Study of imitative micro pit morphology evaluation

This study examined the imitative micro pit formation of pitting corrosion on 304 stainless steel under electrochemical control in a 3.0% NaCl solution. Results demonstrated that the micro pits changed from a conical to a cap shape during growth, indicating that their evolution in shape and growth did not follow the same morphology over time. Additionally, the piecewise polynomial method was employed to compare real-time micro pit growth morphology evolution with current density, which was consistent with cross-sectional images of actual micro pits, enabling the separation of the pitting corrosion stages. Furthermore, 3D images obtained from SRXTM were similar to cross-sectional images of real micro pits, further validating the technique’s usefulness in studying micro pits.


Introduction
Corrosion is a significant cause of structural deterioration of food, dairy, and beverage equipment. It affects the life of the process equipment and piping and can result in structural failure, leakage, equipment damage, and food contamination. Pitting corrosion is considered one of the most hazardous components of food, dairy, and beverage equipment. The total loss of the structure might be very small, but the local attack rate can be very large, leading to early catastrophic failure. Stainless steel is extensively used in the food and beverage industries because of its corrosion resistance to the atmosphere. This resistance is related to forming a thin oxide layer (chromium oxide Cr 2 O 3 ), which protects the surface against aggressive environments and can easily be formed into complex shapes, such as bulk storage tanks and mixing tanks. Furthermore, stainless steel can be cleaned and sterilized without deterioration. In addition, it does not impart color or flavor to food, making it better for the food industry. In the food and beverage industries, a wide variety of tanks and piping is used for the hygienic manufacture of food products. Most of this equipment is fabricated from austenitic stainless steel [1][2][3] and is usually built or produced from thin sheet stainless steel [4,5]. In the production process for food processing, it is essential to use clean-in-place (CIP), a method of cleaning the interior surfaces of pipes, tanks, processing equipment, filters, and associated fittings without disassembly to remove fouling deposits, which is essential to help avoid the cross-contamination of food when various products come into contact with the same surfaces as well as to prevent the growth of microorganisms. However, cleansing agents, commercial cleaning agents, and sanitizers can chemically damage the metal surfaces of the equipment.
The corrosive environment in the food and beverage industries involves moderate to highly concentrated chlorides [6]. The food-contact side of the processing equipment varies from chemical cleaning. Aggressive ions, hypochlorite, and chloride may be concentrated because of the evaporation of cleaning solutions on specific localized areas of the metal surface, inducing pitting corrosion. Therefore, the importance of cleanliness in food processing plants is exceptionally high. To eliminate bacteria from surfaces and equipment, it is imperative to adopt effective cleaning and sanitization programs. Most sanitizers are corrosive to metals [5]. Pitting corrosion can occur when the local breakdown of the passive film layer occurs in an aqueous solution. When the passive film does not recover sufficiently, a micro pit grows, leading to pitting corrosion, accelerated corrosion, and material deterioration. It is widely accepted in the food industry that corrosion causes problems by negatively affecting product quality. Corrosion is the deterioration or destruction of metals and alloys in an environment by chemical or electrochemical means. Pitting corrosion is a localized form of corrosion in which cavities or 'holes' are produced, interring the material layer surface. It is considered more dangerous than uniform corrosion damage because it is more difficult to detect, predict, and design. Pitting corrosion can produce pits with open holes or pits covered with a semi-permeable capsule of corrosion products.
Pitting corrosion is one of the most damaging types of corrosion. Many researchers have studied the pitting corrosion of stainless steel. Such studies have focused more on the initiation and propagation mechanisms than the dissolution rate in chloride-containing solutions [7][8][9][10][11][12][13][14][15][16][17]. It is well known that pits in stainless steel tend to grow with depth. Ernst and Newman [7] experimentally observed the propagation of pits from 2D imaging at the edge of a stainless steel foil. The pit depth was 100 μm, and the pit width was 500-700 μm. They successfully simulated 2D pit growth by considering the effects of the surface roughness and potential solution chloride concentration. Majid Ghahari [9,10] conducted an experimental observation with 3D microtomography under different applied current and cell potential conditions to successfully demonstrate how pits evolve in a stainless steel foil at a pit depth of 12-100 μm and a pit width of 12-100 μm.
This research is on imitative micro pits, simulated pits created on stainless steel material. The purpose was to investigate the characteristics and behavior of micro pits that occur under electrochemical control in a sodium chloride solution. The analysis aimed to understand how the morphology of micro pits changes over time as their shape and structure evolve with increasing duration. The research provides valuable into the behavior and characteristics of these microscopic structures on stainless steel and focuses on the initiation and growth mechanisms of pitting corrosion. We studied the pitting corrosion of the micro pit in foil stainless steel and various stages of micro pit morphology from passive film breakdown, metastable pitting, and pit growth. These stages may be considered the most critical for different durations and using the piecewise polynomial method for segmenting the current density during micro pit propagation. Synchrotron Radiation x-ray Tomographic Microscopy (SRXTM) at BL1.2W of the Synchrotron Light Research Institute Thailand has implemented micro pit morphology in 3D [18,19].

