Why AM internal surfaces are an extreme starting condition for electrochemical polishing

Internal channel surfaces produced by laser powder bed fusion (L-PBF) and selective laser melting (SLM) arrive at the electrochemical polishing (ECP) stage with surface roughness values (Ra) commonly ranging from 3 to 50 µm — driven by balling effects, staircase stepping, partially sintered powder adhesion, and melt pool overlap irregularities. This is not a single, uniform defect: balling produces large hemispherical protrusions, while staircase effects produce periodic ridges aligned to build orientation. The co-existence of multiple defect morphologies at different scales means that no single polishing voltage or current density regime can address all of them simultaneously.

The microstructural heterogeneity of AM parts compounds this multi-scale defect problem. Research on Inconel 718 from Texas A&M University (2019) demonstrated that ECP can reduce as-printed Ra from 17 µm to 0.25 µm under optimal pulsed current density conditions — but further polishing fails because micro-level pores, cracks, non-conductive phases, and carbide particles dissolve at different rates. The study explicitly recommends homogenising and hot isostatic pressing before ECP to achieve uniform electrochemical dissolution, adding process complexity that is especially problematic for sealed internal channels that cannot be easily inspected or accessed, as reported by bodies including NIST in their additive manufacturing process standards work.

Build orientation sensitivity adds a further layer of difficulty that is particularly acute for heat exchanger channels. Work on IN625 from École de technologie supérieure (2019) found that even after 4 hours of electropolishing, surface finish remained strongly dependent on build angle. While Ra ≤ 6.3 µm was achievable across all orientations, material removal rates differed substantially between build angles. This orientation-dependent removal rate directly translates to dimensional non-uniformity in heat exchanger channels where hydraulic diameter tolerance is critical to thermal performance.

Tianjin University (2021) confirmed the same phenomenon for L-PBF 316L stainless steel, showing that surfaces built along different orientations respond differently to electrolyte temperature and current density. Heat exchanger channels frequently feature walls oriented at 0°, 45°, and 90° to the build platform within the same part — meaning a single parameter set cannot uniformly polish all channel wall orientations simultaneously.

“A single polishing voltage or current density regime cannot address balling, staircase effects, and powder adhesion simultaneously — because they exist at different scales and respond to dissolution differently.”

The electrode accessibility barrier: getting a cathode inside a sealed channel

The most technically demanding aspect of scaling ECP to complex internal channels is delivering a cathode electrode that is geometrically proximate to all target surfaces. In sealed, curvilinear channels typical of additively manufactured heat exchangers, no line-of-sight access exists, and rigid electrodes cannot navigate bends — making electrode design the defining geometric barrier to the entire process.

The most comprehensive solution in the patent literature is the co-printing of the cathode with the workpiece. The University of Science and Technology Beijing (2020, US filing 2023) describes a method where a coaxial cathode is designed into the 3D model and printed simultaneously with the part. After polishing, electrode reversal is used to break the co-printed cathode, enabling its extraction. This approach solves the insertion problem but introduces two new challenges: ensuring sufficient gap uniformity between cathode and channel wall to prevent short-circuit contact during polishing, and cleanly removing cathode fragments from channels without leaving debris that would degrade heat exchanger performance.

Shanghai Heland Power Technology (2024) addresses the short-circuit risk of co-printed cathodes directly by filling the gap between the electrode and channel wall with solid particles that act as stand-offs, maintaining gap uniformity while conducting electrolyte. The patent acknowledges that abrasive flow methods suffer from over-polishing at channel bends and dead zones in complex cavities, while chemical polishing lacks the leveling capacity for high-roughness AM surfaces — justifying the electrochemical approach despite its implementation complexity.

For channels too complex for rigid co-printed cathodes, flexible electrode concepts have emerged. Nanjing University of Aeronautics and Astronautics (2024) discloses a hollow helical cathode made from conductive metal wire with selective insulating coatings. The uncoated regions act as active cathode areas, while the coated regions form physical standoffs preventing short circuits. The hollow spiral structure enables simultaneous internal and external electrolyte flushing, and the electrode undergoes both rotational and reciprocating motion during polishing to improve spatial uniformity. The natural step height between coated and uncoated regions enhances mass transport by creating turbulence at the electrode-workpiece interface.

