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Article

Corrosion Mechanisms of a Biodegradable Zn-0.4Li Alloy in Simulated Gastrointestinal Environment

by
Manli Wang
1,2,
Lingbo Yang
1,
Xinglong Zhu
1,
Lijing Yang
1,*,
Jianwei Shen
3,
Ting Lu
3,
Huinan Liu
4 and
Zhenlun Song
1
1
Key Laboratory of Marine Materials and Related Technologies, Zhejiang Key Laboratory of Marine Materials and Protective Technologies, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, China
2
University of Chinese Academy of Sciences, Beijing 100049, China
3
Ningbo Medical Center Lihuili Hospital, Ningbo 315041, China
4
Department of Bioengineering, University of California, Riverside, CA 92521, USA
*
Author to whom correspondence should be addressed.
Coatings 2023, 13(3), 529; https://doi.org/10.3390/coatings13030529
Submission received: 24 December 2022 / Revised: 13 February 2023 / Accepted: 24 February 2023 / Published: 28 February 2023
(This article belongs to the Special Issue Surface Function Enhancement Film and Coating Technology)
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Abstract

:
Zn-Li alloys have been demonstrated to be potential biodegradable materials because of their favorable biocompatibility and exceptional strength. The corrosion behaviors of Zn–0.4Li in simulated gastric fluid (SGF) and simulated intestinal fluid (SIF) were investigated. Compared with samples in SIF, those in SGF were severely corroded locally. Pepsin and pancreatin participated in the formation of degradation products. After immersion in SGF and SIF, the corrosion process presented two interfaces. Lithium (Li) preferentially reacted in the near-surface region to form a Li-rich region. Simultaneously, there were two Li-poor regions around the Li-rich region. Then Zn-rich products gradually became dominant with time. Li+ releasement dominated over Zn2+ releasement throughout the immersion process in SGF and SIF. These results can guide the development of biodegradable gastrointestinal anastomotic nails in the future.

1. Introduction

Gastrointestinal cancer has become one of the cancers with the highest morbidity and mortality. When removing a diseased intestinal tract, mechanical anastomosis is employed to reconstruct the continuity of the intestinal tract. In gastroenterostomy, surgical staples are widely used instead of traditional sutures for the healing and reconstruction of the gastrointestinal tract to greatly improve the surgical efficiency, reduce postoperative complications and relieve patients’ pain [1]. Most suture nails at present are made of titanium (Ti) alloy; they are not degradable and may cause adverse reactions, such as allergic reaction, foreign body reaction and tissue adhesion in the body [2,3]. Given the high X-ray absorption coefficient of Ti, metal artifacts may be produced during computed tomography, and these may interfere with the postoperative examination of patients and result in misdiagnosis.
Zinc (Zn) alloys, as new candidate materials for biodegradable metals, have attracted much attention due to their moderate degradation rate, outstanding mechanical properties and benign biocompatibility [4,5,6,7,8]. Studies on Zn alloys have achieved progress in various areas, such as vascular stents and orthopedics [5,9,10,11,12,13]. Guo et al. studied the residual stress and biocompatibility of Zn–Mn–Li anastomotic nails in simulated gastric fluid (SGF) immersion [14]. Guo et al. investigated the mechanical mismatch of Zn–0.1Li in simulated intestinal fluid (SIF) [9]. But there is still no clear understanding of how Zn alloys degrade in the digestive tract environment, although such a mechanism is crucial for the development, design and application of degradable Zn alloy nails in the future. The mouth, pharynx, esophagus, stomach, small intestine, and large intestine make up the majority of the digestive tract. These components play different roles in digestion, and their internal environments differ [15,16,17]. The stomach is primarily used to grind large pieces of food into small pieces and break down the large molecules in the food into small ones for further absorption. Therefore, hydrochloric acid, Cl-, mucus, sodium salt, pepsin, and other substances are primarily found in gastric juice with a pH value of 1–3 [16]. After the initial digestion and decomposition of food in the stomach, it enters the small intestine, which is the main place for the human body to digest food and absorb nutrients. The intestinal fluid contains many pancreatic enzymes and buffers to neutralize gastric acid and shield the intestinal membrane from acid damage. The pH value in the small intestine ranges within 5.0–7.7 [17,18]. The difference between the stomach and intestinal environment exerts different effects on the degradation of Zn alloys.
The mechanical characteristics of biodegradable metal scaffolds need to meet the following criteria in order to be used in clinical settings: yield strength (YS) > 200 MPa, ultimate tensile strength (UTS) > 300 MPa, and elongation at break (EL) > 15%–18% [18]. According to previous studies [19,20,21,22,23,24,25,26,27], among binary Zn alloys, only Zn-Mg [23] alloys and Zn-0.8Li alloy [27] can surpass the standard in certain circumstances. Moreover, Zn–Li alloy has superior strength among all binary Zn alloys and has an appropriate degradation rate and satisfactory biocompatibility [5,28,29]. In addition, 1.0 mg of Li per day is recommended for an American adult weighing 70 kg in order to meet their daily requirement, which can be met by diet [30]. For more than 50 years, Li has been the first-line treatment for bipolar disorder. Li may have cardioprotective effects in addition to its neuroprotective effects, according to recent clinical and laboratory investigations [31]. Mazor et al. discovered that Li had a particular tumor-inhibiting impact [32]. However, there are some potential health hazards associated with consuming excessive amounts of Li. Li is not harmful in the therapeutic range of 0.6 to 1.0 mM, but toxicity levels start to appear at 1.4 mM or higher, according to clinical experience [33]. Zn levels in human serum should range between 12.4–17.4 μmol/L. Zn is a trace element that is vital for the body’s immune system and bone growth [34]. However, neurotoxicity issues may result from excessive Zn [35]. Studies conducted in vivo and in vitro by Yang et al. revealed that the effect of Zn–0.4Li alloy on cell proliferation and osteogenic activity was much superior to that of pure Zn. Therefore, in this study, immersion experimentation and electrochemical testing were used to identify the mechanism of extruded Zn–0.4Li degradation in intestinal and gastric environments with significant pH differences. The degradation mechanism of Zn alloy in simulated digestive tract environments was revealed, providing reference for the development of degradable Zn alloy nails and digestive tract stents in the future.

