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Posted by Greg Spitz on Aug 24, 2015 6:48:00 PM

chlorine in elemental form as a green gas

Chlorine is one of the most common elements found on Earth's crust. The name comes from the Greek word for light green, which is how the gas appears in elemental form. It has 17 protons and two stable isotopes giving it a standard atomic weight of 35.45, which makes chlorine the second lightest halogen. It also has the highest electron affinity of any element making it a very strong oxidizer. This means that chlorine will readily steal electrons from other elements. In fact the vast majority of chlorine found on Earth is in the form of the chloride anion (a chlorine atom which has already stolen an extra electron), which will form ionic compounds with many cations (like metals). It is in this form that humans are most familiar with chlorine, as in ionic compound sodium chloride, which we know of as table salt. Chloride ions are important to many chemical and industrial processes including the making of usable chlorine and sodium hydroxide, and desalination and testing of potable water.

Chlorine corrosion on chromium plate steel.
Chlorine corrosion on chromium plate steel (stainless steel).
An abundance of chloride in air or water will seriously increase the possibility of corrosion, specifically the most menacing type of corrosion, pitting corrosion.

There is also a dark side to the chloride ion and its reactivity. An abundance of chloride ions in air or water - by the way the ocean is 1.94% chloride ions - will seriously increase the possibility of corrosion, specifically the most menacing type of corrosion, pitting corrosion. Pitting corrosion is a type of corrosion that occurs on metals which are protected with a passive layer of usually intentional oxidation on the outside of the metal. An example of this is stainless steel, which has chromium oxide as its protective layer. Pitting corrosion occurs when that layer is locally removed (i.e. removed in one, potentially very small, spot) either chemically or with as little as a scratch. For aluminum, the oxide layer protecting the rest of the metal can be removed by the chloride ion in the form of hydrogen chloride, which is outgassed from seawater. Hydrogen chloride can also affect the chromium oxide on stainless steel with varying degrees of success, depending on the amount of chromium used in the metal. After the initial breakthrough, corrosion then begins to attack the local area, digging in to the exposed metal. The particularly terrible thing about pitting corrosion, as opposed to other types, is that it creates a micro-environment as it digs into the metal. This micro-environment becomes more and more acidic, increasing the rate of corrosion. This allows pitting corrosion to dig deep into the metal creating what can turn into a devastating weak point for the material. And of course because it happens so locally, and the products of the corrosion tend to cover over the corroded area, pitting corrosion is notoriously hard to detect.

Materials and the chemicals which degrade them.

More from our series on pitting corrosion, the more specific topic of aluminum corrosion and the related topic of corrosion of electronics.

More about corrosion in this video:

And about Intercept Technology™ in this one:

Corrosion Video

View Intercept Video

 Intercept Technology Packaging products fit within a sustainability strategy because they are reusable, recyclable, do not contain or use volatile components (No VOCs, Not a VCI) and leave a smaller carbon footprint than most traditional protective packaging products.  

 

Gas in bottle picture care of W. Oelen via Wikipedia.

 

 

Topics: corrosion, corrosion intercept, pitting corrosion, chloride

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