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Coating thickness can change whether surface roughness is associated with better or worse pitting resistance. A 2026 study of steam-grown boehmite on A6061-T6 aluminum found that, for specimens with root-mean-square roughness (SQ) of about 600–1,080 nanometers, the modeled interaction between roughness and film thickness changed sign near a thickness (FT) of approximately 2,200 nanometers. Below and above that point, the model associated the same roughness range with different interaction behavior.
This is not a universal 2,200-nanometer limit, and it does not show that every thicker coating resists pitting less effectively. The result applies to the studied aluminum, boehmite process, salt solution, measurement definitions and statistical model. It shows why roughness cannot be interpreted without thickness and the rest of the coating system.
What the study actually found
Researchers led by Professor Takahiro Ishizaki at Shibaura Institute of Technology analyzed 90 steam-coated A6061-T6 aluminum specimens. The coating was a boehmite film grown with water vapor. Their report was published online in npj Materials Degradation on 18 August 2026.
The team modeled pitting resistance using four descriptors:
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- Film thickness (FT): the thickness of the boehmite layer.
- Surface morphology (SQ): root-mean-square surface height, used as the roughness measure.
- Crystallite size (CS): a descriptor of the film’s crystal structure.
- Substrate dislocation-density index (FS): an index representing defects in the aluminum substrate.
They used random-forest modeling, SHAP (Shapley additive explanations) and accumulated local effects to examine how those descriptors related to pitting potential. In the higher-roughness group, approximately SQ 600–1,080 nm, the modeled roughness–thickness interaction changed from positive to negative near FT 2,200 nm. The lower-roughness group did not show the same reversal.
Professor Ishizaki described the objective this way: “We wanted to understand which features of the coating most strongly affect its ability to prevent pitting corrosion,” and “Our analysis showed that the importance of these features changes as the coating grows.”
The key result is therefore conditional: thickness altered the modeled role of roughness in one roughness range. It was not a finding that thickness alone determines corrosion performance.
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The response variable was pitting potential, measured in a 5 wt.% sodium chloride solution at room temperature. Under those test conditions, a higher pitting potential indicates greater resistance to pit initiation. The reported roughness values are SQ, an RMS surface-height metric; they are not a generic statement about every possible roughness parameter.
These definitions matter when applying the result. A value measured with a different roughness parameter, electrolyte concentration, temperature, potential-scan method or substrate cannot be placed directly on the same 2,200-nm relationship without checking comparability.
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| Study element | Reported condition | Why it limits interpretation |
|---|---|---|
| Substrate | A6061-T6 aluminum | Alloy composition, temper and defects influence pit initiation and film growth. |
| Coating | Steam-grown boehmite | Its chemistry and structure differ from polymer, conversion, ceramic and vapor-deposited coatings. |
| Specimens | 90 | The model describes this sample set; it is not a universal population threshold. |
| Roughness metric | RMS surface height, SQ | Profile shape, spacing and skew can matter even when SQ is similar. |
| Reversal range | SQ approximately 600–1,080 nm | The sign change was not reported across all roughness values. |
| Thickness near sign change | FT approximately 2,200 nm | This is a modeled interaction location, not a design limit. |
| Corrosion test | 5 wt.% NaCl, room temperature | Other electrolytes and temperatures can produce different rankings. |
What “the interaction changed sign” means
In a model with both roughness and thickness, an interaction describes whether the effect associated with one variable depends on the level of the other. A positive interaction does not simply mean “rougher is better,” and a negative interaction does not simply mean “rougher is worse.” The sign says how the modeled contribution of roughness changes as thickness changes.
For the higher-SQ specimens, the model found one relationship on one side of roughly 2,200 nm and the opposite interaction behavior on the other side. That does not identify a sharp physical boundary in every specimen. It identifies a region where the fitted relationship between two descriptors changed direction.
The distinction is important because overall pitting resistance can still vary with crystallite size, substrate condition, coating defects and other factors. A specimen above 2,200 nm is not automatically less resistant than one below 2,200 nm. The study did not establish such a ranking.
Why thickness could alter roughness effects
The authors offer a mechanism that is plausible but not yet proven experimentally.
Earlier growth: roughness may track protective coverage
At lower film thickness, surface features may reflect how the boehmite layer develops over the aluminum. In this interpretation, some roughness could accompany more complete or more protective coverage. A rough profile might indicate growth that has filled or covered vulnerable areas rather than simply exposing larger peaks and valleys.
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Greater growth: irregularities may create transport paths
As the film becomes thicker, the same roughness measure may instead be associated with structural irregularities. Valleys, pores, cracks or poorly covered regions could provide longer or more tortuous paths for water and chloride ions. If corrosive species reach the metal through those paths, local defects can become sites for pit initiation or for corrosion spreading beneath the film.
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This coverage-to-irregularity explanation remains a hypothesis. The report explicitly says that further experimental testing is needed to establish causality. The statistical reversal alone cannot prove that a particular valley, pore or interface path caused a pit.
Why roughness alone is an unreliable corrosion rule
Roughness affects several competing properties at once. A rough surface can increase mechanical interlocking and support adhesion, but it can also make cleaning difficult and leave peaks or valleys incompletely covered. Corrosive attack can proceed through the coating itself or laterally along the metal–coating interface after a defect exposes the substrate.
