Effect of nano-grain carbide formation on electrochemical behavior of 316L stainless steel

The effect of low oxygen-partial pressured carburizing on relaxation process for 316L stainless steel is reported. Phase, morphology, and amount of compound formation during initial stage of carburizing are investigated using X-ray diffractometry (XRD) and X-ray photoelectron spectroscopy (XPS). The results show formation and development of surface multilayer with nano-grain-carbide (Cr7C3, Fe7C3, and/or Cr3C2) generation in the layer located below outermost protective layer. The relaxation process has been investigated using electrochemical impedance spectroscopy (EIS). Formation of nano-grain carbide(s) during carburizing causes deterioration effect on the electrochemical behavior of steel. However, the steel with large amount of carbide generation (carburized for 30 min) tends to have higher corrosion resistance (indicated by higher values of Rcl and Rct) than the smaller ones (10 and 20 min) due to the effect of phase, grain size, morphology, and amount of compound formation.

between two copper electrodes in a glass tube. The chamber was evacuated in order to achieve a condition of low vacuum with absolute pressure of ~ 66 kPa and subsequently fed with 99.99%-purity Ar gas with a flow rate of 50 ml/min. DC current with a power of 300 W was applied to specimens for 10, 20, and 30 min for carburizing which resulted in carburizing temperatures of ~ 350, 550, and 600 °C, respectively. After treatment, the sample was removed and cooled down to ambient temperature in air.
Surface morphology of sample was analyzed by X-ray diffractometry (XRD) and X-ray photoelectron spectroscopy (XPS). XRD was performed using Mini Flex II (Rigaku) diffractometer with a Cu Kα X-ray source (1.5406 Å, 30 kV, and 15 mA) at 0.02° step size and 0.3 s step time. The area of 3 × 3 mm on surface of sample was sputtered by Ar + with 1 keV ion energy for 20 s for removal of some contamination before characterized by XPS. The samples were examined using Kratos Axis ULTRA DLD (Kratos) spectrometer equipped with a monochromatic Al Kα X-ray source (1486.6 eV). The base pressure in analysis chamber was approximately 5 × 10 -9 torr. The X-ray source was used with the incidence angle of 45° to surface plane. The operation was done at 150 W (15 kV and 10 mA) with a spot size of 700 × 300 µm 2 and initial photo energy of 1.4 keV. The binding energy of adventitious C 1 s peak at 285 eV was used for calibration of wavelength shift. The spectra were acquired (at a constant take-off angle of 90°) with the pass energy of 20 eV and analyzed with the energy step of 0.1 eV using VISION II (version 2.2.9) software. Impedance of samples were measured by electrochemical impedance spectroscopy (EIS). EIS measurement was performed using the Autolab-PGSTAT302N potentiostat (Metrohm Autolab B.V.) in aerated 3.5% NaCl solution at ~ 25 °C with a Ag/AgCl reference electrode and a platinum counter electrode incorporated with NOVA (1.11.0) software. The exposed surface area of working electrodes was ~ 0.9-1.0 cm 2 . Before EIS measurement, the open circuit potential (OCP) of working electrode was monitored for 2000 s or until a stable OCP was achieved. The amplitude of applied sinusoidal potential was 10 mV (r.m.s.) around the OCP and the frequency was controlled to be of between 100 kHz and 0.1 Hz. For each analysis condition, the EIS measurement was performed for at least three times by using a new sample in fresh solution. The closest single result to the average of multiple data was chosen to be the representative of each analysis condition. The electrochemical parameters of steels were determined by fitting the EIS experimental data using NOVA (1.11.0) software.
