Effect of milling parameters on the surface integrity and corrosion behaviour of austenitic stainless steel

Abstract

This study investigates the influence of milling parameters on the surface integrity and corrosion behavior of austenitic 304 stainless steel (SS). The experiments were designed using a Taguchi L16 orthogonal array to evaluate the effects of feed rate and cutting speed on surface characteristics. Commercial-grade 304 SS was subjected to milling under varied conditions, followed by microstructural characterization and electrochemical corrosion testing. Surface integrity was assessed through surface roughness (Ra, Rz, Rt, etc.) and Vickers microhardness (HV200), while corrosion behavior was evaluated via potentiodynamic polarization in a 0.1 mol/L NaCl solution using a three-electrode setup. The results revealed that lower feed rates and moderate cutting speeds tended to produce smoother surfaces and increased microhardness due to work hardening, which contributed to enhanced pitting resistance. A polished specimen was included for comparison and showed the lowest surface roughness and microhardness values. However, the corrosion potential and pitting resistance of the polished specimen were not significantly better than those of the milled specimens under optimal parameters. This suggests that surface roughness alone does not dictate corrosion behavior; rather, a combination of topographical and microstructural factors, such as the presence of a work-hardened layer and nanocrystalline grain structure, plays a critical role. The study demonstrates the complex interaction between milling conditions, surface integrity, and corrosion performance, offering valuable insight for selecting machining parameters that optimize both surface quality and corrosion resistance in industrial applications.

Keywords:
Work hardening; Pitting resistance; Surface roughness; Microhardness; Taguchi method

1. INTRODUCTION

Commercial austenitic stainless steels (SS), such as 304 SS and 316 SS, are widely used as structural materials for pressure vessels and nuclear power plant equipment due to their superior general corrosion resistance, which emanates from the passive film. However, these steels’ susceptibility to localised corrosion, including pitting and stress corrosion cracking (SCC), in aggressive environments exacerbates the risk of cracking which has been reported in recent decades. Pitting behaviour is closely related to SCC where chloride is present [¹]; cracks emerging from locations where pitting corrosion provide the combination of a high stress field and the aggressive, acidic environment critical for initiation of SCC [²]. Pit growth induced dynamic plastic strain and the coalescence of small embryonic cracks from pits may be the key factors determining the transition from a pit to a SCC [³, ⁴]. It is possible to predict SCC tendency from early stage corrosion behaviour, such as pitting and metastable pitting, only if the pitting behaviour and its relationship to SCC are well-understood [⁵, ⁶]. There have been several notable studies on the factors concerning pitting corrosion, including surface roughness [⁷, ⁸, ⁹], residual stress and applied stress [², ¹⁰, ¹¹], inclusions [², ¹²], metallurgical heterogeneities [¹³], and temperature [¹⁴].

Many of these factors are comprised in surface integrity, a term which defines the near-surface physical, mechanical, and metallurgical properties of a material. The surface integrity characteristics of austenitic SS are strongly dependent on the processing method (cutting, grinding, and rolling operations) and, consequently, lead to different corrosion behaviours of austenitic SS [¹⁵, ¹⁶, ¹⁷]. Milling is an important processing method that can be used to obtain components with all imposed dimensional and geometrical requirements [¹⁸]. Because of advances in machine tools and cutting tool materials, using milling as the final operation of work pieces has become a trend that will not only affect the required dimensional accuracy but also the surface integrity of the material [¹⁹] and the corrosion behaviour of fabricated equipment in service. The effects of processing on the corrosion behaviour of SS have only been qualitatively studied in the form of different processing methods [⁹, ¹⁶, ²⁰, ²¹, ²², ²³, ²⁴, ²⁵, ²⁶, ²⁷, ²⁸, ²⁹]; few investigations have examined the role of milling parameters in the corrosion performance of austenitic SS. In this study, an effort is made to evaluate the effect of prior milling parameters on surface integrity and, subsequently, on the corrosion behaviour of austenitic SS.

