Surface and Corrosion Properties of Electrolytic Polished 316L Stainless Steel by Double Melting (VIM and VAR)

Abstract

In this study, STS316L produced by a double-melting process involving vacuum induction melting (VIM) and vacuum arc remelting (VAR) was subjected to extrusion and drawing to form a tube and was subsequently electrolytic polished (EP). The grain size of the obtained STS316L without EP was approximately 55 ㎛, with no difference found after EP. The thickness of the EP layer was measured by AES and TEM, showing values of approximately 10 nm and 15 nm, respectively. After EP, the Cr/Fe and CrO/FeO ratios of the passive layer increased from 1.48 to 1.62 and from 2.15 to 2.26, respectively, while the surface roughness decreased significantly from 0.255 to 0.024 ㎛. Consequently, the corrosion rate decreased in both NaCl and HCl solutions after the EP process. Additionally, the amounts of eluted Cr and Fe ions were reduced from 1.2 to 0.8 ppb and 10.3 to 0.8 ppb, respectively. Furthermore, polarization tests revealed that STS316L treated with EP required a lower current density to reach a passive state, indicating that corrosion behavior was retarded.

1. Introduction

With the growth of the IT industry, there has been a significant expansion in manufacturing plants for semiconductor production, along with related facilities and equipment. Consequently, the demand for high purity tubes designed for the transport highly corrosive gas in semiconductor processing has increased [1-2]. Theses tubes used in semiconductor industry require an ultra-clean state, and stainless steel has been traditionally employed for this purpose. In particular, 316L stainless steel (STS316L) exhibits excellent corrosion resistance attributed to its chemical compositions, which include a higher nickel content (10~14%) and lower chromium content (16~18%) compared to STS304L [3-5]. In addition, the oxide layer formed on the surface of STS316L, composed of CrO and FeO, acts as a protective barrier to prevent reactions under corrosive environments such as moisture and oxygen pollution [2-9].

In order to fabricate much higher quality STS316L, vacuum induction melting (VIM) and vacuum arc remelting (VAR) have been extensively employed [10-12]. VIM uses high purity gas to enhance the surface quality of ingots and minimize the contamination during processing. Additionally, VIM contributes to improve alloy homogeneity through the inherent stirring action of the induction field and precise control of melt superheat [13-16]. On the other hand, VAR process serves as a refining method used as a secondary processing for the homogenization of oxygen sensitive materials with high melting point, such as seamless tube used in semiconductor manufacturing. It also operates in the vacuum atmosphere during melting, which helps to remove impurities and reduce porosity in the ingots [17-20].

Additionally, electrolytic polishing (EP) is widely adopted technique across multiple industries, including semiconductor manufacturing, medical applications, and ultra-clean gas handling, among other. EP produces excellent results when the polished surfaces of tubes and pipes contact with a pure gas. In the semiconductor industry, the performance of semiconductor strongly depends on the purity of gas passing through the STS316L tubes. Therefore, it is required not only to decrease surface roughness but also to enhance corrosion resistance by removing the damaged layer and impurities related to the surface corrosion. As a result, electrolytic polishing is considered as the most effective machining technique to achieve these requirements [21-29].

Previous studies have evaluted the properties of STS316L that did not undergo double melting through EP. However, in this study, the STS316L produced by double melting was used to reduce impurities, and the surface properties of double melted STS316L before and after EP were investigated by measuring the surface roughness, Cr/Fe and CrO/FeO ratios. Furthermore, corrosion characteristics were analyzed in relation to the surface characteristics.

2. Experimental procedure

In this study, the stainless steel 316L (Korean Industrial Standards: STS316L) was used following a precise double melting (DM) through vacuum induction melting (VIM) and vacuum arc remelting (VAR). The compositions of ASTM STS316L and double melted STS316L are presented in Table 1. The DM process resulted in the increased contents of Ni, Cr and Mo, while reducing impurities such as Mn, P and S compared to STS316L specifications listed in ASTM.

Table 1. Chemical composition of STS316L

The double-melted STS316L was subjected to an extrusion and drawing process to obtain tubes with an inner diameter of 1/4 inch. Subsequently, the tubes were heat treated for 1 h at 1258 K and then electrolytically polished using a solution of H₃O₄P (Phosphoric acid), H₂SO₄ (Sulfuric acid) and distilled water.