The XRD and XPS results reveal surface morphology of steel in surface and near surface regions (by providing depth of analyses in micro-and nanoscales). For XPS results, it should be noted that Ni 2p signal was covered with background and is not present in this work. This reflects very low content of Ni in surface region and instability of Ni-containing compound in this carburizing condition. The XRD and XPS results show that the surface of untreated steel consists of a metallic bulk (solid solution of Cr in γ-Fe) which located under oxide-containing layers of Cr 2 O 3 , FeCr 2 O 4 , and FeO. Formation of FeO and FeCr 2 O 4 generally occurs in a low chromium-content region. This region is produced by a slow diffusion of chromium at low temperature. The formation of FeCr 2 O 4 is given by 15 www.nature.com/scientificreports/ Fe 2 O 3 was not detected due to possession of lower thermodynamic stability than Cr 2 O 3 at ambient temperature 34 . A presence of Cr 23 C 6 reflects a precipitation of this phase which generally observed in austenitic stainless steel and aged 316L stainless steel 6 . The precipitation resulted from a reaction 35 The carbon-oxide compound (with binding energy corresponding to carboxyl ((CO*)OH)) observed in the spectra was due to surface contamination. Observation of hydroxides resulted from the exposure of steel to moisture as given by 36 where M denotes Fe or Cr. The Fe(OH) 3 peaks are proposed to be not observed in the untreated steel due to Fe(OH) 3 was less stable than Cr(OH) 3 when the uncarburized steel exposed to air 37 . The peak observed at ~ 532.6 eV (O 1s) contributed by M(OH) 3 is therefore proposed to be of Cr(OH) 3 located at the outermost surface of untreated steel. The results are in agreement with previous research 38 39 . Formation of M(OH) 3 is proposed to be given by For the steel treated for 10 min, no adding carbide produced by the carburizing process was observed at near surface by XPS. However, the nano-grain carbide was observed at a higher depth by XRD where the concentration of carbon in steel reached a critical level for Cr 7 C 3 formation as given by 35 This equation indicates higher stability of Cr 7 C 3 (in comparison with Cr 23 C 6 ) when the steel contains larger amount of carbon as reported by previous research 4, 23 . Observation of additional Fe 2 O 3 simultaneously with disappearance of Fe and Cr indicates oxidation of these metals which results in larger amount of oxide formation. The formation of Fe 2 O 3 resulted from the reaction between outward-diffusing Fe 2+ (through Cr 2 O 3 ) 40 and inward-diffusing O 2− at outermost surface of steel. The reduction of peak intensity in O 1s spectrum of low oxygen containing oxide such as FeO was due to possession of lower stability of this phase than the untreated steel. The reduction reflected larger amount of oxygen content in the surface of steel treated for 10 min. The high oxygen content also resulted in oxidation of Cr 2 O 3 to form CrO 3 . The formation of carbides in steel treated for 20 min resulted from the reactions between C and compounds in steels 15,35,41 : The formation of Cr 3 C 2 was difficult to be confirmed solely by XPS due to the binding energies of Cr 2p (~ 575.8 eV) and C 1s (~ 286.7 eV) for Cr 3 C 2 are very close to other chromium carbide 23,41 . This carbide is proposed to be Cr 23 C 6 which observed in uncarburized steel as described by Eq. (3). In thermodynamics theory of phase transformation, the phase existence and stability associate with formation (nucleation and growth) and decomposition of phase which can be indicated by amount of phase and grain size provided by XRD results. The XRD results show lower thermodynamic stability of Cr 7 C 3 (indicated by the lower peak intensity and smaller grain size as described in Supplementary material 1) for the steel treated for 20 min when compared to 10 min due to some reasons. Cr 7 C 3 in steel treated for 20 min decomposed and reacted with carbon to form Cr 3 C 2 as given by reaction (10). The expense growth of Cr 3 C 2 nanograins was able to suppress the Cr 7 C 3 neighbors. The relative amount of Cr 3 C 2 was larger than Cr 7 C 3 after carburizing time of 20 min.The contents of Cr 7 C 3 and Cr 3 C 2 increased with increasing carburizing time. The results show that the highest carbon-containing carbide, Cr 3 C 2 , formed due to possession of high thermodynamics stability when chromium-containing material possessed large amount of carbon content at the suitable temperature for carbide formation. The large amount of carbon is reflected by a presence of a peak of graphite in which resulted from deposition of unreacted excess carbon. Disappearance of FeO and formation of Fe 7 C 3 when the steels possessed high oxygen and carbon contents are proposed to be due to the reaction 1,42 : Some nascent carbon (from carbon source) had not been involved in carbide formation and existed as free carbon. Some free carbon was amorphous and not seen in the XRD scan. Amount of free carbon on the steel Carbon can diffuse through the α-Fe as an additional path. The formation of carbide was therefore attributed to a combined effect of traditional diffusion, grain boundary diffusion, and diffusion through the α-Fe particles. The XPS and XRD results therefore reveal surface morphologies of uncarburized steel and steels carburized under low oxygen-partial pressure as multilayer reported in Table 1. The multilayer of treated steels consisted of nano-grain carbide(s) generated in the compound layer that located between outermost protective layer and inner base stainless steel. The steel possessed development of phase and amount of carbide formation as carburizing time increased from 10 (Cr 7 C 3 ) to 20 and 30 min (Fe 7 C 3 + Cr 7 C 3 + Cr 3 C 2 ).