2. Materials and Methods

2.1. Alloy Preparation

The alloy in this study was prepared by Ningbo Powerway Alloy Material (Ningbo, China) Co., LTD. The ingots were annealed at 260 °C for 2.5 h. The annealed ingots were then extruded at 220 °C with a 20:1 extrusion ratio. The diameter of the extruded alloy bars was 11 mm. The actual chemical composition of the nominal alloy of Zn–0.4Li (wt%) was measured to be Zn–0.373Li (wt%) via inductively coupled plasma optical emission spectrometry (ICP/OES). The Zn/Li ratio in Zn-0.4Li was 26.8. The metals were cut into a size of φ11 mm × 2 mm. All samples were ground with P2000 grit SiC sandpaper separately and then cleaned in distilled water and absolute ethanol for 5 min in turn.

2.2. Immersion Tests

According to ASTM G31-72, immersion tests were carried out for 1, 3, and 7 days in SGF and SIF at a temperature of 37 °C and an immersion ratio of 0.05 cm2/mL. Chinese Pharmacopoeia Iv contains information on the precise methods for making SGF and SIF. SGF was composed of 10 g/L of porcine pepsin and 16.4 mL/L of dilute hydrochloric acid with the pH adjusted to 1.2. SIF was composed of 10 g/L of porcine pancreatin, 6.8 g/L of KH2PO4 and 0.1 mol/L of NaOH with the pH adjusted to 6.8. KH2PO4 (AR), HCl (AR) and NaOH (AR) were purchased from Sinopharm Chemical Reagent Co. Ltd. Pepsin (1:3000) and pancreatin (1:250) were purchased from Shanghai Macklin Biochemical Technology Co., Ltd.
The tubes and specimens were sterilized with ultraviolet light to avoid bacterial and fungal growth, and liquid penicillin-streptomycin was injected into SGF/SIF at a ratio of 1:100. Five samples were prepared in parallel for each experiment. The staples were taken out at various immersion durations, then dried in a vacuum furnace for 1 h at 100 °C. In order to eliminate the degradation products from the samples, they were submerged in 200 g/L of Cr2O3 at 40 °C for 5 min. The mass loss method was then used to calculate the degradation rates. The corrosion rate was calculated as follows [36]:
C R = K Δ m ρ A t
where CR is the corrosion rate in mm/year, the coefficient K = 8.76 × 104, Δm is the mass loss in g, ρ is the density of the material in g/cm2, A is the initial surface area in cm2 and t is the immersion time in h.

2.3. Microstructure Observation and Corrosion Product Analysis

The microstructure of the material was observed through scanning electron microscopy (SEM; FEI Quanta FEG 250, Hillsboro, OR, USA) before and after immersion. The samples were immersed and analyzed utilizing X-ray diffraction (XRD; Bruker D8 ADVANCE, Karlsruhe, Germany) with Cu Kα radiation between the 2θ values of 10° and 90° to determine the composition of the samples and degradation products. An X-ray photoelectron spectrometer (XPS; Axis Ultra DLD, Shimadzu, Kyoto, Japan) was used to analyze the composition of the corrosion products. To understand the corrosion mechanism, ICP/OES was employed to detect the element concentration in the immersion solution at different times. The extent to which the samples were corroded by the solution was determined through glow discharge-optical emission spectroscopy (GD-OES; GDA, Spectruma 750HP, Frankfurt, Germany) with minimum resolution depth of 25 nm.

2.4. Electrochemical Measurements

Electrochemical tests were performed on the Zn–0.4Li alloys in SGF and SIF with an M237A electrochemical workstation (Princeton M273A). A typical three-electrode cell was employed; the specimen served as the working electrode, a platinum-sheet acted as the counter electrode, and the saturated calomel electrode worked as the reference electrode. The sample, which was encased in the resin, had a surface area of 0.95 cm2 to touch the electrolyte. Data on open-circuit potential (OCP) were gathered for a maximum of 7 days. With a potential fluctuation range of ±10 mV, electrochemical impedance spectroscopy (EIS) was carried out in the frequency band from 100 kHz to 10 MHz. The potentiodynamic polarization (PDP) curves were obtained in a range of ±2 V vs. OCP at a constant scan rate of 0.5 mV·s−1. Additionally, samples were submerged in solutions for 30 min to allow OCP to stabilize before the test. Three replications of each test were run in order to confirm the consistency of the outcomes.

3. Results

3.1. Microstructure Characterization

Figure 1a shows that the extruded Zn–0.4Li alloys were mainly composed of Zn and LiZn4 phases. Figure 1b presents the microstructures of the Zn–0.4Li alloys. The composition of Zn-0.4Li was near the eutectic point of 0.44% Li. According to the equilibrium phase diagram of Zn-Li [22,37], the eutectic structure of Zn + LiZn4 is developed after the main Zn phase is first established. The replacement of Zn atoms with Li atoms made the standard electrode potential (E°) of Zn highly negative due to the E° of Li (i.e., −3.04 V) being much lower than that of Zn (i.e., −0.76 V). Thus, LiZn4 particles were more vulnerable to the corrosion, resulting in a recessed topography and a darker color [27,38,39]. Zn phase should be used in regions that are brighter and more resistant to corrosion. The eutectic phase (Zn + LiZn4), in contrast, should be the darker region with more severe etching properties, as indicated by the yellow arrows in the high-magnification SEM image (Figure 1c).

