging temperature range and the effect of increasing aging time is so clear. The curves show divergence at the temperature of 550  C and subsequently, this divergence becomes less at the temperature of 750  C. Increasing of aging temperature from (550–850  C) increases the current density almost linearly from (1.8–2.42 mA.cm2) for samples those aged for 24 h while from (2.1– 2.5 mA.cm2) for those aged for 48 h. During the polarization test, the metal oxidation takes place when cathodic scan ends and the anodic scan starts (i.e. the upper part of polarization curve). The oxide film starts to form very slowly and when the surface of metal exposed to the test

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Fig. 13. Polarization Behavior of the Isothermally Aged Samples at 48 h.

Fig. 14. Corrosion Potential Variation with Aging Temperature.

Fig. 15. Corrosion Current Density Variation with Aging Temperature.

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Fig. 16. Variation of Passivation Current Density with Aging Time.

has completely covered the corrosion current becomes independent of potential, Ipass appears. When the potential continues to increase to high value, the passive break down and allows high current density to pass. It is found that the Ipass remained constant with aging temperature change. But it increases in its value when the aging time increases. The material that easier to passivate (i.e. easier to lose its electrochemical reactivity) that has lower Ipass. Fig. 16 clarifies the change of the Ipass with aging time. It is clear that the increase of aging time produces passive films have not the same stability. Easier passivation observed at the as-received material and this decreases with increasing the aging time. As a concluded result of polarization test that the corrosion resistance of AISI204 stainless steel decreases with increasing the aging time and temperature. This result in close agreement with results obtained from the OCP test and weight loss measurements, as well as it supports the microstructure analysis results. 3.3. Mechanical properties Aging of AISI204 stainless steel at different temperatures and for different periods produced a change in its mechanical properties. This change varied between slight and notable depending on aging conditions. One of the mechanical properties that showed a change as a result of the aging process is yield strength as shown in Fig. 17a. Initially, little effect of aging process observed at temperature 550  C, where the yield strength of the samples those aged for 24 h decreased by 7.8% while those aged for 48 h decreased by 3.6%. Thereafter, as the aging temperature and aging time increased, the yield strength decreased drastically, wherein the samples those aged at (750 and 850  C) for 24 h showed decreased by (18.5 and 27%) respectively. Meanwhile, more pronounced decrease observed for the samples those aged for 48 h at the same temperatures, where exhibited decrease by (27 and 46%) respectively. Almost identical percentage values of decrease at temperature 650  C, which is 7.7%. The change continued to comprise the tensile strength property as shown in Fig. 17b. It can be seen, the tensile strength is lower than it was in annealed condition (i.e. as-received material) at first. Then, varies with aging temperature between rising and fall. The samples exhibited the same behavior during the two aging times, but those aged for 48 h produced less value for all aging temperatures. Aging of samples at 550  C for 24 and 48 h resulted in a decrease in their tensile strength by (2.5 and 3.9%) respectively. This value of the decrease is growth slightly once the temperature reached 650  C, where becomes (3.7and 4%). Then, the tensile property decreased by (2.7 and 3.3%) at 750  C. Finally, the increase is back again at 850  C to be (3.8 and 5.09%), for aging times (24 and 48 h) respectively. The ductility also had a share from the effect of the aging process, which will be discussed in term of its two forms which are %EL and %RA. The effect of aging process on %RA and %EL is shown Fig. 17c and d respectively. From Fig. 17c, it can be seen that the %RA increased slightly at the temperature 550  C, then negatively affected at 650  C. After that, increased again to reach its maximum value at temperature 850  C by (18 and 21%) of samples those aged for (24 and 48 h) respectively. Increasing of the aging time to 48 h resulted in a decrease in %RA value in the temperatures range (550–750  C), and its positive effect only appears at temperature 850  C. On the other hand, the %EL showed a slight decrease at the temperature 550  C as shown in Fig. 17d. Then, presented an improvement during aging temperatures range (650–850  C) except the samples those aged for 24 h at the temperature of 650  C, they showed a decrease in %EL by 23.26%. The increase of %EL reached (64.8 and 78.7%) at 850  C for aging times (24 and 48 h) respectively. Increasing the aging time to 48 h resulted in a positive increase of %EL in the temperatures range (650–850  C). Aging process affected the microhardness of AISI204 stainless steel as shown in Fig. 18. It is clear from this figure that microhardness of the matrix decreases with increase aging temperature at all aging times. This result in close agreement with that obtained by Yuan et al. [21]. A very little effect of aging observed on the microhardness at temperature 550  C and time 24 h. After that, the effect increases where the microhardness decreases linearly from temperature 650  C to reach the value 215 HV at temperature of 850  C for the same time. Increasing the aging time to 48 h produced further effect, wherein the microhardness continued to decline till reached its minimum value 210 HV at the maximum temperature.

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Fig. 17. Effect of Aging Process on the Mechanical Properties.

The variation of mechanical properties (all were averaged values) is attributed to the microstructural changes during the aging process. These changes resulted in M23C6 carbide phase formation along the grain boundaries. The amount of this phase coarsened and agglomerated with increase the aging time and aging temperature as seen in section of microstructure analysis results. One of the mechanical properties that highly affected was the yield strength property. The lack of this property was subtle at the temperature of 550  C due to few amounts of carbides phase precipitated along grain boundaries. Then, this lack became wider when the aging temperature and aging time increase as a result of an excess amount of precipitation was formed.

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Fig. 18. Effect of Aging Process on the Microhardness.

The reason why precipitation caused degradation of mechanical properties is that the precipitation of M23C6 carbide phase along grain boundaries resulted in depletion of the most important strengthening element in the matrix, which is the carbon [22]. Affected of tensile strength property as a results of precipitation is less than the yield, because of the effect of the second strengthening element, which is the nitrogen. This interstitial solute element did not participate in the formation of carbides during aging process and remained in the matrix. Tensile strength showed a small increase at temperature of 750  C for the two aging times because of at this temperature the precipitation became continuous and caused extra increase for resistance of dislocations motion at grain boundaries. This effect appeared only on tensile strength and did not include other properties. Despite the %EL exhibited a decrease at the temperature of 550  C but in return, the %RA showed an increase at the same temperature. The %RA gives better indication about the ductility than %EL because of it related to the plasticity of material after the starting of necking [11]. The ductility not much affected at the temperature of 650  C although the increase of carbon concentration as mentioned previously but it may be that the nitrogen slightly activates to return the ductility close to the as-received state. The absence of carbon from the matrix was clear at temperatures 750 and 850  C. Despite the existence of another interstitial element, however, depletion of carbon element from the matrix during the aging process caused microhardness decrease. 4. Conclusions The most prominent points that can be drawn from the current study are: 1 Aging of AISI204 stainless steel produced carbide precipitation type M23C6 along grain boundaries and its kinetics was associated with aging time and temperature appreciably. 2 Aging process had a little effect on room temperature tensile strength but severely affected yield strength, ductility, and the microhardness. 3 Increased aging time and temperature resulted in extra deterioration of stainless steel properties. 4 AISI204 stainless steel exhibeted decreasing in corrosion resistance with increasing aging time and temperature as a result of M23C6 carbide precipitation. 5 The passivity of AISI204 stainless steel decreases with increasing aging time (i.e passive current increased with aging time).

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