Sample preparation
Samples of austenitic stainless steel foil grade 304 with thicknesses of 25 and 50 μm [10] were used in the asreceived condition. The chemical composition of steel is listed in table 1. Before the electrochemical tests, a masking technique was implemented on the surface with an opening area of 0.282 cm 2 using a making technique.

Determination of pitting potential (E pit )
Potentiodynamic polarization measurements were conducted using a three-electrode electrochemical cell with an Ag/AgCl reference electrode. The counter electrode was a platinum wire, and a stainless steel foil sample was used as the working electrode [12,20,21]. All potentials in this study are quoted against the Ag/AgCl reference electrode. Measurements were conducted at room temperature (25 ± 2°C). The 3.0% NaCl solutions were prepared using analytic-grade chemicals and distilled water [22].
The corrosion properties were observed using the potentiodynamic polarization test technique according to ASTM G5 to determine the pitting potential (E pit ). DY2300 Potentiostat with DY2322 software was used to evaluate corrosion properties and set up an initial parameter voltage of −1.5 to 2 volts, a scan rate of 0.02 volts per second, and a sensitivity of 1 × 10 -3 A/V. The pitting potential was obtained from the polarization of the polarization curve.

Generation of micro-pitting corrosion
The generated imitative micro pits on stainless steel foil, simulated pitting corrosion was generated by applying a direct current at pitting potential to the specimens for different durations in a 3.0% NaCl solution to obtain micro pits of various sizes on stainless steel foil grade 304 with thicknesses of 25 μm and 50 μm. The methodology used for the generation and growth of corrosion pits and determination of the micro pit depth followed that of J. González-Sánchez [8]. The current generated by the polarization of the specimens for different durations during the potentiostat polarization formation and growth of the micro pits was recorded as the current density and time. The electrochemical generation of pits was performed using the amperometric i-t mode. It was recorded as the current density and piecewise polynomial technique for segmenting the micro pit shape evolution. The electrochemical generation of micro pits was performed using the same equipment, electrodes, and solution to dining the pitting potentials. Micro pits generated under potentiostat polarization control were carried out with a higher pitting potential at 0.9 Volts as transpassive by polarization. They were applied to specimens for nine sets of durations: 5 to 45 seconds for stainless steel foil with a thick, and samples for ten sets of durations: 5 to 60 seconds for stainless steel foil with a thickness of 50 μm.

Macro cross-section
The micro pits evolution and morphological characteristics were determined using cross-sectional optical microscope (OM) images available after pitting corrosion of the micro pit of the stainless steel foil. The micro pit-shaped morphology at the center of the cross-section of the micro pit was obtained, and a polished direction was obtained to offset the center to evaluate pit shapes, as shown in figure 1, which shows a close-up image of a micro pit cross-section.

