Water utilities often do not know the specific cause of external corrosion observed on their water mains, and consequently, the chosen preventative measure may not work effectively. Historically, these choices are based on data from other industries (e.g., gas and oil) and may not be suitable for the water industry. Corrosion of metallic pipes can be caused by a variety of mechanisms, each of which requires a different solution. Determining which corrosion mechanism is at work is not a simple matter, because the resulting pipe damage looks similar for all of them. The failure to properly identify corrosion sources may produce prevention systems that are ineffective or do not last. For example, it is not effective to install an anode bag on a main that has a bacteriological corrosion problem. Similarly, an anode bag installed to reduce corrosion caused by a stray impressed current would be quickly used up and would provide only short-term protection. Much recent research on corrosion has focused on internal corrosion, primarily related to water-quality issues, such as lead and copper control and red water. This project will examine external corrosion, which affects the structural integrity of the pipe and makes it vulnerable to leaks and breakage. After identifying the causes of external corrosion, the study will find economical solutions for each type of corrosion and verify them through field trials.