Tag: Corrosion Testing

Why ASTM G50 Should Be the Gold Standard for Atmospheric Corrosion Testing

When evaluating how metals and alloys will perform in real-world conditions, choosing the right test method is crucial. While accelerated laboratory tests have their place, the superior method for determining true, long-term atmospheric corrosion resistance is ASTM G50: Standard Practice for Conducting Atmospheric Corrosion Tests on Metals.

This testing helps product manufacturers, metallurgical companies, and end users understand the wear and corrosion behavior of specified materials.

What is ASTM G50?

ASTM G50 is a comprehensive standard that outlines the general procedures and conditions for exposing metal panels and alloys to real-world weather and atmospheric conditions.

• Objective: To evaluate the corrosion resistance of metals when exposed to the weather and to assess the relative corrosivity of the atmosphere at specific testing locations.
• Methodology: It involves placing test panels in defined outdoor exposure sites to accumulate data for comparison purposes.
• Key Consideration: Recognizing the highly variable and complex nature of the atmosphere (weather effects, industrial factors, natural elements), the standard recommends a multi-year exposure period to minimize the influence of short-term variations and provide a reliable assessment.
• Quality Control: It mandates the use of control specimens—materials with established weathering characteristics—to ensure the test results are meaningful and comparable.

The Real-World Relevance of ASTM G50

The most significant advantage of ASTM G50 is its direct correlation to how a material will actually perform in service.

• True Corrosivity Data: By exposing materials to actual rain, humidity, temperature cycles, UV light, pollution, and natural salt deposits, the test captures the synergistic effect of these variables. This provides data that is inherently more relevant than data from conditions simulated in a controlled environment.
• Long-Term Performance: The test accounts for seasonal variability and requires multi-year exposure. The results are highly reliable for predicting a material’s long-term durability and lifespan in a specific atmospheric environment.
• Material Selection and Site Evaluation:
  • It helps determine the most suitable metal or protective coating for products used outdoors (e.g., in construction, automotive, or infrastructure).
  • It can also be used to evaluate and compare the inherent corrosivity of different geographical locations before large-scale projects are built.

Why ASTM G50 is Superior to ASTM B117 (Salt Spray)

When the goal is to predict real-world atmospheric performance, ASTM G50 offers a level of fidelity that the commonly used ASTM B117, Standard Practice for Operating Salt Spray (Fog) Apparatus, cannot match.

While ASTM B117 (often called the salt fog or salt spray test) is a valuable tool for quality control and detecting gross material defects, it is fundamentally an accelerated laboratory test with severe limitations for performance prediction.

Feature	ASTM G50 (Atmospheric Corrosion)	ASTM B117 (Salt Spray/Fog)
Test Environment	Real-world weather (rain, sun, temperature cycles, natural contaminants).	Controlled, high-humidity fog of 5% neutral salt solution at constant temperature.
Relevance	Highly correlated with real-world, long-term outdoor performance.	Seldom correlated with performance in natural environments when used as stand-alone data.
Test Duration	Typically multi-year (e.g., 1, 3, 5 years).	Typically measured in hours or days (e.g., 24, 100, 1000 hours).
Purpose	Evaluates true corrosion resistance and atmospheric corrosivity.	Provides relative corrosion resistance information for comparison in a given test chamber.
Limitation	High variability if exposure time is too short.	Poor predictability for long-term outdoor use without corroborating field data.

As the ASTM B117 significance section notes, “Prediction of performance in natural environments has seldom been correlated with salt spray results when used as stand-alone data.” Furthermore, correlation should be considered “only in cases where appropriate corroborating long-term atmospheric exposures have been conducted.”

The ultimate “corroborating long-term atmospheric exposure” is, in essence, an ASTM G50-style test. The salt spray test simply does not replicate the complex, cyclic drying/wetting, UV exposure, and contamination effects that occur naturally. It creates a highly specific, controlled environment, which is why its results often fail to translate to the real-world performance engineers truly need to know.

Coat Zone and ASTM G50

Despite completing 6,000 hours of B117 testing, we believe the G50 evaluation provides a stronger case for the efficacy of CoilSafe®, which is why we conducted an ASTM G50-style marine exposure evaluation. This evaluation included 25,000 hours of natural marine exposure on Galveston Island, one of the harshest coastal environments. Third-party reporting showed that CoilSafe® exhibited complete corrosion resistance, whereas the uncoated coil on the same rooftop showed extensive deterioration consistent with chloride-driven corrosion. Learn more about the results of this evaluation here.

Final Thoughts

For any material designed to withstand the elements, relying solely on an accelerated lab test such as ASTM B117 is a risk. ASTM G50 provides the necessary rigor and realism by testing materials in their actual operating environment: the open atmosphere. By adhering to its procedures—including using controls and committing to multi-year exposures—companies can gain confidence that their material’s performance data is not only fast but also accurate.

Sources

• ASTM International. (2020). ASTM G50-20: Standard Practice for Conducting Atmospheric Corrosion Tests on Metals. Retrieved from https://store.astm.org/g0050-20.html
• ASTM International. (2019). ASTM B117-19: Standard Practice for Operating Salt Spray (Fog) Apparatus. Retrieved from https://store.astm.org/b0117-19.html
• ASTM International. (n.d.). Corrosion and Wear Standards. Retrieved from https://store.astm.org/products-services/standards-and-publications/standards/corrosion-standards-and-wear-standards.html