An anode of zinc, aluminum alloy, magnesium or another metal more active than the metal to be protected, connected electrically so that it dissolves first and protects the metal as the cathode.
Japanese name: 犠牲陽極
Sacrificial anode appears in 1 practice question across 1 topic, including Corrosion and surface treatment.
Cathodic protection comes in 2 forms: the galvanic (sacrificial) anode system, which connects a metal more active than the one to be protected and uses the current produced as it dissolves, and the impressed current system, which drives current from a DC power supply. Sacrificial anodes need no power source or wiring management, are simple to install, and carry little risk of sending stray currents into other structures. On the other hand, the voltage they provide is small, so the current tends to be insufficient in high-resistance soil or fresh water, and impressed current systems are chosen for large structures and high-resistance environments.
In seawater, anodes of zinc and of aluminum alloys containing In and Zn are mainly used. Pure aluminum forms a passive film and stops dissolving readily, so elements such as In are added to hinder film formation and let it dissolve steadily. Magnesium has the most active potential and provides a large driving voltage, so it is used for pipelines in high-resistance soil and for freshwater equipment. In seawater, however, too much current flows, it is consumed quickly and can overprotect the steel, so it is seldom used there. Anodes are chosen by potential, charge available per unit mass and price.
The charge an anode can deliver is set by Faraday's law; the theoretical values are about 820 Ah/kg for zinc, about 2980 Ah/kg for aluminum and about 2200 Ah/kg for magnesium. In practice, part of the anode is lost to self-corrosion without contributing to protection, so the usable charge is less than theoretical; for magnesium in particular it is said to be about half. For example, supplying 1 A of protection current for 1 year requires 8760 Ah, so even on the theoretical value about 11 kg of zinc is needed. In design, the number of anodes and their replacement interval are determined from the required current, the number of years and the usable fraction of the anode.
Whether sacrificial anodes are protecting a structure is judged by measuring its potential with a reference electrode. Common protection criteria for steel are a potential more negative than −0.80 V against a silver/silver chloride electrode in seawater, and more negative than −0.85 V against a saturated copper sulfate electrode in soil. If the potential becomes too negative, hydrogen evolves on the surface and can cause hydrogen embrittlement of high-strength steel or disbonding of coatings, so overprotection must also be avoided. Anodes are consumables, so the remaining amount is inspected regularly and they are replaced before they are used up.
Answer: A. About 0.32 V; zinc dissolves and the steel is protected
The emf is the potential of the cathode side (the more noble iron) minus that of the anode side (the more active zinc): E = −0.44 − (−0.76) = 0.32 V. The active zinc becomes the anode and dissolves as Zn → Zn²⁺ + 2e⁻, and the electrons flow into the steel, making it the cathode, so dissolution of the steel is suppressed. This is cathodic protection by sacrificial anodes (the galvanic anode method), used for ship hulls, harbor steel structures and buried pipelines; the anodes are consumed and are replaced periodically. About 1.20 V is the mistake of adding the magnitudes of the two potentials. The choice of about 0.32 V with the steel dissolving reverses noble and active. Actual potentials in seawater differ from standard electrode potentials, but zinc is still more active than steel.
Yes. Galvanizing can be thought of as a layer of sacrificial anode over the whole steel surface; even if a scratch exposes the steel, the surrounding zinc dissolves first and protects it.
It depends on the current needed for protection and the mass of the anodes, and designs range from a few years to several decades. On ship hulls, it is common to inspect wear and replace the anodes each time the ship is drydocked.
Because an anode produces current by dissolving itself, and covering its surface with paint stops the current and destroys its protective action. When a hull is repainted, the anodes are left unpainted.
This is cathodic protection by sacrificial anodes (the galvanic anode method), used for ship hulls, harbor steel structures and buried pipelines; the anodes are consumed and are replaced periodically.
A zinc sacrificial anode is attached to a steel structure i…Faraday's law of electrolysis Galvanic corrosion Local cell Passivity Pitting corrosion Standard electrode potential Stress corrosion cracking