"316SS and Deionized Water Corrosion"

Question:

"My boiler tank made in Stainless Steel 316 is corroded by deionised water. The heater elements made in Incoloy 800 are corroded too. This boiler tank and heater elements are part of an humidifier. Do you know about the deionised water, and what sort of metal should I use to replace my boiler tank and heater elements? I heard that there are some different sorts of deionised water, more or less corrosive. Is it true? 1- the Resistivity of the deionised water for this specific application is 18 MegaOhms, which means that the water is highly pure (0.0278 TDS) the purest water is 26 MegaOhms. 2-the corrosion start at the welding of the tank, and rapidly (1 week) we observe pitting corrosion all over the surfaces of sheet metal of SS 316. The tank is cubic shape made with 2 sheets, so we have 3 cords of weld at the bottom and at two corners. the fabricator of the tank, told us that the welding is a fusion welding, after welding they perform an etancheity test and they reworked the welding where it's necessary, so it's not a perfect - one shot - weld. Also they have no annealing operation after welding. Could Anealing operation prevent the corrosion? 3-The fabricator of tank suggest to nickel plate the tank. Do you think that can help? 4-About your last answer, Do you mean that we would have to prefer the SS 330 rather than SS 316 for the deionised water application?"

Answer:

316SS is commonly used to handle and store deionized water. 316SS and alloy 800 should not be corroding to the extent that you suggest. Deionized water can be corrosive to carbon steel due to its low hardness content. Harder waters, (based on calicum carbonate) can be protective due to the protective scale they deposit on the steel surface. Deionized waters are also commonly handled and stored using austenitic stainless steels, as the carbon steels will contaminate the relatively pure water. You should not have to result to nickel plating in your stated service. Perhaps, the 316SS is pitting at sites of iron contamination. Also, I am assuming that you are using the low carbon grade of 316SS; therefore, solution annealing after welding should not be necessary (it is also not practical). At this time, your situation warrants a more formal investigation, which we can help with if you want. Regarding the 330SS vs. 316SS, I would not use 330SS in place of 316SS.

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