Chemical Conversion Coating

It depends on the part. Processes like anodizing can be for either cosmetic purposes or to increase wear resistance on aluminum parts. Other parts may have a process done to help with electrical conductivity or resistance.

Do you have a specific part or application in mind or is this a homework question?

 

Are they aluminum? Anodizing is a great example of something to help this scenario.

 

I would suspect the coating is to improve chemical/corrosion resistance of the parts depending on what is flowing through them, or if they are two dissimilar metals it could be to help prevent a galvanic corrosion scenario. However, it doesn't say what the ducts are made of in the first place.

As a side note, in optics a black anodize on aluminum parts is used to help reduce reflection and absorb stray light. I doubt this is an optical application however.

 

It sounds like you have a case of relative motion causing wear. The first thing I would make sure is that the V-band clamp is doing it's job and providing the proper clamping force.

I really think you should look into anodizing the parts.

 

So you are saying that the rest of the part, EXCEPT the part that is "fraying" is anodized? Or are you saying that the surfaces, at one time were anodized, but the anodizing has worn through? Saying something is chemically coated is not sufficient information. Be specific.

 

And while you're at it, what are these ducts being used for? You asked why they are coated in the first place, but we have no answer for you without some idea of the application first...