Solving for Atmospheric Corrosion Rates Using ISO 9223
Author:Tom Hayden, CTO, Engineering Director, Inc.
Industry Context and Standards
In the fields of protective coatings and corrosion management, two ISO standards serve as foundational references: ISO 12944-2 and ISO 9223.
ISO 12944-2 guides the selection of coating systems based on the environmental conditions a structure is exposed to.
ISO 9223, on the other hand, offers a structured framework for classifying and quantifying atmospheric corrosivity—a critical metric for estimating material degradation and service life.
ISO 9223 defines corrosivity categories from C1 (very low corrosivity) to CX (extreme corrosivity). These categories are determined using a dose-response function based on four key environmental parameters:
T – Ambient temperature
RH – Relative humidity
S_d – Chloride (sea salt) deposition
P_d – Sulfate (SO₂) deposition
This classification system forms the backbone of standardized corrosion risk assessments and plays a key role in material selection, asset protection planning, and lifecycle management across multiple industries. Building on this framework, our platform applies ISO 9223’s methodology to deliver actionable corrosion rate estimates across multiple materials and environmental conditions.
Corrosion Rate Estimation Using ISO 9223 Methodology
Our platform applies ISO 9223 principles to estimate corrosion rates (μm/year) for Steel, Zinc, Copper, and Aluminum, leveraging annually updated five-year trailing averages of environmental data. This approach provides a robust and standardized basis for assessing material performance under atmospheric exposure.
While steel and zinc underpin the ISO corrosivity categories due to their predictable response to environmental factors, copper and aluminum behave differently. Their corrosion rates are highly influenced by factors such as alloy composition, surface finish, and microclimatic conditions. As such, they are not used in ISO 9223 to define corrosivity categories, though the standard does provide reference corrosion rates and dose-response formulas for these materials as contextual guidance.
To enhance predictive accuracy, our model incorporates additional environmental variables, including:
Time of Wetness (TOW)
Wind speed and directional resultant (0°–360°)
Terrain ruggedness index
Bathymetric and coastal proximity indicators
By combining ISO-based methodology with expanded environmental intelligence, our platform supports engineers, asset managers, and corrosion professionals in making strategic decisions for coating selection, maintenance planning, and infrastructure lifecycle management.
ISO 9223 Support
Challenges in Real-World Implementation
Despite the value of ISO classifications, few facilities know their true corrosivity category. This is largely due to the difficulty of collecting site-specific data. As a result, stakeholders often overdesign coating systems to extend service life—an expensive and inefficient approach.
To address this, we developed a method that leverages global public datasets to estimate ISO 9223 classifications. This offers a practical and sustainable way to assess environmental conditions and recommend coatings according to ISO 12944-2.
Introducing the ISO 9223 Global Corrosion Map
Since 2019, we've worked on building a high-resolution, global map of atmospheric corrosivity. Initially, we had reliable temperature and humidity data, good sulfate data from the EPA, but limited chloride deposition data—particularly via ISO 9225's Wet Candle Method. Most of what we had came from the respected ISOCORRAG publication.
We have updated the chlorides for our 2024 model to include:
- Wind Power Density Multiplier — capturing high-energy wind effects using a cubic relationship to chloride transport.
- Wind Resultant (0° to 360°) - Vector-averaged wind direction. Used as a summary, though it may not capture seasonal shifts in wind patterns.
- Air Density Adjustments — accounting for changes in elevation and temperature using dynamic air density factors.
- Bathymetry Integration — combining onshore elevation and offshore depth for precise coastal and inland decay modeling.
- Refined Distance-to-Coast Decay — applying steeper PM10-style decay near the coast and PM2.5-style gradual decay inland.
- Terrain Ruggedness Adjustment — using the Ruggedness Index (RIX) with reduced penalties for complex landscapes.
- Humidity and Temperature Modifiers — reflecting chloride solubility, evaporation, and deposition near water bodies.
- Solar Radiation Sensitivity — enhancing deposition predictions in regions with high evaporation potential.
- Sulfate Amplification — boosting deposition values in areas with industrial sulfate emissions.
- Peninsula Detection Logic — adding a 2.5x multiplier in areas with negative coastal distances, low elevation, moderate temperatures, and high humidity.
- Dynamic, Uncapped Multiplier Stack — removing the artificial cap to reflect extreme conditions, with a floor to prevent underestimation.
- ArcGIS Compatibility — fully structured to run in ArcGIS Pro using field calculator scripts with fallback defaults for missing data.
These enhancements result in more accurate, high-resolution predictions for corrosion risk management, infrastructure design, and environmental planning, especially in coastal, industrial, and complex terrain zones.
Final Product and Application
By integrating our chloride, sulfate, temperature, and humidity datasets into the ISO 9223 function, we generate complete global corrosivity classifications. This data feeds both a digital platform and a printed wall map.
📌 Want a map? Contact us—we’ll project you a link to download our most current Map ready to take to the printer!
🧪 Try it yourself: Use our “Score Your Facility” tool www.iso9223.com enter an address or coordinates, and receive the corrosivity classification instantly.
Environmental Shifts Over 30 Years
One of the most dramatic changes since ISO 9223 was introduced has been the reduction of airborne sulfates in North America. Coal-fired power plants have been phased out or modernized with scrubbers, reducing SO₂ emissions significantly.
Example:
Chicago's classification dropped from C4 to C3, and may soon reach C2, due to reduced sulfate sources. See the EPA’s animated data showing this trend—a clear environmental win in the post–acid rain era.
Reduction in SO₂ emissions (USA)
The increased use of natural gas and renewable energy sources, such as wind and solar, has reduced the need for coal-fired power plants, which are among the largest sources of SO2 emissions. This shift has helped to reduce the overall emissions from the energy sector, which accounted for 80% of total SO2 emissions in 1990.
Furthermore, advancements in technology and pollution control equipment have also helped to reduce SO2 emissions. For example, flue gas desulfurization (FGD) systems, which are designed to remove SO2 from power plant emissions, have become more widespread and effective in recent years. These systems have allowed power plants to reduce their emissions without sacrificing energy generation.
Climate Change Considerations
As global climate patterns shift, so too do ISO variables like temperature and humidity. The original ISO 9223 response functions may need to be revisited under these new conditions.
🎓 Are you researching this? We’d love to collaborate. Contact us or connect with Tom Hayden on LinkedIn.
Limitations: Excessive Localized Emissions (ELE)
Our model best suits ambient environments. It may not apply where Excessive Localized Emissions occur—examples include:
Proximity to outdated coal-fired power plants
Urban zones with concentrated chloride sources (e.g., road salts, bird waste)
These require local measurements or alternate modeling approaches. See CBOT™ IoTSensor for localized corrosion rate measurements.
Data Sources
We use the following sources to build our atmospheric corrosivity platform:
Global Wind Atlas (v3) – High-res wind speed data
NOAA Integrated Surface Hourly – Temperature and humidity via GitHub-based pipeline
EPA National Atmospheric Deposition Program (NADP) – Gold-standard for wet SO₂ deposition
NASA Giovanni & MERRA-2 – Chloride and sulfate reanalysis datasets
Contact
Tom Hayden
Chief Technology Officer, Engineering Director, Inc.
📧 Email: thayden@engineeringdirector.com
🔗 LinkedIn:Connect with Tom