2. MATERIALS AND METHODS

2.1. Work piece material

The material selected in this research was commercial grade type 304 SS. The chemical compositions of the 304 SS plates are presented in Table 1. The elemental composition was initially obtained from the supplier’s material certificate and subsequently verified using optical emission spectroscopy (OES, Spectro Maxx LMX06, SPECTRO Analytical Instruments, Germany). The instrument provides a measurement accuracy of ±0.05 wt.% for major alloying elements. This verification ensures reliability in material specification for experimental consistency. As received 304 SS plates were first cut into dimensions of 300 mm in length, 30 mm in width and 30 mm in thickness using wire cut electrical discharge machining (WEDM). The plates were solution annealed at 1050 °C for 60 min, followed by water quenching. This heat treatment was applied to dissolve chromium carbides and restore a uniform austenitic structure, thereby improving plasticity and corrosion resistance. It also ensured that all specimens started with consistent and reproducible metallurgical conditions, free from prior work hardening or sensitization. Then, the plates were roughly milled to remove a thin oxide layer induced by heat treatment processing. Next, 100-mm long, 30-mm wide and 5.5-mm thick specimens were machined from the plates using WEDM for milling testing.

Table 1
Chemical composition of 304 stainless steel (wt.%).

Si	S	Cr	P	Ni	Mn	Fe
0.85	0.03	18.68	0.02	7.83	1.73	balance

2.2. Milling tests

An experimental orthogonal Taguchi array was used in the end mill tests to determine an empirical model of surface integrity variation with milling parameters. In general, there are three main machining parameters in end mill processing including the cutting speed Vc (m/min), feed rate f (mm/rev), and cutting depth (mm). In the present study, the cutting speed Vc and feed rate f were set to two variables. The cutting depth was set to 0.5 mm for all tests, which is a typical machining parameter used in the finish milling. The specifications of the tested Taguchi orthogonal array are listed in Table 2. The cutter used in the tests had a fixed diameter of 120 mm equipped with 6 edges, the cutting tool rake angle was 0° and the clearance angle was 8°. The adopted cutting speeds were between 120 and 450 m/min; the minimum (120 m/min) was slightly higher than the cutting speed, which leads to the build-up edge during SS machining. The Vc with 450 m/min was a typical high speed milling parameter. The feed rates were between 0.1 and 0.4 mm/rev. All the milling tests were performed in the face mill mode on a 3-axis CNC machining center equipped with a FANUC 0i control system. A BT40 taper tool holder was used to mount uncoated carbide cutting inserts. The milling was conducted in the longitudinal direction at only one face for each specimen, which refers to the length of the specimen.

Table 2
Taguchi orthogonal arrays.

F (mm/rev)	VC (m/min)
	120	220	315	450
0.1	1#	5#	9#	13#
0.2	2#	6#	10#	14#
0.3	3#	7#	11#	15#
0.4	4#	8#	12#	16#

For comparison, a specimen was manually polished with 400, 600, 1200 and 2400 grit diamond paste, gradually, after being roughly milled. The polished specimen was designated as 17#.

All the specimens were then machined into two different forms with WEDM. Specimens with dimensions of 10 mm (length) × 10 mm (width) × 5 mm (thickness) were used for the corrosion resistance tests; specimens with dimensions of 50 mm (length) × 10 mm (width) × 5 mm (thickness) were used for the surface integrity measurements.

2.3. Surface integrity measurements

Among the parameters included in surface integrity, typical surface alterations such as surface roughness and micro-hardness were evaluated and correlated with corrosion behaviours of specimens in this study.

Surface amplitude parameters and spacing parameters including the maximum profile valley depth (Rv), maximum height of the profile (Rz), arithmetic mean deviation of the assessed profile (Ra) and total height of the profile (Rt), as well as the mean width of the profile elements (RSm), were measured for each specimen using a Taylor-Hobson stylus profilometer according to ISO 4287–1997. All the surface roughness measurements were operated across machining marks. Surface hardening was characterised by Vickers microhardness testing, using an indenter load of 200 gf (HV0.2). To capture the mechanical response of the actual surface exposed during corrosion testing, microhardness measurements were performed directly on the milled surface rather than on cross-sectional profiles. Measurements were carefully taken on relatively flat areas to mitigate the influence of surface roughness, with three indentations per specimen averaged for analysis.

