Mapping the Accouterment Effects of Plasma Nitriding on AISI 316L in Biomedical Applications

Abstract

The present study aimed to critique the corrosion resistance of plasma-nitrided films of AISI 316L stainless steel with regards to their biomedical applications. The plasma nitriding process improves austenitic stainless steel’s micro-hardness and corrosion resistance. Austenitic stainless steel was treated at a temperature of 470 °C for 12, 24, and 36 h, to observe the outcomes of plasma nitriding. The corresponding microstructure, microhardness, depth of the nitrided layer, and electrochemical parameters were systematically characterized. The corrosion resistance of the plasma-nitrided specimens was gauged using the weight loss method in simulated body fluids (Phosphate-buffered saline (PBS), saline water, and ringer solution) by static immersion for 9, 18, and 27 days. Optimization was catalogued using the Taguchi method L27 orthogonal array to determine the optimum combination of plasma nitriding time and immersion time in simulated body fluid. The material characterization showed that the corrosion resistance of the plasma-nitrided specimens improved with longer nitriding times by Tafel polarization curves. Microhardness was observed at 12, 24, and 36 h as 1060, 1150, and 1220 HV_0.1. SEM, with an energy-dispersive X-ray analysis (EDS) used to characterize the surface before and after plasma nitriding testing. It was concluded that CrN, which precipitates during processing and contributes to the loss of chromium from the surrounding matrix and the onset of a corrosive environment, is the primary cause of this behaviour.

1. Introduction

Commercial metals, such as aluminium and titanium alloys and stainless steel, are used in various industries due to their high strength, corrosion resistance, and the filming that forms on the surface of these metals. These materials are particularly useful in body implants, offshore industries, aerospace, transportation, undersea vehicles, and infrastructures [1,2,3]. They are also used in other applications where corrosion resistance and high strength are essential, such as constructing bridges and buildings. Using commercial metals allows for the creation of durable structures that can withstand harsh environmental conditions. AISI 316L is a suitable option as a biomaterial because of the steady and inert passive oxide film on its surface [4]; however, its implant life must be improved for applications requiring great strength, such as orthopaedic implants. For these reasons, pure AISI 316L should use surface-modification techniques. Diffusional and coating treatments are the two primary categories of applied surface actions. Diffusional surface modification techniques are used to increase the material’s mechanical strength and tribological resistance. Plasma-assisted oxidation and nitriding procedures are the most used techniques for these treatments [5]. However, these surface treatments modify the corrosion characteristics of AISI 316L. The latter’s components primarily use methods to increase corrosion resistance and bone bonding capabilities. Bone ingrowth into permeable implant surfaces happens when the implants are inserted into bone tissue. Due to increased osteoblast cell adherence to the substrate, osseointegration may be improved [6].

Enhanced osseointegration offers mechanical stability by fusing the implant with the surrounding bone tissue. Plasma surface nitriding of 17-4 PH martensitic precipitation-hardening stainless steels was carried out by M. Esfandiari et al. [5] for 10 h at 350 °C, 420 °C, and 500 °C. Under non-lubricated circumstances, all three treatments significantly increased surface hardness and sliding wear resistance. High-temperature (420 °C and 500 °C)-treated materials also showed better corrosion and corrosion-wear performances due to creating surface compound layers. In a 0.9% sodium chloride solution at a pH of 6.3 and a temperature of 37 °C, to mimic the natural tissue environment, Linda Gil et al. [7] evaluated the effects of ion nitriding on the corrosion performance of AISI3l6L stainless steel using electrochemical tests, such as dynamic and linear polarisation, in both nitrided and untreated AISI 316L steel conditions. AISI 5140 low-alloy steel was subjected to experiments by Akgun Alsaran et al. [8] under various process conditions, such as duration (1, 4, 8, and 12 h), temperature (400, 450, 500, and 550 °C), and gas mixture ratio (0.05, 0.33, 1, and 3 N₂/H₂). Using a Taguchi experimental design, the ideal operating conditions were found for surface hardness, compound layer thickness, and case depth. Ratna Kartikasari et al. [9] studied the corrosion behaviour of plasma-nitrided SS316L biomaterial. After the plasma nitriding process, the findings observed were that the percentage of nitrogen atoms concentrated on the surface of the SS316L ranged from 7.61 to 21.73%, with the maximum number occurring at 500 °C. Fe₄N, CrN, and Cr₂N phases of metal nitride were produced on the surface of SS316L following the nitriding process at all temperature variations [10]. M. Godec et al. [11] studied the additive manufacturing of nitride 316L, using a laser powder-bed fusion process and other solutions, when treated at 1060 °C for 30 min, at low-temperature plasma nitriding at 430 °C, or both. The corrosion and wear resistance of 316L was improved, and when nitriding the wear volume was significantly decreased, although it did not reach the value of the conventional fabricated material. Y. Sun et al. [12] investigated AISI 316L stainless steel that had been low-temperature-plasma-carburized and exhibited tribocorrosion. Specimens were subjected to unidirectional sliding in a solution of 1 M H₂SO₄. The tribocorrosion rate was measured using a pin-on-disk with a potentiostat for electrochemical control.

The novelty of the effects of plasma nitriding on AISI 316L in biomedical applications lies in the unique combination of improved surface properties. Plasma nitriding is a surface-modification technique widely used in the biomedical field to enhance metallic biomaterials’ mechanical, tribological, and biological properties. AISI 316L is a commonly used stainless steel in biomedical applications due to its excellent corrosion resistance, high ductility, and good biocompatibility. Novel research uses immersion tests and Taguchi techniques to test biomaterials’ properties. By combining these two techniques, the studies provide a comprehensive analysis of the effects of plasma nitriding on the AISI 316L material. They also identify the optimal process parameters for achieving the desired properties. This information can be helpful in the development of new biomedical implants and devices that require high biocompatibility and corrosion resistance.

In a review of the literature, it appears that some researchers have worked on improving biomaterials’ properties such as titanium, copper, and magnesium alloys. Still, the cost of the above materials is comparable to AISI 316L. Therefore, in developing countries, high-cost implants will not be affordable for poor people. In this research, we have decided to work on the corrosion properties of AISI 316L; improving its properties will make it beneficial for use as a biomaterial to replace high-cost body implants.