3.2. Immersion Test

Figure 2 presents the corresponding corrosion rates and pH of samples immersed in SGF and SIF at different times. The corrosion rate in SGF was much higher than that in SIF. The corrosion rate of the sample in all of the solutions reached the maximum value at the initial stage of immersion and then decreased with the increase in time. Guo et al. also observed the same phenomenon [9]. The phenomenon suggests the formation of a corrosion product layer on the surface of the sample. The SIF’s pH was determined to be reasonably stable, with changes not exceeding 0.1. The samples were extensively corroded, and a significant number of alkaline compounds were released into the SGF under the overall influence of a substantial amount of H+ and Cl− in the solution. (In the Section 4, the specific content was explained.) The large amounts of H+ ions were consumed at the same time, causing a significant shift in the pH value of SGF, which rose with time.

3.3. Surface Morphology and Chemical Composition

The phase compositions of the corrosion products (on the alloy surface) in SGF and SIF are shown in Figure 3 and Figure 4. The SEM morphologies of the corrosion products formed on the Zn–0.4Li surfaces during immersion in SGF and SIF for 1, 3 and 7 days are displayed in Figure 3a. No obvious difference was observed in the morphologies of Zn–0.4Li after immersion in SIF at different times (Figure 3a). The sample immersed in SIF mainly exhibited uniform corrosion. After immersion in SIF, a layer of the corrosion products with dark micropores covered the sample surface, as indicated by the high-magnification area in Figure 3a. The density and depth of the micropores varied with immersion time. Notably, round flower-like corrosion products occupied most of the sample surface after immersion in SIF for 3 days, as shown in Figure 3a. These round flower-like corrosion products were formed because the sample surfaces absorbed abundant PO43- in the solution during immersion in SIF [40].
Figure 3b presents SEM images of Zn–0.4Li after the removal of the corrosion products. The morphologies of the samples immersed in SIF showed little difference between the samples with and without the corrosion products removed, possibly due to the short immersion time and minimal accumulation of corrosion products. According to Figure 3a,b, after immersion in SGF, the surface of the sample was covered with multitudes of dark-grey corrosion pits, and large-diameter corrosion pits appeared in local areas, indicating intensive localized corrosion. The longer the immersion time was, the more obvious the non-uniform corrosion was. The number and depth of corrosion pits increased with time. After immersion for 7 days, the maximum diameter of the corrosion pits exceeded 150 μm. In the SGF, the severe pits and localized corrosion of Zn–0.4Li were due to the attack ion of Cl−. The localized corrosion due to galvanic corrosion resulted from segregation of the secondary LiZn4 phase on grain boundaries for Zn–0.4Li alloy [39]. As we knew, the phase boundaries were the interface of galvanic coupling for the dual-phase zinc alloys. The corrosive Cl− preferentially attacked at the interface of the LiZn4 phase and Zn phase. Then, the localized corrosion initiated from the phase boundaries [41].
The main phase compositions of the corrosion products of the samples after 7 days of immersion in SGF and SIF were analyzed through XRD, as shown in Figure 3c. Figure 3c suggests that the main phase compositions of the corrosion products in SIF included Zn5(CO3)2(OH)6, ZnO, Zn3(PO4)2•4H2O and Li2CO3. Figure 3c reveals that the corrosion products in SGF included Zn5(CO3)2(OH)6, ZnO and Li2CO3. Notably, Zn(OH)2 was not detected probably because of its unstable properties and rapid regeneration to ZnO [42].
Figure 4 displays the XPS results, which served as supplements to further analyze the phase composition of the corrosion products in SGF and SIF. The peaks for Zn 2p, O 1s, N 1s and C 1s were detected on the sample surface in SGF after 1 and 7 days of immersion, as shown in Figure 4a. The peak for P 2p was also detected in SIF (Figure 4a). As depicted in Figure 4b, for SGF, within 1 day, Li was abundant (i.e., roughly 27.6 at%) and Zn was minor (i.e., <5.1 at%) in the corrosion products. They then underwent opposite trends of change. Zn rose to 16.6 at% on day 7, while Li fell to 0 at%. These trends signified the transformation from Li-rich corrosion products to Zn-rich corrosion products [38,39,43]. However, Li was not found in the XPS spectrum of samples immersed in SIF. These indicate that the SGF and SIF corroded the LiZn4 phase in the surface layer more than other phases [43]. The existence of the N element proved that enzymes played a role in the development of products on the sample surfaces.
Additionally, Figure 5(a–c,a1–c1) shows the high-resolution XPS spectra of Zn 2p, C 1s, and N 1s for Zn-0.4 Li alloys following 7 days of immersion in SGF and SIF. In Table 1, the binding energies of the various substances taken into consideration for XPS spectral analysis are provided. The Zn 2p spectra are depicted in Figure 5(a,a1). ZnO and Zn5(CO3)2(OH)6 were the dominant corrosion products within 7 days in SGF. We also found Zn3(PO4)2•4H2O in SIF. Figure 5(b,b1) shows the fitting curves of the C 1s spectra of the corrosion products of the samples after immersion for 7 days in SGF and SIF, respectively. The peaks at 7 days showed good fitting with five contributions at 284.5, 285.4, 286.2, 287.5 and 288.3 eV. The peak at 284.5 eV was ascribed to the C-H/C-C bond, and the high binding-energy signals were separately ascribed to C-N, C-O, C=O and O=C-N. The peak at 286.2 eV was attributed to the protein backbone’s NH-CHR-CO carbons, and the peak at 288.3 eV was attributed to the protein’s -CO-NH- peptide bond [44,45]. The C-N and C=O-NH bonds that came from the amine or amide groups in enzymes supplied two components at 399.4 and 400.0 eV, respectively, according to the deconvolution of the N 1s spectra (Figure 5(c,c1)) [46,47]. These characteristic bands illustrated the presence of enzymes on the surfaces of the samples after immersion [48].
XPS is a non-destructive measurement technology that detects material surface information through a photon beam incident onto the sample surface at a depth of 3–10 nm. The thin films formed on the samples in SGF/SIF in this study were analyzed via GD-OES, as shown in Figure 6. Almost no Li element was detected within 0.5 μm from the surfaces of the samples after 7 days of immersion in SGF/SIF (Figure 6c,d). As a result, Li was not detected in XPS spectra for samples immersed 7 days (Figure 4).
Figure 6a,c demonstrates the distribution of elements along the depth of the sample immersed in SGF for 1 and 7 days, respectively. The insert (Figure 6a) was divided into Li-rich and matrix regions based on the reaction of Li and Zn element. The reactions of interfaces 1 and 2 are discussed in detail in the Section 4. The results of immersing in SGF for 1 day were consistent with the results of XPS (Figure 4b), indicating that the corrosion products on the surface contained considerable amounts of Li-rich products. At day 7, the weight percentage (wt%) of Li was approximately 0 wt% within 0.5 μm of the sample surface (Figure 6c). On the other hand, Zn exhibited a different pattern, rising from 96 wt% on day 1 to 97.25 wt% on day 7. A similar phenomenon was also found from the elemental analysis (Figure 4b). The insert for immersion 7 days in SGF was divided into Zn-rich region, transition region, Li-rich region and matrix region. Li-poor and Li-rich regions can be seen in Figure 6a,c. Figure 6b shows the concentration of each element with the distance from the sample surface after immersion in SIF for 1 day. At 0.5 μm from the surface, a Li-rich region was observed in the sample immersed in SIF for 1 day. The Li-rich region moved to 1 μm away from the surface after immersion for 7 days (Figure 6d). In the Li-rich region, the content of Zn increased sharply. At about 0.25 μm, the content of N decreased rapidly, indicating that pancreatin was involved in the formation of corrosion products on the first day (Figure 6b). Additionally, two Li-poor regions were also observed around the Li-rich region. The reason for the formation of the Li-poor region near the sample surface above the Li-rich region after immersion in SGF is that the region is in direct contact with the solution, and Li is directly corroded and dissolved into the solution [38,39]. Li ions quickly emerge from the matrix and gather close to the surface. Then Li+ ions react with CO32- primarily to produce substantial quantities of Li2CO3(Figure 3b) and deposit to form Li-rich region. The Li-poor region of the substrate beneath the Li-rich region was due to Li atoms in the deeper part of the matrix that could not diffuse to the surface in time. These reasons also apply to the phenomenon of Li-rich and Li-poor regions in the samples in SIF. Subsequently, when near-surface regions became Li-poor regions, the reaction between Zn and H2O became dominant and resulted in the formation of ZnO and Zn5(CO3)2(OH)6 (Figure 3c and Figure 5a), resulting in the dominant Zn-rich products in the later stages of immersion. With the prolongation of immersion time, the corrosion of Li ions on the surface of the sample worsened, and the Li-poor region expanded to the interior of the sample, making the Li-rich area migrate to the interior of the sample. The range of the Li-rich region widened at 7 days due to the accumulation of Li ions. The total eroded depth after immersion for 7 days in the GD-OES experiments was about 1.9 μm in SIF and 2.7 μm in SGF. The range of Zn-rich and Li-rich regions grew with the lengthening of immersion time, indicating the buildup of corrosion products on the samples’ surfaces, as found by comparing the depth profiles of GD-OES samples immersed in SGF and SIF for 1 and 7 days.
Compared with the situation in SGF, the Li-rich and Zn-rich regions of the samples immersed in SIF for 1 day were wider. In addition, the diagram of the element distribution on the surfaces of the samples immersed for 1 and 7 days shows an obvious stage of sudden decrease in O element in SIF, but this stage was not observed in the whole process of SGF immersion. No effective oxidation layer was formed on the surface of the sample immersed in SGF with a low pH value, and most of the corrosion products dissolved into the solution.

