The galvanic couple mechanism
When two metals of different electrochemical potential are in contact in an electrolyte (the cooling water), the less noble metal (anode) corrodes to protect the more noble metal (cathode). This is galvanic corrosion. In a shell-and-tube exchanger, the carbon-steel tube sheet is the anode, and the cupro-nickel, brass, titanium or stainless tubes are the cathodes. The chloride-charged water activates the current.
The metals involved
The galvanic series in seawater ranks metals from most noble (cathode, protected) to least noble (anode, corroded). For heat exchangers, the problematic couples are:
- Titanium (very noble) vs carbon steel (less noble) → very aggressive attack
- 316 stainless vs carbon steel → moderate to severe attack with chlorides
- 90/10 Cupro-nickel vs carbon steel → moderate attack
- Brass vs carbon steel → moderate attack
How to recognise the damage
Galvanic corrosion of the tube sheet shows as deep craters around the tube holes, a flaring of the holes (tubes loosen), localised rust deposits and sometimes tube-to-sheet leaks. At an advanced stage, the sheet is perforated and full re-tubing becomes unavoidable.
Coating-based prevention solutions
The most effective solution is to electrically insulate the tube sheet from the tubes with a barrier coating. The epoxy metal-rebuild mastic restores lost geometry, then a ceramic-epoxy topcoat protects the sheet from the electrolyte. The result: the sheet is isolated from the galvanic couple, tube holes stay sealed, and exchanger service life is extended by 10 to 20 years.
Conclusion
Galvanic corrosion of heat exchangers is predictable and avoidable. A properly applied barrier coating on the tube sheet stops the electrochemical attack at its source, avoiding costly re-tubing and unit downtime.
Hicham M, P Eng, PMP
Engineer and project manager (PMP) at Induscoat Canada. Over 16 years of experience in industrial coatings, composite repairs and wear protection on mining, energy and petrochemical sites in Canada and internationally.