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When trying to deposit materials on substrates such as SiO2, one commonly uses adhesion layers such as Ti or Cr. This is mainly due to the chemical reactivity of the two elements, allowing them to bond easily with the substrate.

After doing some reading into to the properties of Chromium, I learned that it is resistant to tarnishing, which I associate with reacting with the atmosphere. (One of the reasons why Cr is the key ingredients of Stainless Steel.) How is it that Cr is said to be chemically "reactive" alongside elements such as Al or Ti, if it doesn't tarnish easily?

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Here is why.

When Cr reacts with the oxygen in the atmosphere, the resulting oxide film sticks very tenaciously to the Cr, contains no cracks or pores, and conforms perfectly to the shape of the metal beneath it. In addition, the diffusion rate for oxygen through the Cr oxide is very low. This means that once the Cr begins to oxidize, the reaction is quickly choked off and the oxide growth rate goes to near-zero. This behavior is called self-passivation.

By increasing the ambient temperature, the diffusion rate of the oxygen increases, and the oxide film thickens slightly before re-equilibrating. Because the oxide is transparent and has a high index of refraction, it is possible for the thickened oxide to hence exhibit colored interference fringes that indicate the highest temperatures seen by the chrome part; this is the mechanism behind the yellow, blue, and orange discoloration that occurs on the chrome-plated exhaust pipes on motorcycle engines.

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  • $\begingroup$ thanks a lot! I think it answered some of the questions in my head i haven't been able to figure out. $\endgroup$ – BlackPenguin Apr 23 '20 at 16:46

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