This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Localized pitting corrosion can cause sudden failure in aluminum components even when overall corrosion rates are low. Steam coating offers a water-vapor-based route to protective boehmite films, but processing simultaneously changes film thickness, surface morphology, crystallinity and substrate defects.
These intertwined changes make it difficult to determine which physical features govern corrosion resistance and how coatings should be optimized. Addressing this challenge, a research team led by professor Takahiro Ishizaki and master's student Kei Masuhara from Shibaura Institute of Technology (SIT) in Japan used an interpretable machine-learning framework to examine 90 steam-coated A6061-T6 aluminum specimens. Four descriptors—film thickness (FT), surface morphology (SQ), crystallite size (CS) and substrate dislocation density index—were analyzed using random forest, Shapley additive explanations and accumulated local effects.
Their findings were published online in the journal npj Materials Degradation on Aug. 18, 2026. The researchers assessed pitting resistance by measuring the pitting potential—the voltage at which localized corrosion begins—in a 5 wt.% sodium chloride solution at room temperature. Higher values indicate greater resistance to pitting under these test conditions.
Surface morphology was quantified as the root-mean-square surface height, a measure of surface roughness denoted as SQ. "We wanted to understand which features of the coating most strongly affect its ability to prevent pitting corrosion," says Ishizaki. "Our analysis showed that the importance of these features changes as the coating grows." The machine-learning model predicted corrosion resistance more accurately than a model based only on coating temperature and treatment time.
The analysis identified surface roughness and film thickness as the leading descriptors in the model. For specimens with SQ values of approximately 600–1,080 nm, the modeled interaction between roughness and thickness changed from positive to negative near a film thickness of 2,200 nm. This reversal concerns the interaction between the two descriptors, rather than the overall corrosion resistance of every coating above or below that thickness.
The same reversal was not observed in the lower-roughness range. The researchers interpret this pattern as a possible change in what surface roughness represents during film growth. In thinner coatings, roughness may reflect the development of protective coverage.
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