3.1.1. Microstructure Characteristics

As shown in Figure 1a–f, a large number of shrinkage holes, which are visible to the naked eye, can be seen on the surface of the ductile iron pipe that did not receive surface mechanical treatment. Additionally, these holes are deep and cover a wide area. These holes are highly likely to become areas of aggressive ion accumulation in a harsh environment. This would accelerate the formation of a local acidified environment, in which the dissolution of the iron matrix would be promoted. After the surface mechanical treatment, these obvious defects on the surface of the ductile iron pipe are removed; the microscopic morphology is shown in Figure 1g–k. It can be seen that the shrinkage holes on the surface of this specimen are significantly reduced. The surface uniformity of this specimen is better, and the graphite balls are uniformly distributed on the surface.
