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Surface Chemistry and Semiconducting Properties Of JMEPEG (2020) 29:6085–6100 ÓASM International https://doi.org/10.1007/s11665-020-05067-3 1059-9495/$19.00 Surface Chemistry and Semiconducting Properties of Passive Film and Corrosion Resistance of Annealed Surgical Stainless Steel Rodrigo Kenji de Oliveira, Olandir Vercino Correa, Mara Cristina Lopes de Oliveira, and Renato Altobelli Antunes (Submitted March 25, 2020; in revised form June 24, 2020; published online September 1, 2020) ASTM F-139 surgical stainless steel was subjected to annealing treatments at 700 °C for different times. The effect of annealing on the chemical composition of the passive film was evaluated by x-ray photo- electron spectroscopy. The correlation of the surface chemistry with the corrosion behavior and semi- conducting properties of the passive film was also investigated. Potentiodynamic polarization tests were conducted in phosphate-buffered solution at 37°C. The semiconducting character of the passive film was assessed by the Mott–Schottky approach. The microstructure of the annealed samples was characterized by optical microscopy. The grain size increased after annealing, but the differences between each annealing condition were not significant and could not be associated with the corrosion behavior of the annealed samples. The corrosion resistance was improved depending on the heat treatment condition due to com- positional changes of the passive film upon annealing. The best corrosion properties were observed after annealing for 8 h which was ascribed to Cr2O3, MoO3 and FeO enrichment in the passive film. triggered by annealing, such as the formation of sigma phase Keywords annealing treatments, corrosion, Mott–Schottky, surface chemistry and chromium nitrides. Pit nucleation sites were, therefore, dependent on the annealing temperature related to the formation of such new phases. Tan et al. (Ref 12) have also found that annealing of super duplex stainless steel has markedly affected its pitting corrosion behavior due to microstructural transfor- mations and also the contents of passivating elements such as 1. Introduction chromium and molybdenum dissolved in the crystalline phases. Grain growth is another important feature arising from Austenitic stainless steels are commonly employed as annealing. Aghuy et al. (Ref 13) reported that grain refinement biomedical devices especially for temporary applications such of the austenitic 304L grade decreased metastable pit frequency as plates and screws during fracture healing (Ref 1, 2). The by improving the stability of the passive film. By contrast, the surgical grade ASTM F-139 is the standard material for this probability of stable pit growth increased and pitting potential usage owing to its adequate set of attributes relying on its was little affected by grain refinement. Bitondo et al. (Ref 14), mechanical strength, reasonable corrosion resistance, ease of in turn, have found that annealing treatments of ferritic stainless manufacturing and low cost (Ref 3-5). It is, though, well known steel grade 444 did not affect the grain size and distribution of that it is prone to pitting corrosion in chloride-containing inclusions. As a consequence, the corrosion resistance was not electrolytes such as the body fluid (Ref 6, 7). Stress corrosion influenced by annealing. cracking and corrosion fatigue of surgical stainless steels are The aforementioned effects of annealing on the corrosion often associated with the nucleation and growth of pits (Ref 8- properties of stainless steels allow one to hypothesize that it 10). It is, therefore, important to decrease its pitting corrosion could be advantageously explored as a mean to improve the susceptibility in order to reduce the risk of premature failures. stability of the passive film by properly controlling its Annealing treatments can affect the corrosion resistance of composition and/or the grain size of the treated material. These stainless steels. Microstructural changes originated from the effects are not deeply known for surgical stainless steels. A heat treatments are closely related to the corrosion properties. great concern would arise from grain growth as the fatigue Deng et al. (Ref 11) investigated the effect of annealing resistance is enhanced for refined microstructures. One further treatments on the pitting corrosion resistance of a super duplex question arises from this scenario regarding the possible stainless steel. Complex microstructural transformations can be influence of annealing on the diffusivity of point defects through the passive film, its semiconductive character and doping density. Such aspects are also reported to deeply affect Rodrigo Kenji de Oliveira, Mara Cristina Lopes de Oliveira, the corrosion behavior of stainless steels (Ref 15, 16) and can and Renato Altobelli Antunes, Centro de Engenharia, Modelagem e be assessed by the well-known