Method of Fabricating Robust Microcapillary for Corrosion Testing Free

Extensive studies have been conducted on the electrochemical behavior of multiphase alloys.^1-3  However, a comprehensive understanding of their localized corrosion behavior, particularly the interaction of microgalvanic couples between the matrix and intermetallic particles (IMPs), remains insufficiently explored. It is widely accepted that the electrochemical activity of multiphase alloys is closely related to the electrochemical behavior of the individual phases, which serves as the driving force for corrosion initiation and propagation.^4-5  Investigating the microgalvanic interaction of individual phases separately is essential for a better understanding of the overall alloy corrosion performance. Moreover, valuable insights on the influence of different phases on corrosion behavior are crucial for developing predictive performance models and designing corrosion-resistant alloys for various applications.

Conventional electrochemical cells with approximately 1 cm² exposed area are unsuitable for studying microscale phases. Therefore, microcapillaries, typically tens of micrometers in size in diameter, offer a promising alternative.^1,6  The electrochemical microcapillary technique was first introduced by Böhni and Suter.^1,7  These microcapillaries can be constructed by drawing borosilicate glass tubes to a fine point and applying a silicone skirt at the tip to prevent leakage. The capillary can be fixed in an optical microscope rotating turret along with both the reference and counter electrode. By pressing the microcapillary tip onto a specimen surface, electrolyte is exposed only at a very small, localized area of the specimen for electrochemical tests. However, several challenges need to be addressed in the successful usage of microcapillaries; (i) the flexible nature of the silicone skirt can lead to changes in the exposed area or even blocking the tip when the microcapillary is placed on the specimen surface, complicating accurate measurements, (ii) the silicone skirt has to be applied meticulously to avoid leakage, (iii) the silicone skirt requires ∼24 h to fully cure, (iv) the thin wall of a small glass tip makes it susceptible to breaking upon contact with the specimen surface, which can limit the reusability and impact reproducibility, and (v) the yield of standard microcapillary fabrication methods is low, particularly for tips with diameter less than 50 µm to 60 µm. However, the size of many phases, such as the intermetallic particles in aluminum alloys, is typically less than 50 µm to 60 µm, which can limit the applicability of microcapillaries for corrosion studies. Therefore, overcoming these challenges is essential for advancing our knowledge of the electrochemical behavior of multiphase alloys and enhancing the reliability of microcapillary-based techniques.

In the current study, a simple approach is introduced to create robust microcapillaries with a diameter of around 20 µm for corrosion testing. These smaller microcapillaries are useful for studying the electrochemical behavior of individual phases of size ≥20 µm in multiphase alloys. Additionally, they overcome the issues related to silicone skirts, ensure reproducible electrolyte contact with the specimen surface, and have a high yield of fabrication when following the provided step-by-step procedure.

A fine-tipped capillary is fabricated from a cylindrical borosilicate glass tube, with an initial outer diameter, inner diameter, and length of 1.1 mm, 0.8 mm, and 100 mm, respectively, sourced from Kimble Chase. The combination of heating and pulling of the borosilicate tube using a glass puller (PUL-1, manufactured by World Precision Instruments) results in it being drawn to a fine closed tip. The main goal of capillary pulling is to achieve a glass tip that is as sharp as possible, which is necessary for producing a small-diameter microcapillary. The heating band in the glass puller should be positioned close to the borosilicate glass while pulling to make the tip smaller.

The second step in the fabrication process involves enhancing the strength of the sharp, closed tip made from brittle borosilicate glass, which is prone to breakage even under minimal force. To address this, a 1.1 mm diameter glass tube with a sealed end is placed inside a metallic cylinder with a hole of approximately the same diameter Figure 1(a). The tube is then filled with epoxy (resin to hardener ratio of 3:1) using a 5 mL syringe, ensuring that no air bubbles are trapped within the viscous epoxy. Then, a second tube with a drawn, sharp, closed tip is carefully inserted into the unset epoxy, as shown in Figure 1(b). The entire assembly is then gently shifted downward to locate the joining section inside the cylinder (Figure 1[c]), thereby aligning both tubes along a single vertical axis. This also allows hardening of the epoxy to occur with the sharp tip close to the center of the epoxy-filled glass tube, as illustrated in the schematic diagram in Figure 2, which corresponds to Figure 1(c). Finally, the complete assembly is left to cure for 24 h to ensure proper hardening.

