A water droplet in contact with the metal then makes a great voltaic cell. The electrons can move through the iron to the edge of the droplet, where they can reduce the atmospheric oxygen:. When they meet, they form a precipitate of iron II hydroxide. Is corrosion an electrochemical process?
Ernest Z. Sep 5, It doesn't have to be an electrochemical process, but it occurs much faster when it is. Consequently, the development of methods for protecting metal surfaces from corrosion constitutes a very active area of industrial research. In this section, we describe some of the chemical and electrochemical processes responsible for corrosion.
We also examine the chemical basis for some common methods for preventing corrosion and treating corroded metals. Under ambient conditions, the oxidation of most metals is thermodynamically spontaneous, with the notable exception of gold and platinum. Some metals, however, are resistant to corrosion for kinetic reasons. For example, aluminum in soft-drink cans and airplanes is protected by a thin coating of metal oxide that forms on the surface of the metal and acts as an impenetrable barrier that prevents further destruction.
Aluminum cans also have a thin plastic layer to prevent reaction of the oxide with acid in the soft drink. Chromium, magnesium, and nickel also form protective oxide films. Stainless steels are remarkably resistant to corrosion because they usually contain a significant proportion of chromium, nickel, or both. Instead, the rust continually flakes off to expose a fresh metal surface vulnerable to reaction with oxygen and water.
Because both oxygen and water are required for rust to form, an iron nail immersed in deoxygenated water will not rust—even over a period of several weeks. Similarly, a nail immersed in an organic solvent such as kerosene or mineral oil will not rust because of the absence of water even if the solvent is saturated with oxygen. The relevant reactions are as follows:. One of the most common techniques used to prevent the corrosion of iron is applying a protective coating of another metal that is more difficult to oxidize.
Faucets and some external parts of automobiles, for example, are often coated with a thin layer of chromium using an electrolytic process. With the increased use of polymeric materials in cars, however, the use of chrome-plated steel has diminished in recent years.
While neither chromium nor tin metals are intrinsically resistant to corrosion, they both form protective oxide coatings that hinder access of oxygen and water to the underlying steel iron alloy. As with a protective paint, scratching a protective metal coating will allow corrosion to occur.
In this case, however, the presence of the second metal can actually increase the rate of corrosion. Return Current Path - The metallic pathway connecting the anode to the cathode. It is often the underlying metal substrate.
Galv KnowledgeBase Is there concern about galvanic corrosion when different types of zinc-coated steel are in contact? Galv KnowledgeBase. Necessary Required. Cookies that the site cannot function properly without. This includes cookies for access to secure areas and CSRF security.
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