Corrosion with Purpose: Electrochemical Machining Parts, Atom-By-Atom

When considering the various skills, procedures, materials, and systems in the field of corrosion engineering, it is reasonable to assume that the bulk of this work is meant to prevent or deter said corrosion from occurring. However, a material removal method exists that utilizes the principles of corrosion and electrochemistry to “reverse engineer” the work of corrosion engineering to perform what can be equated to ‘controlled corrosion’ on metallic parts as a material removal process in-and-of-itself. This unique process is known as electrochemical machining (ECM), as well as its more advanced version called pulsed electrochemical machining (PECM).

By Kirk Gino Abolafia, Technical Marketing and Sales Manager, Voxel Innovations

This article will discuss how this unique material removal process utilizes the principles of corrosion to its advantage, the distinctive benefits of electrochemical machining, and how electrochemistry can be applied for machining critical parts serving the energy, aerospace, and medical device industries.

In simplified terms, corrosion is the chemical process in which a refined metal converts into a more chemically stable oxide. For the overwhelming majority of applications, corrosion is an undesirable outcome, as it can cause weakening of the metal’s chemical bonds leading to discoloration, loss of material to the environment, and material weakness.

Electrochemical machining, on the other hand, takes advantage of the properties of corrosion and electrolysis to create unique benefits not offered by conventional machining methods, such as tough material machining, non-heat, non-contact material removal, high repeatability, and super finished surfaces.

• Tough material machining: ECM is not concerned with a material’s hardness, but its chemical properties—while it cannot machine non-conductive materials such as plastics, it is capable of machining tough materials like nickel superalloys at a similar rate to copper.
• Non-heat, non-contact material removal: As no heat or contact is involved in the electrochemical machining process, it can machine thermally sensitive areas of a part, or thin-walled features that may otherwise be susceptible to tool distortion.
• High repeatability: The lack of heat or contact also causes little-to-no tool wear, providing ECM a high level of repeatability, making it especially advantageous for machining high part volumes.
• Superfinished surfaces: The aforementioned lack of heat or contact also provides ECM the ability to simultaneously machine and finish parts, which can reduce the steps required to machine a completed component; ECM is capable of producing surfaces down to .005.4um Ra.

In order to understand how the ECM process works, it is best to first understand four key components of the technology: cathode, anode, electrolytic fluid, and power supply.

• Cathode (tool): Also called the electrode, the tooling in ECM is designed as the approximate inverse of the intended shape to be machined on the workpiece. In other words, each project has its unique, custom-built tooling. It is lowered onto the workpiece during machining but never comes into contact with it.
• Anode (workpiece): The anode can be comprised of any conductive material, but electrochemical machining is best utilized when machining tougher materials, such as high-entropy alloys, chrome-based alloys, and nickel super-alloys, as the process is only concerned with the anode’s chemical properties rather than its toughness. While ECM is often used on near-net shape parts (be it stamped, rough machined, or cast), it can also be used on bar stock or raw plates to machine the desired shape.
• The electrolytic fluid is flushed through the microscopic gap between the cathode and anode during machining (also called the inter-electrode gap, or IED), and this fluid serves two crucial purposes simultaneously. First, the charged electrolytic fluid acts as the catalyst for the electrochemical reaction to occur. Furthermore, it acts as a flushing agent that removes both waste products and heat from the area, leaving a smooth surface finish.

The power supply provides the electrolytic fluid with the necessary current to conduct the electrochemical reaction, and the current is directly correlated with the process’s material removal rate. ECM is a low voltage (5-50V), high current (25-150 A/cm2 or 160-970 A/in2) process. This is the biggest distinction between traditional ECM and more advanced PECM; ECM power supplies ap-ply a continuous DC current while PECM provides more precise “pulses” of power for improved precision.

With this foundational understanding of the technology, users can now understand how electrochemical machining removes workpiece materials atom-by-atom.

During ECM, the anode surface gets “machined” by first becoming oxidized in which a given atom loses one or more electrons, changing its atomic structure and removing it from the other atoms. Although this material removal occurs on a microscopic scale, the electrochemical machining process can both machine and finish a part in a fairly short period of time.