The literature review shows that along with various statistical techniques, there is a lot of progress in machine learning and deep learning. Nowadays, many researchers are working on different machine-learning models for research. Aman Garg et al. [13] have investigated the use of machine-learning models for predicting the compressive strength of concrete-containing nano-silica. The goal of that work was to suggest substitute models based on machine-learning methods, such as Gaussian Process Regression (GPR) and Support Vector Machine (SVM), that can forecast the compressive strength (CS) of concrete-containing nano-silica. Aman Garg et al. [14] have studied the prediction of the elemental stiffness matrix of FG nanoplates using a Gaussian Process Regression-based surrogate model in the framework of a layerwise model. The researchers used an elemental stiffness matrix and GPR surrogate model to predict FG nanoplate’ behaviour. The above machine learning, and Gaussian Process Regression with a stiffness matrix model, can be used in future work involving immersion techniques for measuring the corrosion resistance of plasma-nitrided AISI 316L.

The current investigation used a DC plasma-nitriding method to modify AISI 316L. Following this, the surface characteristics of the plasma-nitrided samples were systematically evaluated with various simulated body fluids. It was essential to investigate the corrosion performance of plasma-nitrided implants as though they were intended for use in the human body [7]. Corrosion behaviour was influenced by the thickness of the nitride layer (46 µm at 470 °C for 12 h to 146 µm at 470 °C for 36 h). The corrosion performance of plasma- nitrided specimens varied with ageing time. The primary aim of this work was to investigate the impacts of plasma nitriding on corrosion behaviour in various simulated body fluids (SBF) by static immersion and Tafel polarization tests. The Taguchi technique was used to optimise conditional process parameters such as nitriding time (h).

2. Materials and Methods

The material used for the present investigation was AISI 316L, primarily used for corrosion-resistant body implants. It was in as-received form, having a microhardness of 334 HV_0.1. This material was manufactured with the help of a lathe machine, and the sample specimen was prepared to have a ϕ5 mm diameter and 30 mm length. The AISI 316L was heat-treated using a plasma nitriding process for 12 h, 24 h, and 36 h at 470 °C. The composition of the electrolyte used in this study is shown in

Table 1. Compositions of the different electrolyte solutions used in this work, and their preparation.

Solution	Compositions
PBS	NaCl (8 g/L) KCl (0.2 g/L) Na2HPO4 (1.44 g/L) KH2PO4 (0.254 g/L)
Saline Water	Dextrose (5% w/v) Sodium Chloride (0.9% w/v)
Ringer’s solution	NaCl (8.6 g/L) KCl (0.3 g/L) CaCl2 (0.33 g/L)

.

Heat Treatment: Plasma Nitriding

Plasma nitriding is a surface-modification process that uses plasma to ionize a gas, usually nitrogen, and then deposits the nitrogen ions onto the surface of a metal workpiece. The metal’s wear resistance, fatigue strength, and corrosion resistance may all be improved by this procedure. The process is typically carried out in a vacuum vessel, a sealed chamber that has been evacuated to create a vacuum inside. The vacuum vessel, typically made of metal, such as stainless steel, is equipped with electrical connections and ports for introducing the gas and plasma into the chamber. The workpiece is placed inside the vacuum vessel and is subjected to the plasma nitriding process under carefully controlled conditions [8,9]. Akgun Alsarana et al. [8] conducted plasma nitriding on AISI 5140 using different process parameters, including time, temperature, and gas mixture. He observed improvements in the strength, hardness, fatigue, and case depth of AISI 5140. The present work makes use of AISI 316L stainless steel. It was thoroughly cleaned using an ultrasonic cleaner to remove any filth and oil, and then the surfaces were dried with a hair drier. For each of the nine sample pieces, solution treatment was performed at 470 °C for 12, 24, and 36 h, followed by air cooling. The samples were cut to a ϕ 5 mm diameter and a length of 30 mm. The cylindrical pin sample pieces were manually ground to a 1200 grade to achieve a smooth surface finish (Ra < b0.1 µm). A DC ion nitriding device (Magnus Industries, DC Pulse Power Supply: 40 Kw Maharashtra, pune, India) was used to perform plasma nitriding at 470 °C for 12 h under a pressure of 5 mbar and a gas mixture of 50%N₂ + 50%H₂ [10,11,12].

3. Experimentation

3.1. Specimen Preparation for Immersion Test

Twenty-seven specimens were equipped to be immersion test samples, with a 5 mm diameter and length of 30 mm, in different simulated body-fluid environments. Nine specimens were treated with plasma-nitriding heat treatments for varying nitriding times of 12, 24, and 36 h, in a nitriding Furnace (Magnus Industries, DC Pulse Power Supply: 40 Kw Maharashtra, pune, India) [12]. Nine specimens were treated with plasma-nitriding heat treatments for varying hourly nitriding times, using the same nitriding as in the Tafel polarization test. Care was taken to prevent an excessive rise in temperature when cutting the specimens. Before the experiment, the specimen surfaces were polished with 1200-grit emery paper and then lapped with thin alumina powder suspended in distilled water on a disc polisher (Magnus Industries, DC Pulse Power Supply: 40 Kw Maharashtra, pune, India). The process continued until a smooth surface was achieved. All specimens were then washed in acetone and rinsed in distilled water. Hot air was used to dry the specimens to ensure no stains on the surface. The initial weight of each specimen was measured by scientific balance with a four-digit precision (Bitizon Make, and Model No. CX220) [13].

3.1.1. Immersion Test

Immersion tests were performed as per ASTM-G31 standard [14], utilizing a variety of simulated body fluids, including PBS, saline water, and ringer solution. All immersion experiments were performed at atmospheric temperature without agitation, liquid transmission, or interference with the corrosive environment. All 27 bottles were prepared with 150 mL simulated body solution of PBS, saline water, and ringer’s solution [15].

3.1.2. Ultrasonic Cleaning

After each immersion test, all specimens were cleaned with an ultrasonic cleaner (Spire Automation Make, and Model No. US-2) for 5 min in acetone. Then, weight loss in each sample was calculated with an accuracy of up to four digits using the same digital balance and specimens. Because it eliminates corrosion products with less base metal breakdown, ultrasonic cleaning was preferable.

4. Plan of Experiments:

Immersion tests were conducted by altering three factors—Nitriding time (h), simulated body fluids, and the immersion duration (days) of the corrosion test—to determine the degree to which process parameters influence the test results. In

Table 2. Process parameters and levels.

