Steel Rusts Slower When Sea Salt Falls Hardest, Ecuador Coast Study Finds

On a windswept stretch of Pacific coastline in Crucita, Ecuador, a team of corrosion scientists has spent a full year measuring something most people never think about: how fast the salt in the sea air eats away at ordinary steel. Their findings, published in Case Studies in Construction Materials, upend a common assumption in atmospheric corrosion science. At six outdoor exposure sites, all placed between 15 and 25 meters from the waterline, the researchers discovered that months with the heaviest chloride deposition did not produce the fastest rusting of low-carbon steel. Instead, the relationship ran in the opposite direction, a counterintuitive result that could reshape how engineers design buildings and infrastructure along tropical coasts.

Low-carbon steel, containing no more than 0.2 percent carbon, is the workhorse metal of the global construction industry, prized for its mechanical strength and low cost. But it is also notoriously vulnerable to atmospheric corrosion, the electrochemical process by which thin films of moisture on a metal surface react with oxygen and dissolved salts to form rust. In coastal zones, airborne chloride particles delivered by breaking waves dramatically accelerate this process, which is why seaside structures so often deteriorate decades before their inland counterparts. Ecuador’s coastline, with enormous building potential and a historically sparse network of corrosion monitoring stations, offered the researchers an ideal natural laboratory to quantify just how aggressive such an environment can be.

The team, led by Juan Carlos Guerra and Francisco Corvo, established six exposure sites along roughly a kilometer of shoreline in the Manabí province, each fitted with wooden racks mounted three meters above the ground and angled at 45 degrees toward the prevailing west-northwest wind. At each site, dry cloth devices captured airborne salt for monthly chloride measurements, while polished steel coupons, degreased in n-hexane and pickled in hydrochloric acid, were exposed to track mass loss over time. Meteorological data on relative humidity, temperature, and wind speed came from the nearby Portoviejo station operated by Ecuador’s National Institute of Meteorology and Hydrology. The study ran from July 2021 through June 2022, spanning both the dry and rainy seasons of Ecuador’s coastal climate.

The environmental readings painted a picture of remarkable atmospheric stability. Relative humidity averaged 82 percent with a coefficient of variation of just 2.7 percent, temperature held near 25 degrees Celsius, and wind speed averaged 1.59 meters per second. That persistent humidity is critical: it means a water film essentially never dries from metal surfaces, sustaining the wetness conditions under which corrosion proceeds continuously. The moderate temperature, hovering around the level at which stable electrolyte films form most readily, combined with high humidity to create nearly ideal conditions for electrochemical attack. Yet the low variability of these parameters may itself have tempered the corrosion rates, since other studies have linked strong temperature fluctuations to higher corrosivity.

The chloride numbers were striking. Annual average deposition rates measured by the dry cloth devices exceeded 126 milligrams per square meter per day, which converts to more than 300 milligrams per square meter per day under the wet candle equivalent used by the ISO 9225 standard. That places the atmosphere firmly in the S3 category, the highest chloride deposition class, spanning 300 to 1500 milligrams per square meter per day. No previous corrosion study in Ecuador’s coastal zones, whether from the last century or recent work across Manabí’s cantons, had reported monthly chloride values this high. Deposition also spiked during the January-to-April rainy season, adding a seasonal signature to the salt record.

Multiple linear regression analysis revealed which meteorological forces drove the salt delivery. Relative humidity emerged as the dominant factor, with positive correlation coefficients above 0.61 and statistically significant p-values at every site. The researchers attribute this to the formation of larger, heavier salt particles in saline solution form within the breaking wave zone, which settle at very short distances from the coastline. Wind speed mattered too, but only above a threshold. The data suggested a critical wind speed of roughly 1.5 meters per second, below the transition between what the modern Beaufort scale calls a Ventolin and a Weak Breeze. That threshold is among the lowest ever recorded worldwide; comparable studies in Brazil, Australia, Spain, and Cuba found thresholds near 3 meters per second, while sites in Russia and Bangladesh reported values above 5.5 meters per second. The implication is sobering: even gentle onshore winds in Crucita are enough to load the air with corrosive salt.