3.4. Competitive Mechanisms of Zn and Li in the Degradation Process

The Zn/Li ratio in Zn-0.4Li was 26.8. The Li+/Zn2+ atomic ratio was more than 75 in all of the solutions during immersion (Figure 7a,b), demonstrating that Li+ releasement dominated over Zn2+ releasement in the anode corrosion reaction [39,50]. Li was actively corroded. In SGF, the Li+/Zn2+ atomic ratio decreased in the first 3 days and then increased gradually. This phenomenon also appeared in the samples immersed in SIF. Initially, the Li atoms in the surface layer were preferentially corroded and dissolved into the solution, and this can be seen in Figure 6a. When Li was consumed, Zn began to corrode, thus reducing the Li+/Zn2+ atomic ratio. As shown in Figure 6c,d, the Li-rich region, where O, C and Li ions together formed a compact oxide layer, widened after immersion for 7 days. Owing to the compact oxide layer accumulation preventing further corrosion of Zn, the Li+/Zn2+ atomic ratio increased in the late period of immersion. Notably, the Li+/Zn2+ atomic ratio in SIF was always higher than that in SGF. The samples immersed in SGF corroded at a greater rate than the samples immersed in SIF (Figure 2). Compared with the degradation rate of Li atoms, the degradation rate of Zn in the samples immersed in SIF was much lower than that in SGF. Obviously, strong acid and plentiful Cl- in the SGF environment made it clearly distinct from SIF. Cl- ions have the potential to cause localized corrosion and the disruption of passive films [51,52]. A low pH value is not conducive to passivation film formation. On the other hand, the corrosion rate can be slowed down by the passivation film that is created by hydrogen phosphate and Zn ions [40,49,53]. Owing to these factors, the corrosion of Zn atoms in the sample proceeded slowly in SIF in the current study. Zhang et al. [54] found that pancreatin and pepsin have different effects on preventing metal corrosion, but their effects on Zn degradation remain to be studied further.