Level	Nitriding Time (h)	Simulated Body Fluids	Immersion Time (days)
1	12	PBS	9
2	24	Saline Water	18
3	36	Ringer’s solution	27

, the factors and their corresponding levels are listed. An L27 orthogonal array with 27 rows and 3 process parameters columns was chosen, as stated in

Table 3. L27 Orthogonal array of the Taguchi design: DOE with response test results.

	Input Parameters			Response
Sr. No.	Plasma Nitriding (h)	Environmental Details	Immersion Time (days)	Corrosion Rate (mpy)
1	12	PBS	9	0.100
2	12	PBS	9	0.120
3	12	PBS	9	0.110
4	12	Saline Water	18	0.130
5	12	Saline Water	18	0.123
6	12	Saline Water	18	0.132
7	12	Ringer’s solution	27	0.440
8	12	Ringer’s solution	27	0.430
9	12	Ringer’s solution	27	0.440
10	24	PBS	18	0.230
11	24	PBS	18	0.240
12	24	PBS	18	0.241
13	24	Saline Water	27	0.231
14	24	Saline Water	27	0.232
15	24	Saline Water	27	0.211
16	24	Ringer’s solution	9	0.441
17	24	Ringer’s solution	9	0.423
18	24	Ringer’s solution	9	0.415
19	36	PBS	27	0.312
20	36	PBS	27	0.321
21	36	PBS	27	0.303
22	36	Saline Water	9	0.311
23	36	Saline Water	9	0.302
24	36	Saline Water	9	0.311
25	36	Ringer’s solution	18	0.451
26	36	Ringer’s solution	18	0.405
27	36	Ringer’s solution	18	0.435

, following the requirement that the degree of freedom for the orthogonal array should be more than or equal to the total of the corrosion parameters. The Taguchi model was created based on the run order, and 27 tests were carried out. The first column in an orthogonal array was devoted to the nitriding period (h), the second to the simulated body fluids, the third to the specimens immersion time (days), and the remaining columns were assigned to their interactions [16,17].

The responses for the model were the corrosion rate (mpy). The S/N ratio characteristics can be divided into three categories, namely “nominal is the best”, “larger the better”, and “smaller the better” features. This research used “smaller the better” attributes to inspect the corrosion rate. The logarithmic transformation of the loss function was used to obtain the S/N ratio for corrosion rate using the “smaller the better” characteristic, the results of which are as follows [18,19,20]:

$$\frac{S}{N}=-10 lo{g}^{\left(\frac{1}{n}\right)} \left(x_1^2+x_2^2+\cdots +x_n^2\right)$$

(1)

where x₁, x₂ … x_n is the response of the corrosion rate, and n is the number of observations.

Three samples from each bottle were submerged in a different place (after being weighed first) for repeatability. The bottles were then hermetically sealed with lead to prevent environmental contamination. The immersion test was conducted for 9 days (216 h), 18 days (432 h), and 27 days (648 h) for investigation of the corrosion rate.

Corrosion Rate by Weight Loss Method

The corrosion rate was calculated using the weight loss formula from the immersion test. The static immersion test weight loss method examined the corrosion rate. By deducting the final weight (obtained after the immersion test) from the beginning weight of each specimen, the weight loss was calculated (before the immersion test). Corrosion rate measurements and changes in surface morphology were used to study corrosion behaviour. Using Equation (2), the corrosion rate was determined in mpy [21,22].

$$\mathrm{Corrosion}rate\left(\mathrm{mpy}\right)=\frac{534W}{DTA}$$

(2)

In this equation, the corrosion rate is expressed in mils per year (mpy), and the other variables are as follows:

• Weight loss (W): the amount of metal that is lost due to corrosion, expressed in milligrams.
• Density (D): the mass per unit volume of the metal, expressed in grams per cubic centimetre.
• Exposed surface area (A): the amount of surface area of the metal exposed to the corrosive environment, expressed in square inches.
• Exposure time (T): the length of time the metal is exposed to the corrosive environment, expressed in hours.

where W is the weight loss (mg), D is the metal’s density (gram/cm³), A is the exposed surface area (square inch), and T is the exposure duration, the corrosion rate (mpy) is expressed in mils (0.001 inches) per year (h). Table 3 lists every outcome of the corrosion rate.

5. Results and Discussion

The experimental design aimed to find essential factors and combinations influencing the corrosion process to achieve the minimum corrosion rate. The tests were designed using an orthogonal array and were intended to correlate the effects of immersion time days, simulated body fluids, and nitriding time (h) on plasma-nitrided AISI 316L. The ANOVA aimed to identify the parameter(s) and parameter combinations that significantly impact the corrosion rate. The Taguchi method implemented a conceptual method to analyse the S/N ratio, which entailed analysing the implications and graphically highlighting the essential elements [23].

5.1. Results of Statistical Analysis of Experiments

The outcomes for different parameter combinations were collected after the experiment was carried out according to the orthogonal array. The measured data were examined using the commercial program MINITAB 18, designed exclusively for DOE applications. The experimental data for corrosion rate are shown in

Table 4. Orthogonal array and corrosion rate results (mpy).

Exp. No.	Plasma Nitriding (h)	Environmental Details	Immersion Time (Days)	Corrosion Rate (mpy)	S/N Ratio	Corrosion Rate (mpy)
1	12	PBS	9	0.100	19.1483	0.100
2	12	PBS	9	0.120	19.1483	0.120
3	12	PBS	9	0.110	19.1483	0.110
4	12	Saline Water	18	0.130	17.8293	0.130
5	12	Saline Water	18	0.123	17.8293	0.123
6	12	Saline Water	18	0.132	17.8293	0.132
7	12	Ringer’s solution	27	0.440	7.1965	0.440
8	12	Ringer’s solution	27	0.430	7.1965	0.430
9	12	Ringer’s solution	27	0.440	7.1965	0.440
10	24	PBS	18	0.230	12.5031	0.230
11	24	PBS	18	0.240	12.5031	0.240
12	24	PBS	18	0.241	12.5031	0.241
13	24	Saline Water	27	0.231	12.9612	0.231
14	24	Saline Water	27	0.232	12.9612	0.232
15	24	Saline Water	27	0.211	12.9612	0.211
16	24	Ringer’s solution	9	0.441	7.4022	0.441
17	24	Ringer’s solution	9	0.423	7.4022	0.423
18	24	Ringer’s solution	9	0.415	7.4022	0.415
19	36	PBS	27	0.312	10.1145	0.312
20	36	PBS	27	0.321	10.1145	0.321
21	36	PBS	27	0.303	10.1145	0.303
22	36	Saline Water	9	0.311	10.2282	0.311
23	36	Saline Water	9	0.302	10.2282	0.302
24	36	Saline Water	9	0.311	10.2282	0.311
25	36	Ringer’s solution	18	0.451	7.3154	0.451
26	36	Ringer’s solution	18	0.405	7.3154	0.405
27	36	Ringer’s solution	18	0.435	7.3154	0.435