There are several challenges in the procedure. Air bubbles may get trapped inside the glass tube during epoxy filling, which is undesirable because bubbles near the tip can enlarge the opening diameter. Using a syringe to create a pressurized flow of the epoxy is beneficial. Also, the sharp tip must be inserted carefully without touching any external surfaces to prevent breakage. Shifting the joint section of the assembly into the center of a metallic cylinder can prevent the tip from touching any external surfaces and thus avoid the tip breaking.

After curing, the entire assembly is carefully removed from the metallic cylinder without causing any damage, and the tube is cut just below the end of the sealed sharp tip, as shown schematically in Figure 3(a). The location of the fine tip is visible through the transparent epoxy. The bottom of the tube is then ground using 400 and 1200 grit emery paper, followed by cloth polishing with 6 µm and 3 µm colloidal silica slurries. During polishing, the use of a surrounding metallic cylinder is recommended, as it ensures perpendicularity of the tube and flatness at the base of the tip and provides adequate support to the glass tube while polishing, preventing accidental breakage Figure 3(b). As the polishing proceeds and material is removed, the sealed, sharp glass tip will eventually be opened. To test if the tip is opened, pressurized water can be flowed through the tube using a syringe. Once the tip is open, further polishing can be done to control the opening diameter. Because the tip has a converging diameter, continued polishing increases the diameter, allowing for the desired opening size to be achieved. The opening diameter of the tip can be determined by an optical microscope.

Several challenges can arise during polishing of the tip: (i) Care is required to control the tip diameter when it is first opened; continued polishing increases the diameter of the opening. It is recommended to begin polishing with 400 grit emery paper while observing the tip position in the transparent epoxy. When the tip is close to the polished surface, 1200 grit size emery paper can be used until the tip is opened. After opening, polishing with colloidal silica allows for a better finish. (ii) During colloidal silica polishing, silica particles may get lodged inside the tip, potentially blocking it, especially when working with small-diameter tips (∼20 µm). The best way to clear the blockage is to use ultrasonication for 2 min to 3 min. (iii) The edges of the outer glass tube at the base of the tip can hinder proper contact of the flat tip bottom with the specimen surface, compromising the seal. Therefore, before starting colloidal silica polishing, it is recommended to bevel the corners of the glass tube around the periphery of the tip base, ensuring that only the epoxy touches the specimen surface, which will create a leak-proof bottom surface. The optical micrograph in Figure 4 shows the bottom view of a successfully fabricated tip with an inner diameter of about 20 µm.

The microcapillary fabricated using the procedure described above was used to determine the electrochemical behavior of second phases on commercial 316L stainless steel (SS316L) (UNS S31603⁽¹⁾), aluminum alloy AA6016, β phase (Al_xFe_ySi) in 6xxx aluminum alloys, and 2205 duplex stainless steel (DSS 2205). After fabricating the microcapillary, it was placed within the turret of a microscope in a standard microcell setup containing a platinum counter electrode and a saturated calomel reference electrode. The placement of the tip on the specimen surface was calibrated to an accuracy of about 5 μm using the microscope objectives. The samples underwent a series of polishing steps with emery papers of varying grit sizes, followed by colloidal silica polishing using particle sizes of 0.6 µm, 0.3 µm, and 0.06 µm. Corrosion experiments on all materials were performed in neutral 0.6 M NaCl solution. The potentiodynamic tests were performed at a scan rate of 2 mV/s. The scan ranges are −150 mV_OCP to 1,200 mV_OCP for 316 L and duplex 2205 stainless steels, −150 mV_OCP to 100 mV_OCP for AA6016, and −500 mV_OCP to 500 mV_OCP for β-phase. The area of attack after corrosion tests was investigated using the Olympus GX53™^† inverted optical microscope.

Figure 5(a) shows the potentiodynamic polarization curves from five repeated tests conducted on SS316L, demonstrating good repeatability. In addition, the tip was reused during these tests without breakage or leakage, which has been a challenge for tips prepared conventionally. The curves and breakdown potentials are similar to those reported in the literature^8-9  and the scatter in breakdown potential is about 150 mV, which is typical for SS316L. Figure 5(b) shows an optical micrograph of the exposed area for one experiment, indicating a localized corrosion attack site within the exposed area of about 20 µm in diameter. The absence of widespread attack outside the tip confirms a fully sealed microcapillary. Leakage is prevented without the use of a silicone skirt owing to the flatness and robustness of the polished epoxy tip and the specimen surface. Simply pressing the epoxy-encased microcapillary on the surface is sufficient to seal the tip without any leakage.