2.4. Corrosion resistance tests

To analyse the effect of the milling parameters on the corrosion behaviour of 304 SS, potentiodynamic anodic polarisation tests were conducted. The specimens were ultrasonically cleaned with acetone, rinsed in distilled water and mounted with epoxy resin to leave a 1cm² milled surface exposed to the experimental solution. The back surfaces of the specimens were connected to the electrochemical workstation through a copper wire. The copper wire was insulated to isolate it from the solution. The potentiodynamic anodic polarisation tests were performed using a CS350 Electrochemical Workstation (Wuhan Corrtest Instruments Corp., Ltd., China) in a conventional three-electrode cell exposed to air at room temperature. A saturated calomel electrode (SCE) was used as the reference electrode, and a platinum plate (Pt) was used as the counter electrode for all tests. The corrosive medium was a 0.1 mol/L NaCl solution. No additional surface preparation was applied to the milled surface before polarising. The specimens were immersed in the solution at open circuit potential for 30 min, and then potentiodynamic scanning was initiated from a potential of −500 mV vs. SCE to 1200 mV vs. SCE at a scan rate of 30 mV/min. For each specimen, the electrode potential vs. log current was recorded. The polarisation testing parameter, corrosion potential (E_corr), corrosion current density (i_corr), passivation potential (E_pass) and pitting potential (E_p) were used to evaluate the corrosion behaviour of the specimens.

2.5. Immersion corrosion tests

To complement the potentiodynamic polarization measurements, immersion tests were carried out to evaluate the long-term corrosion behavior of milled and polished 304 SS specimens in 0.1 mol/L NaCl solution. The same milled specimens used for electrochemical testing were immersed in sealed glass containers with 100 mL of solution, kept at ambient temperature (25 ± 1 °C) for 168 hours (7 days). Prior to immersion, the specimens were ultrasonically cleaned with acetone and distilled water, and their initial mass was recorded using an analytical balance with a resolution of 0.1 mg. After immersion, the samples were rinsed, chemically cleaned using a 10% HNO₃ solution to remove corrosion products, and dried before final weighing.

3. RESULTS

3.1. Effects of milling parameters on the surface roughness of 304 SS

Figure 1 demonstrates the effect of the milling parameters on the Ra value of 304 SS. In general, for milling of the austenitic SS, an increase in Ra from 0.227 µm to 10.5 µm could be observed when the feed rate increased from 0.1 mm/rev to 0.4 mm/rev at various cutting speeds. Note that the average value of Ra of 16# specimen was 10.5 µm, which is much rougher than that of the other tested specimens. Therefore, the surface roughness result of 16# specimen was not plotted in Figure 1 for the reason of figure scale. The effect of the cutting speed on the surface roughness was complex, with the higher cutting speed (Vc: 220, 315, 450 m/min) resulting in a smoother surface compared with the lower cutting speed (Vc: 120 m/min) when the feed rate was under 0.2 mm/rev; however, an increasing Ra value was evident at the higher cutting speed when the feed rate exceeded 0.2 mm/rev. This phenomenon was most likely due to the combination of increasing the strain rate, undeformed chip thickness, cutting temperature and the cutting forces [³⁰, ³¹, ³²]. The effects of the milling parameters on the other surface amplitude parameters followed the same tendency as the Ra value; the measured surface roughness parameters are listed in Table 3.

Figure 1
Roughness variation with feed rate and cutting speed.

Table 3
Surface roughness parameters of specimens.

Specimen	Ra (µm)	Rv (µm)	Rz (µm)	Rt (µm)	Rsm (µm)
1#	0.626	1.809	3.42	4.477	0.101
2#	0.669	3.329	5.242	8.148	0.039
3#	0.483	1.3	2.774	3.297	0.09
4#	0.699	1.618	4.05	4.693	0.107
5#	0.227	0.804	1.479	1.672	0.031
6#	0.468	1.593	3.009	3.408	0.073
7#	0.693	1.289	2.982	4.886	0.187
8#	1.512	2.586	5.574	6.332	0.284
9#	0.339	1.059	1.94	2.301	0.056
10#	0.685	1.98	4.411	5.863	0.106
11#	1.723	4.252	7.341	8.77	0.259
12#	0.808	2.97	4.757	5.861	0.083
13#	0.491	1.943	3.252	4.372	0.07
14#	0.559	2.062	4.088	4.951	0.095
15#	1.083	2.983	5.346	5.92	0.138
16#	10.505	28.192	47.589	51.356	0.429
17#	0.072	0.339	0.569	0.852	0.008

The Ra of the comparison specimen, 17#, was 0.072 µm. The surface roughness of the specimen polished to 2400 grit paste was obviously smoother than that of the milled specimens.

