Under Deposit Corrosion in Cooling Water

Under deposit corrosion in cooling systems have been one of the most prevalent problems in our industry.  It is also one of the least understood problems with field level engineers and clients alike.

Deposits occur in cooling systems due to colloidal sediment, environmental debris, scale, old corrosion deposits re-depositing on metal surfaces and of course biofilm.

When deposits form on metal surfaces, the corrosion inhibitors added to systems simply cannot do their job, since they cannot get to the metal surfaces they are supposed to protect.  Further, a dissimilar surface is set up on the metal providing the makings of a corrosion cell.

Scale formations provide a dissimilar surface and allow for the anode/cathode corrosion cell to start.  Further, scale insulated the metal surface from the corrosion inhibitors from protecting metal surfaces, most likely on the cathode surface.

Environmental debris such as garden variety dirt and organic material also presents a dissimilar metal surface allowing a corrosion cell to start, same as scale formations.  Environmental debris also provides a great start to both scale and biofilm formation.  Cooling system components most in danger from environmental debris are chillers with enhanced tubes.  Enhanced tubes are already prone to corrosion due to their rifled nature.  Cooling tower systems with enhanced tubes should all be filtered through a sand or multimedia filter to protect the rifling of these tubes.

Biofilm:  Causes the same dissimilar metal surfaces as noted above.  An added feature of microbiological corrosion is the acidic nature of the biofilm wastes.  pH’s of the underside of a biofilm has been measured as low as 2!

How to prevent under deposit corrosion?

 Make sure flow rates in all areas of your system are high enough to prevent sediment from settling on piping.

  1. Avoid intermittent flow conditions.
  2. Clean up all existing deposits.
    1. Descale fouled systems
    2. Remove old biofilm
    3. Physical remove old corrosion products where possible
  3. Consider an appropriate filter for your system.
  4. Keep your biocide program up and consider a bio dispersant, especially in chiller systems with enhanced tubes.

These deposits can be virtually eliminated from most systems.  Be sure to evaluate your systems to determine appropriate cures.

Keith Morgan/ CWT/ Upstate division