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

Every beach on Earth carries a faint trace of the planet’s geological origins. Primordial radionuclides, including uranium-238, radium-226, thorium-232 and potassium-40, have existed since the formation of the Earth and are incorporated into the minerals that make up sand, rock and soil. In most coastal environments, these isotopes occur at modest concentrations that pose little concern. But where heavy minerals accumulate, sands can become strikingly radioactive. The Visakhapatnam coast, with its complex geology and active sediment transport, proved to be exactly such a place, and measuring it required laboratory instrumentation of the highest sensitivity.

The research team collected beach sand samples from locations along the coastline and analysed them using a high-purity germanium gamma spectrometry system, a technique prized for its ability to resolve the characteristic gamma-ray signatures emitted by individual radionuclides. High-purity germanium detectors, cooled to reduce thermal noise, can distinguish the faint energy lines of uranium, radium, thorium and potassium decay chains from the background of ambient radiation, allowing precise quantification of each isotope’s activity concentration in units of becquerels per kilogram. The researchers also measured ambient radiation levels directly on the beaches to connect their laboratory findings to real-world exposure conditions.

The results were remarkable. Activity concentrations of uranium-238 ranged from 8.9 to 868 becquerels per kilogram, with a mean of 167. Radium-226 spanned from 2.8 to 1,282.8, averaging 292. The most dramatic findings concerned thorium-232, which ranged from 6.9 to a staggering 13,691.7 becquerels per kilogram, with a mean of 2,614, while potassium-40 ranged from 77.2 to 1,191.1 with a mean of 449. Each of these mean values exceeds the global averages reported for ordinary soils and sediments, and the thorium figures in particular place the most enriched samples firmly in the territory of monazite-bearing sands, the mineral that has made parts of the Indian coastline famous among radiation scientists for decades.

Thorium’s dominance in these sands is no accident. Monazite, a phosphate mineral rich in thorium and rare earth elements, is a hallmark of India’s eastern and southwestern coastal placer deposits. When waves and longshore currents sort sediments by density and grain size, heavy resistant minerals such as monazite, zircon and ilmenite become concentrated into placer deposits, while lighter quartz is winnowed away. Because thorium and its decay products are locked into monazite’s crystal lattice, these deposits act as natural concentrators of radioactivity. The measured absorbed gamma dose rates on the Visakhapatnam beaches, the study found, are comparable to those in monazite-rich high background radiation areas in India and worldwide, confirming that the same mineralogical processes are at work here.

One of the study’s most valuable contributions is its mapping of spatial variability. Rather than presenting a single average figure, the researchers generated spatial distribution maps that visualize the heterogeneity of radionuclide concentrations along the entire study area. These maps revealed a clear pattern: radioactivity levels rise consistently toward the northern sector of the coastline. This gradient likely reflects the interplay of sediment sources, wave-driven transport directions and the geomorphology of the coast, which ranges from sandy beaches to rocky headlands between Visakhapatnam and Bhimunipatnam. The authors also observed that radioactivity varied across the width of individual beaches, meaning that a visitor standing at the waterline may be on sand with a different radiological character than sand collected near the dunes or backshore.

To translate raw activity concentrations into statements about human risk, the team computed a battery of internationally recognized radiological hazard indices. These included radium equivalent activity, which combines the contributions of the uranium and thorium series and potassium into a single comparable quantity; the absorbed gamma dose rate, which estimates the energy deposited in air per unit time; the annual effective dose equivalent, which converts that dose into a projection of yearly health-relevant exposure; the gamma level index; and the external and internal hazard indices used in building-material and land-use assessments. Together, these indices allow the Visakhapatnam data to be compared directly with safety benchmarks established by bodies such as the United Nations Scientific Committee on the Effects of Atomic Radiation and the International Commission on Radiological Protection.

The researchers went beyond measurement and mapping by applying statistical analyses to the relationships among the radionuclides themselves. Correlation analysis indicated a strong relationship between the radium series and the thorium series, a signature that the authors interpret as evidence of a common geological origin for the two decay chains. This makes geochemical sense: both uranium and thorium series isotopes tend to reside in the same heavy mineral phases, so their concentrations rise and fall together as those minerals are concentrated or diluted in the sand. Such correlations transform a set of isolated numbers into a coherent narrative about where the sand comes from and how the coast has sorted it over time.

It is important to keep the findings in perspective. Natural radioactivity in beach sand is not the same as contamination from human activity; these radionuclides are part of the Earth’s crust and have always been present. Millions of people worldwide live safely in high background radiation areas, including the well-studied monazite sands of Chhatrapur in Odisha and regions of Brazil, China and Kerala. What the Visakhapatnam study provides is a rigorous, spatially resolved baseline against which any future changes, whether from coastal engineering, sediment redistribution, or industrial development, can be judged. Baseline data of this quality are the foundation of long-term environmental monitoring and credible radiological risk assessment in coastal ecosystems.

The study also carries a broader message about how science sees ordinary landscapes. A beach that appears uniform to the eye is, to a gamma spectrometer, a patchwork of geological history written in decaying atoms. By combining sensitive nuclear instrumentation, spatial mapping and statistical interpretation, the researchers have shown that radioactivity along a single coastline can vary by three orders of magnitude within short distances. For coastal managers, public health authorities and the millions who visit these shores, the work of Ramesh and his colleagues offers both a caution about localized hotspots and a model of how careful, transparent measurement can turn invisible hazards into mapped, understood and monitorable features of the environment.

Subject of Research: Natural radionuclide distribution and radiological hazard assessment in beach sands of the Visakhapatnam coast, India

Article Title: Spatial distribution of natural radionuclides and radiological hazard assessment in beach sands of the Visakhapatnam coast, Eastern India

Article References: Ramesh, B., Savitri, P. P., Sudhakar, J., Kumar, R. B., Sahoo, S. K., Saradhi, I. V., & Pulhani, V. (2026). Spatial distribution of natural radionuclides and radiological hazard assessment in beach sands of the Visakhapatnam coast, Eastern India. Environmental Monitoring and Assessment, 198(10), Article 1113. https://doi.org/10.1007/s10661-026-15926-6

Image Credits: AI Generated

DOI: 10.1007/s10661-026-15926-6

Keywords: natural radioactivity, gamma spectrometry, beach sand, thorium-232, radium-226, monazite, Visakhapatnam, radiological hazard, spatial distribution, coastal environment, high background radiation, Environmental Monitoring and Assessment