Does weathering steel runoff affect water quality?

Weathering steel develops a protective patina that improves corrosion resistance and enables long service life with minimal maintenance. However during the early stages of exposure rainwater runoff from weathering steel surfaces can take on a characteristic brownish appearance. This visible phenomenon often raises questions about whether the runoff could affect water quality.
 
A study by researchers from KTH Royal Institute of Technology and SSAB investigated this question through both one-year field exposure tests in Stockholm and controlled laboratory experiments.

The researchers investigated the release of iron and alloying elements commonly found in weathering steels, including chromium (Cr), copper (Cu), manganese (Mn) and nickel (Ni).
 
The study found that the release of alloying elements was very limited. Any measurable release occurred primarily during the initial exposure period and was associated with the outermost layers of the developing patina. As the patina matured, the amount of metals released decreased.
 
These findings indicate that the visible color of runoff water during the early stages of weathering steel exposure is not necessarily associated with significant levels of released alloying elements.

Although weathering steel runoff may initially show a visible brownish color, a KTH and SSAB study found the release of alloying elements (Cr, Cu, Mn, Ni) is very limited. The photo shows a weathering steel bridge in Tampere, Finland.

As the patina develops, the interaction between the steel surface and rainwater becomes increasingly limited.

To put the results into context, the measured concentrations were compared with the World Health Organization (WHO) guideline values for drinking water. Throughout the study, the measured levels of released alloying elements remained well below the relevant WHO guideline values.
 
This means that while brownish runoff may be observed during the first years of exposure, the associated concentrations of chromium, copper, manganese and nickel remained low.

From an engineering perspective, the formation of a stable patina is a key characteristic of weathering steel.

The patina serves two important functions:

• It reduces the corrosion rate of the steel.
• It acts as a barrier between the steel surface and the surrounding environment.

As the patina develops, the interaction between the steel surface and rainwater becomes increasingly limited. This contributes both to the long-term durability of the structure and to lower levels of metal release.

The study demonstrates that metal release from weathering steel is highest during the initial stages of patina formation and decreases as the protective layer develops. Although runoff may have a visible brownish color during the first years of exposure, measured concentrations of alloying elements remained well below the WHO drinking water guideline values.
 
These findings provide additional data for engineers, infrastructure owners and designers evaluating weathering steel applications in environments where water quality is an important consideration.
 
Source
Herting, G. et al. Patina Formation and Diffuse Dispersion of Alloying Metals from Weathering Steels at Urban Atmospheric Conditions – A Combined Laboratory and Field Investigation. Materials and Corrosion (2024).
https://doi.org/10.1002/maco.202414430