We are planning on installing a sacrificial anode on a granite water feature with stainless steel grating. The grating is rusting/deteriorating, possibly at weld sites and looking to slow down the degradation. Is Zinc the best metal to use?
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Thanks Terry, this is great information.
The feature does have a liner of unknown material and the light fixtures are plastic fiber optic. No other metals are in contact with the feature outside of the pool equipment including a Pentair NT400. We use low dose (0.5-1.0ppm) chlorine (tri-chlor tabs and di-chlor granules) along with Ozone & Nature 2 mineral. The pH tends to drift upwards as the make-up water has a pH of 8.0+ along with 500 ppm Calcium.
With your permission, I will forward your comments to the builder to gather more info.
Thanks again, more to follow.
Does the fountain comply with NEC Article 680 for an equipotential bonding grid connecting all metal parts? Understand it's granite so maybe no rebar, but the metal equipment housings and all metal parts in touch with the water (including encased rebar) should be bonded with a # 8 AWG copper bonding conductor which prevents metal degradation as well as reduce the potential for electrical shock. NEC does exempt small metal parts. You could also change the grate to a non-metal alternative. Usually sacrificial anodes are not needed if metals are bonded properly. There might be stray current promoting the oxidation that should be looked into. The chlorination scheme may also be overly aggressive -- does it use acidic low pH trichlor pucks or extreme free available chlorine levels? I have seen many non-bonded stainless steel grates that have held up for over 10 years in chemically supervised pools, but welds are generally or more susceptible to rusting. It could be that the granite (mostly metal oxides) and stainless steel create a short circuited battery next to one another. Here is something I found from www.neetrac.gatech.edu that might help. “Some pools are lined with insulating materials (vinyl, tiles etc.) and do not have any metal parts such as a ladder or lighting fixture in contact with water. In such a case, intentional bonding of the water is necessary to equalize the water-to-deck voltages. The 2008 NEC requires the installation of conductive metal fitting(s) or equipment totaling a minimum surface area of 9 square inches (equivalent to metal strip in underwater pool light fixture) in contact with water and bonded with the equipotential grid to mitigate the neutral-earth voltages. The metal fitting must consist of a corrosion resistant material such as stainless steel or aluminum.” I suggest bonding both the granite and water (per the above) and the SS grate to the equipotential equipment bonding grid. I have doubts that a traditional sacrificial anode would work if this is what is going on.