1. Introduction
Grain size refinement leads to an improvement in mechanical strength, as described by the Hall–Petch relation [
1]. This can be achieved by severe plastic deformation (SPD), which brings about substantial microstructural changes, primarily grain size reduction and an increase in dislocation density, without changing the chemical composition [
2,
3,
4]. Different SPD methods have already been successfully applied to the processing of stainless steel [
5,
6,
7,
8,
9,
10]. One of them is hydrostatic extrusion (HE), which gives rise to a uniform refinement of microstructure throughout the whole volume of relatively large samples [
11]. Significant improvements in the mechanical strengths of 316L and 316LVM stainless steels following HE have previously been reported [
8,
9,
10]. Although a great deal of attention has already been paid to the mechanical properties of stainless steels after bulk SPD, there are still few studies on the effect of structure refinement on their corrosion behavior.
In an environment that enables the formation of a passive film, microstructural refinement generally enhances the passivation ability of materials [
12,
13,
14]. The defects in the structure (dislocations, grain boundaries, boundary junctions, shear bands, and deformation) act as active sites for passive film nucleation [
13,
15,
16,
17] and are multiplied in the refined structure. The higher density of defects accelerates passive film formation and changes its nucleation mechanism from progressive to instantaneous [
13,
18,
19]. However, it also increases the number of defects in the passive film, which compromises its stability [
19,
20,
21,
22,
23,
24]. In fact, the improvement in passive behavior linked to higher Cr enrichment of refined materials is a result of a faster dissolution of Fe [
13,
25,
26]. For warm-rolled 304 stainless steel [
24], the deterioration in passive film stability was shown to be related to a high density of dislocations. After annealing, which reduced dislocation (but did not change the grain size), a significant improvement in the passivity of the refined 304 stainless steel was observed. Similar observations have been noted for materials after surface nanocrystallization [
18,
27,
28] or bulk refinement [
29], where, before annealing, the refinement process increased the passive current density while, following annealing, it improved the materials’ compactness and stability.
The pitting corrosion resistance of stainless steel is usually enhanced after grain refinement [
26,
30]. The refined structure promotes the occurrence of metastable pitting events while retarding stable pit growth due to its remarkable repassivation ability [
31]. However, these findings were mostly observed with nanocrystalline stainless steel thin films [
12,
18,
19,
32,
33,
34], which were characterized by increased surface homogeneity and a lower number of inclusions (or their absence) [
33,
34]. Pan et al. [
34] reported that Mn and S dissolved in a solid solution of α-Fe phase in a magnetron-sputtered thin film, and that pitting initiated at the grain boundaries. For bulk commercial 316L stainless steel, it is well recognized that manganese sulfide (MnS) and oxide inclusions trigger the onset of pitting corrosion [
35,
36,
37,
38]. During SPD, these inclusions are refined, but their size reduction is lower when compared to the refinement of the matrix. Results on the pitting corrosion resistance of bulk refined materials are often contradictory. A deterioration in pitting corrosion resistance was noted for 316L steel after dynamic plastic deformation (DPD) [
29] and for 303 steel after HE [
39]; it was linked to the fragmentation of non-metallic inclusions. However, 316L stainless steel, after HE, has shown improved resistance to localized attack [
40]. Other studies have revealed that grain size had no effect on the breakdown potential of cold-rolled 304L steel [
41].
So far, the influence of non-metallic inclusions on the corrosion behavior of austenitic steels after SPD has gained little attention. However, it seems that alteration in their size and distribution plays an important part in the corrosion resistance of SPD-refined materials. In our study, the uniform and localized corrosion of 316L stainless steel after HE is investigated, focusing on the effect of MnS and oxide inclusions in the refined structure.
2. Materials and Methods
2.1. Materials
The material studied was 316L austenitic stainless steel purchased from Sandvik Ltd. (Stockholm, Sweden). A nominal composition of 316L stainless steel is shown in
Table 1. The chemical composition was measured using an X-ray fluorescence spectrometer (XRF) by Bruker S4 EXPLORER is listed in
Table 2.
The material was delivered in the form of hot-extruded rods and was annealed to homogenize the microstructure.
