Rust

Rust is scientifically called oxidation, which occurs when oxygen comes in long-term contact with certain metals. Over time, the oxygen combines with the metal at an atomic level, forming a new compound called an oxide and weakening the bonds of the metal itself. If the base metal is iron or steel, the resulting rust is properly called iron oxide. Rusted aluminum would be called aluminum oxide, copper forms copper oxide and so on.
It is a process that doesn't just occur over night, and because of this the process can be used as a medium for art, to reveal something. The process could be recorded as what ever is hidden is slowly revealed.
Here are some examples I found:-

 

Found this artist who deals in 'weathered art' called Danno. His technique of rusting steel to create art has generated much interest. His weathered works seduce people to take a closer look, to make sense of what they are seeing.

"Predominantly created on steel, my art is about weather, about snow, rain, and sun, about how these elements work together to create rust which I work into textures, shapes, and forms. I’m completely smitten with the look and feel of rust and my technique of corroding metal seems to have generated a lot of interest.

"I find inspiration in the natural forces around me. From the weather systems that co-create with me to the plant life of the West Kootenays where I live. By rusting steel, I am able to create works that have elements of organic chaos about them. No two pieces ever corrode the same."

 

Beautiful and amazing!