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arxiv: 2606.24524 · v1 · pith:HWCRHL4Tnew · submitted 2026-06-23 · ⚛️ physics.ins-det · hep-ex

Hydrofluoric acid-free titanium etching for rare-event searches

P. Knights , I. Manthos , K. Nikolopoulos , G. Rogers , D. Spathara , P. Walters This is my paper

Pith reviewed 2026-06-25 22:02 UTC · model grok-4.3

classification ⚛️ physics.ins-det hep-ex
keywords titaniumetchingsulphuric acidhydrofluoric acidrare event searchesradiopuritysurface treatmentpassivation
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The pith

Sulphuric acid can etch titanium as a safer alternative to hydrofluoric acid for rare-event search materials.

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

The paper aims to show that sulphuric acid provides an effective way to etch titanium without using dangerous hydrofluoric acid. This is important for rare-event search experiments that need high-purity titanium parts but must avoid hazardous chemicals, especially in underground labs. Experiments tested different acid concentrations and temperatures, finding that 40% sulphuric acid at 40 degrees Celsius removes the most material over 24 hours. The process also works at lower settings, and a passivation layer forms that might help control how much is removed.

Core claim

Sulphuric acid effectively etches titanium, removing up to 3.5 ± 0.3 mg/cm² for an unagitated 40% solution at 40°C over 24 hours. It remains effective at lower concentrations and temperatures. The formation of a passivation layer during etching may allow control of the total mass removed. These results were obtained through mass change measurements, surface roughness analysis, and scanning electron microscopy on grade 1 titanium samples.

What carries the argument

Sulphuric acid etching process for titanium, with mass removal quantified under controlled conditions of concentration, temperature, and time.

If this is right

  • Titanium structural elements can be prepared for use in rare-event searches using only sulphuric acid.
  • Etching is possible without heating or high concentrations, simplifying the process.
  • The passivation layer offers a potential way to limit or regulate the amount of material removed.
  • Surface analysis shows the etching modifies the titanium surface in measurable ways.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • This method could reduce the need for specialized safety equipment in underground laboratories.
  • Similar acid-based etching might be tested on other metals used in low-background detectors.
  • Long-term stability of the etched surfaces in experimental conditions would need verification beyond initial imaging.

Load-bearing premise

Measurements of mass removal and surface appearance alone are enough to verify that the sulphuric acid etching does not add radioactive contaminants or harm the material's performance in rare-event searches.

What would settle it

Finding higher levels of radioactive background in titanium samples etched with sulphuric acid than in those etched with hydrofluoric acid or left unetched would show the method is unsuitable.

Figures

Figures reproduced from arXiv: 2606.24524 by D. Spathara, G. Rogers, I. Manthos, K. Nikolopoulos, P. Knights, P. Walters.

Figure 1
Figure 1. Figure 1: CAD model of a 3D-printed polypropylene fixture, in white, used to hold the coupons upright during etching. The [PITH_FULL_IMAGE:figures/full_fig_p002_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: Mass change, and estimated thickness removed, of etched Ti coupons in H [PITH_FULL_IMAGE:figures/full_fig_p003_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: Imaging Ti surface x1000 of (a) a pre-etched surface and etched surfaces, etched using 40% H2SO4 at 40 ◦C after (b) 30 minutes, (c) 2 h, (d) 3 h, (e) 9 h and (f) 24 h. Surface roughness parameters were obtained for different etching conditions, using the microscope’s roughness module. The variation of the roughness parameter Sa, defined as the average height of the 5 [PITH_FULL_IMAGE:figures/full_fig_p005… view at source ↗
Figure 4
Figure 4. Figure 4: Imaging x1000 of Ti coupons, etched at 20 ◦C, after 24 h, using H2SO4 solution of (a) 20% and (b) 40% [PITH_FULL_IMAGE:figures/full_fig_p006_4.png] view at source ↗
Figure 5
Figure 5. Figure 5: Imaging x1000, 5x5 frames in 3D mode of Ti pre-etched surface (top left), and etched using 40% H [PITH_FULL_IMAGE:figures/full_fig_p006_5.png] view at source ↗
Figure 6
Figure 6. Figure 6: Surface roughness parameter Sa, extracted from KEYENCE VHX-7000 microscope images of Ti coupons etched in [PITH_FULL_IMAGE:figures/full_fig_p007_6.png] view at source ↗
Figure 7
Figure 7. Figure 7: Imaging of Ti surface using SEM JEOL 7000F with x2000 magnification: pre-etched [PITH_FULL_IMAGE:figures/full_fig_p008_7.png] view at source ↗
Figure 8
Figure 8. Figure 8: (a) BSE image of Ti surface prior to etching and the EDX maps of (b) Ti, (c) O, and (d) Fe. The dependence of etched mass and surface roughness on etching time was quantified. Furthermore, the results indicate that minor surface contamination present prior to etching is removed during the process, an important consideration for low-radioactivity applications. This hydrofluoric acid-free etching method is p… view at source ↗
read the original abstract

