【Weekly Topic】2.15~2.21 Corrosion Case Analysis (Automatic +5 wealth for replies; additional 3-10 wealth for wealth analysis)
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There is a condenser, and the material of its tube bundle and tube sheet is naval copper. Since the flow velocity at the inlet exceeded the critical flow velocity, corrosion occurred very quickly. The technical upgrade changed the material to Monel alloy, but the operators continued to use the previous critical flow velocity criteria, resulting in pitting. Please analyze the causes of corrosion and propose improvement measures. (The critical flow velocity of Monel alloy is greater than that of naval copper.)
The conditions provided by the moderator are insufficient; information on the shell and tube side fluids, pressure, temperature, and material is needed.
Since the flow velocity at the inlet exceeded the critical flow velocity, corrosion occurred very quickly
Control based on the previous critical flow velocity led to pitting.
1. Primarily, structural design is used to prevent the formation of gaps and dead zones; the backside deep holes are sealed by welding, the expansion joint area is increased as much as possible, sealants are used to fill in gaps, and these gaps are enlarged to allow the solution to flow through, thereby eliminating conditions that could lead to blockages. 2. Minimize the content of harmful impurities, use materials resistant to pitting, improve heat treatment, and reduce microstructural defects and precipitates. 3. Minimize the concentration of halide ions in the medium; use surface treatment processes such as soft nitridation, oxidation, polishing, and further oxidation to eliminate structural dead zones, along with cathodic protection.
This is due to abrasion; find a way to change the inlet condition, such as by using dual inlets.
This corrosion example shows that flow rate is not always the lower, the better; for passivized metal materials that form a protective film on their surface, too low a flow rate can lead to liquid stagnation and deposition of solid substances, thereby causing pitting and crevice corrosion. Protective measures: Keep the flow rate within an appropriate range.
Keep the flow rate within an appropriate range
Control the inlet flow rate within an appropriate range or use dual inlets
1) Pitting corrosion in Monel alloy may be caused by dew point corrosion; in the case of erosion corrosion, the corrosion pattern is generally river-like or gully-like. 2) At the inlet location, dew point formation is a common issue; it is possible to line the inlet with high-alloy materials or coatings