**1. Introduction**

With increasing demand for water resources, the laying of water transmission pipelines has been gradually accelerated [1]. Additionally, the demand for water transmission pipelines such as steel pipes, ductile iron pipes, pre-stressed steel cylinder concrete pipes (PCCP), and sandwich glass steel pipes has increased sharply. In the application of cast pipes with cement lining, harmful ions such as Cr6+ often enter into the water from contaminated cement and endanger human health. With the increased use of cement-free lined cast iron pipes in plumbing lines, the fluid in the pipeline comes into direct contact with the line material. In these circumstances, the flow rate and shear stress destroy the resulting corrosion film on the material's surface [2], and elevate the potential risk of the

**Citation:** Wang, B.; Liu, T.; Tao, K.; Zhu, L.; Liu, C.; Yong, X.; Cheng, X. A Study of the Mechanisms and Kinetics of the Localized Corrosion Aggravation of Ductile Iron in a Harsh Water Quality Environment. *Metals* **2022**, *12*, 2103. https://doi.org/10.3390/ met12122103

Academic Editor: Renato Altobelli Antunes

Received: 31 October 2022 Accepted: 5 December 2022 Published: 7 December 2022

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corrosion-based deterioration of pipelines [2–8]. This may endanger the safe operation of the entire water supply system and lead to a series of problems, such as water quality deterioration, and ecological and economic losses.

In non-cement-lined water supply pipelines, the water temperature, pH [1,9], oxygen content, and chloride ion concentration are important factors that lead to the immersion corrosion of the transmission pipeline; these factors exist in addition to the flow immersion corrosion effect. Stefano [3] described the effect of scaling ions (Ca2+ and Mg2+) in water quality on the corrosion process of geothermal galvanized steel pipes. The concentration of Zn2+ and OH− has a clear effect on the formation of the nuclei of corrosion products in a solution with a higher concentration of scaling ions, which results in the presence of spherical corrosion products and scales on the surface of the pipe. This corrosion product mainly comprises zinc oxide, calcium carbonate, and magnesium carbonate, which could decrease the corrosion rate. Georgii [6] showed the effect of different flow rates on the corrosion process of mild steel, whereby, at higher flow rates (0.19–0.45 m/s), the protective properties of the metal surface layer were brought about by enhanced mass transport. In this condition, crystals of calcite and iron oxide could be formed in the cathodic and anodic regions, respectively, leading to the formation of a self-healing barrier film that blocked the oxygen supply and prevented the release of iron into the water.

Ductile iron, as the material most widely used in aqueducts, can lead to the accelerated damage of aqueducts due to microstructural defects such as microscopic shrinkage loosening on the material's surface, which can increase the corrosion area of castings. These defects are also more likely to cause the formation of pits. However, these defects could be improved by suitable surface mechanical treatment techniques [10,11], thus enhancing the corrosion resistance of ductile iron castings in aqueduct services.

Studies of the corrosion resistance of ductile iron are currently dominated by single influencing factors [12–16]. The corrosion of ductile iron [14] under cyclic salt spray exposure consists of three processes: (i) the pitting of ferrite, (ii) the inward oxidation of the austenitic ferrite matrix, associated with the formation of a dense inner layer, and (iii) the mixed oxidation of the matrix, associated with the formation of a rust layer on the sample surface, where chloride ions are transferred and become involved in the corrosion reaction by changing the properties and composition of the rust layer. The levels of corrosion resistance of carbon steel and ductile iron castings show significant differences at different levels of chloride ion content [15]. An increase in the chloride content leads to an increase in the corrosion rate, and carbon steel is more susceptible to damage from chloride than ductile iron is. However, the differences in the mechanisms of the carbon steel and ductile iron castings in a multi-factor coupled environment remain unclear.

This study investigated the influence of surface cracks on the corrosion resistance of ductile iron castings. In this work, the main causative factors for the acceleration of the corrosion of ductile iron materials were investigated, and a variety of water quality environments that may exist in water pipelines were simulated. We offer a quantitative ranking of the degree of influence of each environmental factor on the corrosion kinetics of ductile iron, and screen out the environmental factors that have the greatest influence on the acceleration of the corrosion of ductile iron at each corrosive stage. The main environmental factors that influence the corrosion resistance of ductile iron castings were identified.

#### **2. Materials and Methods**
