Characterisation of single microdischarges during plasma electrolytic oxidation of aluminium and titanium

Andrew R. Gibson; Florens Grimm; Jan-Luca Gembus; Lars Sch\"ucke; Nikita Bibinov; Peter Awakowicz; Vera Bracht

Characterisation of single microdischarges during plasma electrolytic oxidation of aluminium and titanium

T0 review · reviewed 2026-08-16 · deepseek-v4-flash

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Pith's one-line read Using a 1 mm wire-tip anode, the authors show that aluminium produces longer-lived microdischarges and bubbles while titanium discharges are faster, and that KOH concentration controls whether titanium forms any PEO coating at all.

desk verdict First systematic Al vs Ti single-microdischarge comparison; the qualitative trends hold up, but fix the abstract/body crack contradiction and verify the pulse-to-bubble one-to-one assignment before accepting. read the letter →

arxiv 2504.12139 v1 pith:H2DZML4X submitted 2025-04-16 physics.plasm-ph cond-mat.mtrl-sci

Jan-Luca Gembus , Vera Bracht , Florens Grimm , Nikita Bibinov , Lars Schücke , Peter Awakowicz , Andrew R. Gibson This is my paper · ORCID

classification physics.plasm-phcond-mat.mtrl-sci

keywords microdischargesmicrodischargealuminiumcoatingelectrolytematerialsinglesurface

verification ladder T0 review T1 audit T2 compute T3 formal

The pith

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The reading

Plasma electrolytic oxidation is a way to grow a hard oxide layer on light metals by passing current through a saltwater bath at high voltage. The process works through thousands of tiny electric sparks, called microdischarges, that melt and re-solidify the metal surface. Because the sparks live for only microseconds to hundreds of microseconds and happen underwater, they are hard to study. This paper uses a special rig in which the metal sample is the tip of a 1 mm wire, so the sparks are mostly isolated. The authors watched the sparks with a fast camera, recorded their electrical current and voltage, measured the light they emit, and examined the resulting coating with an electron microscope.

Extended reading notes

Core claim

The study demonstrates that microdischarge behaviour is significantly influenced by the substrate material, treatment time, and electrolyte concentration, all of which impact the coating morphology. Under the conditions studied in this work, aluminium exhibits longer microdischarge and bubble lifetimes, with fewer cracks on the top layer of the coating, whereas titanium showed faster, shorter-lived bubbles due to more rapid microdischarge events. Supporting quantitative claims include surface temperatures of 2000-3750 K on Al and 2000-2500 K on Ti, electron temperatures of 5000-12200 K on Al and 4300-8600 K on Ti, and bubble pressures of 0.5-3 bar from Rayleigh-Plesset fitting.

Load-bearing premise

The assumption that each resolved current pulse corresponds to a single microdischarge on the 1 mm tip, and that the gas bubble imaged by the high-speed camera belongs to that same microdischarge. The paper states in §2 that triggering on the microdischarge current synchronizes measurements, and in §4.1 that 'Each current pulse represents an individual microdischarge,' citing Troughton. However, §4.1 also reports multiple simultaneous ignitions at some KOH concentrations, and §4.2 notes bubbles that could not be detected due to overlapping discharge events or noise. If current pulses are not single events, the reported lifetimes, radii, and pressure values would not be assignable to individual microdischarges.

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Desk Editor's Note

read the letter

Quick read on arXiv:2504.12139.

First the punchline: it's the first systematic single-microdischarge comparison of aluminium and titanium across KOH concentration. The genuinely new result — Ti forms no PEO coating above 2 g/l KOH, with voltage and current behaviour that support the explanation — is believable and useful for process control.

What is good: careful experimental work. The single-tip setup is inherited from Bracht's thesis and Troughton's earlier studies, but extending it to Ti and running electrical, high-speed imaging, spectroscopy, and SEM/EDX on both metals is real work. The qualitative trends hang together: bubble lifetime and radius increase with treatment time on both metals; Al bubbles are much bigger and longer-lived (200–550 µm, hundreds of µs) than Ti's (≤100 µm, tens of µs); bubble pressure stays roughly 0.5–3 bar regardless of substrate or concentration. Three independent measurement channels agree. The authors are candid about the large electron-temperature error bars and the model assumptions in the continuum fit.