Suzhou University (2026) takes this concept further by using liquid metal droplets as the cathode, exploiting their self-adaptive shape to conform to U-shaped, S-shaped, and variable cross-section channels that defeat any solid electrode geometry. Rolls-Royce PLC (EP, 2026) discloses a shuttle-and-guide-cable system where fluid flow through the passageway itself transports a shuttle that pulls a flexible electrode into position — particularly relevant for heat exchanger channels with hydraulic diameters below 2 mm where mechanical manipulation of an electrode is impractical. This type of precision manufacturing challenge is also tracked by ISO through its technical committees on additive manufacturing and surface finishing standards.

Electrolyte flow, mass transport, and the uniformity problem in sealed channels

Electrolyte management within closed internal channels presents challenges fundamentally different from open-bath polishing. In sealed channels, the electrolyte becomes saturated with dissolved metal ions and reaction byproducts locally, creating concentration gradients that produce non-uniform anodic dissolution — polishing faster near inlets and slower at channel midpoints and dead zones. Passive bath immersion is simply insufficient for internal channel ECP.

Harbin Institute of Technology (2024) directly addresses this by implementing a two-stage voltage protocol. The first stage applies a voltage 10–50% above the limiting current plateau, rapidly removing large protrusions through a selective over-potential mechanism. The second stage applies a voltage at 90–100% of the plateau for precision smoothing. Electrolyte is driven through the channel by a peristaltic pump at 100–500 mL/min or by magnetic stirring at 300–1000 rpm during the first stage, ensuring continuous electrolyte renewal. This protocol achieved Ra ≤ 0.8 µm on complex flow channel components.

The same research group’s 2024 conformal electrode patent confirms that electrolyte flow alone cannot compensate for geometric electrode mismatch. A titanium mesh cathode used without conformal geometry leaves inner surfaces at Ra ≥ 5 µm even with the same dual-stage protocol — while a 3D-printed conformal cathode with the identical flow conditions achieves Ra ≤ 0.8 µm. This finding establishes that electrode geometry and electrolyte flow are not interchangeable: both are necessary conditions for uniform internal channel polishing.

For channels with multiple parallel flow paths — the standard architecture in aerospace heat exchangers — Hefei University of Technology (2025) proposes a parallel circuit architecture where multiple flexible tool cathodes are threaded simultaneously through all channels, each controlled by a separate sliding rheostat on a shared power bus. The cathodes undergo synchronised reciprocating rotation via dual rotating shafts, with electrolyte supplied by a nozzle assembly. The sliding rheostats enable independent current adjustment per channel, allowing compensation for channel-to-channel geometric variation — a capability critical for heat exchangers where hundreds of cooling channels must be polished to consistent surface quality, as documented in engine combustion chamber applications.

Passivation film formation further complicates electrolyte management for titanium alloys, a common heat exchanger material in aerospace and energy systems. Nanjing University of Aeronautics and Astronautics (2022) demonstrated that neutral salt electrolytes — preferred for environmental compliance — form dense passive films on titanium surfaces that severely impair polishing efficiency. The solution employs high-velocity abrasive particle flow to mechanically disrupt the passivation layer while electrochemical dissolution proceeds. This hybrid approach, however, introduces abrasive particle concentration gradients along channel length and abrasive settling in low-flow regions as additional uniformity concerns — a trade-off documented in the electrochemical machining literature tracked by ASME.

“Electrolyte flow alone cannot compensate for geometric electrode mismatch. A titanium mesh cathode leaves inner surfaces at Ra ≥ 5 µm even with the same dual-stage protocol that achieves Ra ≤ 0.8 µm with a conformal cathode.”

Electrolyte chemistry, toxicity, and the path to industrial deployment

Conventional ECP of AM superalloys relies on acid-based electrolytes containing hydrofluoric acid, perchloric acid, concentrated sulfuric acid, or glacial acetic acid — formulations that are corrosive to ECP equipment, hazardous to operators, and environmentally problematic. These constraints become acute at manufacturing scale, where regulatory compliance and worker safety requirements impose hard limits on what can be deployed in a production environment.

