It Is Sacrificial Metal.
Babbitt metal (sometimes just called Babbitt or white metal) is a soft, low-friction alloy used for the working surface of plain bearings. Isaac Babbitt, a goldsmith and inventor from Taunton, Massachusetts, came up with it in 1839.
It’s usually a mix of tin or lead as the main ingredient, plus some antimony and copper. The high-tin versions (around 80-90% tin) handle higher speeds and loads better. The cheaper lead-based ones work fine for slower, heavier stuff.
What makes it special is the structure: hard little crystals of copper and antimony sit in a soft tin or lead matrix. As the bearing wears, the soft stuff wears away a bit and leaves oil channels between the hard spots. That keeps things from seizing up and lets the bearing swallow dirt without scoring the shaft.
Originally people poured it solid into the bearing housing. These days it’s almost always a thin lining bonded to a stronger steel or bronze shell.
You’ll still find it in turbines, big industrial machines, some railroad gear, and older engines. Modern car engines mostly moved on to other materials, but the stuff still does the job where you need a forgiving, oil-friendly surface.
Pretty clever 19th-century fix that never really went away.