To further elucidate the influence of milling parameters on surface integrity, scanning electron microscopy (SEM) was conducted on selected specimens (1#, 9#, 16#, and 17#). The SEM images, shown in Figure 2, reveal significant differences in surface morphology attributable to feed rate and cutting speed. Specimen 1#, processed with the lowest feed rate (0.1 mm/rev) and cutting speed (120 m/min), exhibited smooth machining marks and minimal plastic deformation features. In contrast, specimen 16#, milled at the highest parameters (0.4 mm/rev and 450 m/min), showed pronounced surface damage, including deep grooves, built-up edge particles, and localized microcracks. The polished reference specimen 17# displayed a uniform, smooth surface with no visible deformation features, while specimen 9# showed moderate surface disruption. The degree of surface damage observed correlated well with Ra and Rz values listed in Table 3 and helped explain the differences in pitting potential and corrosion rate. These SEM observations support the assertion that rougher surfaces with machining-induced defects act as stress concentrators and chloride ion accumulation sites, thus lowering the resistance to pitting corrosion.

Figure 2
SEM images of representative 304 stainless steel specimens subjected to different milling parameters and surface treatments: (a) Specimen 1# (Vc = 120 m/min, f = 0.1 mm/rev) shows smooth machining marks with minimal surface damage. (b) Specimen 9# (Vc = 315 m/min, f = 0.1 mm/rev) exhibits moderate roughness with visible grooves and slight particle adhesion. (c) Specimen 16# (Vc = 450 m/min, f = 0.4 mm/rev) reveals severe surface degradation, including deep grooves, built-up edges, and microcracks. (d) Specimen 17# (polished reference) presents a smooth and uniform surface, free from machining-induced defects.

3.2. Effects of milling parameters on the microhardness of 304 SS

The Vickers microhardness results are presented in Figure 3. The microhardness value increased rapidly as the feed rate increased. The measured microhardness of 17# was 241 HV200; however, microhardness values of milled specimens far exceed 241 HV0.2. The results indicate that extensive microstructure modification or so-called work hardening occurred for the milled specimens, and the increase in microhardness was mainly associated with plastic deformation and local martensitic transformation, such as the formation of strain-induced α’-martensite [³³, ³⁴, ³⁵, ³⁶]. Although direct phase identification was not conducted in this study, the observed increase in microhardness and reference to prior studies suggest the probable presence of α’-martensite in the deformed surface layers. Future work will aim to explicitly characterize such phase transformations using techniques such as X-ray diffraction or EBSD to confirm their role in affecting both mechanical and corrosion properties.

Figure 3
Micro-hardness variation with feed rate and cutting speed.

3.3. Effects of milling parameters on the corrosion behaviours of 304 SS

Figure 4 presents the potentiodynamic polarisation curves of typical specimens. Such polarisation curves were recorded at a data acquisition rate of 2.0 Hz, and these curves supplied information on the susceptibility of the specimens to various corrosion. In general, the polarisation curves of specimens coincided with the typical polarisation plot of 304 SS. A cathodic region, active region, active–passive transition region, passive region and transpassive region existed in the polarisation curves. There was no obvious difference in the cathodic current between specimens; however, the difference in anodic behaviour indicted the effect of the machining method and parameters. Table 4 presents the electrochemical parameters obtained from the potentiodynamic polarisation curves. Notably, E_corr of the polished specimen shifted to a less noble value (−215 mV/SCE), and i_corr increased compared to most milled specimens. Therefore, the general corrosion resistance of the milled 304 SS specimens was superior to that of the polished specimen. Therefore, the general corrosion resistance of milled 304 SS was superior to that of polished 304 SS.

Figure 4
Potentiodynamic anodic polarization curves for representative specimens in 0.1 mol/L NaCI solution.

Table 4
Electrochemical parameters obtained from potentiodynamic polarization curves for milled and polished specimens.

