CP Design in Variable Salt Water Concentrations

[cod]

We are designing a sacrificial anode type of cathodic protection system for a sheet pile wall in an estuary that changes salinity according to the seasons. Salinity values range from a minimum of 1 ppt to a maximum of 20 ppt. The problem arises because the low salinity periods can last for up to 6 months with the remaining time averaging around 10-15 ppt.

An aluminum anode immersed in the lower salinity water will have little current output and yet the corrosion rate of steel in this brackish water is almost the same as in pure seawater (see curves by FW Fink reprinted in the old AISI "Handbook of Corrosion Protection"). For this reason, I would surmise that the current density for CP would be about the same as for seawater. We typically use a current density of 5-10 ma/sqft for seawater but this is hard to achieve in the low salinity environment with aluminum. On the other hand, using magnesium anodes, which work better in the low salinity water, are consumed rapidly in the higher salinity water. It would seem that initial polarization will not occur with aluminum at the low salinity levels but would with magnesium.

The question is, what is the best approach for designing a sacrificaial CP system in this type of environment? For several non-technical reasons, an impressed current type of system cannot be considered.

Any comments would be greatly appreciated.

Charles Daniell

 

[NACECP]

Use corrosion coupons to monitor the actual corrosion rate and current demand.

It is possible the results let you conclude that the Zinc's are good enough. This may save the expense of designing and installing a system for when the salinity is low and the output from the zinc is low.