2.5.
Synchrotron radiation x-ray tomographic microscope (SRXTM) of micro pits SRXTM of the micro pits was carried out at the XTM beamline (BL1.2W), Siam Photon Source Facility, Thailand. The x-ray beam was generated from a 2.2 T multipole wiggler source. The x-ray imaging was performed using a filtered polychromatic x-ray beam with a mean energy of 12 keV. The x-ray imaging system included a YAG-Ce scintillator, an objective lens-coupled microscope (Optique Peter, France), and a PCO.edge 5.5 camera (Excelitas PCO GmbH, Kelheim Germany). All x-ray projections were acquired at a pixel size of 1.44 μm in a 16-bit greyscale format. The experimental setup is illustrated in figure 2.  The stainless steel sample was mounted on a rotary stage at 34 m from the radiation source for x-ray tomography data collection. A total of 1,800 x-ray projections of the sample were obtained from 0°-180°r otation with an angular increment of 0.1°. Preprocessing of the x-ray projections included flat-field correction, noise filtering, and beam intensity normalization. Tomographic slices presenting the sample's cross-sectional details were reconstructed using the Octopus Reconstruction software [23]. The reconstructed images were used in the Octopus Analysis software [23] to determine the volume of the micro pits. Finally, 3D visualization and presentation of micro pits were generated by using Drishti [24] and Image J [25].

Polarization test and micro pits generated
The results of potentiodynamic polarization tests of stainless steel foil with polarization curves for samples of 25 and 50 μm were tested in a 3.0% NaCl solution, as shown in figure 3. The pitting potentials (E pit ) of the stainless steel foils of 25 and 50 μm were approximately 0.7 Volts. Pitting corrosion occurs as transpassive corrosion, as shown beyond this point. The pitting potential values were then used as the reference for potentiostat polarization to generate micro pits on stainless steel foil samples of 25 and 50 μm. Therefore, the pitting potential value was used as a reference to apply transpassive pitting potential to generate simulated micro pits on the specimens.
A specimen's passive current density (I pass ) is shown for a stainless steel foil of 25 μm. Its value was calculated to be 17.7726 mA cm −2 , and the passive current density of the stainless steel foil of 50 μm was calculated to be 0.0355 mA cm −2 , indicating a lower passive current density. This indicates that the passive film of the stainless steel foil specimen of 50 μm was stronger than the passive film of the stainless steel foil of 25 μm. The stronger passive film of 50 μm stainless steel foil could result in the size and number of pitting initiations [20], which may be attributed to the high passivating properties of the Cr and Mn alloying elements.
The micro pits generated under potentiostat polarization control were carried out with the pitting potential at 0.9 Volts as transpassive the critical pitting potential above which new micro pits will initiate and existing micro pits will propagate. The potentiostat polarization formation and growth of the pits as the specimen current density and time curves for different periods during the potentiostat were recorded and are shown in figure 4. The propagation time varied between 5 and 45 seconds for the specimen of stainless steel foil of 25 μm and from 5 to 60 seconds for the specimen of stainless steel foil of 50 μm, which showed a high passive current density and started to propagate after the breakdown of the passive layer owing to the aggressiveness of chloride ions. An increase in the current characterized the first 5 seconds of pit propagation which could be discussed in detail. An increase in the current characterizes the assumption of pit growth.
Integration of the current versus time curves obtained from the potentiostat generated by pits provided the total charge induced y pit growth. Figure 5 and table 2 summarize the samples' electric charge (Q in Coulomb) increases with time and current. This difference in the electric charge value as a function of the passive current density was consistent for both stainless steel foils. It can be seen that the electric charge obtained from the 25 μm sample is higher than that of the 50 μm sample because the stainless steel foil of 50 μm had a stronger passive film affected by the low electric charge of the sample. In addition, the tests showed that the experimental setup was reproducible for pits of different thicknesses and the same material. Varying the stainless steel foil did not change the reproducibility of the pit.
After the simulation of the imitative pits, the sample was removed from the electrolyte, washed, rinsed using soap water, and dried with hot air. An optical microscope was used to determine the number of actual micro pits on the surface of the sample, as shown in figure 6 and table 2. It was found that good evidence of the actual  number of micro pits was present more in samples of 25 μm than in the 50 μm samples. It can be seen in the sample of 25 μm that the current density was high. The current density increased as more pits propagate, affecting the morphology of the micro pits. Following the results of the passive current density effect on the passive film, these results affected the number of pitting initiations.