3.5. Electrochemical Characterization

Figure 8a,b shows the anodic and cathodic PDP curves of Zn–0.4Li at different times in SGF and SIF. Figure 8c,d provides a list of the relative electrochemical corrosion parameters that were identified by PDP. The values of corrosion potential (Ecorr) and corrosion current density (icorr) of the sample soaked in SIF were calculated using Tafel extrapolation of the cathodic region, while Ecorr and icorr of the sample soaked in SGF were obtained using the Tafel extrapolation method with both the anodic and cathodic Tafel regions [55,56,57]. icorr is a kinetic parameter used to assess the rate of corrosion. In this study, on the first day, the icorr value increased compared with that on day zero and then decreased with immersion time in SGF (Figure 8c), which corresponded with the corrosion rate obtained using the mass loss approach (Figure 2a). When the samples were immersed in SGF, Ecorr changed very little with time. With the exception of a minor fluctuation, the Ecorr of the samples immersed in SIF drifted towards positive values with duration. This finding showed a decreased sensitivity of the sample to corrosion. For the anodic branch, a passive region appeared in the samples in SIF, indicating that a film provisionally formed on the surface and impeded corrosion, whereas the Zn–0.4Li alloy in SGF was corroded continuously. Moreover, the range of the passive region of SIF increased with the extension of immersion time, revealing a high kinetic barrier effect on anodic dissolution with immersion time. The corrosion pattern discovered by the electrochemical test was similar to the corrosion trend discovered by the immersing experiment.
The curves in Figure 9 represent the variation of the OCP values of the Zn–0.4Li electrode with time in SGF and SIF. The value of OCP was more negative in SIF than in SGF. For SIF, the corrosion potentials shifted from approximately −1.1 V and increased quickly during the first 0.5 days, with fluctuations in the range of approximately 10%. This behavior might be connected to the collaborative activity of oxides and enzymes on the sample surfaces during immersion [58]. Then, some sharp drops in corrosion potentials were observed from the first 0.5 day to 1 day. After the first day, the corrosion potentials remained stable with a low fluctuation frequency until 2.5 days. The OCP of SIF changed periodically within a period of about 2.5 days; this may have resulted from the adsorption and subsequent desorption of enzymes or the shedding of corrosion products during immersion [48]. For SGF, OCP shifted cathodically with small fluctuations. The phenomenon in SGF was mainly due to the pH value of the solution being 1.2, and the amount of H+ in the solution was presumably too large to form an effective protective film.
EIS is a non-destructive method for examining the reactions of corroding electrodes to small-amplitude alternating potential signals with a wide range of frequencies in order to understand the electrochemical corrosion process in metals [59]. Figure 10 displays the comparison of EIS as Nyquist plots of Zn–0.4Li alloys measured for the samples in SGF (Figure 10a) and SIF (Figure 10b). To derive comprehensive insights into the surface reaction during immersion measurement, Bode plots were created for the impedance data (Figure 10c,d). The diameter of the capacitive loop is related to the corrosion rate; the higher the corrosion rate is, the smaller the diameter is [60,61]. The changes in the capacitance circuit diameters of the samples immersed for 1, 3 and 7 days corresponded exactly to the corrosion rate trend of SGF in Figure 2a. As can be seen in the insert in Figure 10a, the values of impedance modulus |Z| shrank within the first day and gradually increased. As is common knowledge in the oxidation community, the majority of metals can instantly produce a very thin oxidation layer when they come into contact with the air [61]. Before the fresh alloy substrate was immersed in SGF, it was quite possible that the natural oxide formed as a result of the substrate’s spontaneous oxidation. On the first day, natural oxide was removed from the surface by the extremely low pH of the solution. With the gradual consumption of H+, the product layer formed and thickened gradually (Figure 6a,c). This phenomenon also explains why icorr increased on the first day of immersion, then decreased afterwards (Figure 8c).
According to the Nyquist plots (Figure 10a), one capacitive loop was evident for the sample in SGF in all intervals. One overlapped time constant was identified from the Bode plots (Figure 10c). The experimental EIS data of Zn-0.4Li immersed in SGF for 0, 1, 3, and 7 days were fitted utilizing the one-time constant model. In Figure 10, the solution resistance is denoted by Rs, and the film resistance and capacity, respectively, are denoted by Rf and CPEf, which define the first capacity loop at medium frequency. Rct and CPEdl, which respectively stand for charge transfer resistance and the electric double layer at the metal/electrolyte interface, characterize the second capacity loop at low frequency. CPE is a constant phase element to compensate for the non-homogeneity in the system [62]. The CPE’s electrical impedance is defined as follows [63]:
  Z C P E = 1 ( j ω ) n Y o     ,
where Yo denotes the general admittance function, ω stands for the angular frequency, and the exponent n denotes a coefficient relating to the departure from pure capacitance. Table 2 contains a list of the fitting parameters. The chi-square values (χ2) were less than 1.6 × 10−3, indicating a satisfactory fit between the measured and simulated values.
In Figure 10f,g, the equivalent circuit models that were employed to match the SIF curves are depicted. After immersion for 0, 1, 3 and 7 days, the Nyquist curves of the samples in SIF exhibited two capacitive loops. The charge transfer resistance of the electric double layer produced the capacitive semicircle at the high frequency. It appeared in the linear zone at the low frequencies and was associated with diffusion impedance. They were adapted using the equivalent circuit with Warburg impedance [40,58,64]. Table 3 displays the corresponding parameters. The value of CPEf decreased gradually, indicating that the corrosion product layer thickened [65]. In the EIS measurements, polarization resistance Rp was obtained by combining Rct and Rf [66]. The Rp values in SIF increased with immersion time, indicating that the corrosion of Zn–0.4Li weakened with time. This result is in line with the PDP curves (Figure 8b,d).

