MIL-DTL-18001L Class 1 Type ZDM zinc anode assembly with brass plug and replaceable zinc elements. Plug NPT sizes 3/8" to 1". Used for cathodic protection of marine vessels.
MIL-DTL-18001L Class 1 Type ZDM is a replaceable zinc sacrificial anode assembly used for cathodic protection of marine vessel hulls, seawater piping systems, and submerged metallic structures. The assembly consists of a brass retaining plug in standard NPT pipe thread sizes (3/8″, 1/2″, 3/4″, and 1″) that installs permanently into a threaded port in the vessel hull or fitting and a replaceable zinc element that screws into the preceding element in a threaded chain configuration. The screw-together element design allows renewal of the zinc anode by replacing spent elements without removing the brass plug from the hull – a significant advantage for in-water anode replacement without drydocking. The zinc elements conform to MIL-DTL-18001L Class 1 composition requirements which specify the zinc alloy chemistry to provide the correct open circuit potential for sacrificial protection of steel hull plating and underwater hardware. Configurations include zinc elements in 0.500″, 0.625″, 0.825″, and 1.050″ diameter by 0.750″ height configurations.
Available Forms
Plug NPT Sizes: 3/8", 1/2", 3/4", 1", Zinc Element options: .500" dia x .750" ht., .625" dia. x .750" ht, .825" dia. x .750" ht., 1.050" dia. x .750" ht
Common technical inquiries regarding MIL-DTL-18001L CL.1 Type ZDM Replacement Anodes
The brass plug is permanently installed in the hull fitting during initial commissioning or at drydocking and remains in place throughout the vessel's service life. The zinc elements thread onto the brass plug and onto each other in a stack. When the outermost zinc element is consumed a diver can unscrew the depleted element underwater and thread on a new element without disturbing the brass plug or requiring the hull port to be resealed. This eliminates a drydocking requirement for routine anode maintenance on vessels with ZDM-style fittings reducing lifecycle maintenance costs significantly.
MIL-DTL-18001L Class 1 specifies a zinc alloy containing aluminum (0.1-0.5%), cadmium (0.025-0.07%), and trace amounts of other elements with strict limits on iron, copper, and lead. Pure zinc develops an insulating oxide film in seawater that reduces its sacrificial current output. The aluminum and cadmium additions prevent this filming maintaining an active steady dissolution potential of approximately -1.03V (vs. Ag/AgCl) - the correct potential to protect steel hull plating by maintaining it in the cathodic protection range. Off-specification zinc anodes may passivate or have incorrect potentials that provide inadequate protection.
ZDM element diameter is matched to the NPT plug size installed in the hull fitting - larger diameter elements fit larger plug sizes and provide more zinc mass per element extending the service interval between replacements. The appropriate element size for a given vessel depends on the current demand of the protected structure (hull area, paint condition, water salinity and temperature) and the desired inspection/replacement interval. Naval architects and corrosion engineers calculate the required zinc mass using cathodic protection design methods then select the element size that provides the required mass within the installation constraints.
MIL-DTL-18001L Cathodic Zinc Anode Type ZEP Style B
MIL-DTL-18001L Type ZEP Style B round zinc anodes, 2″ to 11″ diameter, 1″ thick. Configurations include 3/8″ or 1/2″ IPS. Used for cathodic protection of steel hulls and tanks.
MIL-DTL-18001L Class 2 Type ZRN zinc rods with full mill certification. Three diameters in 72″ lengths. Used as pencil anode stock and for marine cathodic protection rod applications.