Xi’an University of Technology (2024) addresses this by developing a novel electrolyte formulation based on glycerol (500–700 mL), ethanol (20–40 mL), phosphoric acid (10–20 mL), acetic acid (5–10 mL), ethylene glycol dibutyl ether (1–3 mL), sodium chloride (10–20 g), phosphoric acid corrosion inhibitor, trisodium phosphate, and p-hydroxybenzoic acid. This formulation avoids methanesulfonic acid, hydrofluoric acid, and perchloric acid. The patent reports micro-channel surface roughness reaching 0.54 µm after polishing with this safer formulation.

The University of Naples Federico II (2023) provides a comprehensive review of deep eutectic solvents and acid-free electrolytes as sustainable alternatives, noting that conventional strong acid baths are incompatible with increasingly stringent environmental compliance requirements in production environments. This aligns with the regulatory trajectory tracked by ECHA under REACH, which has progressively restricted perchloric acid and hydrofluoric acid use in industrial finishing processes.

Plasma electrolytic polishing (PEP) has emerged as a candidate for internal channel finishing due to its use of environmentally benign aqueous salt solutions without hazardous acids. However, as documented by Bern University of Applied Sciences (2022), bath-mode PEP yields inhomogeneous polishing and cannot reach complex cavity geometries. Jet-mode PEP improves localisation and increases polishing rate by a factor of six, but delivering a controlled PEP jet to a sealed internal channel of millimetre-scale diameter remains an unresolved engineering challenge.

For complex pump body and heat exchanger internal cavities, Jiangsu Jianghang Aircraft Engine Component Research Institute (2026) takes a multi-parameter adaptive approach: the system segments the internal cavity into zones by geometric complexity, measures real-time machining current, inter-electrode voltage, and back-pressure, computes a “comprehensive machining difficulty coefficient,” and selects electrolyte type from a database matched to each zone’s difficulty. This adaptive control approach acknowledges that no single electrolyte composition or parameter set can uniformly address the varying geometric difficulty across a complex heat exchanger internal network — a conclusion consistent with the broader electrochemical machining research published through Elsevier‘s Journal of Materials Processing Technology.

Who is solving this: global patent landscape and innovation trends

The global innovation landscape for ECP of AM internal channels is concentrated in a relatively small number of highly active research institutions and industrial actors, with the dataset spanning over 40 patent documents and peer-reviewed publications from 2008 to 2026, originating from assignees across China, the United States, Germany, Ireland, Italy, Canada, and Taiwan.

Harbin Institute of Technology is among the most technically advanced contributors, with two 2024 patents covering both dual-stage over-potential ECP and 3D-printed conformal cathode strategies. University of Science and Technology Beijing holds active US patents for the co-printed coaxial cathode method (2023). Nanjing University of Aeronautics and Astronautics leads in flexible electrode design and abrasive-assisted ECP. Hefei University of Technology has the most recent multi-channel simultaneous polishing patent (2025), addressing the scaling problem directly with a parallel circuit architecture.

On the industrial side, General Electric Company has built a multi-jurisdiction portfolio on additively manufactured electrodes with internal flushing passages for electro-machining processes, active in EP, CA, US, and WO jurisdictions, establishing the precedent of using AM itself to solve electrode accessibility problems. Rolls-Royce PLC represents the aerospace OEM perspective with its 2026 EP filing on shuttle-assisted flexible electrode deployment for internal passageways, signalling industrial-level commitment to solving the channel access challenge.

The trend across these actors is clear: innovation has moved from bath-mode ECP of external surfaces toward increasingly sophisticated internal channel tooling — co-printed electrodes, flexible and liquid-metal cathodes, shuttle deployment systems — paired with multi-stage voltage protocols and adaptive process control. The patent activity tracked through WIPO‘s PATENTSCOPE database confirms a sharp acceleration in filings related to internal channel finishing from 2020 onward, with Chinese research institutions accounting for the majority of technical innovation volume.