cast iron vs iron-clad

cast iron

noun

  • A hard and brittle, but strong, alloy of iron, carbon, and silicon, formed by casting in a mould. 

adj

  • Inflexible or without exception. 

  • Made of cast iron. 

  • Durable; tough; resilient. 

iron-clad

How often have the words cast iron and iron-clad occurred in a corpus of books? (source: Google Ngram Viewer )