Soft spots, in proportion:

1. The abstract contradicts the body on cracks. The abstract says Al shows fewer cracks on the top layer; §4.4 says the Al outer layer has large cracks, tens to hundreds of µm long, and that Ti shows no visible outer-layer degradation. Small textual fix, but a factual inconsistency.

2. The one-to-one assignment of current pulse to microdischarge to bubble. §4.1 asserts each resolved pulse is a single microdischarge, but the same section reports simultaneous ignitions (Al at 0.5 g/l, Ti above 1 g/l) and §4.2 admits some bubbles were untrackable due to overlapping events. No spatially resolved check is shown that the pulse and the imaged bubble are the same event in the conditions used for the quantitative bubble values. This is a caveat, not a demonstrated error, and the data to check it already exist. Ask them to verify or soften the claim.

3. The quantitative estimates are model-dependent. Temperatures come from fitting a spectrum assuming a Maxwellian EEDF, ε = 1, and n0 >> ni; pressures come from a second-degree polynomial fit of the radius through Rayleigh-Plesset. The paper says most of this, so it is not concealment, but the abstract's confidence does not match the method's.

4. No raw data deposited; several claims rest on a few representative events.

The central qualitative claim holds up. The Al/Ti contrast is large and consistent; multi-ignition might explain part of the Ti numbers but not an order-of-magnitude difference in bubble size and lifetime. The paper is for people tuning PEO parameters and for the single-discharge-in-liquids diagnostics community. It deserves a serious referee; the issues are addressable. My recommendation: send to review, and require the abstract fix, the pulse-to-bubble verification or softening, and a data availability statement.

Assumptions & free parameters 5 free parameters · 8 assumptions · 0 invented entities

The central qualitative observations of lifetimes and morphology rest on the single-wire setup and the pulse-to-discharge association, while the quantitative temperatures rest on a spectral model with two free scaling factors and several physical assumptions. No new physical entities are introduced.

free parameters (5)

C1 (Bremsstrahlung scaling factor in Eq. 7) = fitted per spectrum, values not tabulated
Absorbs electron and neutral densities and instrumental proportionality; fitted to the continuum spectrum to obtain Te.
C2 (black-body scaling factor in Eq. 7) = fitted per spectrum, values not tabulated
Scales Planck radiation to the measured intensity; fitted together with C1 and the temperatures.
Te (electron temperature) = Al: 5000-12200 K; Ti: 4300-8600 K
A fit parameter in Eq. 7, not independently measured; the paper acknowledges errors up to 3700 K.
Ts (surface temperature) = Al: 2600-3750 K; Ti: 2000-2500 K
A fit parameter in Eq. 7 under the black-body and epsilon=1 assumptions; treated as a lower bound for the gas temperature.
Bubble radius polynomial fit coefficients = not stated
A second-degree polynomial fit of the measured bubble radius is used as input to the Rayleigh-Plesset equation for pressure; the coefficients are not reported.

assumptions (8)

domain assumption n0 >> ni approximately ne(max) of 1e17 cm-3, so electron-ion free-free and free-bound terms are neglected in Eq. (2).
Invoked in §3.2 before Eq. (3), based on values from Troughton and Bracht. If the ion density is not negligible, the continuum fit omits recombination and ion-bremsstrahlung components.
domain assumption The electron energy distribution function is Maxwellian and the elastic scattering cross-section has a weak energy dependence.
Used to reduce Eq. (3) to Eq. (4) in §3.2. A non-Maxwellian EEDF would change the wavelength shape of the Bremsstrahlung component and the extracted Te.
domain assumption The hot substrate surface radiates as a black body with emission coefficient epsilon = 1.
Stated in §3.2. If epsilon is less than 1, the Planck component and hence the surface temperature would shift; the paper calls Ts a lower boundary but does not quantify the emissivity correction.
domain assumption Liquid water absorption follows the Beer-Lambert law with literature values of alpha(lambda) over a path of about 1 cm.
Used in Eq. (7); the absorption coefficient is from Hale and Querry. Uncertainties in the absorption coefficient and path length affect both temperature fits.
domain assumption The gas bubble can be modeled by the spherical Rayleigh-Plesset equation with an incompressible Newtonian liquid, no gravity, and constant properties.
Assumptions enumerated in §3.1; the bubble is actually hemispherical and the paper states that no hemispherical model exists. This affects the pressure values in §4.2.
domain assumption Electrolyte conductivity remains constant over the 10-minute treatment, so voltage rise indicates coating resistance growth.
Invoked in §4.1 for aluminium based on Bracht's thesis; not directly verified here for titanium, where voltage behaviour is less stable.
domain assumption The 1 mm wire-tip anode under 1.27 A/cm2 produces isolated microdischarges representative of PEO.
Basis of the single-microdischarge method from §2; the paper itself reports multiple simultaneous ignitions for some concentrations and side ignitions behind the O-ring, so the isolation is not always achieved.
standard math The continuum emission formulas from the cited literature, such as Bílek et al. and Burm, correctly describe the discharge radiation.
Equations (3) through (5) are taken from the cited literature without derivation; if the model constants or cross-sections are inaccurate, the Te estimates inherit the error.

