The chemical is ethyl chloride also known as chloroethane and the material of the tank is carbon steel. We connect these tanks in series and push the chemical from tank to tank in our transfer system using ultra high purity nitrogen gas. The manufacturers of the chemical vent the residual gas inside the tank to the air with their gas side valve while filling the tanks with ethyl chloride. Our tank pressures are usually double what is expected at 68 degrees Fahrenheit. Could there be a substitution reaction at the surface of the interior of the tank for the chlorine of the molecule to yield trace amounts of hydrogen chloride? Can corrosion cause a build up of extra pressure? Could non-condensables be at play here? Any insight would be appreciated.