rust vs strengthening

rust

noun
  • A similar substance based on another metal (usually with qualification, such as "copper rust"). 

  • A disease of plants caused by a reddish-brown fungus. 

  • Damage caused to stamps and album pages by a fungal infection. 

  • The deteriorated state of iron or steel as a result of moisture and oxidation. 

  • A reddish-brown color. 

verb
  • To (cause to) degenerate in idleness; to make or become dull or impaired by inaction. 

  • To oxidize, especially of iron or steel. 

  • To cause to oxidize. 

  • To be affected with the parasitic fungus called rust. 

  • Of a black cat or its fur, to turn rust-coloured following long periods of exposure to sunlight. 

strengthening

noun
  • The process by which something is strengthened. 

How often have the words rust and strengthening occurred in a corpus of books? (source: Google Ngram Viewer )