junkyard vs slag heap

junkyard

noun
  • A place where rubbish is placed. 

  • A business that sells used metal or items. 

slag heap

noun
  • A man-made mound or heap formed with the waste material (slag) as a by-product of coal mining. Can also refer to the waste by-product from a foundry or furnace, formed into such a mound. 

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