Zinc Anodes and Zinc Sheet

When two metals touch in seawater or wet soil, the more active one corrodes and the more noble one is protected. Zinc is more active than steel, bronze or stainless steel, so when it is electrically connected to them in the same electrolyte, current flows from the zinc through the water to the steel. The steel becomes the cathode of a small cell and stops rusting; the zinc dissolves in its place.

Two conditions must be met. The anode needs a good metallic connection to the structure, usually by welding a steel insert or bolting to clean metal, and it must sit in the same water or soil. A zinc block in air protects nothing.

ASTM B418 is the standard most anode specifications refer to. Type I anodes are for seawater, brackish water and other saline electrolytes; their chemistry meets the US military specification MIL-A-18001, with small controlled additions of aluminium and cadmium that keep the zinc active instead of building up an insulating crust. Type II anodes are high-purity zinc for fresh water, soils and backfill, where that alloy would not suit.

Using the wrong type is a classic failure. Ordinary commercial zinc in seawater can coat itself with a hard white film and stop working within weeks while still looking almost new.

• Ship and boat hulls, rudders, propeller shafts and sea chests
• Ballast tanks and cargo tanks in vessels
• Seawater-cooled heat exchanger and condenser water boxes
• Jetty piles, sheet piling and other marine steelwork
• Buried pipelines and tanks in low-resistivity soils
• Water heaters and storage tanks, though magnesium is more common there

All three are used for sacrificial protection, and each fits a different environment. Magnesium has the highest driving voltage and is used in fresh water and high-resistivity soil, where zinc cannot push enough current. Aluminium-zinc-indium alloys give the most charge per kilogram and are now common on offshore structures and large ships. Zinc sits in between: dependable in seawater and brackish water, tolerant of mud and sediment, and simple to specify.

Zinc anodes lose effectiveness in warm water, roughly above 50 °C, so they are not used on hot condenser outlets or hot water tanks. Do not mix anode types on one vessel without advice from a corrosion engineer.

How much zinc a structure needs depends on the bare steel area, the coating condition, the water and the design life. Naval architects and cathodic protection engineers calculate it; as a buyer, the useful thing is to replace anodes with the same type and weight unless that calculation says otherwise, and to record the date each set was fitted.

Zinc sheet and plate are used for fabricated anodes and earthing straps, as lining and flashing in some building work, for photo-engraving and printing plates, and as anodes in electrogalvanising lines, where zinc is plated onto steel strip or small parts. Zinc dissolves in acids and in strong alkalis, so it is not a structural material for chemical service.

Galvanising works and die casters usually buy zinc as ingot rather than sheet; see zinc ingots. In India, zinc ingot is specified to IS 209, with Special High Grade at 99.995 % zinc as the top grade.

For anodes, tell us the environment (seawater, fresh water or soil), the anode type to ASTM B418, the shape and dimensions, the insert or strap detail and the quantity. For sheet, give thickness, sheet size and purity. Sizes are confirmed at enquiry, and every supply comes with a test certificate through a Govt. recognised test house. Marine buyers will find related materials on our marine and offshore page; zinc anodes are also the usual partner for nickel-copper alloy and bronze fittings on steel hulls.