I replaced my anodes last year when I did my impeller. I honestly don't know if I bought aluminum or Zinc. It was a package designed for the alpha 1 gen 2 and came as a kit. I feel like the package said it was for brackish water. I will be storing the boat in salt water this year, so obviously need to make sure they are zinc, but they are still in like new condition, so would rather not replace if I do not have to.Is there any way I can tell by looking at them what material they are?