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Pith. "Pith review of Characterisation of single microdischarges during plasma electrolytic oxidation of aluminium and titanium." pith.science (2026). https://pith.science/paper/H2DZML4X

@misc{pith2026250412139,
  author       = {Pith},
  title        = {Pith review of: Characterisation of single microdischarges during plasma electrolytic oxidation of aluminium and titanium},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/H2DZML4X}},
  note         = {Machine review of arXiv:2504.12139}
}

read the original abstract

Plasma electrolytic oxidation (PEO) is a technique used to create oxide-ceramic coatings on lightweight metals, such as aluminium, magnesium, and titanium. PEO is known for producing coatings with high corrosion resistance and strong adhesion to the substrate. The process involves generating short-lived microdischarges on the material surface through anodic dielectric breakdown in a conductive aqueous solution. To investigate single microdischarges during PEO, a single microdischarge setup was developed, where the active anode surface is reduced to the tip of a wire with a diameter of 1 mm. In this work the focus is on the effect of electrolyte concentration, anode material, and electrical parameters on the microdischarges. The electrolyte is composed of distilled water with varying concentrations of potassium hydroxide (0.5 - 4 g/l). High-speed optical measurements are conducted to gain insights into the formation and temporal evolution of individual microdischarges and the induced gas bubble formation. Optical emission spectroscopy is used to estimate surface and electron temperatures by fitting Bremsstrahlung and Planck's law to the continuum spectrum of the microdischarges. To evaluate the impact of the microdischarges on coating morphology, the resulting oxide layers on the metal tips are analysed using scanning electron microscopy. The study demonstrates that microdischarge behaviour is significantly influenced by the substrate material, treatment time, and electrolyte concentration, all of which impact the coating morphology. Under the conditions studied in this work, aluminium exhibits longer microdischarge and bubble lifetimes, with fewer cracks on the top layer of the coating, whereas titanium showed faster, shorter-lived bubbles due to more rapid microdischarge events.

Figures

Figures reproduced from arXiv: 2504.12139 by the authors.

**Figure 1.** Schematic of a single microdischarge (SMD) setup. It allows the observation of single microdischarges during a PEO process by reducing the anode to the tip of a wire with a diameter of 1 mm. The anode is immersed in an electrolytic cell. A quartz glass window in line-of-sight to the substrate tip allows an investigation of single discharges. KOH (≥ 85 %) (g/l) Molarity (M) Conductivity (S/m) pH 0.5 0.0089 0.23 12.1 … view at source ↗

**Figure 2.** Time-resolved current-voltage measurements at the start and after 10 minute PEO treatment with an aluminium anode. The voltage is shown in orange (start), purple (10 min) and the current in red (start), blue (10 min). a) with 1 g/l of KOH and b) with 3 g/l of KOH in distilled water. where the effect was less strong with half of the current density. Side ignition can likely be reduced by exchanging the fluorine rubb… view at source ↗

**Figure 4.** Voltage behaviour and corresponding standard deviation during a PEO treatment with an aluminium anode and different electrolyte (KOH) concentrations, while the current supply is fixed at 1.27 A/cm2 . Higher voltages are measured for a lower KOH concentration and also for increasing treatment time. 0 2 4 6 8 1 0 0 250 300 350 400 450 500 550 600 650 700 v o l t a g e / V time / mi n 0. 5 g/l KOH 1 g/l KOH 2 g/l KOH 3… view at source ↗

Figures from the paper (7 more)

