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

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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  which is nickel by other elements, when nickel prices have been relatively high over the early 1930s . 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 .

This type of stainless steel is widely used for general applications and for pressure vessels where the high temperature and moisture are present . 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. However, chromium-rich  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% . The presence of these precipitates reduces the durability of the material for a certain time. Simmons studied the effect of nitrides on mechanical properties of high nitrogen stainless steel and reported that these nitrides cause embrittlement. Bansod et al. 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 . 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 . All Specimens were isothermally aged at [550, 650, 750, 850 °C] for 24 and 48 h.

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.

For microstructural observations, specimens subjected to standard grinding and polishing processes according to ASTM E3 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 μm 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) . 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 .(1)${w}_{}$${}_{t}=\frac{{W}_{0}-{W}_{t}}{S}$Where wt is the weight loss, $\text{}{\text{W}}_{}$${\text{}}_{0}$ the weight of the specimen before test, ${\text{W}}_{}$${\text{}}_{\text{t}}$ 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  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 , where the values were 9 and 229.22 Ω.cm. The samples were buried at a depth of (50 cm) and all experiments conducted according to ASTM G162  during the winter of 2018.

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

Electrolyte Solution pH Conductivity μS/cm TDS mg/L
Brackish water 7.46 4460 2240
RO water 7.4 140 69
Sea water 7.5 75200 54060

Simple humidity test is adapted to perform wet environment experiments (the forth medium). The constructed corrosion test apparatus as shown in 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 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 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 s−1.Vickers microhardness test of the matrix of specimens, those used for optical microscopy examination conducted according to the ASTM E92 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.