The 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).