. The data provided are an average across three repetitions. The signal-to-noise ratio was developed using the experimental results to calculate the quality attributes. Signal-to-noise response tables have analysed how control factors such as nitriding time (h), simulated bodily fluids, and immersion time affect corrosion rate. Akgun Alsaran et al. [8] studied how five factors, each with four levels (values), were used to create an experimental plan using an orthogonal array (OA) experimental design approach (L16). Looking at the amount of variation in a response might help to rapidly pinpoint the control aspects that can help minimize variance (better quality). The S/N ratio combined several repeats into one value depicting the variance degree. The ranking of process factors using signal-to-noise ratios obtained at different parameter levels of corrosion rate is given in Table 4. The simulated body fluids influenced the corrosion rate the most, followed by immersion duration (days) and plasma nitriding (h), and the control parameters were statistically significant in the signal-to-noise ratio. Analysis of these experimental results using S/N demonstrated optimum conditions resulting in a minimum corrosion rate. As shown in Table 4, the optimal corrosion rate conditions comprise a plasma nitriding time of 36 h using ringer’s solution as the simulated body fluid, for an immersion time of 27 days. Through these findings, the ideal control parameters for maximising the corrosion resistance of plasma-nitrided AISI 316L were discovered [24]. It was also observed that with increasing nitriding time(h), the corrosion resistance of AISI 316L increased due to the formation of a CrN layer.

Table 5. Response table for the “smaller is better” signal-to-noise ratio (corrosion rate).

Level	PN (h)	ED	IT (Days)
1	14.725	13.992	12.260
2	10.956	13.673	12.549
3	9.219	7.305	10.091
Delta	5.505	6.617	2.459
Rank	2	1	3

shows response table for the “smaller is better” signal-to-noise ratio (corrosion rate) of plasma nitriding AISI 316L in various simulated body fluids.

5.2. ANOVA of Corrosion Test (Immersion Test) Results

The experimental data were analysed using ANOVA to examine the impact of the considered corrosion characteristics, including the simulated body fluids, the plasma nitriding duration (h), and the immersion period (days), on the performance measures. An ANOVA is used to identify which independent variable outperforms the others and to what percentage of the result variable it contributes.

Table 6. ANOVA for corrosion rate (mpy); ANOVA: analysis of variance for means.

Source	DF	Seq. SS	Adj. SS	Adj. MS	F	P	P%
Plasma Nitriding (h)	2	0.02362	0.02362	0.01181	3.02	0.249	18.73%
Environmental Details	2	0.89136	0.89136	0.04456	11.38	0.089	70.62%
Immersion time (days)	2	0.00561	0.00561	0.002810	0.72	0.582	4.44%
Residual Error	2	0.00783	0.00783	0.003917			6.20%
Total	8	0.12621

Notes: DF—degrees of freedom; Seq. SS—a sequential sum of squares; Adj. SS—adjusted sum of squares; Adj. MS—adjusted mean of squares; F—Fisher’s test; p—probability statistic.

shows the ANOVA findings for the corrosion rate in the three components modified at three levels and their interactions. This analysis was conducted using a 95% confidence interval and a significance level of 0.05. The last column of Table 6 displays each parameter’s percentage of contribution (p) to the total variance, showing how much each parameter influenced the outcome. It can be observed from the ANOVA in Table 6 that the simulated environment (electrolyte) had the maximum influence (p = 70.62%) on the corrosion rate. Hence, the simulated environment (electrolyte) is a vital control factor to be taken into consideration during plasma nitriding (h) (p = 18.73%), followed by immersion time (days) (p = 4.44%) [25]. The pooled error was less influential, accounting for only 6.20%. From the ANOVA and S/N ratio, it is inferred that the simulated environment (electrolyte) has the highest contribution to corrosion rate, followed by plasma nitriding (h) and immersion time (days).

5.3. Multiple Linear Regression Models

A multiple linear regression model was developed to establish a correlation between the significant terms obtained from ANOVA analysis, namely, simulated body fluid environment (electrolyte), nitriding time (h), and immersion time (days). Equations (3)–(5) give the corrosion rate of plasma-nitrided AISI 316L in PBS, saline water, and Ringer’s solution, respectively.

The regression equation for the corrosion rate is as follows:

Regression Equation

For PBS Corrosion rate (mpy) = 0.0516 + 0.005213 × NT + 0.00239 × IT

(3)

For saline water Corrosion rate (mpy) = 0.0522 + 0.005213 × NT + 0.00239 × IT

(4)

For Ringer’s solution Corrosion rate (mpy) = 0.2630 + 0.005213 × NT + 0.00239 × IT

(5)

6. Confirmation Experiment

The last stage of the design was a confirmation experiment. The statistical analysis was validated by carrying out an immersion test with a particular set of parameters and levels. The values used for the immersion test are shown in

Table 7. Confirmation experiment for corrosion rate.

Exp. No.	Nitriding Time (h)	Simulated Body Fluids	Immersion Time (Days)
1	13	PBS	8
2	28	Saline Water	16
3	35	Ringer’s solution	25

, and the confirmation test results are shown in

Table 8. Results of the confirmation experiment and their comparison with the regression model.

	Regression Model Equation (3)
Exp. No.	Exp. Corrosion Rate (mpy)	Corrosion Rate (mpy)	% Error
1	0.162	0.148	8.61
2	0.249	0.235	5.30
3	0.312	0.293	5.83
	Regression model Equation (4)
1	0.154	0.148	3.59
2	0.249	0.236	5.32
3	0.462	0.447	3.30
	Regression model Equation (5)
1	0.374	0.359	3.90
2	0.463	0.447	3.41
3	0.542	0.505	6.78

. A comparison between the experimental and computed values derived from the regression model was made. It was discovered that the observed corrosion rate differed from the corrosion rate calculated in the regression equation, by an error percentage ranging from 3.30 to 8.61%. The corresponding inaccuracy was negligible. As a result, the regression model showed that it is possible to forecast the rate of corrosion of plasma-nitrided AISI 316L.

