I was told by the Volvo dealer not to use zinc other than the original from Volvo for the IPS (the price are more expensive that any others like boayzinc.com any suggestions or should I use the originals next time again and paid a lot more)
This may not be very timely, just read your post, but anodes (zincs) need to be purchased for the intended water they are used in. i.e. salt vs fresh. Short answer, Magnesium should be good for fresh, Al or zinc for salt. Best to consult an ABYC certified marine electrician.
I bought a 43 Sovran with IPS and the previous owner skimped on the Engine zincs mounted on the transom...had to buy a new set of props ($3500) because of the electrolysis. For me, only Volvo zincs on the transom for the IPS protection and only Volvo zincs in the lower unit. Yeah Volvo zincs are expensive but it’s cheaper than props.
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Zinc purity is not guaranteed to be optimal for the ips on aftermarket zincs at least that's what they will say.