4. Discussion

This work studied the influence of the gastrointestinal environment on the degradation behavior of Zn–0.4Li alloys by using simulated gastric and intestinal fluids. Similarly to albumin, pepsin and pancreatin are proteins made of many amino acids connected by peptide bonds. The Langmuir adsorption isotherm has been utilized to describe the adsorption of albumin onto metal surfaces [67]. The value of Gibbs free energy of adsorption obtained from the Langmuir adsorption isotherm can reveal whether albumin molecules are adsorbed on pure Zn surfaces through chemisorption or physisorption [40]. However, no adsorption isotherm can be used for pepsin and pancreatin. The proteins are commonly perceived to have a chelating and barrier function. Delayed corrosion was confirmed due to the physical barrier created by proteins adhering to the metal surface [40,68,69,70]. The GD-OES and XPS spectra in the current study indicated the existence of organic components in the corrosion layer. Both enzymes affected the formation of the degradation products. Specifically, Li+ was released prior to Zn2+ during immersion in SGF and SIF from the releasement of the Li+/Zn2+ atomic ratio in Figure 7. The first reason is that the Li atom in the corrosion layer exhibits superior mobility and activity and readily combines with the aqueous environment to generate Li-containing compounds in the external corrosion layer [71]. The second reason is that the E° of Li (i.e., −3.04 V) is much lower than that of Zn (i.e., −0.76 V) [27]. The corrosion mechanisms of Zn–0.4Li immersed in SGF and SIF are shown in Figure 11.
For the samples immersed in SGF, the near-surface area of the sample was divided into Li-rich and matrix regions (Figure 6a). At the interface 1 of direct contact between matrix and solution (Figure 10a), Li reacted first and released an amount of Li+ ions, alkaline hydroxyl anions and hydrogen gas (Formulas (3) and (4)).
Li(s) → Li+(aq) + e−
2H2O + 2e− → H2(g)↑ + 2OH−(aq)
2Li+ + CO32− → Li2CO3(s)
The released Li+ ions could react with CO32− ions in the SGF according to Formula (5) to form Li2CO3. The Li2CO3 nuclei grew continuously, and Li-rich products thickened because there were adequate Li+ and CO32− ions available.
Subsequently, after the near-surface region became Li-poor, the reaction between Zn and H2O at the interface 2 became dominant and resulted in the formation of Zn(OH)2, parts of which were likely to be dehydrated to form ZnO [72] or react with the dissolved CO2 in SGF to form Zn5(CO3)2(OH)6 [73].
According to XRD and XPS (Figure 3c and Figure 5a), the corrosion products were formed according to the following reactions:
Zn + 2H2O → H2(g)↑ + Zn(OH)2(s)
Zn(OH)2(s) → ZnO(s) + H2O
Zn(OH)2(s) + 4Zn2+ + 4OH- + 2CO32- → Zn5(CO3)2(OH)6(s)
During the initial immersion in SGF (Figure 10a), hydrogen evolution and adsorption of enzymes occurred simultaneously. From XPS and GD-OES (Figure 5 and Figure 6), pepsin contributed to the development of corrosion products and mainly existed in the Zn-rich product region. The surface became uneven due to the combined effect of chlorine and hydrogen ions, and severe localized corrosion occurred, then extended further (Figure 3a). Additionally, the surface of the sample was covered with a layer of loose corrosion products. The adsorption of pepsin and the corrosion layer could not alleviate the Cl- attack in SGF due to the extremely low pH of the solution, resulting in the diminution of the |Z| value (Figure 10a). A Li-poor region of the substrate could be discovered below the corrosion layer as a result of the deeper substrate’s Li atoms being unable to diffuse to the surface in a timely manner (Figure 6). With the increase in immersion time, the Li-rich region migrated to the interior of the sample, and the range was enlarged. The weight percentage of the element N in the dissolved region of the sample surface decreased from 0.733 to 0.309 (Figure 6a,c), indicating that the amount of the enzyme adsorbed on the sample surface decreased, possibly due to enzyme shedding or hydrolysis during the immersion process (Figure 11a). However, because of the accumulation of the corrosion products, the corrosion product layer increased the film resistance, and the corrosion resistance of the samples was improved, as demonstrated by the corrosion rate, polarization curve and impedance spectrum (Figure 2a, Figure 8c and Figure 10a).
For SIF, the dissolution of Zn–0.4Li and the formation of Zn phosphate are presented in Figure 3c and Figure 5(a1). Notably, the deposition of phosphate helps decrease the corrosion rate and uniform corrosion of metals [52,74,75]. According to the SEM images, the samples were uniformly corroded (Figure 3b). Although the adsorbed pancreatin was implicated in the production of the corrosion products, it was less dense than the Zn phosphate layer, which allowed the aqueous solution to penetrate it easily [70]. As a result, Li-poor regions and a Li-rich region can be observed in Figure 6b,d. With the accumulation of phosphates, the charge transfer at the interfaces was hindered. Hence, the corrosion resistance of Zn–0.4Li improved. The corrosion mechanism of the Li-rich and Li-poor regions close to the sample surface area was similar to that of SGF when samples were immersed in SIF. During immersion in SIF (Figure 11b), the reaction type of Zn and H2O at interface 2 differed from that in SGF. Dissolution of Zn takes place in accordance with Equations (8) and (9):
Zn → Zn2+ + 2e−
O2 + 2H2O + 4e− →4OH−
In the Zn-rich region of the sample in SIF, Zn and the ions in the solution formed Zn carbonate and Zn phosphate. The OCP of SIF changed periodically within a period of about 2.5 days (Figure 9). The phenomenon may have resulted from the adsorption and subsequent desorption of pancreatin or the shedding of corrosion products during immersion (Figure 11b). The Zn-rich and Li-rich regions of the samples immersed in SIF were always thicker than those of the samples immersed in SGF (Figure 11).

