Our client has a pump station that will primarily be pumping water at pressures around 1,000 psi. However, at certain times it may be subjected to a highly corrosive water that may have chlorides over 150,000 mg/L and TDS > 275,000 mg/L. pH around 6.3. Total acidity = 390 mg/L, total alkalinity = 40 mg/L. Sulfate = 150 mg/L. Temperature ranges between 37 deg F and 68 deg F. It is anticipated that the water line could see the corrosive water for a period of 3-7 days at a time and would then convert back to seeing surface water. There are quite a few valves on this line and we are looking at both 316SS and super duplex valve bodies. Since cost is an issue for our client, is it possible to use the 316SS valves and then rinse the line using surface water? Or will we definitely need to upgrade to a super duplex-type material? Also, for some of the piping, it was originally specified as carbon steel (at the station). In lieu of replacing that piping with FRP, could the carbon steel be coated with something for corrosion resistance?
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Brian Dalder Forum Moderator 177 Posts
Re: Do I have to worry about chloride-induced corrosion of 316 stainless steel?8 September 2010 at 1:29pmThe main concern here appears to be chloride induced corrosion of 316 stainless steel. The corrosivity of a certain concentration of chlorides can be strongly affected by the other constituents present including dissolved oxygen and carbon dioxide. Chloride attack of austenitic stainless steels, like 316, occur in three ways: stress corrosion cracking (SCC), pitting corrosion, and crevice corrosion. SCC is usually not a problem in the temperature range you have described. Pitting corrosion is associated with stagnant conditions and usually not an issue where continuous flow is occurring. Crevice corrosion can be minimized by eliminating crevices through proper design, So, depending on how your system is designed and operated, the risk of corrosion may not be as high as it might first appear. The best way to answer this question is to search for corrosion data from a similar system that is already in operation or short of that, design and run laboratory corrosion tests to simulate the actual process conditions and streams. If 316 stainless steel is not sufficient, there are intermediate choices, for example Alloy 2205, that might work before jumping to a super duplex alloy such as Alloy 2507, or even a super austenitic stainless steel like AL6XN. For carbon steel coating options, I would consult a coatings manufacturer for recommendations. Having said this, coatings would seem to have several disadvantages in this application. These include incomplete coverage of the substrate as a result of improper initial application or damage after application and the potential for short-lived protection due to wear (erosion).