Cathodic protection of tubes in Shell & Tube Heat Exchanger Our $\ce{Cl2}$ liquifier is of the following type:



It is entirely made of mild steel (shell as well as tubes plus the connecting pipelines). At shell side there is Freon (R-12) with its refrigeration cycle (not shown here); at the tube side there is dried &amp; compressed $\ce{Cl2}$ gas from compressor section (~2bar max). One must realise that drying of $\ce{Cl2}$ is done physically via passing it through packed glass wool columns &amp; then washing it in $\ce{H2SO4}$ scrubbers &amp; then again through glass wool columns...Compression is done via $\ce{H2SO4}$ liquid ring compressors.

Problem we face is excess corrosion of tubes which thus lose their thickness &amp; may in time compromise...For instance every 2-3 weeks we open up this liquifier &amp; wash the tubes (via high pressure demi/RO water) &amp; we found quiet an amount of dark-green/brown sludge indicating ineffectiveness of our drying section. Lab test are yet to be done so that I can clearly say what this sludge is made of. From the WIKI what I understand the compounds of such colour can either be oxides or chlorides of Iron because sulphates of iron are yellow in colour...

In a nutshell what I can do is increase the number of drying towers or increase the number of glass wool columns, but the most active measure I am thinking of cathodic protection via impressed currents. Have any user here applied impressed currents to a S&amp;T heat exchanger &amp; how can it be done? I would love any literature on it.

As a side note our liquifier can be electrically isolated using gaskets &amp; insulated teflon bolts on any flange fitting.
Cathodic protection requires an electrolyte to work. You could just pass current through the tubes but how is it going to return to the power supply? If there is no electrolyte to conduct ions it cannot provide any protection to your system. Have you considered replacing your heat exchanger with glass or glass-coated metal? If the capital costs of this are prohibitive, I would think your best bet would be to try drying the gas some more, as you mentioned. Do you happen to know the water content of the gas going into the exchanger now?