Billets were subjected to an HE process, according to the procedure described in [
42]. The 316L rods, with initial diameters of 8.9 and 11.4 mm (
Table 3), were hydrostatically extruded in oil, in a multi-step process, to a final diameter of 6 mm. This process is characterized by high strain rates at a relatively low temperature. Plastic deformation was achieved accumulatively (step-by-step). A schematic diagram of the HE process is shown in
Figure 1.
The accumulated strain was calculated using Equation (1):
where
dp = initial diameter and
dk = final diameter. The accumulated strain is listed in
Table 3.
The sample, after annealing, was designated as coarse-grained (CG), and the extruded samples were designated as HE0.8 and HE1.3, where the numbers refer to the accumulated strain. Microscopic observations and electrochemical measurements were performed on cross-sections (cs) perpendicular to the HE direction and, on longitudinal sections (ls), parallel to the HE direction. The hydrostatic extrusion was carried out atthe Institute of High Pressure Physics, Polish Academy of Sciences, Warsaw, Poland. All the extrusion experimentswere conducted at ambient temperature.
2.2. Microstructural Observations
A Nikon Epiphot 200 light microscope (Nikon, Tokyo, Japan) was used to evaluate the microstructures of the investigated materials. For microscopic observations, the samples were polished to a mirror finish and etched with a solution of 50 mL 38% HCl, 25 mL 60% HNO3, and 25 mL H2O.
To characterize the inclusions formed in the material, a field emission scanning electron microscope (FE-SEM SU-70, Hitachi, Tokyo, Japan), equipped with an energy-dispersive X-ray spectrometer (EDS, Thermo Fisher Scientific, Waltham, MA, USA) was used. The inclusions were quantitatively described on both the cross and longitudinal sections in terms of their size and their number per area. The size of the inclusions observed on the cross-section was determined using an equivalent diameter, defined as the diameter of a circle with a surface area equal to the measured grain. In the case of inclusions on longitudinal sections, their minimum and maximum diameters were determined. At least 150 inclusions were measured in each case. The number of inclusions per unit area was calculated according to Equation (2):
where
N—number of inclusions and
A—area.
2.3. Electrochemical Testing
Potentiodynamic polarization measurements were recorded using Autolab PGSTAT302N potentiostat/galvanostat (Metrohm, Herisau, Switzerland). A standard three-electrode setup, with a platinum sheet as the counter electrode, a saturated calomel electrode SCE as the reference electrode, and the sample as the working electrode, was used. Scans were recorded after 30 min of immersion, starting at the EOCP with a 1 mV/s scan rate. Each measurement was repeated at least three times to ensure the reproducibility of the results. The measurements were carried out at an ambient temperature in unstirred test solutions.
Before each experiment, the surface of the samples was ground with 2500# SiC papers and then polished using polishing cloths with diamond suspensions (from 3 to 1 µm) until a mirror-bright surface was obtained. This surface preparation procedure was performed using an ATM Saphir 550 automatic polisher/grinder machine (ATM, Berlin, Germany). Finally, the surfaces were ultrasonically cleaned in ethanol.
Prior to each experiment, all solutions were freshly made from analytical grade reagents and distilled water. The chosen solutions covered a wide range of pH. Chloride ions were added to a buffer solution to evaluate the resistance to pitting corrosion, which ensured that a constant pH was maintained during measurements:
- ○
0.5 M H2SO4 of pH 0.4
- ○
0.11 M H3BO3 + 0.027M Na2B4O7 of pH 8.4
- ○
0.5 M NaOH of pH 13
- ○
0.11 M H3BO3 + 0.027M Na2B4O7 + 0.1 M NaCl of pH 8.4. Post-corrosion morphology observations were performed using a Nikon Epiphot 200 light microscope and TM1000 SEM (Hitachi, Tokyo, Japan).
5. Conclusions
The role of non-metallic inclusions in the electrochemical behavior of ultrafine-grained 316L stainless steel was investigated by potentiodynamic polarization and microstructural characterization. Based on the obtained results, the following conclusions can be drawn for ultrafine-grained 316L stainless steel:
(1) The stability of the passive film decreased with higher accumulated strain due to the multiplication of MnS and oxide inclusions over a wide range of pH; the refinement of the matrix has a minor effect on the passive film stability.
(2) The pitting potential was higher for the HE samples; therefore, the resistance to localized attack after HE is better compared to CG sample. At the same time, the frequency of metastable pitting events is higher for HE processed materials; however, the repassivation of metastable pits is more probable, and their stable growth starts at a higher potential.