Then came the surprise. When the team correlated monthly chloride deposition with monthly corrosion rates, the relationship was consistently negative, with correlation coefficients as strong as −0.96 and p-values well below 0.05 across all six sites. Linear models fitted to the data, such as the averaged equation relating corrosion rate to chloride deposition with a coefficient of determination of 78 percent, confirmed that months with less salt deposition saw more steel loss. The proposed explanation lies in oxygen chemistry. When chloride concentrations in the aqueous layer on the steel surface become extremely high, the solubility of dissolved oxygen drops, suppressing the cathodic oxygen reduction reaction, the essential electron-accepting step that drives atmospheric corrosion. In essence, the salt load becomes so concentrated that the corrosion reaction is partially starved of the oxygen it needs.

This mechanism, though statistically grounded here, echoes earlier observations elsewhere. A study at the Cabo Vilano wind farm in Spain found higher steel corrosion at a sheltered site with lower salt deposition than at an unsheltered one, and laboratory work has shown that corrosion of low-carbon steel in immersion conditions decreases once sodium chloride concentrations exceed about 10 grams per liter. The researchers are careful to note that their interpretation rests on statistical analysis alone; electrochemical techniques such as polarization curves, impedance spectroscopy, and direct dissolved oxygen measurements would be needed for definitive confirmation. Still, the consistency of the inverse relationship across all six sites lends the finding considerable weight.

Scanning electron microscopy of the rust layers added further texture to the story. The one-year corrosion products revealed amorphous ferrihydrite, grass-like lepidocrocite, hummock-shaped maghemite, and ring-shaped magnetite, the latter two forming under oxygen-deficient, chloride-rich conditions and driving cracking and exfoliation of the oxide layers. Energy-dispersive X-ray spectroscopy confirmed elevated chloride and oxygen mass fractions in many samples, and numerous embedded salt particles were visible across most specimens. Notably absent was akaganeite, a phase often found in severely chloride-contaminated rust, which the authors attribute to the lack of the accelerated internal drying that elevated temperatures would normally provide. Bird-beak-shaped feroxyhyte and rosette-shaped goethite completed the mineralogical inventory, a suite consistent with a high-salinity tropical atmosphere.

For engineers, the practical takeaway is a formal corrosivity classification. All six exposure sites earned a Very High (C5) rating for low-carbon steel, confirmed visually by the deteriorated specimens, and ISO-based modeling predicted that this classification would hold across exposure periods from 2 to 20 years. That is paradoxically less severe than the Extreme (CX) category the team had anticipated given the sites’ extreme proximity to the surf, likely for the same oxygen-solubility reasons that suppressed monthly corrosion peaks. Previous Ecuadorian studies had capped out at C4 or found C5 and CX only at isolated near-shore locations. By demonstrating that the highest salt loads do not automatically translate to the fastest metal loss, the Crucita study gives coastal planners in Ecuador and similar tropical environments a more nuanced, and ultimately more reliable, foundation for specifying materials and protection strategies before the first concrete is poured.

Subject of Research: Atmospheric corrosion of low-carbon steel and chloride deposition in a high-salinity tropical coastal environment

Article Title: Chloride deposition and atmospheric corrosion behavior of low-carbon steel in a high-salinity tropical coastal environment

Article References: Guerra, J. C., Corvo, F., Pech, I., Gorety, M., Castañeda, A., & Bastidas-Arteaga, E. (2026). Chloride deposition and atmospheric corrosion behavior of low-carbon steel in a high-salinity tropical coastal environment. Case Studies in Construction Materials, 25, Article e06546. https://doi.org/10.1016/j.cscm.2026.e06546

Image Credits: AI Generated

DOI: Not provided

Keywords: atmospheric corrosion, low-carbon steel, chloride deposition, coastal environment, Ecuador, corrosivity category, relative humidity, wind speed threshold, rust morphology, ISO 9223, marine aerosols, infrastructure durability