When to replace the Anodes ?

[Idiens]

What is the advice, in voltage terms, for correctly anoding a bronze propeller on a SS shaft?

[GordMay]

Underwater bronze, such as the propeller, is considered to be fully protected by the zinc on the prop shaft if the voltmeter reads between 500 and 700 mv, vs an Ag/AgCl reference.

[GordMay]

Excerpted from “The Future for Sacrificial Anodes” ~ by Bob Crundwell
Goto:http://www.jcse.org/Volume4/Preprints/V4Preprint2.pdf

The calculation of the weight of anode alloy required to protect a structure is given by a simple calculation:

W = (A x C x L) ÷ Z

Where:
W = Weight in Kg
A = Area to be protected in M2
C = Polarisation Maintenance Current Density in Amperes / M2
L = System life in Hours
Z = Anode current capacity in Ampere Hours per Kg
The individual components of this equation, with the exception of design life, are known but only approximately*.

* It is a brave designer that claims to know the true surface area of the structure. Dimensional tolerances of rolled sections of the sizes from which offshore structures are made are known quite accurately but it is surprising what bits get left out of the calculation let alone any correction for surface irregularity (one authority quoted the difference to be a factor of x2).
* Maintenance current density is variously quoted at figures between 0.140 A/M2 and 0.040 A/M2 for the same location, a factor of almost 4 times.
* Anode current capacities for Al-Zn-In alloys are variously quoted between 2550 Ahrs/Kg & 2750 Ahrs/Kg In general the lower figure is on the basis of long term field tests and the higher figure is on the basis of short term lab tests.