β phase (Al_xFe_ySi) is a common constituent phase in AA6016 Al-Si-Mg aluminum alloy.¹⁰  Previous work with a 50 µm diameter microcell showed that the β phase is cathodic to the alloy matrix phase, with the possibility of driving the potential above the matrix breakdown potential in chloride solution as the result of microgalvanic coupling.¹⁰  The β-phase components in the cast analog specimen studied, shown in Figure 6(c), were much larger than β-phase IMPs in AA6016 but were smaller than the size of the 50 µm diameter microcell that was used in the work and was fabricated conventionally. Therefore, the tested area also contained a matrix, which might have affected the measurements because of microgalvanic interactions. Here, we use a 20 µm diameter microcell for improved testing of this matrix/β-phase galvanic couple. Figure 6(a) shows the potentiodynamic polarization curves for the AA6016 matrix and the cast β-phase analog. The matrix measurements were made by positioning the tip in a region of the alloy with no evident large intermetallic particles. Furthermore, as the size of the dendritic β phase in the analog is larger than 20 µm, we could land the tip on β-phase regions without interference from the surrounding phase. Figure 6(a) shows that the measurements were highly repeatable and the results were essentially identical to those made with the larger diameter microcell.¹⁰  These experiments show that the cathodic kinetics measured in the previous work on an area containing β phase and surrounding phase were dominated by the β phase, so the use of the larger microcell in that work was inconsequential. The optical micrographs in Figures 6(b) and (c) show that the exposed regions of the AA6016 matrix and the β-phase analog, respectively, were in single-phase regions.

A final example in Figure 7 illustrates potentiodynamic polarization curves for the austenite and ferrite phases of the DSS 2205. Prior work studying the behavior of the individual austenite and ferrite phases in DSS required a complex procedure, such as selective phase dissolution and subsequent epoxy resin coating.^11-12  The strong similarity of the polarization curves in Figure 7(a) to previous findings^11-12  further validates the effectiveness of the microcell for electrochemical evaluation of small, precisely targeted areas. The ferrite phase is seen to be slightly cathodic relative to the austenitic phase in these conditions. Additionally, the i_pass and pitting potential of the ferrite phase is marginally lower than that of the austenite phase, in agreement with previous findings. Figures 7(b) and (c) show the localized corrosion attack in the targeted area of the austenite and ferrite phases, respectively, further demonstrating the superior effectiveness of the fabricated microcapillary.

These examples indicate the benefits of the new small-diameter microcell fabrication approach for studying small regions of a multiphase microstructure. However, as with any method, the limitations must be understood. This method achieves a leak-proof seal at the tip, without the use of a silicone skirt. This is done by mating the flat, smooth epoxy around the microcapillary with the smooth surface of the specimen. No evidence of leaking was observed. The interface between the microcapillary and the specimen surface was visually monitored during the experiment using an optical inspection lens. If any minor leakage occurred, the solution would have accumulated around the periphery of the contact area, but none was observed in any of the experiments. The corroded area was also examined after the experiment, and there was no evidence of any leakage outside of the exposure area. However, if surfaces are rough, liquid might seep between surface asperities and spread across the surface, contacting regions other than the desired phase. Localized corrosion resistance is known to depend on surface roughness, but this method is only valid for polished surfaces. Tilting of the tip relative to the specimen surface as a result of the positioning of the turret or an uneven specimen surface can also cause the liquid to reach beyond the targeted area. Thus, to achieve a leak-proof setup, it is crucial that the tip is properly aligned and lands flat on the specimen surface.

A new method has been introduced to fabricate durable and reliable microcapillaries with diameters of 20 µm. The capability of the 20 μm microcapillary is demonstrated by conducting highly repeatable measurements on SS316L, AA6016, β-phase cast analog, and DSS 2205. In addition, the method successfully addresses several common challenges faced in the use of microcapillaries through the following approaches:

• A flat epoxy layer around the tip eliminates the need for a silicone skirt. This ensures the exposed area remains consistent across multiple experiments and prevents blockage by silicone.

• The tip can be fabricated with a high yield. The epoxy-reinforced tip is highly durable, preventing breakage when it touches the specimen surface during experiments. This greatly enhances the reusability of the tip and improves the data quality by using the same tip consistently.

This work was supported by Novelis Inc., Kennesaw, GA. The assistance of Dr. P. Adapala is greatly appreciated.