Unexpected Behavior Of Sodium Sulfate Observed In Experimental Freezing And Corrosion Studies

This article reports results from two distinct, originally unrelated studies that show unexpected behavior with respect to sodium sulfate solutions in contact with flat surfaces. On the one hand, we investigated immersion freezing of sulfate containing solutions (target salt was CaSO4 and Na2SO4 was used as one control) in the presence of flat sapphire-0001 crystals using second-harmonic generation (SHG) spectroscopy. Further control experiments were carried out with neat water and CaCl2. The SHG signals from CaSO4 and Na2SO4 solutions behave somewhat differently from those sensed in both cases, the neat water and CaCl2. The common pattern shows a decrease of the SHG signal with decreasing temperature until about -15 degrees C, whereupon the signal sharply drops, indicating freezing. With the Na2SO4 solution, although the signal initially follows closely the neat water curve, the trend increases sharply at about -10 degrees C followed by a gradual decrease with further cooling. There is no plausible explanation for the distinct behavior of the Na2SO4 solution, and thermodynamic calculations do not suggest any precipitation of a sodium sulfate solid phase. At the end of the freeze-thaw cycle, the initial SHG for each system is retrieved, suggesting reversibility. On the other hand, in a separate investigation unrelated to the freezing study, it was repeatedly observed that sodium sulfate precipitates from solution on flat steel surfaces at room temperature. As in the freezing experiments, thermodynamic calculations for the bulk solution solubility of sodium sulfate indicate that precipitates should not be forming under the conditions of the studies. The form of the crystals observed on the steel samples has snowflake shapes similar to one literature report. We conclude that Na2SO4 in the presence of flat surfaces shows unexpected behavior that should incite further detailed studies in the future to elucidate the phenomenon.