The microstructure was observed by an optical microscope (OM). The Cr/Fe and CrO/FeO ratios were measured through surface composition analysis conducted with X-ray photoelectron spectroscope (XPS, Thermo VG) and were calculated by the following equation.

\(\begin{aligned} \operatorname{Total} \frac{C r}{F e}= & \frac{(\Sigma C r \text { peak areas }) /(\text { Number of } C r \text { scans })}{(\Sigma F \text { peak areas }) /(\text { Number of Fe scans })} \\ & \frac{/(\text { Cr sensitivity factor })}{/(\text { Fe sensitivity factor })}\end{aligned}\)       (eq 1)

\(\begin{aligned} \operatorname{Total} \frac{\mathrm{CrO}}{\mathrm{FeO}}= & \frac{(\Sigma \mathrm{CrO} \text { peak areas }) /(\text { Number of } \mathrm{Cr} \text { scans })}{(\Sigma \mathrm{FeO} \text { peak areas }) /(\text { Number of Fe scans })} \\ & \frac{/(\mathrm{Cr} \text { sensitivity factor })}{/(\mathrm{Fe} \text { sensitivity factor })}\end{aligned}\)       (eq 2)

The component and thickness of oxide layer were investigated by Auger electron spectroscope (AES, PHI 700Xi) with a sputtering rate of 0.47 nm/min. The thickness was determined by multiplying the sputter time by the sputter rate at a half of maximum oxygen intensity. Furthermore, these results were compared with measurements obtained by transmission electron microscope (TEM, JEM-F200).

In addition, surface roughness was measured by both a confocal microscope and a surface profiler (Alpha step). Corrosion tests were conducted by the immersion method with two solutions: 40% HCl + 60% distilled water solution and 5% NaCl + 95% distilled water solution. The relative weight loss was measured according to immersion time. Also, the corrosion stability was conducted by measuring the amount of eluted ions over the course of a day using distilled water and 40% HCl + 60% distilled water solution, respectively. A polarization test was carried out at 323 K using a mixed solution of phosphoric acid, sulfuric acid, and distilled water under the following conditions: from - 2 to 3 V in power supply and from - 0.1 to 1 A/cm² in current density.

3. Results and Discussions

Fig. 1 shows the microstructure of STS316L before and after EP. Both samples exhibited the typical microstructure with twin and high angle grain boundaries as shown in the heat-treated STS316L [30]. Furthermore, the sample before EP demonstrated a grain size of 55 µm, which showed no significant difference compared to the sample after EP, where the grain size showed 57 µm.

Fig. 1. Microstructure of STS316L; (a) before EP and (b) after EP.

The XPS results of the passive layer in the samples before and after EP are presented in Fig. 2. The surface of both samples showed the presence of Cr, Fe, Ni, Mo and some oxygen. Moreover, it was noted that the chromium-to-iron ratio (Cr/Fe) was 1.48 and chromium oxide-to-iron oxide ratio (CrO/FeO) was 2.15 before EP. However, after EP, the ratio slightly increased to 1.62 and 2.26 in Cr/Fe and CrO/FeO ratios, respectively. Also, after EP, the thickness of oxide layer measured by AES and TEM is presented in Fig. 3. The thickness by AES was approximately 10 nm. In case of TEM, it was approximately 15 nm, which was compatible with the result from AES. These indicate that the passive films formed after EP are effective in preventing the invasion of other impurities into the materials.

Fig. 2. Cr/Fe ratio and CrO/FeO ratio analyses of STS316L by XPS.

Fig. 3. Thickness of EP layer measured by (a) AES and (b) TEM.

Fig. 4(a) displays the 3D modelling results of the surface before and after EP, as observed by confocal microscope. It is evident that the surface became smoother after EP. This reduction in surface roughness is verified by the results from surface profiler, as presented in Fig. 4(b), which show a significant decrease from 0.25 to 0.02 µm after EP. Therefore, the EP effectively prevents damage caused by the flow of highly corrosive gas, leading to improvement in the performance of tube in the semiconductor production process.