Micro pits shape evolution and morphological characteristics cross-section
Micro pits shape evolution and morphological characteristics Cross-Section, in figures 7 and 8, showed crosssectional optical images after imitative micro pit experiments, which presented pictures of the micro pit for   investigating and visualizing the internal characteristics of a pit generated in a stainless steel foil thickness of 25 and 50 μm. An optical microscope was used to determine the measurement accuracy of the actual micro pit diameters. The average diameter of the micro pit was determined by calculating the percentage of relative accuracy. Based on the calculation results, the results for all specimens had a relative accuracy value of more than 90%. The results indicated that the methods used in this study were accurate. The micro pit depth of the samples obtained from the cross-section of the micro pits was listed in table 3. The longer time during simulated imitative pitting increased the diameter of the micro pits. The results showed that the micro pits were wider than deeper because the passive layer on the surface was destroyed, thus forming localized deterioration on the surface. Anodic zones were concentrated in these pits. Electron transfer is driven through the electrolyte to the cathodic zone on the adjacent non-corroding surface (cathode). Because the anodic/cathodic area ratio was often low, the corrosion inside the pit was intense suggesting that the metal dissolution rate was higher at the pit wall compared to at the pit bottom. In figure 7, the micro pit of the 25 μm specimen showed micro pit growth; at a duration time from 5 to 10 s, the micro pit propagation characterized the morphology of a nearly-conical shape; the duration time from 15 to 45 s presented the morphology of a nearly cap-shape with a combination cylindrical shape. Observation for more than 30 s duration showed that the pit depth broke through the thickness of the stainless steel foil owing to the thickness limitation. Consequently, the pit can only expand the mouth pit (diameter), resulting in a final cylindrical shape. In figure 8, the micro pit of specimen 50 μm did not have the same morphology for a longer duration but changed with time during micro pit growth. The first stage presented a conical morphology from 5 to 10 s of the micro pit. As it entered the second stage, metastable growth at the duration time from 15 to 60 s showed that micro pit propagation was presented as a cap-shape. Micro pits had the growth evolution and did not have the same morphology with a longer duration but changed with time during micro pit growth. Table 3 listed the average micro pit width and depth under the experimental conditions. It was found that, within 5-30 sec, micro pits grew at near rates in both samples. The average diameter of the micro pits and micro pits morphology continued to develop. In addition, the depths of the micro pits continued to increase. At a 5-30 s duration in the 50 μm sample, the pit depth was deeper than in the 25 μm. When stainless steel was exposed to an oxygenated aqueous solution containing sodium chloride as the electrolyte, the pit acted as an anode (metal oxidation), and the metal surface acted as the cathode (oxygen reduction). Therefore, it could be explained that the anode area of the sample of 50 μm was smaller (lower pit number). In other words, the corrosion rate of the anode metal was greater. Consequently, the pit depth was higher.
The cross-sectional shapes for the same duration of both samples were shown in figure 9. The pit evolution of the stainless steel foil of 25 μm presented the micro pit evolution of cap shape and transited to a cylindrical  shape as the pitting corrosion progressed, as shown in figure 9(A). In addition, it was found that the evolution of the micro pit geometry exhibited a stainless steel foil of 50 μm present in the cap shape, as shown in figure 9(B). These results confirmed the micro pit evolution of the shape in both samples.
A comparison of the current density with real-time micro pit growth morphology evolutions was shown in figures 10 and 11. A piecewise polynomial method was presented for segmenting the current density during micro pit propagation. As the connecting points were visible in the graph as they jump, the separation of the current sections could be detected. This could also help evaluate the micro pit evolution stage, which could explain the micro pit morphology evolution from the current density and duration. Under potentiodynamic polarization, stainless steel in sodium chloride solution showed a breakdown in the transpassive region (pitting potential). However, a longer duration significantly affected the increase in current density potentials (figure 4) due to the passive film'srapid breakdown and the development of local micro pits. The metal surface acted as a cathode, whereas the pit acted as an anode. The local production of metal cations in the pit created a positive charge that attracts the chloride anions in the electrolyte. The view of the passive film as a dynamic structure was critical for the proposed mechanisms of passive film breakdown and pit initiation. This was possible on passive film breakdown and pit initiation had been devised to segregate the stages of the pitting corrosion. The piecewise polynomial method was illustrated in figure 10. The sample stainless steel foil of 25 μm could be considered to have a three-stage, first-stage duration time of 5-7 s after the passive film breakdown. Furthermore, micro pits were initiated after the metal surface was exposed to sodium chloride, showing a conical shape. The second stage duration was 8-24 s after the metal surface was exposed to sodium chloride-containing aggressive anions. The stage propagation of pitting shows a cap shape, and the third stage duration time of 25-45 sec. The continuous stage propagation showed a cylindrical shape.
The morphology of stainless steel foil of 50 μm was shown in figure 11. It could be considered that the micro pit morphology evolution was evaluated to have a three-stage, first-stage duration time of 5-10 s. The initiation of the micro pits showed a conical shape. In the second stage, a duration of 11-38 s after the metal surface was exposed to sodium chloride, it contains aggressive anions. The stage propagation of pitting showed a cap shape and the third stage duration time of 39-60 s. The continuous stage propagation showed the cap shape to prepare for the transformation to a cylindrical shape, as seen from the continuous propagation of the pit mouth.
These results had a root mean squared error (RMSE) of 4.657e-06, R-square of 1 and RMSE of 7.891e-07, and R-square of 1. In both RMSE samples, the results for the polynomial model indicated good results when fitted to the data. The results from the two samples were in excellent agreement to explain the evolution of the micro pit morphology with the current density. These results were also consistent with the cross-sectional image results. The Analysis confirms that it was possible to separate the stages of pitting corrosion, including the initiation and propagation of pitting. It developed in four successive stages: (1) It represents an un-attacked metallic surface completely covered with passive films. Stage (2) Initiation by the breakdown of the passive film protecting the metal surface from oxidation, Stage (3) growth of metastable pits, and Stage (4) growth of larger and stable pits [26].

