Corrosion Behavior And Corrosion Inhibition Of Typical Metal Materials In Disodium Hydrogen Phosphate/sodium Sulfate Hydrated Phase Change Materials

Inorganic hydrated salt phase change materials(PCM)have attracted much attention in thermal energy storage area due to their low phase change temperature and high thermal energy storage capacity.However,hydrated salt PCM are disposed to induce corrosion problems and in return the structural integrity of the container material would be compromised.Therefore,addressing the corrosion issues of hydrated salt PCM so as to achieve metal protections become the essential parts of study to improve the safety and reliability of heat storage system.The main objective of this work is to conduct a series of electrochemical experiments along with surface inspection techniques of three typical metal materials,namely stainless steel 316(SS-316),copper(Cu)and aluminum 1060(Al-1060)in the composite molten hydrated salt of Na₂HPO₄·12H₂O and Na₂SO₄·10H₂O medium so as to provide useful information in realizing corrosion mechanisms.Besides,the corrosion inhibition of metal materials in the studied PCM was achieved by introducing effective organic inhibitors,i.e.Schiff base compounds and imidazoline derivatives.Together with electrochemical measurements,the inhibition mechanism of inhibitors in the PCM medium was further investigated by quantum chemical calculations and molecular dynamic simulations.The experiments reveal that SS-316 readily gets passivated in the composite PCM melts and forms a dense passive film with no considerable corrosion,whereas Cu and Al-1060 behaves incompatibility with the studied PCM to different extent.Cu suffers an initial rapid anodic dissolution which is further impeded by the formation of spot-like salt precipitates on the substrate,mainly consisting of Cu HPO₄·n H₂O,Cu SO₄·n H₂O and Cu₂O.Al-1060 exhibits a continuous large dissolution in the studied PCM with a porous corroded surface morphology due to the complete destruction of passive film under the attack of OH^-ions.The detected corrosion products mainly are Al(OH)₃,Al PO₄·n H₂O,and Al₂(SO₄)³·n H₂O.Among the three,SS-316 is the most reliable one owning the highest interfacial charge transfer resistance,the lowest corrosion current density and thereby the most compatibility with the studied PCM.Four Schiff base compounds with different functional groups were introduced to Na₂HPO₄·12H₂O-Na₂SO₄·10H₂O melts and exhibit satisfying corrosion inhibition efficiency.Experimental results indicate that the inhibitory effect of T33,T45,T46 and T49Schiff base compounds depends on the addition concentration.As the concentration of inhibitor increasing,the reaction rate at Al/PCM interface slows down and the polarization resistance of Al-1060 surface increases.As compared to T33,T45 and T49,T46 Schiff base compound owns the highest corrosion inhibition efficiency.The recovered Al-1060 specimen after immersing in T46-PCM system shows the most intact surface and the least mass loss as well as the most hydrophobic property.Quantum chemical calculations in conjunction with Monte Carlo simulations reveal that O and N heteroatoms in each Schiff base molecular are the nucleophilic sites,which have high reactivities and therefore are designated as preferential adsorption sites.The adsorption energy between Schiff base molecule and aluminum substrate follows T46>T49>T45>T33.Ascribing to the high global softness,the large dipole moment and the low frontier orbital energy gap of molecular structure,T46 Schiff base compound owns the best corrosion inhibitory activity among the four inhibitors.Moreover,the corrosion inhibition of copper in Na₂HPO₄·12H₂O-Na₂SO₄·10H₂O melts was successfully achieved by a newly synthesized imidazoline derivates(PDIE).The imidazoline compound,2-(2-pentadecyl-4,5-dihydro-1H-imidazol-1-yl)ethanol,was prepared via solvent-free microwave synthesis method and its inhibitor activity shows highly sensitive to addition concentration,which is maximized as the dosage up to the optimal concentration of 0.5m M.In comparison to inhibitor-free PCM system,the addition of PDIE greatly decreases the mass transportation at Cu/PCM interface and reduces the corrosion rate of copper by adsorbing on the copper surface and physically covering the metal substrate.The adsorption process of PDIE on copper surface is thermodynamically preferred including both physisorption and chemisorption which obeys Langmuir isotherm.Quantum chemical calculations and molecular dynamics simulations show that the imidazoline ring and the heteroatoms are the active sites of PDIE molecule most responsible for the adsobing and bonding process with copper surface.Driven by the solvation effect from PCM particles,the long nonpolar alkyl tail of PDIE molecule tends to deform and hand over the copper substrate,contributing to offer a physical barrier and limit the further penetration of aggressive PCM species.