Relaxation process and impedance
Electrochemical behavior of the steels carburized at 300 W for 10 to 30 min and the uncarburized steel was investigated using EIS. The results (Fig. 4) and parameters (Table 2) obtained by curve fitting using the equivalent circuit with two time constants 43,44 are shown. EIS parameters including constant phase element (CPE)-capacitive parameter (Q), capacitance associated with the CPEs (C CPEcl and C CPEdl ), passive (resistive) impedance of electrode (Z′), and α were calculated and reported. The value of α associates with microscopic surface roughness. The values of 0, 1, and -1, correspond to resistive, capacitive, and inductive behaviors, respectively 45,46 . Z′ is a summation of a polarization resistance of steel (R p ) and Ohmic resistance (R e ). R p is obtained by summation of chemical layer resistance (R cl ) and charge transfer resistance (R ct ). C CPEcl and C CPEdl were calculated from Q CPEcl , Q CPEdl , R p , and α (using normal distribution) as given by 47 The relaxation process was clarified by Faradaic impedance of electrode which was simulated using the equivalent circuit with two time constants as shown in Fig. 4a. This circuit is appropriate for fitting of experimental impedance data of 316L stainless steel obtained in aerated media 44 . The impedance was constituted of real (Z′) and imaginary (Z′′) components corresponding to passive (resistive) and reactive (capacitive and/or inductive) contributions, respectively. The Faradaic impedance of steel was divided into two parts: chemical layer and double layer impedances which each contributed by Z′ and Z′′. These two impedances each consist of resistance (R) and capacitive impedance of a constant phase element which is given by 48 where Q, j, and ω are CPE-capacitive parameter, √ −1 , and angular frequency, respectively. The Nyquist plots of steels in Fig. 4a,b each show one time constant that indicates one relaxation or rate determining process for each steel. The relaxation frequencies (f r ) of processes were in the range of below 1 Hz which indicated the rate determining relaxation process of oxygen adsorption on electrode surface 44,48 . f r values for the carburized and uncarburized steels are in the order of magnitude o 0.1 and 0.01, respectively, as shown in Table 2. The adsorbed oxygen influenced on conversion and uncharged diffusion process. The conversion process associates with redox reaction that occurs in material-surface region. In redox reaction, metal atoms are oxidized and converted to metal ions and electrons. O atoms dissociated from O 2 gas molecules (in electrolyte) adsorbing onto the surface are reduced by these electrons which results in generation of O 2−7,49 . In the testing environment of electrolyte with no carbon potential, Cr 3 C 2 was less thermodynamically stable than Cr 7 C 3 and underwent a larger amount of decomposition. The decomposition gave rise to the redox reaction or conversion process due to Cr 3+ decomposed from the carbide was able to promote the reduction of adsorbing O atoms. The rising of redox reaction resulted in reduction of R ct of the steel treated for 20 min when compared to 10 min. The uncharged diffusion process associates with O 2 diffusion (perpendicular to the surface) in a stagnant gas layer that establishes an O 2 concentration gradient (producing diffusion impedance) over the surface 7,50 . As active species, O 2 diffused down the concentration gradient which arisen between the material surface and the location from material surface where the O 2 concentration equal to a bulk solution. The bulk solution outside the stagnant layer possessed the higher O 2 concentration than at the material surface. However, slow dissociation of O 2 on the surface (which implied slow reduction rate) resulted in the accumulation of O 2 and decrease in concentration gradient which gave rise to R ct of the steel treated for 30 min. R cl of the treated steels were lower than R ct for two orders (12) (13)   www.nature.com/scientificreports/ of magnitude which indicated small contribution of migration process in the corrosion of treated steels. The migration process associates with ion migration in chemical layer, and chemical layer impedance which depends on the structure of layer including phase, amount of compound, and grain size. The higher value of R cl (~ 4.5 times) for the steel carburized for 30 min when compared to 10 and 20 min reflected the larger contribution of chemical layer impedance as reported in literature 8,51,52 . The results are in agreement with previous research 53 . XRD results show that Cr 7 C 3 is the major compound formed in the carburized steels. The grain size of Cr 7 C 3 therefore influenced on migration of metal ions which was largely contributed by a preferential diffusion pathway such as grain boundary. The small grain size in steel treated for 20 min contributed to the fast diffusion which corresponded to low R cl . However, R cl of the steel treated for 20 min was higher than 10 min. It is proposed that the large thickness of compound layer and large grain size of Cr 3 C 2 had some contribution in retardation of both traditional and