Corner Why are chloride ions and principal Consultant, ph.D.ait,tW. StephenChief Science Officer found at corrosion pair O Docs professionals, LLC sites on spray packaging metals? His found around or in corrosion sites. This month I’d like the corrosion site. In other words, water, formula ingredients andtoward and into the corrosion site to decrease the concentrationof the metal ions and balance electrical charge around and insideello everyone. It is often erroneously concluded thatchloride ions cause spray package corrosion when chloride to discuss how chloride ions are actually involved in spray package negative chloride ions are drawn toward the positively-charged corrosion. Let’s start by defining two terms commonly used in corrosion site to dilute the concentrated metal ion solution formed corrosion engineering and science. A corrosion site is the area(s) by corrosion and balance electrical charge. It should be noted that on the spray package metal where electrons are removed from the any negative ion or molecule could also fulfill the need to balance metal and the metal ions are ejected from the metal. Corrosion electrical charge. In other words, chloride ions are not exclusively sites can be very large areas or small localized areas, such as pits. needed to support spray package metal corrosion. A corrosion product is a complex mixture of metal hydroxides and oxides found around and inside a corrosion site. Corrosion product What is the origin of the concept that chloride ions cause spray colors typically range from black to red for steel and white to black package corrosion? for aluminum. The actual color of corrosion products depends on a Metals and alloys that spontaneously passivate—such as stainless number of chemical factors, such as pH and the amount of oxygen steel alloys—form a very thin, transparent metal oxide film barrier dissolved in the formula water. between the metal and the environment (e.g., your formula). Passive films are approximately 20 Angstroms thick and their How are chloride ions found around and inside corrosion sites? compositions are very close to the theoretical oxygen-metal ratio Scanning electron microscopy (SEM) is often used to examine for a metal oxide. The aluminum, steel and tin metals used for corrosion sites to determine the type of corrosion and help spray packages do not spontaneously form passive films.Chloride determine the most likely cause for the corrosion. A secondary ions damage passive films in small spots by replacing the oxygen SEM technique—energy-dispersive x-ray spectroscopy (EDS)—is ion in the metal oxide film via ion exchange. The resultant metal often used during an SEM examination to determine the chemical chlorides dissolve in water leaving a hole in the passive film. composition of the corrosion product around and inside the Damaged areas of a passive film will quickly regenerate. However, corrosion site. EDS is often erroneously referred to as a surface pitting corrosion occurs whenever the chloride ions damage a analysis method. However, the electrons bombarding the corroded passive film faster than it can regenerate. Hence, chloride ions metal penetrate into the metal and the emitted x-rays are from cause pitting corrosion with metals that spontaneously passivate, both the corrosion product and the bulk metal. Chloride ions are but do not cause corrosion of metals that do not spontaneously be detected by EDS if they are present around or in the corrosion passivate, such as the aluminum, steel and tin used for spray site and corrosion products. packaging. What is the actual role chloride ions play in spray package Research confirms the actual role of chloride ions in corrosion metal corrosion? The supporting role of chloride ions for pitting corrosion of non- Electrochemically active ions and molecules cause metallic passivating steel was confirmed several decades ago with auger corrosion by removing electrons from the metal. Chloride ions have electron spectroscopy studies on the stages of pit initiation and a saturated valen ce shell—much like noble gases. Hence, chloride growth. It was found that chloride ions diffused into a pit after ions have no room in their valence shells for more electrons and pitting corrosion fully developed. I requested that the researchers are not electrochemically active. In other words, chloride ions repeat their measurements with tinplated steel supplied by me. can’t remove electrons from metals to cause corrosion. However, They granted my request and found that the same phenomenon chloride ions are negatively charged. applied to tinplated steel. A corrosion site has a concentrated solution of positive metal Please send your questions/comments/suggestions to rustdr@ ions. This solution produces a large osmotic pressure between the pairodocspro.com. Back issues of Corrosion Corner are available corrosion site and the bulk of your formula.This pressure enhances on CD from ST&M. Thanks for your interest and I’ll see you in diffusion of water, ions and polar molecules from your formula December. 28 Spray November 2012
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