7. Electrochemical Behaviour of AISI 316L in Different Simulated Body Fluids

Open Circuit Potential

Open circuit potential measurement provides efficient information about surface electrochemistry conditions in different simulated body fluids. Using the Ivium potentiostat multi-channel device, electrochemical experiments were conducted in various simulated body fluids to compare the corrosion behaviour of the plasma nitride and untreated samples [25,26,27,28,29,30]. The procedure was carried out utilizing a three-electrode setup, with the working electrode being the samples (both as-received and plasma nitrided), the counter electrode being platinum (Pt), and the reference electrode being Ag/AgCl (stored in KCl solution). Open circuit potential (OCP) was measured for 10 min.

The electrochemical circumstances affected how the material responded to corrosion and how quickly surface layers deteriorate. In the PBS, saline water, and Ringer’s solution, Figure 1 displays the change in the equilibrium potential over time for all the examined alloys. An excellent passivation behaviour of the alloy in Ringer’s solution is shown by the OCP plot (Figure 1), which clearly demonstrates that there are no potential fluctuations at the specified duration of 10 min under static conditions. After 10 min of immersion, the OCP quickly rose from very low to steady-state levels. The OCP levels increased from around −0.165 V to approximately −0.045 V.

The corrosion potential (E_corr) values were derived directly from the polarization curves using passive anodic branches, reflecting the control of the anodic dissolution rate by the passive current density (with passive anodic branches reflecting the control of the anodic dissolution rate by the passive current density). A 1.5 mV/s scan rate was used for all the experiments. The potentiodynamic curves of the plasma-nitrided AISI 316L tested in PBS, saline water, and Ringer’s solution are shown in Figure 2; potential domains can be seen in both solutions’ polarisation curves. The first is the cathodic domain, which is made up of all the potentials below the corrosion potential E_corr, where the reduction of water and dissolved oxygen determines the current density. The possible range around the E_corr is represented by the second domain (−0.20 V). This domain defines the change from cathodic to anodic current at the corrosion potential. The passive plateau above −0.18 V is the third domain. The passive oxide film develops in this area, and the metallic dissolution is carried out via this. Many electrochemical characteristics were derived from the Tafel polarization curves and are shown in

Table 9. Electrochemical parameters of plasma-nitrided AISI 316L obtained for PBS.

	12 h PN	24 h PN	36 h PN
Ecorr (V SCE)	−4410 ± 0.002	−2241 ± 0.002	−1981
icorr (μA/cm2)	0.0232	0.01277	0.009566
Rp Ohm	4.944 × 104	1.235 × 105	3.823 × 105
βa (V/dec)	0.131	0.258	0.160
βc (V/dec)	0.044	0.040	0.083
Corrosion Rate (mile/year)	0.01045	0.005734	0.00429

,

Table 10. Electrochemical parameters of plasma-nitrided AISI 316L obtained for saline water.

	12 h PN	24 h PN	36 h PN
Ecorr (V SCE)	−4622 ± 0.002	−2211 ± 0.002	−1181 ± 0.002
icorr (μA/cm2)	0.0136	0.0009393	0.00064
Rp Ohm	3.823 × 105	1.235 × 105	4.944 × 104
βa (V/dec)	0.160	0.198	0.131
βc (V/dec)	0.083	0.040	0.044
Corrosion Rate (mile/year)	0.00613	0.00421	0.00287

and

Table 11. Electrochemical parameters of plasma-nitrided AISI 316L obtained for Ringer’s solution.

	12 h PN	24 h PN	36 h PN
Ecorr (V SCE)	−3112 ± 0.002	−1911 ± 0.002	−1081 ± 0.002
icorr (μA/cm2)	0.0126	9.293 × 10−5	0.0001718
Rp Ohm	4.944 × 104	1.235 × 105	3.823 × 105
βa (V/dec)	0.132	0.186	0.158
βc (V/dec)	0.053	0.040	0.045
Corrosion Rate (mile/year)	0.00568	0.000417	0.0000771

. The 12 h plasma-nitrided AISI 316L had the lowest i_corr of the three solutions, and plasma nitride at 24 h and 36 h showed the greatest i_corr.

The overall corrosion rate of the plasma-nitrided AISI 316L is shown against the nitriding time in different simulated body fluids in Figure 3. When chemical reactions occur concurrently under electrochemical circumstances, material heat treatment is crucial. Figure 3 shows that the corrosion rate (mpy) of plasma-nitrided AISI 316L decreases with increasing plasma nitriding duration (h). AISI 316L offered more corrosion resistance at 36 h of aging time compared to at 12 h aging time in Ringer’s solution. The corrosion resistance of AISI 316L increased due to the formation of diffusion layers of nitrogen on its surface. The corrosion rate of AISI 316L SS was found to be higher in the PBS solution and saline water compared to Ringer’s solution due to the presence of more chloride ions [31]. An electrochemically destabilized passive layer below the thickness of the diffusion layer may have been the source of the anodic shift, which was shown to move in the direction of increased current flow. The corrosion resistance of AISI 316L was higher in the Ringer’s solution than in the other two simulated body fluids. In the PBS solution, the corrosion rate of AISI316L at 12 h from plasma nitriding was 0.01045 mpy, and was 0.00429 mpy at 36 h from plasma nitriding. In saline water, the corrosion rate at 12 h from plasma nitriding, was 0.01045 mpy, and 0.00287 mpy at 36 h from plasma nitriding. In Ringer’s solution, the corrosion rate at 12 h from plasma nitriding was 0.01045 mpy, and at 36 h from plasma nitriding was 0.00287 mpy. The measured corrosion current density at 12 h after AISI 316L was plasma-nitrided was greater than that 36 h after AISI 316L had been exposed to a body fluid environment. The corrosion rate of plasma-nitrided AISI 316L decreased due to the formation of a nitrogen diffusion layer on the surface of AISI 316L. During electrochemical corrosion a passive layer of CrN was generated, due to which corrosion resistance decreased.

