Ferrules, Fittings, and Tubing - Material Selection
Swagelok Tech Tip: Ferrules, Fittings, and Tubing
The short answer is like goes with like. Match stainless steel ferrules and fittings with stainless steel tubing, brass fittings and ferrules with copper tubing, and plastic fittings and ferrules with plastic tubing. You will head off the problem of dissimilar materials fighting each other, and you will be assured that your ferrules are going to adequately swage your tubing.