Most elements and materials when are exposed to the atmosphere, there appearance changed. This change is due to a certain chemical reaction between these materials and the surroundings. Corrosion is one the chemical reactions that is considered as a Redox reaction.

http://fire.nist.gov/bfrlpubs/fire94/PDF/f94026.pdf

First I suggest to know what material you have. You can do chemical or elemental analsis to know what corroded product you have. Then you could explain possible corrosion mechanism (whether it is atmospheric oxidation or chemical reaction from some liquid medium or hot corrosive gases)along with some microscopic characterization techniques.

Dear Dr. Tailor,

Reason behind corrosion is always due to the presence of corrosive medium. Now to find the medium, you can simply cut a piece of sample and do the EDS observation on it. From there, you can find out the presence of corrosive particles in form of ions. As an example, If you find Cl ion by EDS, you can say it is a chloride induced corrosion. Also, you can take some rust in powder form and may do the X-ray diffraction analysis to find your solution. From the analysis report of EDS, you may be able to know all the elements which are present inside the material. On your own prediction you may also try microscopic study for knowing which material it is. You can perform EIS also to characterize the same.

Hope this will help you.

Cheers

Soumya