Illustrated Case Studies in the Maintenance Reliability Engineering World of Failure Analysis, Predictive Maintenance, and Non Destructive Evaluation |
 The report said "... oxidative attack that has caused outward diffusion of metal resulting in a spongy structure." How in tarnation does a material get spongy? Does that simplified description mean exactly what I think it to mean?
This description simplifies a very important electrochemical reaction. It is important to understand this from a material properties standpoint. Especially when trying to understand how oxidation and corrosion work to form a deadly tag team. The picture to the left shows a dark oxide scale (top) and the metal (bottom). The oxide layer had penetrated the spongy material (wavy appearance). This sample came from a furnace wall. It was 309L stainless steel. The oxide layer was on the "fire side" of the vessel. It operated at about 200C. Electrochemistry in simple terms is nothing more than a battery. That's right, a battery. One electrode wants to oxidize and give up electrons, while the other wants to reduce, or accept electrons. To facilitate this marriage, there needs to be a closed circuit with an electrolyte. That's where the oxide scale comes into play. Metals want to oxidize. Why, because metals were originally simple oxides down in the earth. Forcing them into being metals forced them into a higher more unstable state of energy. By the very nature of what we did, metals want to return to their natural lower energy oxide state. It's a fundamental law of thermodynamics. Everything wants to return to a more stable, less excited state, that's why we invented Prozac. OK, so what's going on? Here's how the battery works. Metal ions are formed at the metal-scale interface, while oxygen is reduced to oxygen ions at the scale-gas interface. The oxide acts as the electrolyte. If you remember your grade school battery test you'll remember that the anode got smaller. Those holes you see in the picture, those are locations where the metal ions were formed and then migrated into the oxide layer, making the oxide layer bigger. Depending upon other variables, the oxide layer may grow more at the gas interface, or the metal interface. That may be one reason for the wavy appearance in the oxide layer. Here's the real clincher. The rate at which that scale forms and continues to grow is dependent upon how easily the ions travel through the oxide layer. Make that oxide layer a lousy conductor, and you have more resistance to ion exchange. That's one way protective oxide layers work. Another thing, whatever product is on the oxidation side, that product has a great influence on the electrolyte. Simply put, it becomes the electrolyte. Whether those elements oxidize or reduce more readily has a great effect on the rate of diffusion of those metal ions. |
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| There is an excellent discussion of this process in the book Corrosion Engineering, by Fontana and Greene, McGraw Hill, 1967. |
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