We have an application that is leaking (weeping) DI water with 12% Ethylene Glycol from 60 degree cone type joints. We have other systems without the Glycol that appear not to be leaking. Just next to the SS-SS cone fitting is a flat-faced joint with dowty washer that is not leaking. We have since been advised it is not good to try to seal against DI water without a compliant seal! The DI/Glycol is being pumped at about 30-40C and 4 Bar g max.. The DI/Glycol is kept low conductivity with a resin de-ioniser cartridge on a bypass circuit under closed loop control. We suspect the following possible mechanisms for leaking at the SS interface; 1) Oxidation of the Glycol causing acid corrosion (crevice corrosion) at the interface. The leaks mean occasional topping up of the syatem with DI water which probably contains dissolved O2 and CO2. 2) DI water may be scrubbed to 0.1uS/cm instead of nominal 1uS/cm which is causing the DI water to dissolve its way out (compared to the systems that are not leaking). We do have copper in the waterways (an Inductor with copper pipe windings) which we suspect is sending copper into the DI /Glycol. Systems without leaks also have copper. We are getting the DI/Glycol samples analysed as well as possible copper build-up on the conductivity probe electrodes. Only materials in the water path are copper, 316L & silicone rubber hoses Any comments?