Some manufacturing processes, such as forging, welding, and cooling, can cause internal stresses in steel, which significantly influence the steel’s mechanical properties, such as fatigue life, corrosion resistance, and strength. Undetected or undamaged, these stresses can lead to catastrophic failures in critical applications.
For example, in shot-peening, spherical media is launched at a surface at a high velocity, creating plastic deformation on the surface, subsequently resulting in biaxial residual stress not only to the surface but also to the subsurface of some hundred micrometers. The depth and distribution of the compressive residual stress profile are critical factors that determine the effectiveness of shot-peening. Since X-rays can only penetrate some micrometers from the surface, it’s common practice to use an electrochemical polisher to remove the surface layer by layer before measuring residual stress by X-ray diffraction.
When used effectively, an electrochemical polisher is an exceptional technique for revealing those internal stresses. It removes layers of material uniformly without introducing new stresses or altering the inherent stress profile of the material, which is a common issue with mechanical removal. An electrochemical polisher also produces a high-quality finish that can be ideal for subsequent stress analysis techniques, such as X-ray diffraction.
Pulstec offers a supplementary electrochemical polisher that can be purchased as an add-on with our μ-X360s residual stress analyzer. In this article, we’ll cover how to use it to measure internal stresses in steel samples.
Table of Contents
Precautions Before Starting
- Always wear protective glasses, rubber gloves, and a protective mask when handling the electrolytic solution.
- Ensure your skin does not come into contact with the electrolytic solution, which may contain hexavalent chromium, a known carcinogen. Generally, the concentration of hexavalent chromium is around 0.5 mg/l, but there may be variations.
- Keep the polisher away from liquid to prevent fires, electric shocks, or equipment malfunctions.
- Keep your workspace at a stable temperature and humidity to prevent inconsistencies in the polishing process. Temperature fluctuations can affect the viscosity and conductivity of the electrolyte, while humidity can influence the rate of corrosion on metal components. We recommend an ambient operating temperature between 50 and 104 degrees Fahrenheit and humidity levels between 20 to 90% without condensation.
- Keep the product at least 4 inches away from a wall and make sure its cooling fan and vent holes aren’t obstructed.
- Wash all accessories with tap water after use. The electrolyte contains salt, which can rust the surface of the accessories.
- Don’t use the polisher if you notice abnormalities, like an unusual smell.
- Never operate the polisher in a dusty environment, in a room with corrosive gas, or in direct sunlight.
Steps by Method Type
Using our electrochemical polisher is relatively simple, but instructions will vary based on the method you’re using to measure internal stresses.
The Wiper Method
The Wiper method is ideal for wide or curved surfaces and is sometimes used for cleanup purposes. In this method, the sample to be polished (anode) and a moving cathode (the wiper) are immersed in an electrolyte bath.
One of the key advantages of this method is that it allows for the constant renewal of the electrolyte at the interface between the cathode and the anode, removing byproducts of the electrochemical reaction, like sludge or metal ions, that could interfere with the polishing process. The “wiper” motion also ensures a consistent concentration of the electrolyte and maintains optimal conductivity at the sample’s surface.
Steps for Using the Wiper Method
- Degrease the sample area and place a mask seal on it. We recommend using an φ8 millimeter mask seal when polishing a depth of 300 μm or more. You can also create a mask yourself with vinyl tape. The electrolyte solution will trickle out from the torch in the wiper method, so you’ll want to protect the seal with curing tape or another similar solution. Be sure to clean the markings with a Sharpie before polishing the target.
- Next, set the current volume and timer. The amount of polishing differs according to the polishing size, metal type, and surface treatment.
- Perform test polishing if your target is being polished for the first time. After testing, if you notice that the surface is rough, the current is too low, but if it’s significantly distorted, the current is too high.
- Fold a sheet of Kimwipe five times and place the folded sheet inside the torch until it makes contact with the electrode.
- Fill the provided pipette up to 3 milliliters with the electrolyte solution and dampen the Kimwipe.
- Apply the torch (with the Kimwipe) to the polishing point. When you notice the solution coming into contact with the sample, turn the mode switch to the timer side to start polishing.
- During the process, check the liquid volume monitor. If the current meter has gone off the scale to the left, add another 3 milliliters to the Kimwipe.
- When you’re done polishing, use tweezers to remove the Kimwipe. Place the Kimwipe in a plastic container. Be very careful when doing so, ensuring the solution does not come into direct contact with your skin, and follow local waste disposal regulations and guidelines.
- Use a depth gauge to check that the target depth has been polished. Change the current volume or use the immersion method if you notice any roughness on the polished surface.
- Perform post-treatments on the polished surface. If you used a neutral solution, wash the surface with water or wipe with a moist towelette, then apply rust-preventive oil. If you used a weak acid solution, use a neutralizer, then wash the surface with water or a moist towelette, then apply a rust-preventive oil.
The Immersion Method
The Immersion method is usually the best technique to use if the surface is flat or irregularly shaped. Sometimes, this method can be used on curved surfaces, but it’s not as ideal as the Wiper method. As the name suggests, this technique involves submerging the entire sample, or part of it, in the electrolyte solution while applying an electrical current. The result is a surface that is very uniformly polished. Additionally, compared to the Wiper method, you won’t have to worry about refilling the electrolytic solution throughout the process.
