Untitled

Stainless steels are the largest tonnage alloy materials used for industrial and domestic purposes. The primary property, which determines their use in many applications, is their corrosion resistance in a wide variety of aqueous environments. There are many varieties of stainless steel, but in all cases they are essentially alloys of iron and chromium to which other elements are added to give specific properties. The addition of nickel results in the well known austenitic stainless steels, and the largest tonnage alloy is an iron chromium nickel alloy (18%Cr 10%Ni) UNS S30400, commonly referred to as Type 304. The corrosion resistance of this alloy is improved by the addition of 2–3% molybdenum giving the often used UNS S31600, commonly referred to as Type 316. This alloy is particularly useful in atmospheric marine environments, but when immersed in seawater it can suffer pitting and crevice corrosion. With this tendency to pit in chloride-containing environments, the use of stainless steels in distillation plants which handle hot concentrated seawater would seem unlikely. However, because the environment is well deaerated, the tendency to pit is greatly reduced and stainless steel grades such as UNS S31603, commonly referred to as Type 316L, are successfully used in most modern MSF plants. This paper reviews the properties of stainless steels relevant to MSF environments, describes their use for various components such as flash chambers, demisters, tube support plates and interstage walls. Also it considers the economic aspects of these alloys when used in clad and solid form, and it indicates where increased usage can be expected.

Despite their attractive corrosion resistance, mechanical properties and ease of fabrication, stainless steels were once considered too expensive for use in the main structures of MSF plants. Improved manufacturing technology has reduced the real cost and from small beginnings in a few components such as weirs, demisters and vent piping the use of stainless steels has grown rapidly and they are now used, in large tonnage, in most modern units. The allstainless-steel plant has not yet been achieved, but work is progressing towards this [9]. The main component missing from the range of available components is a suitable heat exchanger tube at a price competitive with the copper base alloys and titanium. With corrosion resistance and manufacturing techniques continually improving for the stainless steel alloys, a suitable stainless steel heat exchanger tube cannot be far away [10,11].