This oxidization occurs because the electrical current from the power source, passing through the fluid, facilitates an imbalance of electrons between the negatively charged cathode (tool) and the positively charged anode (workpiece)

As a result of the electrical charge, the positively charged cations are removed from the anode surface (alongside their electrons) to be deposited on the negatively charged cathode surface. However, these cations bond with oxygen and hydrogen to form neutralized metal hydroxides rather than be deposited on the negatively charged cathode. Simultaneous to this oxidation is a reduction in which electrons form with hydrogen ions on the surface of the cathode to form hydrogen gas—another byproduct of the electrochemical reaction.

This illustrates a set of reactions by showing a hypothetical equation in which Copper (Cu) is dissolved in the electrochemical reaction into cupric cations and its 2 electrons. At the end of the ECM process, these will become copper hydroxides and hydrogen gas, which are the waste products that will be removed.

This is the oxidation process—the primary reaction in ECM – in which the copper molecules on the surface are broken down into cations (in this case, a cupric cation) and its spare electrons:

Cu→Cu⁺²+2e^–  

This is the reduction process, in which the spare electrons combine with hydrogen ions and form hydrogen gas, which is another byproduct of the reaction:

2e^(-) +2H⁽⁺⁾→H₂

Here is the “redox” formula accounting for both of these simultaneous reactions:

Cu+2H⁺→Cu⁺²+H₂

The positively-charged metal cations are attracted to the negatively-charged cathode (tool), and–under normal circumstances—would attach to the surface of the cathode. In electrochemical machining, however, they become neutral hydroxides while interacting with the hydrogen and oxygen in the water in the electrolyte solution—which is usually comprised of water and a salt, such as NaCl.

Here is the final formula in which the cupric cations, spare electrons and hydrogen/oxygen in the water becomes copper hydroxides and hydrogen gas:

Cu⁺²+2_e-+2H₂O→Cu(OH)₂+H₂

The lack of contact between the metal cations and the cathode surface is another reason the cathode (tool) is not worn in the ECM process. The fluid holds these neutralized hydroxides as waste products and acts as a flushing agent, removing the waste products.

For example, Voxel Innovations’ PECM technology takes advantage of Faraday’s Law, which states that chemical reactions will occur the quickest wherever current density is highest; by creating unique, customized cathodes and varying the inter-electrode gap (IEG), PECM has an additional layer of control over the electrochemical reaction to develop specific geometries on the workpiece.

Critical Applications for PECM

Electrochemical machining has applications in several critical industries including the energy, medical device and aerospace industries in which the tolerances, material and surface quality of parts require advanced manufacturing processes.

Aerospace manufacturers, for example, are seeking new ways to optimize the manufacturing process for turbine blades, rocket nozzles, heat exchangers, blisks, and more. As these ECM applications often require unique materials capable of withstanding extreme environments— such as high temperature flux and withstanding high loads, while simultaneously meeting low weight and high surface quality requirements— these parts may have challenging design and material properties which thereby warrant alternative manufacturing methods like PECM.

Heat exchangers for aerospace applications are a good example of critical components with complex part designs that may call for the use of PECM. Manufacturers are working to improve the heat transfer capabilities of these parts by increasing the quantity of cooling features per unit volume, adding additional complexity to part designs.

Conventional machining processes are not always ideal for these heat exchangers, as the increasingly thinner-walled features may be subject to thermal distortion or tool vibration, amplified by the toughness of the materials (such as nickel superalloys including inconel). PECM, however, is an effective means of creating high-density, thin-walled cooling features on critical heat exchangers, as the chemical-based machining process ignores the toughness of the workpiece material and imposes no heat or friction on it, allowing it to machine thin-walled inconel features with relative ease.

For instance, Voxel Innovations showed the formation of <0.075mm or <0.003″ thick walls with a 20:1 aspect ratio in stainless steel.

Another valuable application of PECM technology can be found in critical parts of the medical device manufacturing industry, as orthopedic fixture device manufacturers are incentivized to:

• Use tough-to-machine materials that are both biocompatible and sturdy, such as certain titanium alloys and stainless steel
• Develop miniaturized parts to improve patient mobility and minimize the invasiveness of the surgical procedures
• Produce parts with superfinished surface quality, to minimize the potential cytotoxic effects of certain materials in parts like nickel

These difficulties are exacerbated due to large part volumes required for product launch, putting additional strain on manufacturers. Companies such as Voxel Innovations, has proven that PECM is a capable machining process for machining high volumes of tough-to-ma- chine, miniaturized fixation devices with excellent surface quality (down to .005-.4 µm Ra). The atom-by-atom removal of the workpiece material allows PECM a unique level of precision, control and repeatability not offered by conventional processes.