8. Surface Morphology

8.1. SEM Analysis of Plasma-Nitrided AISI 316L

Multiple plates were observed using optical microscopy (OM), scanning electron microscopy (SEM), and energy-dispersive spectroscopy to see how surface morphology affected heat treatment, and after the immersion test. The microstructure of plasma-nitrided specimens and the nitrided surfaces were studied using a Carl Zeiss microscope (Model No. A-2) and an optical microscope. Scanning electron microscopy (SEM) and EDS evaluation were conducted using a scanning electron microscope (Carl Zeiss Make, Model No. Sigma 4 HV).

It was observed that the kind of treatment affected the microstructure produced during plasma nitriding. Figure 4 shows an optical microscopic image of plasma nitriding after 12, 24 and 36 h. The microscopic surface samples were examined by L. Nosei et al. [30]. It is obvious that the ion nitriding process does not change the austenitic grain fundamental structure; instead, it creates sliding bands that are parallel within each grain and exhibit various orientations in different grains. The original grain boundaries were identified, and the grain size ranged from 50–120 μm. In addition, inside the grains were numerous parallel deformation twins. It was observed that extensive precipitation occurred at the grain boundaries, in the grain interior, and on the twin boundaries. Figure 5 depicts optical micrographs of the diffusion layer after plasma-nitrided AISI 316L samples for 12, 24, 36 h. SEM micrographs in Figure 6 illustrate the diffusion layers of AISI 316L samples subjected to plasma nitriding at 470 °C for 12, 24, and 36 h, respectively.

8.2. Optical Microscopic Analysis of Plasma-Nitrided AISI 316L in Various Simulated Body Fluids after the Immersion Test

Using a Carl Zeiss microscope (Model No. A-2) and optical microscope, the microstructure of heat-treated plates after immersion and observed corroded surfaces were analysed. Every image that was captured had a resolution of 400× and covered the whole field of the optical microscope. Optical micrographs are presented in Figure 7, Figure 8 and Figure 9, illustrating the appearance of plasma-nitrided AISI 316L samples following immersion in PBS, saline water, and ringers solutions for durations of 12, 24, and 36 h.

Figure 7a, Figure 8a and Figure 9a show that for 12 h, plasma-nitrided samples of AISI 316L had higher numbers of pits generated in the PBS. Small pitting points or relatively larger pits were seen in the AISI 316 steel layers that had been deposited, and these pits appeared to spread superficially around the pores. Negligible corrosion pits are observed in the 24 h plasma-nitrided AISI 316L sample after 27 days of immersion testing. According to research, porosity significantly impacts a material’s corrosion behaviour, and layers of 316L stainless steel produced by plasma nitriding commonly exhibit morphological surface corrosion. From Figure 7b,c and Figure 8b,c it can be observed that at 24 h and 36 h plasma-nitrided samples of AISI 316L had fewer numbers of pits generated in Saline water and Ringer’s solution. Nitrogen diffusion had more impact on corrosion resistance, as observed in Table 9, Table 10 and Table 11. No apparent corrosion pits were found in post-36-h plasma-nitrided AISI 316L sample image after 27 days of incubation. After the electrochemical tests, optical microscope images observed a localised attack in the PBS solution as a product of the Cl- ions present. The corrosion current density of AISI 316L decreased from 12 h to 36 h, showing that the material was protected from corrosion in the electrolytes. In the electrolyte of H₂O, the corrosion mechanism was uniform; however, the electrolyte generated a more significant dissolution of the oxide layer formed by the AISI 316L stainless steel. Overall, the oxide and CrN layer were observed on the plasma-nitrided samples, which plays an essential role in improving materials’ corrosion resistance.

8.3. Scanning Electron Microscopy Analysis of Plasma-Nitrided AISI 316L in Various Simulated Body Fluids after Immersion Test

The morphology of heat-treated plates after immersion and the observation of corroded surfaces were analysed using a Carl Zeiss Scanning electron (Model No. A-2). Each picture was high in resolution and spanned the whole field of the SEM.

Morphologies of the Plasma-nitrided AISI 316L corroded surfaces were studied using a Carl Zeiss microscope (Model No. A-2). Figure 10a,b, Figure 11a,b and Figure 12a,b show the morphology of the after-immersion surface, pits, and transfer film formed on AISI 316L following corrosion at the tested potentials. According to the morphology of the corroded surface in Figure 7, some small pits can be seen inside the surfaces of AISI 316L when an anodic potential is applied. Still, no pits are seen for AISI 316L, which has been plasma-nitrided for 36 h (Figure 10c, Figure 11c and Figure 12c), demonstrating that this material is more susceptible to pitting corrosion. It was found that the addition of plasma nitriding time increased the anodic current density in physiological PBS, saline water, and decreased in Ringer’s solution, because the surface of the material is protected due to the formation of an oxide layer and CrN layer. Both meta-stable and stable pitting corrosion occurs at lower potentials in the presence of all electrolytes in the 12 h and 24 h plasma-nitrided AISI 316L than in the 36 h plasma-nitrided specimens (Figure 2). The pitting potential is regarded as the point where the current increases suddenly without dropping to a passive current again. It was the highest in physiological Ringer’s solution and lowest in the presence of PBS and saline water.

8.4. Microhardness and Case Depth of Plasma-Nitrided AISI 316L

The Vickers hardness tester determined the Vickers microhardness HV_0.1 and bulk hardness HV_0.1 of the metallographic materials. For the nitrided samples, the microhardness depth profile was assessed. The microhardness values for specimens subjected to plasma nitriding are presented in

Table 12. Microhardness of Plasma-nitrided specimens.

Sr. No.	Plasma Nitriding Period (h)	Micro-Hardness HV0.1
1.	0	334
2.	12	1026
3.	24	1150
4.	36	1220

.

The optical and scanning electron microscope (SEM) was employed to quantify the thickness of the nitriding diffusion layer. Figure 13 shows the nitriding diffusion layer thickness variation with increased ageing time.

The quantitative microscopic examination supported the qualitative claims that the nitriding layer thicknesses on the AISI 316L substrate grow with increasing ageing duration [32]. Figure 13 shows that the nitriding layer thickness at 470 °C and 12 h nitriding time is approximately 46 μm, at a nitriding time of 24 h about 104 μm, and at a nitriding time of 36 h about 146 μm. The specified values of the different thicknesses of the compound and diffusion layers are additionally given in Figure 5 and Figure 6.