A review covering multiple coating systems describes both routes and warns that roughness effects are not straightforward. Its examples include polymer barriers, so they provide context rather than proof of the boehmite result.
A 2019 study of machined and epoxy-coated steel likewise found that roughness magnitude alone did not explain coating stability. Profile shape and mechanical interlocking were important. Those steel-and-epoxy results should not be transferred directly to steam-coated aluminum.
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A separate 2026 study of CrAlN-coated H13 steel found that reducing substrate roughness improved corrosion indicators under its test conditions while reducing measured coating critical load. That trade-off illustrates why corrosion protection and adhesion cannot be inferred from the same roughness trend in every material system.
How the modeling improved on a process-only view
The report says the descriptor-based model predicted corrosion resistance more accurately than a model using only coating temperature and treatment time. Its reported root-mean-square error was 0.292 V. A Monte Carlo analysis estimated that uncertainty in measuring the descriptors contributed approximately 0.080 V to prediction variability.
Those figures characterize this model and dataset. The 0.292-V RMSE is not a general corrosion uncertainty, and the 0.080-V estimate is not a universal instrument error for roughness or thickness measurements. They show why measured film and substrate characteristics can add predictive information beyond nominal process settings, while also showing that measurement uncertainty remains material.
The use of SHAP and accumulated local effects helped the researchers examine changing feature importance and interactions instead of treating each descriptor as having one fixed, independent effect. Such tools can reveal where a model’s behavior changes, but they do not replace controlled experiments for proving a mechanism.
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Use the following sequence before transferring a roughness–thickness result to another part or coating:
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- Match the substrate. Check alloy, temper, heat treatment and defect state. A6061-T6 is not interchangeable with another aluminum grade or with steel.
- Match the coating chemistry and process. Steam-grown boehmite, epoxy, CrAlN and other films have different growth mechanisms, porosity and failure modes.
- Match the roughness definition. Confirm that SQ means RMS surface height and record the measurement area, direction, filtering and scale. Similar SQ values can hide different peak spacing or valley shapes.
- Match thickness and its uncertainty. Record the actual FT distribution rather than only the nominal process target. A sign change in a model is meaningful only within the thickness range represented by the data.
- Match the corrosion environment. Compare salt concentration, temperature, exposure time and electrochemical protocol. Pitting potential in 5 wt.% NaCl at room temperature is not equivalent to service exposure in another electrolyte.
- Check validation. Look for replicate specimens, held-out predictions, uncertainty analysis and direct microscopy or electrochemical evidence. A fitted interaction should be tested independently before becoming a manufacturing rule.
Within the Shibaura study, crystallite size and the substrate dislocation index were also included. The report notes a region-dependent interaction between roughness and crystallite size, another reason not to reduce performance to two numbers alone.
Practical implications for coating development
Do not set a universal maximum thickness from 2,200 nm
The reported value is near the modeled interaction change for one roughness range on one boehmite-coated alloy. It is not a specification that all boehmite films, all aluminum alloys or all protective coatings should remain below.
Measure the surface after coating
Substrate roughness and finished-film roughness are not necessarily the same. Growth can smooth, replicate or amplify features. Measuring only the untreated substrate can miss the morphology that chloride actually encounters.
Pair topography with cross-sectional evidence
Confocal laser scanning microscopy, X-ray diffraction and cross-sectional electron microscopy were among the characterization methods mentioned in the report. Their roles differ: surface microscopy describes topography, diffraction informs crystal structure, and cross-sectional imaging can reveal thickness and internal defects. The methods are characterization approaches, not endorsements of particular equipment.
Test around, not only at, a target thickness
If a process is being optimized, include several thickness levels below and above the proposed transition and several roughness bands. Measure pitting potential with replicates and inspect the failure sites. This can determine whether an apparent interaction persists or was specific to a narrow sample distribution.
What remains unknown
- Whether the approximately 2,200-nm interaction location repeats with independent A6061-T6 batches.
- Whether the same reversal occurs with different steam-treatment temperatures, times or water-vapor conditions.
- Which physical defects, if any, connect higher-thickness roughness to chloride transport.
- Whether the pattern persists under cyclic exposure, different chloride concentrations or elevated temperature.
- How profile shape, wavelength, skewness and coating porosity contribute when SQ is held constant.
- Whether the model transfers to other alloys, substrates or coating chemistries.
Answering those questions requires experiments that deliberately vary thickness and morphology, then connect model predictions with microscopy, transport measurements and electrochemical results.
Bottom line for engineers and readers
Surface roughness does not have one fixed effect on pitting corrosion. In the 2026 study of steam-grown boehmite on A6061-T6 aluminum, the roughness–thickness interaction changed sign near 2,200 nm for SQ values of about 600–1,080 nm, while the lower-roughness range did not show that reversal. The result is best used as a warning against single-variable rules: interpret roughness together with film thickness, profile structure, coating chemistry, substrate condition and the corrosion test. The proposed shift from protective coverage to defect-related transport is promising, but it remains a hypothesis awaiting experimental validation.
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