Passivation of Zinc Anodes in lakewater

As you stated the anodes have lost there electrical coupling to what ever they are try to protect.. We used a lot of anodes where the formation of zinc oxide buildup isolating the anodes. We had to periodically remove and clean them up. If the anodes are on an insulator and coupled thru the bolts check and see if someone has put resistance washer under the nut. Make sure you use clean nut and washer when replacing. If not just clean the anode up up and reinstall.
To be absolutely sure if any other anode will work you will probably need a quick survey around what you are protecting. The anodes around the water where I worked were of a mixed variety and all seemed to work.
Offsite I have seen offsite a tremendous variability in marine anodes. If there is any doubt, as I mentioned, get a survey of the potential around equipment.
If they are protecting the equipment stay with zinc and kept them maintained.

Thank you for the info. We plan to do a survey of the flood gates that we are trying to galvanically protect.
Periodic cleaning of the anodes is not practical though. This would require heavy lifting equipment and significant manpower. I think a better alternative would be to go with a different anode material - something that would not react with the dissolved oxygen in our lakewater.

I always understood that zinc anodes work in salt water applications but magnesium was necessary in fresh water systems.
Cathodic protection may be a better option.
I would contact a specialist in this area.