9. Energy-Dispersive X-Ray Spectroscopy(EDS) Analysis

On the other hand, in EDS Analysis, it was seen that elements such as Fe, Cr, Ni, Mo, and O from one inside alloys were determined, while the Na, Ca, Si, and Cl elements came from simulated body fluids [33]. In the EDS, when nitriding time (h) increased, the nitrogen percentage of metal samples increased from 8.94% to 19.05%. Increment nitrogen showed the chromium nitride layer’s (CrN) development during the material’s plasma nitriding [34].

Table 13. List of EDS reports of the plasma nitriding specimen.

Elements	Series	Weight%	Atomic%	Weight%	Atomic%	Weight%	Atomic%
		PN 12 h AISI 316L		PN 24 h AISI 316L		PN 36 h AISI 316L
Fe	K	60.41	44.31	58.4	41.84	62	44.52
Cr	K	11.49	9.46	13.2	8.22	10.2	8.52
Ni	K	8.53	6.54	5.51	6.51	8.62	5.5
C	K	5.18	17.39	6.27	14.13	5.19	16
N	K	8.94	15.21	9.14	16.75	9.95	19.05
Mn	K	1.78	1.3	1.4	1.03	0.46	0.33
O	K	1.69	4.26	4.11	10.15	1.96	4.89
Mo	L	1.25	0.53	1.42	0.45	1.12	0.47
Si	K	0.4	0.57	0.33	0.56	0.3	0.45
P	K	0.18	0.24	0.12	0.17	0.1	0.15
S	K	0.15	0.19	0.1	0.19	0.1	0.12

displays a record of EDS reports for the plasma nitriding specimen. SEM image for EDS analysis of plasma-nitrided AISI 316L is illustrated in Figure 14. The EDS analysis spectroscopy of plasma-nitrided AISI 316L after immersion test samples underwent treatment for 12 h, 24 h, and 36 h is displayed in Figure 15.

In the EDS

Table 14. List of EDS figures of the plasma-nitrided AISI 316L specimen after the immersion test in PBS.

Elements	Series	Weight%	Atomic%	Weight%	Atomic%	Weight%	Atomic%
		PN 12 h AISI 316L in PBS		PN 24 h AISI 316L in PBS		PN 36 h AISI 316L in PBS
Fe	K	61.41	45.33	59.4	41.77	61.5	43.45
Cr	K	10.66	10.4	12.26	8.31	10.42	8.4
Ni	K	8.44	6.01	5.22	6.26	8.45	4.53
C	K	4.99	16.12	6.33	14.33	5.22	12.5
N	K	9.01	15	9.22	16.75	9.96	18.07
Mn	K	1.79	1.3	1.39	1.03	0.47	0.34
O	K	1.7	4.26	4.15	10.15	1.96	11.45
Mo	L	1.23	0.54	1.45	0.46	1.45	0.49
Si	K	0.44	0.59	0.34	0.56	0.35	0.48
P	K	0.19	0.25	0.13	0.18	0.12	0.16
S	K	0.14	0.2	0.11	0.2	0.1	0.13

, when nitriding time (h) increased, the nitrogen percentage of metal samples increased from 9.01% to 19.05%. Increment nitrogen showed the chromium nitride layer’s (CrN) development during the material’s plasma nitriding. Figure 16 shows SEM image for EDS analysis of plasma-nitrided AISI 316L after immersion test in saline water. Corrosion products of plasma-nitrided AISI 316L are CrO₂, and Oxygen percentage increased after 27 days in plasma-nitrided AISI 316L from 4.26% to 11.45% in the PBS solution. Figure 17 depicts the EDS spectroscopy analysis of plasma-nitrided AISI 316L samples following an immersion test in a PBS for 12, 24, and 36 h.

In

Table 15. EDS figures of the plasma-nitriding AISI 316L specimen after immersion test in saline water.

Elements	Series	Weight%	Atomic%	Weight%	Atomic%	Weight%	Atomic%
		PN 12 h AISI 316L in Saline Water		PN 24 h AISI 316L in Saline Water		PN 36 h AISI 316L in Saline Water
Fe	K	61.31	44.31	58.4	41.01	62.18	42.5
Cr	K	10.01	9.46	13.2	8.23	10.1	7.53
Ni	K	8.58	6.54	5.51	6.1	8.3	3.33
C	K	5.13	17.39	6.27	14.13	5.5	16
N	K	8.99	15.21	9.14	16.7	9.95	18.04
Mn	K	1.75	1.3	1.4	1.19	1.1	0.32
O	K	1.64	4.26	4.11	11.3	1.25	11.88
Mo	L	1.65	0.53	1.42	0.4	1.07	0.48
Si	K	0.6	0.57	0.33	0.57	0.31	0.46
P	K	0.18	0.24	0.12	0.18	0.12	0.14
S	K	0.16	0.19	0.1	0.19	0.12	0.13

, when nitriding time (h) increased, the nitrogen percentage of metal samples increased from 15.21% to 18.04%. Increment nitrogen showed the chromium nitride layer’s (CrN) development during the material’s plasma nitriding. Figure 18 shows SEM image for EDS analysis of plasma-nitrided AISI 316L after immersion test in saline water. The corrosion product of plasma-nitrided AISI 316L is CrO₂, and Oxygen percentage increased after 27 days in plasma-nitrided AISI 316L from 4.26% to 11.48% in the saline water. Figure 19 depicts the EDS spectroscopy analysis of plasma-nitrided AISI 316L samples following an immersion test in a saline water for 12, 24, and 36 h.

In the EDS

Table 16. EDS figures of plasma-nitrided AISI 316L after immersion test in Ringer’s solution.

