Why is the phosphate dosing pipe in power plants corroding frequently?
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In the early days of the plant’s operation, the phosphate dosing pipes were made of 321 stainless steel, and there were no leaks for 8–9 years; however, leaks have started to occur frequently in recent years. All the 321 stainless steel pipes were replaced during this year’s maintenance period; however, more than a month after the machine was put back into operation, three spots of pitting corrosion appeared – two at positions 2 nm away from the welds, and one on the pipe itself. What is the chlorine corrosion resistance threshold for 321 stainless steel pipes? The chlorine content in the chemical dosing tank is 260 ppm – is this due to a high concentration of chloride ions? Dear experts, I seek your advice.
Chloride ions cause the most severe corrosion of stainless steel; 321 is simply 304 with titanium added. 316L is a better option, and passivation treatment must be carried out properly
But when the factory was built, using 321, 8-9 worked fine; now it doesn’t, and I’m puzzled as to why.
If the operating conditions remain unchanged, then the reason lies in a change in the composition of the solution
Has there been any inspection revealing issues with the quality of the suppliers’ products, or have there been any changes in the thickness specifications? Does the presence of dull metal adhering, forming an electrical circuit, accelerate corrosion? The leakage may be caused by contact during welding repairs, or by non-compliant welding techniques, as well as by the failure to perform purging after welding