1966 Cessna 172 Control Yokes --- What Alloy?

I need to treat them against corrosion.

The yokes are rough looking, corroded from the hands of white-knuckled
sweaty students, and the paint chipped so I stripped the paint off to
refinish them. The metal doesn't act like the usual aluminum alloy. When
exposed to a drop of phosphoric acid, it turned dark. In my experience
typical aircraft aluminum alloy gets silvery and shiny when dipped in a
phosphoric acid bath. Could it be magnesium?

Great airplane, has mogas STC and flies on $1.89/ gallon gasoline: the
envy of the other ramp queens. Just like the airplane Kyle learned to fly
in too, making it historic and a keeper. Slow but sure and a pleasure to
fly.