**Figure 3.** Figure 3: Time-resolved current-voltage measurements at the start and after 10 min of PEO treatment with a titanium anode. The voltage is shown in orange (start), purple (10 min) and the current in red (start), blue (10 min) for a) 0.5 g/l of KOH and b) 1 g/l of KOH, in distille…

**Figure 6.** Figure 6: Time-resolved current measurement with the corresponding bubble radius for an aluminium substrate and 1 g/l of KOH. Bubble formation is shown in the top images in b). The numbers correspond to the marked areas in the graphs below. a) shows the bubble at the beginning, …

**Figure 8.** Figure 8: An example of the pressure profile for a bubble formed in 1 g/l of KOH on an aluminium substrate. The bubble forms at the beginning of the process. The pressure is calculated using the Rayleigh-Plesset equation, with a seconddegree polynomial fit for the radius as the…

**Figure 9.** Figure 9: Calculated surface temperature and corresponding standard deviation for an aluminium substrate a) and a titanium substrate b) during the PEO process. It is obtained by fitting black body radiation to the measured continuum spectrum, based on several assumptions, such a…

**Figure 11.** Figure 11: SEM images at 450x magnification showing the inner layer of Al (left) and Ti (right) after a 10 minute PEO treatment with 1 g/l KOH. For Ti, no combination of inner and outer layers was observed. of an inner and outer layer, which is in agreement with Hussein et al. […

**Figure 12.** Figure 12: SEM images with a magnification of 450x for Al substrates and different KOH concentrations: a) 1 g/l KOH, b) 2 g/l KOH, c) 3 g/l KOH and d) 4 g/l KOH. The images show the outer layer of the coating. ies are necessary to confirm this observation. Furthermore, the vari…

**Figure 13.** Figure 13: SEM images with a magnification of 4500x for Ti substrates and different KOH concentrations: a) 1 g/l KOH, b) 2 g/l KOH, c) 3 g/l KOH and d) 4 g/l KOH. Titanium substrate Aluminium substrate Element Atom (at. %) Element Atom (at. %) O 64.83 O 57.89 Ti 32.32 Al 38.59 A…

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Cited by 1 Pith paper

Reviewed papers in the Pith corpus that reference this work. Sorted by Pith novelty score. Full citation record

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Reference graph

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Introduction: plasma electrolytic oxidation Light metals like aluminium, titanium, magnesium, and their alloys are extensively used in transport and medical applications [1, 2]. With the densities of alu- minium (2.7 g/cm 3) and magnesium (1.7 g/cm 3) be- ing less than one-third that of iron (8.9 g/cm 3), they are suitable for weight reduction in car manu...

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This system was originally described in de- tail in the work of Bracht [27]

Experimental setup An experimental system specialised for the study of single microdischarges (SMDs) during PEO has been developed. This system was originally described in de- tail in the work of Bracht [27]. The reduction of the anode/substrate to the tip of a wire with a diameter of 1 mm ensures ignitions of mainly single discharges on the wire tip, eac...

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Diagnostic methods Different diagnostic tools are applied to observe the substrate tip and investigate individual microdis- charges and bubble dynamics during the PEO process (sections 3.1, 3.2). In addition, a post treatment anal- Characterisation of single microdischarges during PEO of Al and Ti 4 ysis is performed with a scanning electron microscope (S...

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For simplicity, a black body radiator is assumed, with an intensity calculated using Planck’s law divided by the energy E = h×c/λ [20]

This results in the final ex- pression for the intensity of Bremsstrahlung: Ien ff (λ,T e) =C ′ 1 (kBTe) 3 2 λhc " 1 + hc λkBTe 2 + 1 # × exp −hc λkBTe (5) As previously noted, thermal radiation from the anode surface also contributes to the continuum radia- tion. For simplicity, a black body radiator is assumed, with an intensity calculated using Planck’...

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The first part investigates the current and voltage behaviour during the PEO process, followed by an analysis of bubble dynamics in relation to microdischarges

Results and discussion This section is divided into four parts comparing the effect of electrolyte concentration and treatment time on an aluminium (Al) and a titanium (Ti) substrate during a PEO process. The first part investigates the current and voltage behaviour during the PEO process, followed by an analysis of bubble dynamics in relation to microdis...

