Early in the operation of a 50,000-gal/D pilot plant with aluminum heat transfer " surfaces, it became apparent that the electrochemical relationships among aluminum alloys were, radically different than at "normal";" conditions. " As a result, a series of laboratory investigations were undertaken to determine the effect of variables upon solution potential of aluminum alloys, as well as on their corrosion characteristics. Sample size, temperature time to equilibrium, and alloy were among the major variables. Factors such as alloy temper, velocity and pressure were of secondary importance.
Marked excursions of solution potential vs. temperature were, found for all alloys tested. The effect of velocity is small in comparison to the effect of temperature. Sample, size has been found to be a critical factor when relating, lab experience to field observations.
Pilot plant inspection data, obtained aft, er21 months of operation, are compared to laboratory data when such comparisons are possible. Major pilot plant problems encountered. were galvanic in nature. The primary problem was with contact to steel, but less, severe effects were found between aluminum alloys. The data from the lab indicate that present operating limits are most severe, for "aluminum and that an increase or decrease in temperature could reduce galvanic interactions.
Based on both lab and field data, the use of aluminum in desalting appears to be nearing engineering reality.
The operation of a 50,000-gal/D multistage flash desalting plant with aluminum heat transfer surfaces indicated that the electrochemical relationships of the alloys" changed markedly at higher temperatures. At temperatures above 180 °F, the sacrificial cladding of alclad alloys was expended so rapidly that alloys suitable for normal seawater are no onger useful. Similarly, anodes used to protect steel evaporator sections were rapidly used up., Galvanic attack to steel was greatly accelerated. Finally, an aluminum-magnesium alloy normally immune to grain boundary precipitation developed this problem under a combination of heat and cold work.
A proposal for a laboratory study of:-the potential vs. temperature relationships of various alloys was made to the Office of Saline Water (OSW). This proposal was limited to studies in recycle brine and product water from the pilot plant. Effects of velocity and oxygen concentration were also to be considered. The proposal was. accepted and became OSW Contract 14-01-0001-2301. This paper describes work done under'; the contract prior to Nov. 1, 1970. The contract was completed Jan. 31, 1971.
Static potential-temperature studies were made in an autoclave depicted schematically in Fig. 1. The autoclave is made of type 304 stainless steel and contains an 8-1 glass vessel in which the test solutions are contained. PVC pipe connects the half cell to the solution. The unit is gas fired with suitable regulators, gauges and safety valves.