Investigation of the Corrosion of Stainless Steels Used for Reverse Osmosis Desalination Plant By Varying Chemicals and Flow Velocity
Authors: Kim Young-Jae , Park Chan-Jin 
Published: February 2020

Desalinated water produced by a reverse osmosis (RO) filtering method forms about 22% of total production of desalinated water in the world. However, the RO environment is very corrosive due to the presence of various chemicals for water treatment and the flow of sand particles leading to corrosion. Recently, there has been much effort to substitute cheaper and more corrosion resistant stainless steels for copper based alloys as a valve material in RO. Nevertheless, the effects of chemicals and particles on the corrosion of stainless steels have rarely been studied. In this study, we report the effects of flow velocity under conditions with and without sand particles and chemicals such as NaOCl, FeCl3, and Na2S on the corrosion behaviors of various stainless steels.

 UNS S31603, S31803 and S32750 based cast stainless steels were used for tests. All electrochemical tests were conducted at 35 oC in artificial sea waters containing various chemicals and flow-velocity with/without sand particles using three-electrode system composed of a specimen of interest as the working electrode, a saturated calomel electrode (SCE) as the reference electrode and a Pt wire as the counter electrode. Prior to measurements, all the specimens were carefully polished using abrasive papers and diamond paste. Influence of chemicals for water treatment in RO, such as NaOCl, FeCl3, Na2S was investigated using electrochemical impedance analysis as well as cyclic-polarization tests. In addition, current noise of the samples in potentio-static test was monitored in solutions with and without sand particles under the jet impingement condition. After the corrosion tests, morphological change of the specimens was examined by an optical microscope and a scanning electron microscope.

 Among the chemicals tested, Na2S exhibited a greater effect on corrosion of the stainless steels under investigation compared with other chemicals such as NaOCl and FeCl3. At cyclic polarization tests, the obtained corrosion current density (icorr), as a measure of corrosion rate, of the stainless steels was found to be almost similar to each other. However, the cast austenitic stainless steel clearly exhibited the lower resistance to pitting corrosion compared with duplex stainless steels. In addition, EIS tests confirmed that the charge transfer resistance (Rct) and the resistance of the passive film (Rf) measured in sulfide-containing solution decreased more significantly compared with those in other chemical-containing solutions. Under various flow-velocity, all the current density of stainless steels increased at passive range. In particular, at the flow rate of higher than 6 m s-1 erosion corrosion was detected on the surface. Increasing the flow-velocity was found to increase the physical damage in the passive film. In seawater containing sand particles, the erosion in stainless steels was accelerated further, and passive current density was more increased than that in the seawater without particle.