Mott–Schottky approach (Ref Cieˆncias Sociais Aplicadas (CECS), Universidade Federal do ABC 17, 18). (UFABC), Santo Andre´, SP 09210-580, Brazil; and The aim of the present work was to investigate the effect of Olandir Vercino Correa, Centro de Cieˆncia e Tecnologia de Materiais (CCTM) – Instituto de Pesquisas Energe´ticas e Nucleares annealing treatments on the surface chemistry, semiconducting (IPEN/CNEN-SP), Sa˜o Paulo, SP, Brazil. Contact e-mail: properties of the passive film and the corrosion behavior of the [email protected]. ASTM F-139 surgical stainless steel. This steel was chosen Journal of Materials Engineering and Performance Volume 29(9) September 2020—6085 Table 1 Samples description Sample Description Temperature, °C Time, h AR As-received …… SA Solution-annealed 1050 1 A-1h Solution annealing at 1050°C followed by annealing 700 1 A-8h Solution annealing at 1050°C followed by annealing 700 8 A-24h Solution annealing at 1050°C followed by annealing 700 24 Fig. 1 Optical micrographs showing the microstructures of the: (a) AR; (b) SA; (c) A-1h; (d) A-8h; (e) A-24h 6086—Volume 29(9) September 2020 Journal of Materials Engineering and Performance based on its widespread use as surgical implants in the form of 0.70 cm 9 1.0 cm were cut from the original plate using a sheet and strips. X-ray photoelectron spectroscopy (XPS) was silicon carbide blade in a conventional cutoff machine. employed to assess the surface chemical composition of as- Solution annealing was carried out in a tubular furnace received and annealed samples. The corrosion properties were under argon atmosphere at 1050 °C for 1 h, followed by water evaluated by potentiodynamic polarization. The Mott–Schottky quenching. The solution-annealed specimens were, next, sub- approach was employed to assess the semiconductive proper- ject to annealing at 700 °C for 1 h, 8 h and 24 h, followed by ties of the passive film. natural cooling outside the furnace. Sample designation is shown in Table 1. 2. Materials and Methods 2.2 Electrochemical Tests Firstly, a copper wire was connected to one face of the 2.1 Material and Heat Treatments specimens using a colloidal silver paste to promote electrical contact. Next, this set was embedded in cold-curing epoxy The ASTM F-139 stainless steel plate (thickness of 2 mm) resin. After complete curing, the specimens were wet ground used in the present work was purchased from Acnis (Brazil). using silicon carbide (SiC) papers up to grit 2400. All tests Small rectangular pieces with approximate dimensions of were carried out in an Autolab M101 potentiostat/galvanostat using a conventional three-electrode cell setup with the ASTM Table 2 Grain sizes of the AR and heat-treated samples F-139 specimens as the working electrodes, a platinum wire as the counter electrode and Ag/AgCl as reference. The tests were Sample Grain size, lm conducted in a phosphate-buffered solution (NaCl 82 g LÀ1, À1 À1 Na2HPO4 10.5 g L , NaH2PO4ÆH2O 3.55 g L )at37°C. AR 24 ± 5 Samples remained immersed for 24 h before the tests to ensure SA 51 ± 10 a steady state condition. A-1h 53 ± 4 One set of experiments consisted an initial monitoring A-8h 63 ± 4 A-24h 65 ± 8 period of the open-circuit potential (OCP) for 1 h. Next, the specimens were subjected to potentiodynamic polarization at a Fig. 2 XPS Cr2p3/2 spectra of the ASTM F-139 stainless steel samples: (a) A-1h; (b) SA; (c) AR; (d) A-8h; and (e) A-24h Journal of Materials Engineering and Performance Volume 29(9) September 2020—6087 sweep rate of 1 mV sÀ1 starting at À 300 mV versus the OCP typical duplex character showing p-type and n-type responses up to + 1.0 VAg/AgCl. At least three different specimens were depending on the potential range (Ref 20-22): tested for each sample. The corrosion morphology after 1 2 kT potentiodynamic polarization was examined by confocal laser 2 ¼À E À EFB À ðEq 1Þ scanning microscopy (CLSM, Olympus LEXT OLS4100). C e Á e0 Á e Á NA e Another set of experiments was conducted to evaluate the semiconducting properties of the passive film by the Mott– 1 2 kT 2 ¼À E À EFB À ðEq 2Þ Schottky approach. In this case, the OCP was monitored for C e Á e0 Á e Á ND e 600 s. Next, Mott–Schottky plots (1/C2 vs. E) were acquired at a fixed frequency of 1 kHz in the potential range from 2.3 Microstructural Characterization 0.5 VAg/AgCl up to À 1.0 VAg/AgCl with a step of 25 mV in cathodic direction. The classic Mott–Schottky equation (Eq 1, The microstructure of the AR and heat-treated samples was p-type and Eq 2, n-type semiconductors) was employed to characterized by optical microscopy (Olympus LEXT determine the acceptors (NA) and donors (ND) density of the OLS4100 confocal laser scanning microscope). The specimens passive film, assuming that the electrode capacitance is equal to were ground using SiC waterproof papers up to grit 2400, the space charge capacitance at sufficiently high frequencies.
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