A journalist who flew over the EC-121 crash site (in the Sea of Japan in April 1969) said he spotted yellow metal in the water. I believe what he saw was zinc chromate. Did 1969-vintage EC-121’s use a zinc-chromate primer?
Quick answer would most likely be yes. Zinc chromate became a very common standard for US aircraft manufacture prior to WWII, it was first developed by Ford in the 1920s. An aircraft like the EC-121 which would be anticipated to see worldwide service would most likely have used it. Also, Lockheed used it in other aircraft that it built prior to the Connie starting production, like the P-38.
Zinc chromate was used post war for some time. If I remember it was sprayed on to aluminium alloy parts inside the aircraft to inhibit rust inside.