5. Conclusions

In the present study, the influence of the gastrointestinal environment on the corrosion of Zn alloys was investigated.
  • In SGF, the samples were severely corroded locally, and the corrosion pits had a maximum diameter of over 150 μm. In SIF, the samples were uniformly corroded.
  • The generation of the degradation product layer involved the digestive enzymes pepsin and pancreatin. The amount of enzyme attached to the surface of the sample decreased with immersion time because of solution flushing or hydrolysis of enzymes. The adsorption and subsequent desorption of pancreatin or the shedding of corrosion products occurred when the specimens were immersed in SIF.
  • Following immersion in SGF and SIF, Li-poor and Li-rich regions were observed in the near-surface of the samples. In the surface of samples where direct contact with the solution occurred, Li reacted first and released an amount of Li+ ions. Then Li+ ions reacted dominantly with CO32− to form significant amounts of Li2CO3 to form the Li-rich region. Due to that, Li atoms in the deeper part of the matrix could not diffuse to the surface in time, and the Li-poor region of the substrate beneath the Li-rich region was formed. When Li was depleted, the reaction between Zn and H2O became dominant and formed Zn-rich products with time. The extent of the Li-rich region increased with immersion time because of the accumulation of the corrosion products.
  • Li+ releasement dominated over Zn2+ releasement throughout the immersion process in SGF and SIF.

Author Contributions

M.W.: Investigation, Methodology, Data curation, Writing—original draft. L.Y. (Lingbo Yang): Data curation. X.Z.: Investigation. L.Y. (Lijing Yang): Writing—review and editing, Funding acquisition, Supervision. J.S., T.L.: Conceptualization. Z.S., H.L.: Conceptualization, Resources, Supervision. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by National key research and development program (2019YFE0118600).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The raw/processed data required to reproduce these findings cannot be shared at this time as the data also form part of an ongoing study.

Conflicts of Interest

The authors declare no conflict of interest.

