An anode is an electrode through which conventional current flows into a polarized electrical device. A common mnemonic is ACID for "anode current into device". The direction of (positive) electric current is opposite to the direction of electron flow: (negatively charged) electrons flow out the anode to the outside circuit.
The polarity of voltage on an anode with respect to an associated cathode varies depending on the device type and on its operating mode. In the following examples, the anode is negative in a device that provides power, and positive in a device that consumes power.
In a discharging battery or galvanic cell (diagram at right), the anode is the negative terminal because it is where current flows into "the device" (i.e. the battery cell). This inward current is carried externally by electrons moving outwards, negative charge flowing in one direction being electrically equivalent to positive charge flowing in the opposite direction.
In a recharging battery, or an electrolytic cell, the anode is the positive terminal, which receives current from an external generator. The current through a recharging battery is opposite to the direction of current during discharge; in other words, the electrode which was the cathode during battery discharge becomes the anode while the battery is recharging.
In a diode, the anode is the positive terminal at the tail of the arrow symbol (flat side of the triangle), where current flows into the device. Note electrode naming for diodes is always based on the direction of the forward current (that of the arrow, in which the current flows "most easily"), even for types such as Zener diodes or solar cells where the current of interest is the reverse current.
In a cathode ray tube, the anode is the positive terminal where electrons flow out of the device, i.e., where positive electric current flows in.
Flow of electrons
The flow of electrons is always from anode to cathode outside of the cell or device, regardless of the cell or device type and operating mode, with the exception of diodes, where electrode naming always assumes current in the forward direction (that of the arrow symbol), i.e., electrons flow in the opposite direction, even when the diode reverse-conducts either by accident (breakdown of a normal diode) or by design (breakdown of a Zener diode, photo-current of a photodiode).
In electrochemistry, the anode is where oxidation occurs and is the positive polarity contact in an electrolytic cell.At the anode, anions (negative ions) are forced by the electrical potential to react chemically and give off electrons (oxidation) which then flow up and into the driving circuit. Mnemonics: LEO Red Cat (Loss of Electrons is Oxidation, Reduction occurs at the Cathode), or AnOx Red Cat (Anode Oxidation, Reduction Cathode), or OIL RIG (Oxidation is Loss, Reduction is Gain of electrons), or Roman Catholic and Orthodox (Reduction – Cathode, anode – Oxidation), or LEO the lion says GER (Losing electrons is Oxidation, Gaining electrons is Reduction).
This process is widely used in metals refining. For example, in copper refining, copper anodes, an intermediate product from the furnaces, are electrolysed in an appropriate solution (such as sulfuric acid) to yield high purity (99.99%) cathodes. Copper cathodes produced using this method are also described as electrolytic copper.
Sacrificial anodes mounted "on the fly" for corrosion protection of a metal structure
In cathodic protection, a metal anode that is more reactive to the corrosive environment of the system to be protected is electrically linked to the protected system, and partially corrodes or dissolves, which protects the metal of the system it is connected to. As an example, an iron or steel ship's hull may be protected by a zinc sacrificial anode, which will dissolve into the seawater and prevent the hull from being corroded. Sacrificial anodes are particularly needed for systems where a static charge is generated by the action of flowing liquids, such as pipelines and watercraft. Sacrificial anodes are also generally used in tank-type water heaters.
In 1824 to reduce the impact of this destructive electrolytic action on ships hulls, their fastenings and underwater equipment, the scientist-engineer Sir Humphry Davy, developed the first and still most widely used marine electrolysis protection system. Davy installed sacrificial anodes made from a more electrically reactive (less noble) metal attached to the vessel hull and electrically connected to form a cathodic protection circuit.
A less obvious example of this type of protection is the process of galvanising iron. This process coats iron structures (such as fencing) with a coating of zinc metal. As long as the zinc remains intact, the iron is protected from the effects of corrosion. Inevitably, the zinc coating becomes breached, either by cracking or physical damage. Once this occurs, corrosive elements act as an electrolyte and the zinc/iron combination as electrodes. The resultant current ensures that the zinc coating is sacrificed but that the base iron does not corrode. Such a coating can protect an iron structure for a few decades, but once the protecting coating is consumed, the iron rapidly corrodes.