S-SM-11-01f / 3
Seaguard Ltd, P.O.Box 2628, Christchurch, New Zealand
Telephone: +64 (0)3 389 9032 Mobile: +64 (0)25 362 767 Fax: +64 (0)3 389 9042
Cathodic protection levels
The Sentry mutizone has LCD and LED (Light Emitting Diode) indication of the cathodic protection
level. Providing the unit has been set up for the correct material and reference electrode type
(factory adjustable only), green LED indication shows that the material is protected from corrosion.
Red LED indication can indicate under protection or over protection. Under protection indicates
corrosion can take place. The rate of corrosion is influenced by a number of variables such as the
type of material, the oxide layer present, the quality of the electrolyte and the potential relative to the
reference electrode. Moderate over protection can be accepted but may be uneconomic for steel,
bronze and brass. Over protection can cause problems with aluminum due to amphoteric attack.
LCD readings can be evaluated from the table below for Seaguard zinc electrodes and typical
materials in use in the marine sector. For Ag AgCl reference electrodes, subtract 1000 from the
numbers below:
Material Freely corroding Protected Over protected
Carbon steel +390 +150
304 Stainless steel (active) +470 +230
Aluminum +260 +50 -100
Bronze +680 +350
Brass +640 +310
Corrosion Theory
The rate of corrosion is influenced by a number of variables such as the type of material, the oxide
layer present, the quality of the electrolyte and the potential relative to the reference electrode.
Moderate over protection can be accepted but may be uneconomic for steel, bronze and brass.
Over protection can cause paint disbondment and hydrogen embrittlement in high strength steel,
and can cause amphoteric attack to aluminum.
Corrosion rates depend upon many factors and, for this reason, the protection should be monitored
regularly under all operating conditions. These should include measuring the protection whilst the
vessel is moored and whilst under way.
If the anodes on the vessel have been set up for seawater usage and the vessel is used in fresh
water, the protection levels may fall and the Sentry will indicate the vessel is under protected. Under
normal circumstances, this should not be a problem as corrosion rates in fresh water are
significantly less than in seawater.
Care should always be taken where dissimilar metals are used next to one another.
Specifications
Dimensions 115 x 65 x 40mm Battery life 2 years (when used once per day)
Mounting bulkhead Ref. electrode Zinc
Power 3 x alkaline AA cells Zones 1,2 or 3 zones
Operating temp. 0 to 500 CResolution +/-5 mV
Range -1300mV to +1300mV Protection IP 65