Elements	Series	Weight%	Atomic%	Weight%	Atomic%	Weight%	Atomic%
		PN 12 h AISI 316L in Ringer’s Solution		PN 24 h AISI 316L in Ringer’s Solution		PN 36 h AISI 316L in Ringer’s Solution
Fe	K	60.9	43.5	58.98	42	61.5	42.72
Cr	K	11.4	9.46	13.2	8.22	10.22	6.57
Ni	K	8.13	6.54	4.93	6.5	8.8	5.22
C	K	5.01	17.35	6.27	14.13	5.19	15
N	K	8.94	16	9.1	16.53	9.95	18.05
Mn	K	1.78	1.3	1.4	1.04	0.46	0.33
O	K	1.69	4.26	4.11	10.14	1.96	10.9
Mo	L	1.35	0.55	1.45	0.49	1.45	0.47
Si	K	0.45	0.59	0.32	0.58	0.25	0.46
P	K	0.19	0.25	0.13	0.17	0.11	0.15
S	K	0.16	0.2	0.11	0.2	0.11	0.13

, when nitriding time (h) increased, the nitrogen percentage of metal samples increased from 8.94% to 18.05%. Increment nitrogen showed the chromium nitride layer’s (CrN) development during the material’s plasma nitriding. Figure 20 shows SEM image for EDS analysis of plasma-nitrided AISI 316L after immersion test in Ringer’s solution. The corrosion product of plasma-nitrided AISI 316L is CrO₂; Oxygen percentage increased after 27 days in plasma-nitrided AISI 316L from 4.26% to 10.9% in Ringer’s solution. Figure 21 shows the results of an EDS spectroscopy examination performed on AISI 316L samples that underwent plasma nitriding, then were subjected to an immersion test in Ringer’s solution for 12, 24, and 36 h.

10. X-Ray Diffraction Analysis

The X-ray diffraction patterns of plasma-nitrided AISI 316L after the immersion test are shown as pins in Figure 22 (Bottom and top, respectively).

The plasma nitriding process significantly alters the lattice structure of the FCC austenitic phase. Two prominent characteristics of the nitrided surface can be seen at 52 °C and 61 °C. The two critical intensity peaks are known as the “γ-N-phase”. It is connected to an enlarged austenite structure created by moving nitrogen atoms to the FCC lattice interstices. The last peak is caused by the austenite phase of the substrate’s FCC-lattice structure diffracting in the (111) plane [35]. The initial γ” (111)” and γ” (200)” peaks are prompted by this expansion to shift downward and broaden following lower values of two [36]. Furthermore, the stainless steel’s nitrided surfaces did not exhibit chromium nitride cubic or hexagonal phases. This observation is especially significant since stainless steel corrosion-resistance qualities are often lost due to the precipitation of Cr-N phases. The peaks are centred at 51 °C and 61.33 °C in the 2θ-values of the β-CrN(200) and CrN(200) planes, respectively. The FCC-lattice structure of CrN demonstrates the diffraction of these planes. The other two peaks, which have a different chromium nitride stoichiometry (CrN and Cr₂N), are relatively broad, showing a complicated phase composition [37,38]. The XRD patterns of plasma-nitrided plasma-nitrided specimens after the immersion test are given in Figure 8. The ß-Cr₂N and Fe₂O₃ phases were observed for the plasma-nitrided surfaces in PBS, saline water and Ringer’s solution. However, at the ß-Cr₂N phase, Fe₂O₃ was more intense in the 12 h, 24 h, and 36 h nitrided specimens in PBS, saline water and Ringer’s solution, and the ß-Cr₂N and Fe₂O₃ was the main phase for the samples plasma-nitrided at a relatively lower process time (12 h, 24 h, 36 h). The OM and SEM images of the specimens showed that uniform corrosion occurred on the plasma-nitrided steels. After the nitriding process, the corrosion mechanism changed to resistance-pitting corrosion. This situation is more apparent in the 24 h-nitrided surfaces incubated in Ringer’s solution. It was reported that Cr₂N and Cr₂O phases obtained by 36 h nitriding time have a more resistant phase in terms of corrosion in simulated body fluids [15,17,18]. These results were supported by SEM images (Figure 10c, Figure 11c and Figure 12c).

A layer generated by austenite expansion because of the presence of nitrogen atoms is followed by a layer formed by austenite extending because of the presence of carbon atoms, according to research by Waldemar Alfredo Monteiro et al. [24] on the materials AISI 304 and 316 Austenitic Stainless Steels. According to the analysis of XRD samples by V. Singh et al. [27], CrN begins to precipitate at processing temperatures of 500 °C and significantly greater nitrogen contents [39,40].

11. Conclusions

This study shows the corrosion of AISI 316L (biomaterial) in various simulated body fluids and compared plasma-nitrided steel at varying nitriding times. It also demonstrates how the heat treatment cycle explicitly made for AISI 316L impacts deterioration. The corrosion in heat-treated (i.e., plasma-nitrided) AISI 316L can be concluded in the following points:

• The microhardness of plasma nitride AISI 316L increased with increasing nitriding time (h). Microhardness was observed for plasma nitriding samples with ageing times of 12, 24, and 36 h at 1060, 1150, and 1220 HV0.1. The microhardness of plasma-nitrided samples improved by four times that of bare AISI 316L.
• The corrosion rate (mpy) of plasma nitride AISI 316L was analysed using Taguchi’s Design of experiments. Corrosion resistance plasma nitride AISI 316L increased with increasing ageing times (h) in various simulated body fluids, specifically in Ringer’s solution, and showed less corrosion resistance. PBS > Saline water > Ringer’s solution.
• The ranking of control factors using the response table for the S/N ratio obtained for different levels showed simulated body fluids as the dominant factor during the corrosion process, then plasma nitriding time (h), i.e., the thickness of diffusion layer.
• From the ANOVA of the immersion test, it was observed that simulated body fluids significantly influenced corrosion rate, followed by immersion time (days) and nitriding time (h). The minimum corrosion rate was observed for the corrosion rate in the following observed parameters: plasma nitriding (h) = 36 h, simulated body fluids = Ringer’s solution, and immersion time (days) = 27 days.
• The Tafel polarization curve showed that the corrosion rate(mpy) was decreased from 12 h plasma nitriding to 36 h plasma nitriding time in different simulated body fluids. In Ringer’s solution, a lower corrosion rate (i.e., 0.0000771 mpy) was observed compared to saline water and phosphate-buffered saline due to the formation of a passive layer on plasma-nitrided AISI 316L.
• A very high stable passivation range for both simulated bodily fluids of saline water and Ringer’s solution was shown at 36 h plasma-nitrided AISI 316L. In the Ringer’s solution, 36 h plasma-nitrided AISI 316L demonstrated much better corrosion resistance (4 times) than 12 h plasma-nitrided AISI 316L SS.