SYDNEY—The world’s $300 billion scrap-metal market is looking beyond junked cars and copy machines to a new prize: the cloud.

Large data centers are mushrooming in number as global demand grows for storage of everything from emails to videogames. Investment in cloud infrastructure has surged since 2015, and the market for data-center equipment is expected to grow at an average annualized rate of roughly 16% this year and next, according to Citigroup Inc.

Cloud...