grain-boundary diffusion in the steel treated for 20 min. The R cl was therefore contributed by larger thickness of compound layer and grain-boundary diffusion associated with small Cr 7 C 3 and large Cr 3 C 2 grains. These contributions were larger than grain-boundary diffusion associated with large Cr 7 C 3 grains in thin compound layer of the steel treated for 10 min. Even though the grain size of steel treated for 30 min was not the largest, the steel possessed the largest amount of Cr 7 C 3 formation (as shown by XRD) and thickest compound layer (Supplementary material 2) which resulted in the highest R cl . A deviation from semi-circle shape of the Nyquist plots shown in Fig. 4b reflects a larger contribution of resistive than a capacitive one. The resistive contribution (corresponding to Z′) was therefore the main contribution in Faradaic impedance. Besides, a capacitance calculated from Q CPE was able to imply the electrochemical susceptibility of steel. CPEcapacitance of chemical layer (C CPEcl ) was attributed to the accumulation of charges (metal ions) 47 Table 2. This reflects that oxygen adsorption had a greater effect than the migration process. The passive impedance reflecting corrosion resistance of steel therefore had an ascending order of the steel treated for 20 < 10 < 30 min. The order is in agreement with the results of potentiodynamic polarization reported in Supplementary material 3. The value of R p for untreated steel was 1 to 2 orders of magnitude higher than the treated steels. The contribution of R cl in R p was larger than R ct which reflected the large contribution of protective layer for untreated steel.
The results show that formation of nano-grain carbide(s) during carburizing had deterioration effect on the electrochemical behavior of steel due to disturbance of formation (as well as self-healing) of protective layer by delaying the growth of layer to achieve the critical thickness of protection 54,55 . However, the steel with large amount of carbide generation (treated for 30 min) tended to have higher corrosion resistance than the smaller ones (10 and 20 min) due to some reasons. Possession of smaller amount of ion migration as indicated by higher value of R cl , and larger amounts of adsorption and accumulation of O 2 in a stagnant gas layer on surface of steel which resulted in reduction of concentration gradient and gave rise to R ct . The electrochemical behavior of carburized 316L austenitic stainless steel in this study exhibits the same trend as the 420 martensitic stainless steel of our previous work 7,54 due to the formation of nano-grain-compound layer. The trend also shows some correspondence to previous research in which corrosion resistance of carburized 316L stainless steel (with formation of micro-grain-compound layer) was superior to the uncarburized steel. The enhancement of micrograin-carburized layer resulted from retardation of mobility of oxygen-vacancy and metal-ion 52,55,56 . Besides, this also reflects the different electrochemical behavior between the carburized steels with nano-and micro-grain carbides. The corrosion resistance associated with electrochemical behavior of stainless steel is therefore mainly influenced by grain size, and amounts of hydroxide, oxide, and carbide formation of Cr. These compounds have an effect on determination of relaxation process of steel. For the sustainable operation of 316L stainless steel parts, the control of wet corrosion in period of formation of nano-grain carbide is highly recommended. The wet-corrosive environment and in-service period should be controlled and extended, respectively, until the nano-grain carbide has been developed to micro-grain carbide.

Conclusions
Carburizing of 316L stainless steel in low oxygen-partial pressure resulted in formation of surface multilayer with nano-grain carbide(s). The carbide(s) generated in the compound layer that located between outermost protective layer and inner base stainless steel. The steel underwent development of phase and amount of carbide formation as carburizing time increased from 10 (Cr 7 C 3 ) to 20 and 30 min (Fe 7 C 3 + Cr 7 C 3 + Cr 3 C 2 ). Formation of nano-grain carbide(s) during carburizing had deterioration effect on the electrochemical behavior of steel due to disturbance of formation (as well as self-healing) of protective layer by delaying the growth of layer to achieve the critical thickness of protection. However, the steel with large amount of carbide generation (treated for 30 min) tended to have higher corrosion resistance than the smaller ones (10 and 20 min) due to some reasons. Possession of smaller amount of ion migration as indicated by higher value of R cl , and adsorption and accumulation of O 2 in a stagnant gas layer on surface of steel which resulted in the reduction of O 2 -concentration gradient and gave rise to R ct . Phase, grain size, morphology, and amount of compound formation during carburizing had the www.nature.com/scientificreports/ effect on relaxation process of steel. For the sustainable operation of 316L stainless steel parts, the control of wet corrosion in period of formation of nano-grain carbide is highly recommended.