Lab 6 Addendum - expected active corrosion rate. The...

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Lab 2 Addendum In order for corrosion to proceed, both an ionic and electronic circuit must be present. If there is some impedance to flow in either of these circuits, the rate of corrosion is reduced. One example of ionic impedance would be the diffusion limit of oxygen from the atmosphere to a cathodic site. Another example is a "passive film". A passive film is a very thin layer of material composed of an oxidized metal and some other component, commonly oxygen or hydroxide. In many cases, this passive film may be only a few atoms thick and can even be less than one atomic layer thick. The films are so thin, they frequently do not take on the structure of a "bulk" oxide. The passive film can be an ionic barrier, an electronic barrier, or a combination of both. A passive film is capable of decreasing the corrosion rate many orders of magnitude below the
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Unformatted text preview: expected active corrosion rate. The passive film is very fragile and can easily be damaged. If the film can be reformed, then it is protective. For example, aluminum in air is protective and can reform if damaged. It can be "healed". If the aluminum is under a drop of liquid mercury, the passive film cannot be healed and the aluminum reacts with the mercury if damaged. In general, passive films form at high interface potentials. They are characterized by a passivation potential above which, the metal is passive. In the lab demonstration, the passivation of steel in concentrated nitric acid will be examined. Make note of the behavior of the steel in the various environments to which it is exposed. Explain the behavior of steel in: 1. Water 2. Concentrated Nitric Acid 3. 50% mixture of water and concentrated nitric acid 4. Copper sulfate solution...
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This note was uploaded on 01/26/2010 for the course ENGLISH 45 taught by Professor Morris during the Spring '10 term at University of California, Berkeley.

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