Corrosion Behavior Of 316L Stainless Steel In Hot Concentrated Artificial Seawater

As the shortage of natural fresh water becomes a worldwide problem, seawater desalination techniques have obtained attention and development widely. Due to the good corrosion resistance,316L stainless steels have been used extensively to construct the seawater desalination plants. In order to provide a theoretical basis for using stainless steels in seawater desalination plants and their long-term safe operation, it is of great significance to study the corrosion behavior and mechanism of 316L stainless steel in the corresponding hot seawater environments. Thus, electrochemical corrosion of 316L stainless steel was investigated here in the seawater with different temperature from 25 to 95℃ and concentration ratios from 1 to 3 times by using anodic polarization curve, electrochemical impedance spectroscopy (EIS) and Mott-Schottky analyses. The corrosion mechanism of 316L stainless steel was discussed in hot concentrated seawater.Temperature and concentration ratio of seawater have great influence on the corrosion of 316L stainless steel. Both pitting potential and repassivation potential of 316L stainless steel reduce linearly with the logarithm of the concentration ratio of seawater in the range of 25 to 95℃, and decrease linearly with temperature in the concentration ratio range of 1 to 3 times, but the change of pitting potential is very slight when the solution temperature is higher than 85℃ in the case of concentration ratio larger than 2 times. The concentration ratio of seawater has a weaker influence on pitting resistance of 316L stainless steel in comparison with temperature.Under working temperature 72℃ condition of the Low Temperature Multi-Effect Distillation (LT-MED),the semiconductor properties of anodic passive films on 316L stainless steel are closely related to the concentration ratio of seawater。The Mott-Schottky curves of anodic passive films show three potential regions where C-2 changed linearly with electrode potential. When the potentials is below -0.4VSCE, the passive films behave as p-type semiconductors of the chromium oxides near the metal/oxide interface, in which the Cr3+ cation vacancies are the main acceptor species and the predominant ionic current carriers. The passive films behave as n-type semiconductors of iron oxides near the outer surface in the potential range from about-0.4 to 0.3 VSCE， Fe2+ species or oxygen vacancies are the predominant donors. When the potentials are above 0.3VSCE, the p-type semiconductor behavior appears again. This capacitance response may be induced by forming an inversion layer. The density values of acceptors and donors are very high with the range of 1020 to 1021 cm-3 and increase linearly with the concentration ratio of seawater. The concentration of donors and Cl- ions may increase with the concentration ratio of artificial seawater, which will promote the absorption of Cl- ions in the passive film and enhance the pitting susceptibility of 316L stainless steelThe corrosion evolution process of 316L stainless steel presents three stages in artificial seawater with a concentration ratio of 2 times at 72℃. In first stage for the pitting induction period,316L stainless steel shows good passive state within about 1150 h of immersion in the solution. The polarization resistance can reach a value of 106Ω cm2. The second stage is the pitting transformation period, which takes about 400h. The defects accumulate in the passive film surface gradually result in the breakdown of the passive films and occurrence of pitting corrosion.The corrosion potential and impedance values decrease significantly. The third stage is the stable pitting corrosion period. Pits continuously form and grow in the active corrosion state. The corrosion potential stabilizes at about -0.51VSCE.In the long-term immersion, the stable pits of 316L stainless steel propagate very slowly due to the alloying effect of Mo, low dissolved oxygen levels and weak occlusion cell effect on pitting corrosion.316L stainless steel has good pitting resistance in hot concentrated seawater.A cerium chemical conversion layer forms on 316L stainless steel surfaces in a mixed solution of citric acid and cerium salt, which is composed of CeO2 with a small amount of Ce2O3. The cerium conversion coating makes the corrosion potential and pitting potential of 316L stainless steel increase greatly. It may form open pits on the steet surface as the passive films are destroyed. These indicate that the cerium conversion layer can improve significantly the corrosion resistance of 316L stainless steel in hot concentrated seawater.