Marine problems are of national importance• At the present time, the U# S* Government is stimulating interest in marine activities in many areas. Examples of problems receiving attention are shore erosion, harbor protection, sea-water intrusion of the fresh-water table, navigation, underwater transportation, oceanographic exploration, hydrofoil ships, nuclear-powered vessels, iceberg movements, seafood, pollution control, etc* One program which has gained national and international interest is that of the Office of Saline Water to develop methods of converting sea or brackish water to fresh water.
For highly saline waters, such as sea water and brines from salt wells, many today favor distillation processes as the most economical method of producing potable water* Processes which can make use of waste heat (often available at shore-based steam plants, at refineries, and at chemical plants) are particularly attractive.
Some of the thermal processes under investigation by the Office of Saline Water include conventional distillation, distillation using Yapor-compression multi-stage flash evaporators, distillation with vapor reheat, distillation without the use of metallic heating surfaces, vapor compression using forced circulation, etc.
This increased activity in the utilization of sea water involves the use of a variety of equipment and focuses attention on the need for knowledge of the behavior of metals and other materials in marine service.
Before choosing metals or other materials of construction for distillation plants handling sea water, many factors have to be considered. Among these are the intial cost of materials, the efficiency of the materials in the intended design, the lifetime of these materials as influenced by corrosion and by other factors, and the amount of servicing required to keep the intended design operational.
The chemistry of sea water is complex and much more information needs to be developed. Many materials behave unpredict-ably in sea water, particularly when incorporated into actual designs. This report is concerned primarily with the corrosion behavior of metals and other materials in sea water, in diluted sea water, and in brackish waters. Corrosion and scaling problems as a result of heating saline waters are given particular attention.
Information presented was obtained from (1) a review of pertinent corrosion literature, (2) consultations with experts in the field of corrosion, (3) reports of marine corrosion research, presented at meetings of The Sea Horse Institute (directed by The International Nickel Company) at Wrightsville Beach, North Carolina, (4）manufacturers1 technical publications, and (5) Battelle*s own marine experience.
The chloride ion is probably the most deleterious ionic constituent occurring in sea water in large quantities* Its corrosive nature comes from the fact that it readily penetrates passive protective films and thus enhances the corrosion reactions.
In addition to chloride ions, the anions found to the greatest extent in sea water are sulfate« bromide9 fluoride, and bicarbonate. Lyman and Abel(2)list a typical analysis for the major constituents of a sample of•North Pacific Ocean water# Their data, tabulated below, also include the major cations present*
Grand Total: 3.449 per cent
Natural processes, operating both at the surface and at great depths, result in a continuous circulation of ocean water, so that the relative proportions of dissolved salts are virtually the same everywhere, although the total salt content (salinity) may show appreciable variations with geography•
The halogen ions, other than chloride, are present only in small amounts, and their corrosive effects in sea water are probably masked by the very high chloride content.
Other corrosion experience would suggest that the sulfate also contributes much less to the corrosion by sea water than the chloride.
The presence of bicarbonate ions in water can help promote corrosion attack on many metals.
Tt should be mentioned that the vE of sea water normally
Since corrosion is dependent on electrolytic processes, it is greatly influenced by the conductivity of the solution. Sea water is a good electrolyte, so it is not surprising that it is corrosive# Figure l〇)shows that the resistivity of sea water is relatively low at normal temperatures. However, it can also be seen that as the water is diluted (as might occur near rivers), the resistivity is markedly increased. Accordingly, the corrosion might be expected to be somewhat less in the vicinity of rivers. Actually, the effect of varying the salinity is inter-related with some of the other variables, as far as corrosion is concerned. For example’, concentrating sea water (as in a multiple-effect distillation process) reduces the oxygen solubility.
For steel and for the temperatures involved in a typical process, it is found that the rate of attack becomes less as the brine becomes more concentrated. The role of oxygen is discussed more completely later.
It is interesting to note that resistivity for sea water in the normal range of salinity is not greatly affected by temperature (see the lower curve in Figure 2)* On the other hand, a solution of only 1*84 parts/thousand (0/00) salinity decreases almost one half in resistivity as the temperature is increased from 32. to 77 F.