Cathodic Protection Of Tubes In Shell & Tube Heat Exchanger

This might be worth getting a membership in NACE (National Association of Corrosion Engineers). I don't know whether your problem is electrolysis versus "simple" corrosion. Imposed current might help with the former, but maybe not the latter. Investigation is likely to be interesting.

Zinc and aluminum sacrificial anodes are used in circulating water systems (Google it), but aren't going to help you. You need to work on the efficacy of your drying system.

How many towers/pumps/coolers, temps, etc.?

I have a gas at tube side is that why you are indicating that impressed currents wont help me? Bcz there is literature available about protecting S&T via active cathodic protection (patents US5513694 & US5515913), but of coarse an electrolye is involved rather a gas!

Cl2 from electrolysers (~1.2 bar & 80Celsius) > Demister > Chilled Condenser > 2-3 condensate seals > Chilled Condenser > 3x Scrubbers > Cyclone separator > Ring Compressor > Cyclone separator > 2x Demister (~2bar & 16 Celsius) > Liquifier

I do not believe that your issue is galvanic corrosion, therefore I do not believe that a cathodic protection system will help you.

For a tubeside stream that may or may not include wet chloride, I believe that you may want to consider:

- C-276 materials on the tubeside - tubes and tubesheet face ( You may find a premium coating for the HX channels) This may be a cheaper solution

- Titanium (grade 2) tubes and tubesheet face. This will be more expensive.

Whomever decided to use mild steel in this service made a dire mistake (MBA candidate possibly ?.... Your boss perhaps ?)

https://books.google.com/books?id=OrlG98AHdoAC&pg=PA159&dq=corrosion+wet+chlorine+gas&hl=en&sa=X&ei=erfGVKPFGoytyATf1oCADg&ved=0CDAQ6AEwAQ#v=onepage&q=corrosion%20wet%20chlorine%20gas&f=false

I agree that this is the point where you should engage a NACE corrosion consultant. Pay his fee....

Titanium would just catch fire under anhydrous Cl2 conditions (Google it). Our problem are highly linked with carryover acid (H2SO4)from compressor section...For this mild steels is OK provided dryness of Cl2 is ensured

It's something to do with carry-over of H2SO4. Consider replacing it with more exotic materials. Graphite? Borosilicate glass? What size is this thing?

15metres in length & ~1m in dia. Yes carryover acid IS a problem, thats why we installed glass wool columns even after liquid ring compressors

I recall a similar experience, we suffered long back. We found the dryness of Chlorine after dryer was not that good, after the compressors further worse. The failure continuum was analysed thus:

• From drying tower Chlorine with some wetness sent to compressors.
• There it got dried, on immense contact with H2SO4 and let to condenser. Until this no big issue since condenser was receiving fairly dry Chlorine.
• But the recirculated acid in compressor continually diluted and corroded the acid cooler tubes. This was the start-up of worse happenings.
• Cooling water migrated to acid side and diluted the acid further.
• Chlorine under compression picked up relatively wet acid along with and carried to dryer.

The established watch points were:

• The top bobble tray in the dryer was of simple mild steel. This tray was meant only to ensure the dryness. Unhealthy appearance of this tray (due to slurry formation) through the window is an indication of poor drying at lower trays and spray drying.
• Frequent inspection of Cl2 at dryer outlet, preferably automate the analysis.
• Frequent acid sampling for the strength and sludge at compressor cooler. (I had a nasty accident in attending the drain valve. That was stuck plug valve, broke at the nipple and flashed acid. I thought worthwhile to mention here so that you may consider changing to a better type if plug valves are used at drain. Needle valve won't work since would get chocked on slight sludge formation. PTFE lined plug valve should do a good job.)

Thanks sir, our valves at Cl2 section section are always PTFE lined. However, the sludge that you referred in your explaination, have you done lab tests was it chlorides/sulphates of iron. Second, have you considered cathodic protection in your scenario

One of the causes might be that the vapours are being forced too hard through the acid scrubbers. Before modifying the exchanger in any way, review the operation of the scrubbers to make sure these are operating within original design parameters; if anything is wrong, correct it here first.

Nightcrawler: IF your packed glass wool is bypassing to any appreciable extent, then some sulfuric acid may be getting through.

Colors: if the sludge being dark-green with brown is present, then (1) if no water vapor is making it through, and no oxygen, you are looking at ferric chloride (brown, most likely) with a smattering of ferrous acid sulphate (typically a lighter colored bluish green material) mixed in with the chloride salts. If water and oxygen are included, then it's Katy bar the door, because corrosion will be totally out of control, as it appears. Are you sure you are changin out the sulfuric acid or adding SO3 to it to keep the moisture content of that as low as possible? Unless the acid is in specification (as to % sulfuric), you cannot achieve "dry" chlorine.

I suspect there might be a more resistant alloy to use (however expensive) such as CN7M or CN8M. I think one of those was formerly referred to as Alloy 20. We have had an alloy 20 chlorine gas transfer line in service for approximately 30 years with no sign of corrosion, much less failure, and from time to time wet or moist air enters the line when chlorine bottles are being changed out.

Nightcrawler: A sidebar, if not completely "dry" chlorine, and you still want to protect the metal, cathodic protection may not work, simply because you may not have 100% success projecting the applied potential over the entire mild steel surface.

I remember reading some company literature on anodic protection of sulfuric acid vessels. This form of protection actually makes the potential of the metal more anodic, but at least in sulfuric acid storage situations apparently results in the formation of a passive oxide/sulfate layer on the metal, and the corrosion slows way down. I have never tried that. It might be worth a google though.

http://www.corrosionservice.com/anodic_protection.htm

The above link is to a commercial website, and mentions only protection of tanks in sulfuric acid storage service.

http://www.corrosion-doctors.org/ProcessIndustry/Process-chemicals.htm

This link above is from corrosion doctors, and mentions a critical velocity maximum to avoid stripping off the passive layer from the piping. It also mentions the absolutely critical necessity of having absolute dry chlorine. Sorry, I did not find anything directly related to cathodic protection and liquid chlorine in mild steel. I think it is OK (obviously works fine for years with dry chlorine liquid in steel cylinders), only if the chlorine is dry, dry, dry. Maybe your sparging system needs a good going over?