SPECIMEN	CORROSION POTENTIAL (MV VS. SCE)	CORROSION CURRENT DENSITY (A/cm2)	PASSIVATION POTENTIAL (mV vs. SCE)
1#	−142	1.6 × 10−8	−80
2#	−130	2.2 × 10−8	−97
3#	−145	7.5 × 10−8	−45
4#	−99	4.9 × 10−9	30
5#	−169	1.4 × 10−8	−91
6#	−130	1.6 × 10−8	−58
7#	−128	8.5 × 10−9	−63
8#	−158	5.7 × 10−9	−23
9#	−101	8.9 × 10−9	−61
10#	−111	1.2 × 10−8	−57
11#	−158	1.1 × 10−8	−83
12#	−141	1.4 × 10−8	−49
13#	−153	1.0 × 10−8	−65
14#	−109	7.5 × 10−9	−25
15#	−173	1.8 × 10−8	−13
16#	−154	3.8 × 10−8	−12
17#	−215	2.6 × 10−8	−14

The final breakdown of the milled specimens was recorded in the curves accompanied by many metastable pitting events and current noise, leading to some difficulty in determining the pitting potential (E_P). This phenomenon was most likely due to the rougher surface produced by milling compared with the conventional polarisation specimens finish processed by diamond paste or abrasion paper [³⁷]. In the present work, E_p was defined as the minimum potential at which the current began to increase continuously. The effect of the milling parameters is demonstrated in Figure 5a and b, where the measured E_p and passive region (the difference between E_p and E_pass) are plotted as a function of the milling parameters. As the E_p and passive region increased, the resistance of the material to pitting corrosion increased. E_p decreased from 370 mV vs. SCE (1#) to 220 mV vs. SCE (16#) while the passive region decreased from 450 mV vs. SCE (1#) to 232 mV vs. SCE (16#) due to the increase in the feed rate and cutting speed. Based on the measured E_p and passive region, the resistance to pitting corrosion of the milled 304 SS decreased when the feed rate and cutting speed increased. The large difference in E_p and the passive region revealed the significant effect of the milling parameters on the pitting resistance of 304 SS. Furthermore, the E_p of 17# polished specimen was 300 mV vs. SCE, and a passive region of 314 mV vs. SCE was observed.

Figure 5
The pitting potential and passive region variation with feed rate and cutting speed.

3.4. Immersion test results

Figure 6 shows the average corrosion rates obtained after 168 h of immersion. The corrosion rates ranged from 0.012 mm/year to 0.089 mm/year depending on the milling parameters. The polished specimen (17#) exhibited a corrosion rate of 0.034 mm/year. Notably, specimens milled at lower feed rates (1# and 5#) exhibited the lowest corrosion rates (0.012–0.018 mm/year), while those processed at higher feed rates and cutting speeds (especially 16#) showed markedly higher corrosion rates up to 0.089 mm/year. Table 5 summarizes the immersion test results, showing a general trend of increasing corrosion rate with increasing surface roughness and decreasing pitting potential (Ep). These results are in good agreement with the potentiodynamic polarization data.

Figure 6
Corrosion rates of selected specimens after 168 hours of immersion in 0.1 mol/L NaCl solution. The corrosion rate increased with milling parameters, consistent with the pitting potential trends obtained from potentiodynamic polarization tests. Specimen 1#, prepared at the lowest feed rate and cutting speed, exhibited the lowest corrosion rate (0.012 mm/year), while specimen 16#, processed at the highest parameters, showed the highest rate (0.089 mm/year).

Table 5
Corrosion rates obtained from immersion tests.

SPECIMEN	RA (µm)	EP (mV vs SCE)	CORROSION RATE (mm/year)
1#	0.626	370	0.012
5#	0.227	345	0.014
9#	0.339	328	0.016
13#	0.491	313	0.019
16#	10.505	220	0.089
17#	0.072	300	0.034

