ferrous vs implacable

ferrous

adj
  • Of or containing iron. 

  • Of compounds of iron in which it has a valence or oxidation number of 2. 

implacable

adj
  • Not able to be placated or appeased. 

  • Impossible to prevent or stop; inexorable, unrelenting, unstoppable. 

  • Adamant; immovable. 

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