Micro pit evolution and morphological characteristics from SRXTM
The micro pits in figures 12(A)-(I) from SRXTM showed 3D images of the reconstructed micro pits for investigating and visualizing the internal characterization of a micro pit sample at 25 μm. The micro pit was grown for an increased duration. It could be seen that the micro pits grew in different shapes. In the first stage, 5-10 sec duration, the passive film breakdown showed a conical micro pit shape in the second and third stages. The pit growth shows the micro pit presented as a cap and a cylindrical shape. 3D x-ray tomography was a powerful tool for visualizing the micrometer scale and internal structure of materials. Good agreement was found between the micro pit shape geometry results obtained via 3D tomography and those obtained from the cross-sectional image geometry. However, the above experiment had a limitation from the x-ray energy. It limited the thickness of the test to a stainless steel foil of 25 μm while the thickness of the stainless steel foil of 50 μm was too thick to allow the transmission of the x-ray energy.

Conclusions
1. The current density with real-time micro pit growth observations confirmed that the micro pits that had grown under electrochemical control on the stainless steel foil metal surface gradually changed the pit shape from a conical shape at the start to a cap shape during micro pit growth. 2. During the initiation stage of micro pit growth, the width of the pit trended to be greater than its depth. As the pits continued to grow, they transited from conical to cap-shaped with increasing duration, indicating the evolution of their morphology. Once the pits reached a stable state, they grew rapidly in size. The shape and size of the micro pits depended on their growth duration. The morphology of the pits did not remain consistent with longer duration, indicating further evolution in their growth and shape.
3. The piecewise polynomial method for segmenting the current density during micro pit propagation was consistent with the cross-sectional image results of actual micro pits. These confirmed the method's efficacy in separating the stages of pitting corrosion for analysis.
4. The morphology of micro pits growth using 3D SRXTM was consistent with that observed in cross-sectional images of actual micro-pits. These presented of 3D x-ray tomography as an effective tool for visualizing materials at the micrometer scale and examining their internal structure.