4. DISCUSSION

There is a multiplicity of changes in the surface integrity induced by varying milling parameters. The effect of the surface roughness and microhardness on the pitting resistance will be discussed below. As a consequence of relying on the Ra value as an indicator of milling, a low feed rate accompanied by a high cutting speed is often applied for SS finish machining in workshops to achieve a smooth surface of SS. In consideration of pitting, the correlation of E_p with different surface amplitude parameters is presented in Figure 7. The four linear fitting equations in Figure 7 exhibit the same trend; the pitting potentials decreased with an increase in the surface roughness. This phenomenon indicates that a rougher surface has a deleterious effect on the pitting resistance of milled 304 SS. This result is consistent with early studies on the effect of surface roughness, in which it was reported that a smoother surface could improve the average electron work function (EWF) [³⁸, ³⁹] and reduce the nucleation of stable pitting [⁷, ⁸, ⁹]. Thus a smoother surface results in superior pitting resistance of SS. However, the four correlation coefficients in Figure 7 indicate that E_p only has a middle-grade correlation with surface amplitude parameters, while Rz exhibits the highest correlation coefficient (R = −0.56) among the measured surface roughness parameters. Rz is defined as the sum of the heights of the largest profile peak height and the largest profile valley depth within a sampling length. This result may be explained as an additional stress concentration effect, which was attributed to the deeper profile valley, and the more difficult diffusion of the dissolved species to the outward solution was attributed to the higher profile peak. Rt also measures the sum of the height of the largest profile peak and the deepest profile valley within a evaluated length but exhibited the lowest correlation coefficient (R = −0.44). In the present study, the sampling length of Rz was 0.8 mm, and the evaluated length of Rt was 4 mm. This result could be attributed to the narrow dimensions of pit mouths, which were usually less than 200 µm, and the size dependency of the correlation between corrosion resistance and surface roughness. Analogous results were reported by LI and LI et al. [³⁸], SHAHRYARI et al. [⁴⁰], and LEE et al. [⁴¹]; in their studies, poor correlations between corrosion resistance and surface roughness were observed when Ra was relatively high. The highest correlation coefficient, R = 0.56, achieved by Rz is not in the range of high linear correlation, indicating E_p of milled SS was not merely dominated by amplitude height parameters (Rz, Rt, Rv, Ra). ZUO et al. [⁸] suggested that the aspect ratio of the surface groove, w/d, be taken as a parameter describing the linear relationship between surface roughness and pitting. However, the measurement and calculation of the aspect ratio were quite difficult in the workshop. In this study, a substitutive parameter RSm/Rz was examined. Figure 8 presents the relationship between RSm/Rz and E_P; however, the poor correlation coefficient R = 0.12 indicates RSm/Rz is not a suitable substitutive parameter for the aspect ratio w/d. E_p was also correlated with parameters other than RSm/Rz, such as RSm/Ra, RSm/Rv, and RSm/Rt. However, poor correlations existed between E_p and these parameters (not shown).

Figure 7
The correlation of E_p with different surface amplitude parameters.

Figure 8
The relationship between RSm/Rz and E_P.

All the microhardness measurement results of the milled specimens were far greater than that of 17# specimen. This result indicates the existence of a near-surface work harden layer induced by the milling process. TURNBULL et al. [¹⁷], GHOSH and KAIN [⁴²], and ACHARYYA et al. [⁴³] presented electron back-scattered diffraction (EBSD) maps of surface machined specimens. The specific features of the microstructure after surface machining were summarised: an intense plastic deformed layer extending 100 μm below the machined surface, the small size of grains down to the nanolevel, the high densities of dislocations and slip bands, and local martensitic transformation. It should be emphasised that 17# specimen was manually polished after being roughly milled, which would remove the extremely deformed work harden layer produced by the rough mill procedure. Thus, the E_P of 17# specimen should be approximately considered as the pitting potential of the 304 matrix after being solution annealed. The Rz value of 17# specimen was 0.569 µm, which was much smoother than the milled specimens. However, the E_P of 17# specimen was only 300 mV vs. SCE, which was lower than the E_P of the specimens produced using small milling parameters (feed rate = 0.1 mm/rev). This phenomenon also suggested that the surface roughness was not the only factor affecting the pitting performance of the milled specimens. Milled austenitic SS consists of a double-layer structure in the thickness direction, a matrix layer beneath and a work harden layer above. Pitting corrosion occurred at the surface layer, and consequently, the actual pitting resistance of the milled SS depended on the character of the work harden layer rather than the matrix layer. The interaction of the strain rate and thermal-mechanical field for varying milling parameters resulted in distinctive work harden layer properties including the surface roughness, microhardness, and pitting resistance. The work hardening effect during austenitic SS machining was mainly attributed to the plastic deformation and local martensitic transformation. It is reported that the local martensitic transformation included in the austenitic phase was detrimental to the pitting resistance and the plastic deformed nanocrystalline grain structure was beneficial to the pitting resistance [¹⁶, ¹⁷, ²³, ²⁴, ²⁵]. Note that the 17# specimens exhibited the smoothest surface and the lowest microhardness among the specimens; however, the E_P of 17# was only at middle-grade. Thus, the relative higher E_P of 1#, 5#, 9#, and 13# should be attributed to the plastically deformed nanocrystalline grain structure produced by the small feed rate; as the milling parameters increased, the local martensitic transformation and rougher surface condition dominated the pitting resistance of the specimens. Furthermore, the superior general corrosion resistance of the milled specimens compared with that of the polished specimen should be attributed to the refine of grain structure.