Fig. 4. (a) Surface by confocal microscope and (b) surface roughness by surface profiler of STS316L.

Fig. 5(a) shows the results of corrosion test in a NaCl solution for 6 weeks. The appearance of the samples remained consistent with increasing the immersion time. However, differences in weight loss behavior were observed between two samples before and after EP. The sample before EP showed a linear increase in relative weight loss up to 432 h, reaching a saturated weight loss beyond that point. In contrast, the sample after EP showed only a minimal weight loss of 0.001 g at 432 h, with the weight loss increased up to 720 h and it was saturated over 720 h. Fig. 5(b) presents the results of the corrosion test carried out with HCl solution for 7 days. After 168 h, the weight loss of sample after EP was 0.1 g, indicating better corrosive resistance compared to the sample before EP, which experienced a weight loss of 0.28 g. The sample before EP exhibited a higher rate of weight reduction compared to sample after EP. These differences can be attributed to the increase in Cr/Fe and CrO/FeO ratios and the thicker oxide layer on surface after EP, which acted as a protective barrier against external impurities.

Fig. 5. Weight loss curves of STS316L in (a) 5 wt.% NaCl solution and (b) 40 wt.% HCl solution.

The corrosion stability of STS316L was assessed by measuring the amounts of ions eluted during immersion in distilled water and HCl solution, respectively. Fig. 6(a) illustrates the difference in amount of eluted ion before and after EP, following 24 h of immersion in distilled water. There was a slight decrease from 1.2 to 0.8 ppb in the amount of Cr ion, while the amount of Fe ion significantly decreased from 10.3 to 0.8 ppb after EP. As shown in Fig. 6(b), the amounts of eluted ions after immersing in HCl solution also decreased after EP, from 3.2 to 1.5 ppb for Cr and from 13.2 to 5.2 ppb for Fe. These results indicate that the EP process played a significant role in improving corrosion resistance and reducing impurities on the surface.

Fig. 6. Corrosion stability of STS316L through measurement of eluted ion: (a) distilled water and (b) 40 wt.% HCl.

Fig. 7 displays the results of the polarization test conducted by a mixed solution of phosphoric acid, sulfuric acid, and distilled water. In Fig. 7(a), when comparing the polarization curves of samples before and after EP, it is evident that the equilibrium potential increased from 2.0 to 2.2 V, and the current density decreased from 0.11 to 0.06 A/cm². These changes indicate a decrease in the corrosion rate after EP. On the other hand, Fig. 7(b) reveals the polarization curves of EPed samples measured over a temperature range of 323 K and 343 K. With increasing temperature, the equilibrium potential decreased and the corrosion rate increased, indicating a decrease in corrosion resistance. Therefore, EP process can promote the development of smooth surface with very low roughness values that resist the adsorption and penetration of impurities. Additionally, the quality of the passive film also improves by an increase in Cr/Fe and CrO/FeO ratios on the surface. Consequently, after EP, the enhancement of surface properties such as surface roughness, Cr/Fe ratio and CrO/FeO ratio helps reduce the impurities and results in better corrosion behavior.

Fig. 7. Effect of (a) EP at 323 K and (b) solution temperature on polarization properties.

4. Conclusions

The STS316L obtained through double melting (VIM and VAR) was subjected to extrusion, drawing and heat treatment at 1258 K for 1 h and subsequently electrolytic polishing (EP). The microstructure of STS316L with / without EP showed no significant difference. However, after EP, the Cr/Fe and CrO/FeO ratios increased from 1.48 to 1.62 and from 2.15 to 2.26, respectively. In addition, the surface roughness decreased to 0.02 µm by EP. The EPed sample showed lower weight loss rates than the non-EPed sample in HCl and NaCl solutions. Also, the amounts of Cr and Fe ions eluted from STS316L after EP significantly reduced in both distilled water and HCl solution. From this results, it was reasonably concluded that the EP layer effectively reduced the corrosion rate and ion elution in STS316L. In addition, the polarization test revealed that STS316L after EP had higher equilibrium potential and lower current density than that before EP, indicating that the corrosion rate decreased. Consequently, it was considered that the EP process was effective to improve the surface and corrosion properties.