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Coatings 13 00529 g001 550
Figure 1. Microstructure characterization of Zn–0.4Li before immersion in solutions. (a) XRD spectra and (b) SEM image. (c) High-magnification image of the yellow frame.
Figure 1. Microstructure characterization of Zn–0.4Li before immersion in solutions. (a) XRD spectra and (b) SEM image. (c) High-magnification image of the yellow frame.
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Figure 2. Corrosion rate and pH of Zn–0.4Li immersed in SGF and SIF at different times: (a) in SGF and (b) in SIF.
Figure 2. Corrosion rate and pH of Zn–0.4Li immersed in SGF and SIF at different times: (a) in SGF and (b) in SIF.
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Figure 3. In vitro degradation behaviors of Zn–0.4Li alloy in SGF and SIF. (a) SEM images of Zn–0.4Li immersed in SGF and SIF at different time, and (b) surface morphologies after the removal of corrosion products. The areas enclosed by white squares are observed at high magnification. (c) XRD analysis of degradation products. (d) Enlarged image of the red frame.
Figure 3. In vitro degradation behaviors of Zn–0.4Li alloy in SGF and SIF. (a) SEM images of Zn–0.4Li immersed in SGF and SIF at different time, and (b) surface morphologies after the removal of corrosion products. The areas enclosed by white squares are observed at high magnification. (c) XRD analysis of degradation products. (d) Enlarged image of the red frame.
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Figure 4. Immersion in SGF and SIF for 1 and 7 days: (a) XPS spectra; (b) atomic concentration of elements detected by XPS.
Figure 4. Immersion in SGF and SIF for 1 and 7 days: (a) XPS spectra; (b) atomic concentration of elements detected by XPS.
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Figure 5. XPS spectrum of the samples immersed in SGF and SIF for 7 days. (a–c) Zn–0.4Li alloy in SGF; (a1–c1) in SIF. (a,a1) Zn 2p, (b,b1) C 1s and (c,c1) N 1s.
Figure 5. XPS spectrum of the samples immersed in SGF and SIF for 7 days. (a–c) Zn–0.4Li alloy in SGF; (a1–c1) in SIF. (a,a1) Zn 2p, (b,b1) C 1s and (c,c1) N 1s.
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Figure 6. GD-OES depth profiles of Zn–0.4Li immersed in SGF (a,c) and SIF (b,d). (a,b) For 1 day, (c,d) for 7 days. ①: interface 1, ②: interface 2. The insert is an enlarged view from the surface of the sample to 2 μm inside.
Figure 6. GD-OES depth profiles of Zn–0.4Li immersed in SGF (a,c) and SIF (b,d). (a,b) For 1 day, (c,d) for 7 days. ①: interface 1, ②: interface 2. The insert is an enlarged view from the surface of the sample to 2 μm inside.
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Figure 7. Releasing Li+/Zn2+ atomic ratio in SGF (a) and SIF (b) at different times.
Figure 7. Releasing Li+/Zn2+ atomic ratio in SGF (a) and SIF (b) at different times.
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Figure 8. PDP curves of Zn–0.4Li after immersion in SGF (a) and SIF (b) for different times. Electrochemical parameters of the samples after immersion in SGF (c) and SIF (d).
Figure 8. PDP curves of Zn–0.4Li after immersion in SGF (a) and SIF (b) for different times. Electrochemical parameters of the samples after immersion in SGF (c) and SIF (d).
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Figure 9. OCP evolution with time in the electrolytes of Zn–0.4Li immersed in SGF and SIF.
Figure 9. OCP evolution with time in the electrolytes of Zn–0.4Li immersed in SGF and SIF.
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Figure 10. EIS of Zn–0.4Li immersed for different times: (a,b) Nyquist plots and (c,d) Bode plots. (a,c) For SGF, (b,d) for SIF. Diagrams (e–g) show the equivalent circuits used for EIS data. Circuit (e) was used to fit the EIS data obtained in SGF at all intervals, and (f) was used to fit the EIS data obtained in SIF on day 0 and (g) for 1, 3, 7 days. The insert shows the change in the values of |Z| and pH along with immersion time in (a).
Figure 10. EIS of Zn–0.4Li immersed for different times: (a,b) Nyquist plots and (c,d) Bode plots. (a,c) For SGF, (b,d) for SIF. Diagrams (e–g) show the equivalent circuits used for EIS data. Circuit (e) was used to fit the EIS data obtained in SGF at all intervals, and (f) was used to fit the EIS data obtained in SIF on day 0 and (g) for 1, 3, 7 days. The insert shows the change in the values of |Z| and pH along with immersion time in (a).
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Figure 11. Schematic illustration of Zn–0.4Li alloy degradation in different solutions as the immersion time increases. (a) Degradation in SGF and (b) degradation in SIF.
Figure 11. Schematic illustration of Zn–0.4Li alloy degradation in different solutions as the immersion time increases. (a) Degradation in SGF and (b) degradation in SIF.
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Table
Table 1. Binding energy and corresponding compounds in the XPS spectra of Zn–0.4Li surfaces after immersion for 7 days.
Table 1. Binding energy and corresponding compounds in the XPS spectra of Zn–0.4Li surfaces after immersion for 7 days.
ElementCharacterized BondsBinding Energy (eV)References
Zn 2pZnO1022[49]
Zn5(CO3)2(OH)61022.3[49]
Zn3(PO4)2•4H2O1022.5[49]
C 1sC-H/C-C284.5[44,45]
C-N285.4[44,45]
C-O286.2[44,45]
C=O287.5[44,45]
O=C-N288.3[44,45]
N 1sC-N399.4[46,47]
C=O-NH400.0[46,47]
Table
Table 2. Fitting data of EIS for Zn–0.4Li after immersion in SGF for different times.
Table 2. Fitting data of EIS for Zn–0.4Li after immersion in SGF for different times.
SolutionImmersion Time (day)CPEf
(10−4Ω−1cm−2sn1)		n1Rf
(Ωcm2)		χ2 (10−3)
SGF00.67 ± 0.080.91 ± 0.0237.5 ± 3.551.610 ± 0.108
13.51 ± 0.320.82 ± 0.048.62 ± 0.890.233 ± 0.021
32.95 ± 0.190.85 ± 0.0513.3 ± 1.010.122 ± 0.011
71.72 ± 0.070.87 ± 0.0216.1 ± 1.980.614 ± 0.037
Table
Table 3. Fitting data of EIS for Zn–0.4Li after immersion in SIF for different times.
Table 3. Fitting data of EIS for Zn–0.4Li after immersion in SIF for different times.
SolutionImmersion Time (day)CPEf
(10−4Ω−1cm−2sn1)		n1Rf (Ωcm2)CPEdl
(10−4Ω−1cm−2 sn2)		n2Rct
(kΩ cm2)		W
(10−4Ω−1cm−2s0.5)		χ2 (10−3)
SIF06.39 ± 0.520.70 ± 0.106177 ± 3545.83 ± 1.440.73 ± 0.088.97 ± 0.65-0.40 ± 0.018
10.50 ± 0.090.69 ± 0.06175 ± 21.84.87 ± 0.200.73 ± 0.119.15 ± 0.872.78 ± 0.120.30 ± 0.022
30.360 ± 0.050.73 ± 0.05339 ± 30.42.05 ± 0.130.68 ± 0.079.28 ± 0.442.74 ± 0.230.44 ± 0.015
70.168 ± 0.010.8 ± 0.096755 ± 4671.29 ± 0.410.63 ± 0.0416.5 ± 0.710.265 ± 0.040.85 ± 0.031
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Wang, M.; Yang, L.; Zhu, X.; Yang, L.; Shen, J.; Lu, T.; Liu, H.; Song, Z. Corrosion Mechanisms of a Biodegradable Zn-0.4Li Alloy in Simulated Gastrointestinal Environment. Coatings 2023, 13, 529. https://doi.org/10.3390/coatings13030529

AMA Style

Wang M, Yang L, Zhu X, Yang L, Shen J, Lu T, Liu H, Song Z. Corrosion Mechanisms of a Biodegradable Zn-0.4Li Alloy in Simulated Gastrointestinal Environment. Coatings. 2023; 13(3):529. https://doi.org/10.3390/coatings13030529

Chicago/Turabian Style

Wang, Manli, Lingbo Yang, Xinglong Zhu, Lijing Yang, Jianwei Shen, Ting Lu, Huinan Liu, and Zhenlun Song. 2023. "Corrosion Mechanisms of a Biodegradable Zn-0.4Li Alloy in Simulated Gastrointestinal Environment" Coatings 13, no. 3: 529. https://doi.org/10.3390/coatings13030529

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