The key observation of this experiment is that the milling parameters exhibited a decisive effect on the corrosion behaviours of 304 SS. 304 SS exhibited superior general corrosion resistance induced by milling. The E_P and passive region of the specimens decreased with an increase in the feed rate and cutting speed. A surface work hardened layer with reinforced corrosion resistance could be achieved at a feed rate of 0.1 mm/rev.

Additionally, the immersion test results supported the findings from electrochemical tests, confirming the superior corrosion resistance of milled specimens under lower feed rate and cutting speed. The lowest corrosion rates were observed for specimens with high pitting potential and smoother surfaces. In contrast, specimens with rough surfaces and low Ep (e.g., 16#) exhibited the highest long-term corrosion rates. The consistency between polarization and immersion tests reinforces the role of surface integrity in determining the overall corrosion performance. From an industrial perspective, optimizing milling parameters not only improves surface integrity and corrosion resistance but also yields significant practical benefits. Lower feed rates and cutting speeds, which were found to enhance pitting resistance, are generally associated with reduced tool wear and lower cutting forces, thereby extending tool life and minimizing maintenance costs. Additionally, achieving smoother surfaces at moderate cutting parameters can reduce the need for secondary finishing operations, thus conserving energy and reducing production time. In terms of sustainability, enhanced corrosion resistance in milled components translates to longer service life and reduced material degradation, which lowers the frequency of part replacements and the associated resource consumption. Minimizing surface defects and corrosion susceptibility also reduces the risk of hazardous material leaching, contributing positively to environmental protection. These observations underscore the potential of controlled milling as a strategy aligned with green manufacturing principles, where process optimization benefits both performance and ecological impact.

5. CONCLUSION

Austenitic 304SS was milled using various cutting parameters, the surface integrity and corrosion behaviours of specimens were examined. The following conclusions were derived from this study.

1. After a series of milling tests using the Taguchi method, it was verified that the milling parameters play a crucial role in the surface integrity of austenitic SS. The surface roughness increased as the feed rate increased. The smoothest surface was produced at a cutting speed of 220 m/min and a feed rate of 0.1 mm/rev; the roughest surface was produced at a cutting speed of 450 m/min and a feed rate of 0.4 mm/rev.

2. After milling, 304 SS exhibited a higher microhardness than that of the polished material. Intensive work hardening effects were measured under an increased feed rate.

3. 304 SS exhibited a superior general corrosion resistance induced by milling compared with that of the polished material.

4. Potentiodynamic polarisation tests generate very different values for E_P; the results differed by as much as 150 mV vs. SCE for the various milling parameters. The pitting resistance decreased with an increase in the feed rate. A surface with reinforced pitting resistance compared with that of original material could be achieved under proper milling conditions with a feed rate of 0.1 mm/rev. The effect of the cutting speed on the pitting resistance was complex; however, greater pitting resistance was achieved at a cutting speed of 120 m/min.

5. The pitting resistance of milled austenitic SS has a tendency to decrease with an increase in the surface amplitude parameters (Rz, Rv, Ra, and Rt), although not overwhelmingly. The microhardness value did not affect the pitting resistance directly; however, the pitting resistance of milled SS could be affected by the plastically deformed nanocrystalline grain structure and local martensitic transformation that was observed in the form of microhardness. Thus, the pitting resistance was dominated by the combination of surface roughness and microhardness. The polished specimen did not exhibit the greatest corrosion resistance, despite having the smoothest surface and the lowest microhardness compared with the milled specimens. Therefore, simple reliance on surface roughness parameters or the microhardness value maybe unsuitable when assessing the corrosion performance of components processed by different methods.

6. BIBLIOGRAPHY

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Publication Dates

• Publication in this collection
  07 July 2025
• Date of issue
  2025

History

• Received
  10 Feb 2025
• Accepted
  24 Apr 2025