Rare-event search experiments require construction materials with high radiopurity to minimise background contributions. Thanks to its high mechanical strength, low density, machinability, and commercial availability in relatively radiopure forms, titanium is a suitable material for structural elements in rare-event searches. In such applications, a chemical etching stage is typically performed to remove surface contamination or to prepare the surface for further treatment. However, due to its chemical resistance, the etching of titanium conventionally requires hydrofluoric acid, posing serious health and safety concerns that are further exacerbated in deep underground laboratory settings. An alternative approach is proposed, which uses sulphuric acid. Grade 1 titanium samples were etched in 20\% and 40\% sulphuric acid solutions at 20$^\circ$C and 40$^\circ$C for up to 24\,h. The effects of etching were quantified through mass change measurements, surface roughness analysis, and scanning electron microscopy. Sulphuric acid effectively etches titanium, with up to $3.5\,\pm\,0.3$ mg/cm$^2$ of titanium removed for an unagitated solution of 40\% sulphuric acid at $40^\circ$C for 24\,h. Furthermore, sulphuric acid is shown to be effective at etching at lower concentration and temperature. The formation of a passivation layer during the etching may enable control of the total mass removed.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

2 major / 2 minor

Summary. The manuscript claims that sulphuric acid offers a viable HF-free alternative for etching Grade 1 titanium components in rare-event search experiments. It reports that 40% H2SO4 at 40°C for 24 h removes up to 3.5 ± 0.3 mg/cm² of titanium (with lower concentrations and temperatures also effective), quantified via mass-loss measurements, surface roughness analysis, and SEM imaging that indicate formation of a passivation layer allowing control of total mass removed.

Significance. If the process can be shown not to compromise radiopurity, the result would provide a meaningful safety improvement for underground laboratories by removing the need for HF handling. The reported mass-loss values with uncertainties and the multi-technique surface characterization constitute concrete, reproducible experimental evidence for the etching efficacy itself.

major comments (2)
  1. [Abstract] Abstract and introduction: the work is explicitly motivated by the need for high radiopurity in rare-event searches, yet no post-etch radiopurity assays (gamma spectroscopy, ICP-MS, or alpha counting) or comparison to HF-etched controls are presented; mass removal and SEM data alone do not establish that the etched surfaces meet the radiopurity requirements that justify the study.
  2. [Results] Results section (mass-loss and SEM data): the passivation layer is invoked to explain control of total mass removed, but no compositional analysis or leaching tests are reported to rule out incorporation or retention of U/Th/K contaminants from the sulphuric acid, which is load-bearing for the claimed application to low-background detectors.
minor comments (2)
  1. [Abstract] The abstract reports the 3.5 ± 0.3 mg/cm² value for the 40% / 40°C / 24 h condition but does not tabulate the full matrix of concentration-temperature-time results; a summary table would improve clarity.
  2. Notation for temperature (20$^ ext{ extdegree}$C) is inconsistent with standard °C usage in the provided text; standardize throughout.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the constructive comments on our manuscript. We address each major comment below, acknowledging the limitations of the current study while clarifying its scope as a demonstration of the etching process.

read point-by-point responses

  1. Referee: [Abstract] Abstract and introduction: the work is explicitly motivated by the need for high radiopurity in rare-event searches, yet no post-etch radiopurity assays (gamma spectroscopy, ICP-MS, or alpha counting) or comparison to HF-etched controls are presented; mass removal and SEM data alone do not establish that the etched surfaces meet the radiopurity requirements that justify the study.

    Authors: We agree that the manuscript is motivated by radiopurity considerations for rare-event searches but does not present post-etch radiopurity assays or comparisons to HF-etched samples. The study focuses on establishing the efficacy of sulphuric acid as an HF alternative through mass-loss, roughness, and SEM characterization. We will revise the abstract and introduction to explicitly note that radiopurity validation via gamma spectroscopy, ICP-MS, or alpha counting is required for application in low-background detectors and lies beyond the scope of this work. revision: yes

  2. Referee: [Results] Results section (mass-loss and SEM data): the passivation layer is invoked to explain control of total mass removed, but no compositional analysis or leaching tests are reported to rule out incorporation or retention of U/Th/K contaminants from the sulphuric acid, which is load-bearing for the claimed application to low-background detectors.

    Authors: The passivation layer is inferred from the mass-loss plateau and SEM-observed surface morphology changes. No compositional analysis or leaching tests for U/Th/K were performed. We acknowledge this limitation for the radiopure application. We will add a clarifying statement in the results or discussion section noting that further tests are needed to confirm absence of contaminant incorporation from the sulphuric acid before use in low-background experiments. revision: yes

Circularity Check

0 steps flagged

No circularity: purely experimental measurements with no derivations or equations

full rationale

The paper reports direct experimental results on sulphuric acid etching of titanium samples, quantified via mass change (e.g., up to 3.5 ± 0.3 mg/cm²), surface roughness, and SEM imaging. No equations, models, fitted parameters, predictions, or derivations are present that could reduce to inputs by construction. No self-citations, uniqueness theorems, or ansatzes are invoked. The work is self-contained against external benchmarks as a set of controlled laboratory measurements.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

The work rests on standard materials characterization techniques and the domain assumption that mass loss and microscopy indicate suitability for radiopure use; no free parameters, invented entities, or ad-hoc axioms beyond experimental setup.

axioms (1)
  • domain assumption Mass change, surface roughness, and SEM imaging are sufficient proxies for etching effectiveness and surface suitability in radiopurity applications.
    Invoked implicitly when concluding effectiveness from the reported measurements.

pith-pipeline@v0.9.1-grok · 5795 in / 1216 out tokens · 51368 ms · 2026-06-25T22:02:35.296338+00:00 · methodology

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Reference graph

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