DIY marine metals corrosion meter

Solder small pencil zinc, without threaded brass adapter, commonly found in marine engine cooling systems, to flexible insulated wire, flexibility over gauge, only milliamps here, lenght appropriate to geometry of use. Seal with epoxy, need only 1/4 inch of exposed metal to clean off any corrosion from use and storage, this end is immersed in sea water. Connect other end to negative lead of small cheap digital multimeter and its positive lead to clean metal of boat. For steel millivolts readings of 0 to 200 good ,below 0 over protected, obove 200 you have problems. In expensive and less sensitive analogue corrosion meters this is "freely eroding"
Inactive zincs can result from poorly conducting connections or an insulating scale from either oxidation and or marine growth, stiff wire brush will solve the latter, hammer often the former. There are many myths of both natural and induced corrosion so be very skeptical of any diagnostic opinions and test them with the meter. The theory behind all this lies in the fuzzy world of the electron, a thead someone else should start.