Permeability of Polymers to Electrolytes

One of the reasons for local corrosion at the metal-polymer interface is sorption of electrolytes by polymers and permeability of the polymer barrier towards electrolytes. Sorption of electrolytes (acid solutions, bases and salts) leads to essential variation in the service characteristics of the protecting polymer coatings and anticorrosion packaging films under mechanical loads. These variations under mechanical loads, especially in seals and friction joints, are much deeper and can affect mechanisms of contact interactions. 

Transfer of aqua electrolytes in polymers has characteristics [22] that distinguish it from the transfer of other low-molecular-weight matter. The presence of water in the medium sorbed may result in formation of aqua associates and electrolyte in the polymer. Depending on the amount of sorbed water, the polymers are subdivided into three groups (i) hydrophobic, i.e. slightly swelling in water (water concentration in the polymer below 0.5%), (ii) mildly hydrophilic (water concentration 0.5-10%), (iii) hydrophilic (water concentration above 10%). This subdivision is very conventional since the permeability mechanism depends not only on water concentration in the polymer but on the nature of their physical-chemical interactions with electrolyte ions and macromolecular active centers as well [23]. 

Hydrophobic polymers (polyolefins, polystyrene, polyethylene terephtha-late, pentaplast, fluoroplasts) contain small amounts of polar groups in their macromolecular composition able to interact with water and electrolytes. To that extent, the permeability mechanism of electrol3des in hydrophobic polymers is similar to that of gases. 

The comparison of permeability values of low-molecular-weight substances diffusing into hydrophobic polymers from water solutions to those from the dry gaseous phase (e.g., HCl, NH3, CO2, SO2) showed that these values were commensurate, providing that the vapor elasticity over the solution and its partial pressure in the gaseous phase were equal. It is believed that volatile electrolytes are transferred in hydrophobic polymers in the form of non-dissociated molecules devoid of hydrate shells. 

It is important for hydrophobic polymers that their permeability to nonvolatile electrolytes is at least three orders of magnitude lower than that for volatile electrolytes. Low permeability for nonvolatile electrolytes is related to their low sorption by the non-polar polymers. 

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