This article describes the Faraday paradox in electrochemistry. There is a different Faraday paradox in electromagnetism: see Faraday paradox

Faraday paradox

The Faraday paradox is an experiment that illustrates Michael Faraday's law of electromagnetic induction. Faraday deduced this law in 1831, after inventing the first electromagnetic generator or dynamo, but was never satisfied with his own explanation of the paradox.-The equipment:The experiment...

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The Faraday paradox was a once inexplicable aspect of the reaction between nitric acid

Nitric acid

Nitric acid , also known as aqua fortis and spirit of nitre, is a highly corrosive and toxic strong acid.Colorless when pure, older samples tend to acquire a yellow cast due to the accumulation of oxides of nitrogen. If the solution contains more than 86% nitric acid, it is referred to as fuming...

 and steel

Steel

Steel is an alloy that consists mostly of iron and has a carbon content between 0.2% and 2.1% by weight, depending on the grade. Carbon is the most common alloying material for iron, but various other alloying elements are used, such as manganese, chromium, vanadium, and tungsten...

. Around 1830, the English

England

England is a country that is part of the United Kingdom. It shares land borders with Scotland to the north and Wales to the west; the Irish Sea is to the north west, the Celtic Sea to the south west, with the North Sea to the east and the English Channel to the south separating it from continental...

 scientist Michael Faraday

Michael Faraday

Michael Faraday, FRS was an English chemist and physicist who contributed to the fields of electromagnetism and electrochemistry....

 found that diluted nitric acid would attack steel, but concentrated nitric acid would not. The attempt to explain this discovery led to advances in electrochemistry

Electrochemistry

Electrochemistry is a branch of chemistry that studies chemical reactions which take place in a solution at the interface of an electron conductor and an ionic conductor , and which involve electron transfer between the electrode and the electrolyte or species in solution.If a chemical reaction is...

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Passivation

The key to resolving the paradox is passivation

Passivation

Passivation is the process of making a material "passive", and thus less reactive with surrounding air, water, or other gases or liquids. The goal is to inhibit corrosion, whether for structural or cosmetic reasons. Passivation of metals is usually achieved by the deposition of a layer of oxide...

. When the acid is concentrated enough, and because concentrated nitric acid is an oxidizing agent, the potential of the metal is raised to the point that a layer of metastable Fe₃O₄

Iron(II,III) oxide

Iron oxide is the chemical compound with formula Fe3O4. It is one of a number of iron oxides, the others being iron oxide , which is rare, and iron oxide also known as hematite. It occurs in nature as the mineral magnetite. It contains both Fe2+ and Fe3+ ions and is sometimes formulated as FeO ∙...

 forms on the surface and protects it from further corrosion, even though the pH

PH

In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...

is so low that stable Fe₃O₄ cannot exist. This explanation is supported by the observation that scratching the surface causes a burst of bubbles. Diluted nitric acid is not as strong an oxidizing agent and hence does not raise the potential of the metal to the extent that metastable Fe₃O₄ forms on the surface. In this case, the metal freely corrodes.