Abstract
The variation with time of the open circuit potentials of six molybdenum containing stainless steels was measured in Arabian Gulf water under conditions simulating operation in multistage flash evaporation distillers. The effects of temperature in the range 22–70°C and of sea water concentrated by cold evaporation to between 100 and 150% were examined in detail. With all steels, plots of potential as a function of logarithm of immersion time consisted of two straight line segments: the first, characterised by a lower slope, was attributed to the formation of a very thin, barrier type oxide layer, whereas the second related to longer times of immersion, indicating the formation of a thicker film. The mean rates of thickening of both forms of oxide (δ¯1 and δ¯2) were computed according to open circuit transient theory. Depending on the type of steel, δ¯2/δ¯1 varied between 4·6 and 11·0 below 40°0C and 1·1 and 1·6 at 70°C. Curves representing the variation of δ¯1 and δ¯2 with temperature exhibited marked increases at ∼50°C. This behaviour did not appear to be the result ofa change in the physical nature of the passivatingfilm (given that molybdenum free 304 stainless steel, which forms a similar oxide layer, shows different variation of δ¯1 and δ¯2), but was attributed to enhancement of MO6+ ion diffusion through the oxide at temperatures ≳50°C. X-ray surface analysis supported this idea, indicating higher concentrations of molybdenum in the films grown at high temperatures. The X-ray maps also revealed the presence of copper in the films of alloys containing this element as a minor constituent. The rates δ¯1 and δ¯2 were practically independent of brine concentration at a given temperature, but did show a combined dependence on temperature and concentration.
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