The research team focused on tension-type prestressed anchors, the workhorse of early slope reinforcement in China. Unlike compression anchors, whose grout stays squeezed and therefore rarely cracks, tension anchors place their grout in a state that invites cracking. Those cracks become highways for groundwater, oxygen, chloride, and sulfate ions to reach the steel tendon, which is itself held under high stress. The result is a particularly vicious form of deterioration: stress corrosion that can transform a ductile steel strand into a brittle one, quietly stripping away the load-carrying capacity that engineers counted on at design time.

To capture this decay mathematically, the researchers built on an electrochemical corrosion rate model originally developed by Hong, adapting it to anchor cables. Their equation links the corrosion rate to ambient temperature, relative humidity, the thickness of the grout cover wrapping the tendon, and the compressive strength of the grout itself. The physics is intuitive once unpacked: warmer conditions accelerate the electrochemical reactions through Arrhenius-type kinetics, moisture drives the cathodic half of the corrosion cell, and a thicker, stronger grout cover slows the ingress of aggressive species. Because fully enclosed anchors corrode far more slowly than those exposed to wet-dry cycling, the team applied a correction factor, effectively assuming the grout remains intact until corrosion products expand and crack it from within.

From this corrosion rate, the researchers derived time-varying expressions for three competing failure modes. The first is debonding at the mortar-rock interface, where the entire grout column pulls out of the borehole. The second is slippage at the tendon-grout interface, where corrosion products and chemical degradation of the cement matrix erode the bond that transfers load from steel to rock. The third is tensile yielding of the free segment, as the effective cross-section of the steel shrinks and its yield strength drops. Crucially, the anchorage system behaves as a series, or weakest-link, system: whichever of the three resistances is smallest at any given moment governs the anchor’s capacity, and failure of that single link means failure of the whole.

The study’s most striking mechanistic finding is that this weakest link does not stay put. In the early decades of service, the mortar-rock interface typically controls the anchoring force. But as corrosion progresses, expanding rust products crack the grout, allowing water, oxygen, chloride, and sulfate ions to flood the tendon surface. The tendon-grout bond then collapses far faster than the other resistances, and control of the anchoring force shifts to that degrading interface. In the case study, a twelve-meter anchor’s tendon-grout bond resistance plummeted from 2,714 kilonewtons to just 291 kilonewtons over a century, while the free segment’s yield resistance fell more gently from 1,316 to 1,033 kilonewtons. This mechanism transition is what produces the alarming acceleration of anchoring-force decay in the later service life.

To translate anchor degradation into slope-scale risk, the team embedded their time-dependent anchor capacities into a pseudo-static limit equilibrium framework based on the horizontal slice method and the Mohr-Coulomb strength criterion. Seismic demand was not applied uniformly: following the Chinese seismic design code for hydraulic structures, the horizontal acceleration coefficient was distributed trapezoidally along the slope height, rising from the toe to the crest to reflect the well-documented topographic amplification of ground motion. A genetic algorithm searched for the critical slip surface, and the First-Order Reliability Method, implemented through the JC algorithm with equivalent normalization of non-normal variables, computed failure probabilities at each service age. Peak ground acceleration was treated as a Gumbel-distributed extreme-value random variable, consistent with standard probabilistic seismic hazard practice.

The case study examined a 28-meter rock slope cut into homogeneous slate at 47 degrees, reinforced with seven rows of high-strength strands and analyzed under four seismic intensity levels ranging from 0.1 g to 0.3 g. The results trace a clear three-act drama. For the first forty years, failure probability stays essentially flat, governed mainly by seismic parameters rather than corrosion. Then, between roughly fifty and seventy years, the curves bend upward: under 0.1 g shaking, failure probability jumps from 8.3 percent to 17.5 percent, a 110 percent increase, while under 0.3 g it climbs from about 50.7 to 59.1 percent. In the final decades, the system enters what the authors call a high-risk state, with failure probability exceeding 38.5 percent at 0.1 g and reaching 67.8 percent at 0.3 g by year one hundred.

Perhaps the most counterintuitive insight concerns how seismic intensity and corrosion interact. The absolute gap in failure probability between the 0.3 g and 0.1 g scenarios over a century is about 29.3 percentage points, but the relative difference shrinks dramatically, from 7.7-fold to 1.8-fold. In other words, long-term corrosion accumulation erodes system reliability most severely in low-seismicity environments, where a corroded anchor system has little residual margin to absorb even modest shaking. The authors also benchmarked their results against Chinese code target reliability indices, finding that under a 0.2 g scenario the failure probability surpasses 40 percent after about 85 years, a reliability index below 0.25 that would be unacceptable for most structures, while early-life indices above 1.0 are only marginally adequate even for low-consequence applications.

The sensitivity analysis delivers a rare piece of genuinely actionable engineering guidance. Increasing the grout cover thickness over the tendon from 20 millimeters to 50 millimeters cuts the hundred-year failure probability by 42 percent, raising the reliability index from roughly 0.31 to 0.82. Thin covers show accelerated failure probability growth beginning around thirty years, whereas thicker covers delay that inflection to around fifty years. Conversely, every 10-degree-Celsius rise in average temperature or 20 percent rise in relative humidity boosts the failure probability growth rate by 15 to 25 percent, a sobering prospect for coastal and monsoon-climate regions and for a warming world. Among all parameters examined, the environmental corrosion coefficient exerts the strongest influence on reliability, followed by humidity and temperature, confirming that durability, not seismic demand alone, dominates long-term performance.

The authors are candid about the limits of their framework. They assume uniform rather than pitting corrosion, treat corrosion across the anchor array as statistically independent rather than spatially correlated, and model fully saturated conditions without the strengthening effect of matric suction in unsaturated soil, all choices that make their baseline estimates conservative. Even so, the practical implications are hard to ignore. The fifty-year inflection point offers a quantitative target for maintenance scheduling, pointing to interventions such as secondary grouting or anchor supplementation well before reliability collapses. For major projects with design lives beyond half a century, the study argues, durability design must incorporate environmental acceleration factors and hygrothermal coupling, and slopes in hot, humid regions with inadequate grout cover may fall below acceptable reliability thresholds well within their intended service life. What emerges is a clear message for infrastructure owners: the anchors holding back the world’s slopes are aging on a schedule that static, time-invariant safety checks were never designed to see.

Subject of Research: Time-variant reliability analysis of corroded prestressed anchor-reinforced slopes under seismic loading

Article Title: Time-variant reliability of anchored slopes considering coupled corrosion and seismic effects

Article References: Wang, H., Meng, X., Yang, X., Jia, J., Tu, B., Zhang, L., Pan, X., & Han, Q. (2026). Time-variant reliability of anchored slopes considering coupled corrosion and seismic effects. Discover Geoscience, 4(1), Article 339. https://doi.org/10.1007/s44288-026-00700-5

Image Credits: AI Generated

DOI: 10.1007/s44288-026-00700-5

Keywords: anchored slopes, prestressed anchors, corrosion, seismic loading, time-variant reliability, slope stability, geotechnical engineering, failure probability, grout cover thickness, limit equilibrium method, FORM, infrastructure durability

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