"Sodium hydroxide Cooler Corrosion"
"We are solving problem connected to corrosion of smoke cooler body made from 18Cr8Ni (0.049 weight % C, 0.33 weight % Mo) stainless steel. Weld metal is made from 18Cr8Ni(0.062 weight % C, 1.40 weight % Mo). The corrosion started in delta ferrite in the weld root. Almost all weld was removed by corrosion attack within approximately 3000 hours of service. The base metal is locally affected by general corrosion, depth of corrosion attack is up to 5 % of plate thickness. The cooling medium is 10-20 % NaOH solution in water. Temperature in cooler vary between 170 and 240°C. The soluble corrosion products taken from inner surface of smoke cooler contained chlorides, sulfur, sodium and oxygen (analyzed in SEM using EDX technique). Here is my question. There is a clear evidence, that the AISI 316 steel has to be replaced by more resistant grade Can you imagine some other ss or alloy we can recommend for smoke cooler body ? We appreciate the cheapest solution."
At your stated sodium hydroxide concentrations and maximum temperatures of 240C, I would suggest you consider the merits of alloys 400 (NO4400) or 600 (NO6600) for the cooler. Type 304SS (your present cooler material of construction) is also susceptible to caustic stress corrosion cracking, in addition to the accelerated corrosion you are experiencing.
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