Flow-Accelerated Corrosion Wear of Power-Generating Equipment: Investigations, Prediction, and Prevention: 2. Prediction and Prevention of General and Local Flow-Accelerated Corrosion

The second part of this review considers physicochemical models and computer codes used for predicting flow-accelerated corrosion wear of power generating equipment. Approaches used to prevent the occurrence of general and local flow-accelerated corrosion that are based on selecting metals resistant to flow-accelerated corrosion and adjusting the water chemistry of power units are also discussed. The existing computer codes use physicochemical models of flow-accelerated corrosion and statistical data on damages inflicted to power units due to flow-accelerated corrosion processes. Advantages and drawbacks of different analytical physicochemical models describing the flow-accelerated corrosion process are pointed out together with the specific features of using them in elaborating flow-accelerated corrosion computing codes. It is shown that the processes lying at the heart of the flow-accelerated corrosion mechanism include, on the one hand, the occurrence of a protective oxide layer on the metal surface and, on the other hand, the dissolution of this layer and carryover of dissolution products in the flow. Differences between the processes through which metal undergoes flow-accelerated corrosion in a single-phase water flow and in a two-phase wet steam flow are analyzed. Thus, the redistribution of admixtures and gases between the phases that takes place in two-phase media may cause a change in the pH values, thereby significantly influencing the flow-accelerated corrosion rate. In addition, the rate with which flow-accelerated corrosion products are carried over into a two-phase stream depends on the liquid film flow mode on the streamlined surface. The flow-accelerated corrosion rate computing codes most widely known around the world, including the COMSY code (Germany), CHECWORKS SFA code (United States), BRT-CICERO^TM code (France), and RAMEK code (Russia) are considered. Their specific features and application limits are pointed out. Information on the effect the content of chromium, molybdenum, and copper has on the flow-accelerated corrosion rate is given. It is shown that the choice of metals resistant to flow-accelerated corrosion is a combined technical and economic problem, and the way in which it is solved has an effect on the safety and reliability of power unit operation. It is pointed out that the liquid phase pH value is essentially affected by the steam wetness degree if the latter exceeds 20%.