On which type of pipe can tubercles form?

Tubercles are small, rounded projections that can form on the inside of pipelines due to corrosion, particularly in water systems. These formations often occur in materials susceptible to corrosion and influenced by factors like water chemistry, pH levels, and the presence of certain microorganisms.

Ductile iron is a type of cast iron that possesses enhanced tensile strength and ductility, making it a popular choice for water distribution systems. However, it can still corrode in certain conditions, especially when subjected to aggressive water chemistry or lacking adequate protective coatings. When this corrosion occurs, tubercles can develop as the corrosion products accumulate. These tubercles can disrupt flow, reduce pipe diameter, and lead to uneven water quality.

In contrast, materials like polyethylene and PVC are generally inert and resistant to corrosion, significantly reducing the likelihood of tubercle formation. Galvanized steel, although it can corrode, typically experiences rust formation rather than tubercle development. Therefore, the susceptibility of ductile iron to tubercle formation under the right conditions makes it the correct answer to the question.