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Gibson: https://orcid.org/0000-0002-1082- 4359

ORCID iDs Jan-Luca Gembus: https://orcid.org/0009-0002-1263- 9218 Vera Bracht: https://orcid.org/0000-0003-4623-7532 Lars Sch¨ ucke: https://orcid.org/0000-0002-7991-853X Peter Awakowicz: https://orcid.org/0000-0002-8630- 9900 Andrew R. Gibson: https://orcid.org/0000-0002-1082- 4359

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2. 3. bubble discharge Figure 6. Time-resolved current measurement with the corresponding bubble radius for an aluminium substrate and 1 g/l of KOH. Bubble formation is shown in the top images in b). The numbers correspond to the marked areas in the graphs below. a) shows the bubble at the beginning, and b) after 10 minutes of operation. which is consiste...

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3. 4. bubble bubble bubbledischarge bubble discharge 2.1. 3. 4. Figure 7. Time-resolved current measurement with the corresponding bubble images for a titanium substrate. Bubble growth and collapse are seen in the top images, where the numbers correspond to the highlighted time points in the graphs below. a) presents the beginning of the process for 1 g/l...

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Pith tools

Reviewed August 16, 2026 · model on record in the stance chip above.

[1] [1]

Introduction: plasma electrolytic oxidation Light metals like aluminium, titanium, magnesium, and their alloys are extensively used in transport and medical applications [1, 2]. With the densities of alu- minium (2.7 g/cm 3) and magnesium (1.7 g/cm 3) be- ing less than one-third that of iron (8.9 g/cm 3), they are suitable for weight reduction in car manu...

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[2] [2]

This system was originally described in de- tail in the work of Bracht [27]

Experimental setup An experimental system specialised for the study of single microdischarges (SMDs) during PEO has been developed. This system was originally described in de- tail in the work of Bracht [27]. The reduction of the anode/substrate to the tip of a wire with a diameter of 1 mm ensures ignitions of mainly single discharges on the wire tip, eac...

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[3] [3]

Diagnostic methods Different diagnostic tools are applied to observe the substrate tip and investigate individual microdis- charges and bubble dynamics during the PEO process (sections 3.1, 3.2). In addition, a post treatment anal- Characterisation of single microdischarges during PEO of Al and Ti 4 ysis is performed with a scanning electron microscope (S...

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For simplicity, a black body radiator is assumed, with an intensity calculated using Planck’s law divided by the energy E = h×c/λ [20]

This results in the final ex- pression for the intensity of Bremsstrahlung: Ien ff (λ,T e) =C ′ 1 (kBTe) 3 2 λhc " 1 + hc λkBTe 2 + 1 # × exp −hc λkBTe (5) As previously noted, thermal radiation from the anode surface also contributes to the continuum radia- tion. For simplicity, a black body radiator is assumed, with an intensity calculated using Planck’...

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The first part investigates the current and voltage behaviour during the PEO process, followed by an analysis of bubble dynamics in relation to microdischarges

Results and discussion This section is divided into four parts comparing the effect of electrolyte concentration and treatment time on an aluminium (Al) and a titanium (Ti) substrate during a PEO process. The first part investigates the current and voltage behaviour during the PEO process, followed by an analysis of bubble dynamics in relation to microdis...

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Gibson: https://orcid.org/0000-0002-1082- 4359

ORCID iDs Jan-Luca Gembus: https://orcid.org/0009-0002-1263- 9218 Vera Bracht: https://orcid.org/0000-0003-4623-7532 Lars Sch¨ ucke: https://orcid.org/0000-0002-7991-853X Peter Awakowicz: https://orcid.org/0000-0002-8630- 9900 Andrew R. Gibson: https://orcid.org/0000-0002-1082- 4359

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2. 3. bubble discharge Figure 6. Time-resolved current measurement with the corresponding bubble radius for an aluminium substrate and 1 g/l of KOH. Bubble formation is shown in the top images in b). The numbers correspond to the marked areas in the graphs below. a) shows the bubble at the beginning, and b) after 10 minutes of operation. which is consiste...

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3. 4. bubble bubble bubbledischarge bubble discharge 2.1. 3. 4. Figure 7. Time-resolved current measurement with the corresponding bubble images for a titanium substrate. Bubble growth and collapse are seen in the top images, where the numbers correspond to the highlighted time points in the graphs below. a) presents the beginning of the process for 1 g/l...

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