Executive Summary

Galvanic corrosion is one of the most prevalent corrosion problems of modern times, where dissimilar metals and alloys are routinely used in industrial applications. This report focuses on galvanic corrosion in bridge applications through a literature review, a comprehensive experimental study, advanced analysis, and the development of an evaluation tool, a guiding procedure, and a set of galvanic corrosion criteria. It aims to provide information and resources to bridge engineers to conduct a better assessment on galvanic corrosion in bridge design and management.

A galvanic series was developed that provides a systematic collection and comparison of the corrosion tendencies of bridge alloys. It has demonstrated that a significant galvanic corrosion tendency warrants attention, but also revealed that a seemly low tendency could lead to substantial galvanic corrosion between two actively corroding metals. These findings support the need of a method for further evaluation: a measurement of galvanic corrosion rate that assesses how fast the anodic alloy loses its mass. A set of criteria to interpret the magnitude of galvanic corrosion risk was established by considering both galvanic corrosion rate, and a new concept of a ratio that takes account of the galvanic corrosion rate in the context of a general corrosion rate (Gal/Corr), providing a consistent assessment for the alloys studied in this project. These advancements subsequently enabled the development of a three-step guiding procedure for assessing galvanic corrosion.

The experimental design and results provided the foundation on which 11 bridge alloys were studied by using quantitative electrochemical techniques, microstructural analysis, and visual inspection of samples tested in a salt-spray chamber. The results also support corrosion-mechanism-based mitigation strategies by countering the necessary conditions for galvanic corrosion to occur and considering the key factors that influence its magnitude; that is, how the assembly and configuration affect the cathode/anode surface area ratio and how an environment affects corrosivity. Another important finding was that the galvanic corrosion rate is not only governed by the driving force (potential difference) but also by individual corrosion behaviours of metals. Two actively corroding metals (e.g., weathering steel and a carbon steel bolt in a chloride-laden environment), despite having similar corrosion potentials, were found to produce a galvanic corrosion rate much higher than two less active metals having very different corrosion potentials (e.g., aluminum and stainless-steel bolt).

While effectively comparing the galvanic corrosion resistance of paired bridge alloys, galvanic corrosion is clearly shown to be materials-environment-configuration specific. For example, the galvanic corrosion rate of a A325 Type I carbon steel bolt coupled with weathering steel was found to be high in chloride-laden environments; however, it would be expected to be low in a chloride-free environment. In the future, developing a better understanding of the corrosion performance of bridge metals and alloys, and systematically examining them in a manner similar to their mechanical properties is recommended. This applies particularly to aluminum bridges. Although aluminum’s corrosion potential indicates a high corrosion tendency, its actual corrosion rate can be low due to the protection from the corrosion products of aluminum oxides. It would be also beneficial to establish information about the individual corrosion properties of bridge alloys as a part of the alloys’ complete characterization, including their interactions with exposure conditions, concerning especially weathering steel since its corrosion behaviour in chloride-laden environments is not fully understood. There is also a need to further develop risk assessment criteria to interpret galvanic corrosion. This study represents an initial effort based on existing data, an experimental study, and a newly developed concept of the Gal/Corr ratio that has put the evaluation of galvanic corrosion in a new perspective and will hopefully spur interest and promote further discussion and research.