Here are other anti-corrosion measures for moisture sensors I found:
A moisture sensor with gold-plated electrodes: https://www.dfrobot.com/product-599.html#.UhflOL-N_H0
Replacing metal electrodes with graphite: http://tuxgraphics.org/electronics/200908/eth-flower-watering.shtml
Capacitive (i.e. not resisitive) moisture sensor: https://www.amazon.com/DFROBOT-Gravity-Capacitive-Corrosion-Resistant/dp/B01GHY0N4K/ref=cm_cr_arp_d_product_top?ie=UTF8
Non-corrosive high quality sensor ($$$): https://www.vegetronix.com/Products/VH400/