Steps for Using the Immersion Method
- Follow the same steps as the Wiper method for placing the mask seal on the sample and setting the current volume and timer.
- Place a gel mat on the mask seal and lightly apply the torch to it, using the torch stand to prevent damage to the torch. If you’ve used the gel mat several times and the adhesive is wearing off, wash it with water.
- Place between 3 and 5 milliliters of the electrolyte solution into the torch.
- Turn the mode switch to the timer side to start polishing.
- Use the pipette to remove the solution from the torch and place the waste solution into another bottle or vessel for disposal. Make sure you keep the torch attached to the gel mat.
- Use the same pipette to add 3 milliliters of water into the torch. Use the pipette to stir the added liquid in the torch. Then, use the same pipette to transfer the dirty water into the waste bottle or vessel.
- Check to ensure the target depth is polished and perform necessary post-treatments, as outlined in the Wiper method.
The Narrow Part Method
The Narrow method, sometimes called “localized” or “spot” electrochemical polishing, is commonly used for polishing a specific, limited area of a sample or areas that are difficult to reach using the torch, such as gear teeth or in applications where only certain features or sections of the material need to be polished.
We’ll explain the process in more detail below, but in this method, the sample is the anode, a wire, rod, or other custom-shaped tool acts as the cathode, and the electrolyte solution is applied locally to the target area.
The primary benefits are that it can achieve microscale and even nanoscale surface finishes and that the rest of the sample is unaffected, preserving the integrity and properties of the material outside of the targeted zone.
Steps for Using the Narrow Part Method
- Follow similar steps for applying a mask seal. In the Narrow method, you’ll use a φ2 or φ4 millimeter seal, depending on the targeted area. You can use curing tape to protect the sample if needed, but it’s generally unnecessary in the Narrow method because only a small area is being polished.
- Follow similar steps for setting the current and time.
- As mentioned earlier, the torch normally used in the Wiper and Immersion methods won’t work in the Narrow method, so we have a special electrode. Fold a cotton swab in half, insert it into the hole in the electrode, and push it far enough in the electrode so it doesn’t come out easily.
- Use a pipette to place 5 to 6 drops of the electrolytic solution onto the cotton swab in the electrode, or until the cotton swab is dampened and the solution accumulates at the spout.
- Apply the cotton swab to the targeted area, wait until drops form on the sample, then turn the mode switch to the timer side to start polishing.
- Add more solution to the cotton swab every 60 seconds or so. If there is a lot of dirt on the swab, replace the swab before the second polishing.
- Once the polishing is completed, dispose of the cotton swabs (in accordance with your local waste disposal guidelines) and check to ensure the target depth has been polished.
- Perform post-treatments, similar to the Wiper and Immersion methods.
Understanding Removal Speed
The removal speed, or the rate at which material is removed from the sample, typically measured in micrometers per minute, is based on the size of the target area, the material’s properties, the current, and the time, and is a critical factor in determining the effectiveness of the electrochemical polishing process.
Target Area Size
The size of the target area inversely affects the removal speed. A larger area disperses the electrical current over a wider surface, potentially reducing the current density unless the total current is proportionally increased. Additionally, larger target areas can dissipate heat more effectively, which can impact the electrochemical reactions.
Material Properties
The material’s electrical conductivity, chemical reactivity with the electrolyte, and crystal structure influence how quickly the material can be removed. Harder materials, or those that form stable oxide layers, might be polished more slowly.
Current
The amount of electric current per unit area, or current density, is a primary factor of removal speed. Higher current densities generally increase the material removal rate because they improve the electrochemical reaction rate at the sample’s surface.
Polishing Time
As you might assume, longer polishing times can lead to greater material removal if the current density and other conditions remain constant. Please note that it’s possible to increase the polishing speed by increasing the current setting, but there is a possibility that the polished surface will not be finished finely.
Setting Current Volume & Time
The material properties of your sample will largely influence the current volume and time you select on the polisher. As already mentioned earlier, the conductivity of the material will determine how easily current passes through it, influencing the amount of current needed. Materials that react more aggressively with the electrolyte may also require adjustments in current volume to control the rate of material removal and avoid excessive dissolution. Additionally, harder materials, or those with complex microstructures, might need different current settings compared to softer or more homogenous materials.
The amount of material that needs to be removed will also influence the current setting and the polishing time. More material removal generally means higher currents or longer polishing times. If you need a higher-quality finish, a lower current and longer time might be preferable to provide a gradual and controlled material removal.
Lastly, the size of the target area will influence the volume and time settings. Larger target areas might dilute the effect of a given current setting, requiring either an increase in current or in polishing time to achieve similar results as with smaller areas.
Here are some recommendations on current settings based on the polishing size/target area:
| Target Area Size | Current Setting |
| φ2mm | 0.2 A |
| φ3mm | 0.24 A |
| φ4mm | 0.4 A |
| φ5mm | 0.6 A |
| φ8mm | 1.6 A |
Have Questions? Contact Pulstec Today!
Pulstec has been an industry-leading manufacturer of X-ray equipment for over five decades.
Please contact us today if you have any questions on how to use our electrochemical polisher or visit our blog for more residual stress resources.
