Found this blog and boy u guys are onto something. I too am battling this question on corrosion. here is my story. I have a hot tub with a swg eco spa generator. Made it one year and the inline heater started to show signs of corrosion around the heater terminal. It was replaced and my pool guy placed a zinc anode disc in the strainer basket and ran a copper wire to the bonding/ground terminal on the side of the balboa heater box. In 3 months the heater was toast. The stainless turned into a rusty dripy mess. The thought was that the swg was putting out too much clorine and maybe aggressive water since he had installed lots of these swg's without very many problems in our area. I questioned the bonding that was done from Coleman since it wasn't a contiuous wire linking all the pumps and heater together. So that heater was replaced and all the bonding was tightened down the way Coleman had installed it since most connections were very loose and dielectric grease placed on those connections. 2 months later now the new heater is destroyed....eaten up. He doesn't know what to think but I know it is stray current in salt water and dissimiar metals causing this. I also think, from my research, that the titanium plates and the stainless in the heater are reacting. The zinc anode goes uneffected since its not bonded to anything, at least thats my thought. One person in the industry suggested to bond everything together but don't hook that bond up to ground or back to the house panel as this is where you get stray current. This way everything is at the same potential. Also hook the zinc anode to the bond wire which it will complete that circuit and the zinc should dissolve and not the stainless heater. Thoughts?
PF: thanks for the great comment!
And, yes. I have quite a few thoughts on this subject. Your description of the problem fits in exactly with what I've come to believe about salt pools; if there is any Stray Current on the grid, proper bonding and salty water will amplify it's damage. Not to say that every electrical appliance attached to a pool shouldn't be properly bonded. They absolutely should be. That's how we keep people safe in water that is filtered, circulated, sanitized and illuminated via electrical devices; through proper bonding.
That's why I can't say that your idea of creating a separate bonding grid for your spa is a good one; because I don't think it would meet code. Check with an electrician in your area to be sure. But the last thing you want to do is to compromise your bonding where water and people are involved. And you may say that you're just creating a separate grid from the other, so it will be safe and isolated from the problems. But if anyone gets hurt, all that will matter is that you may have violated code to try to create a less susceptible bonding grid.
Second, Ground is Ground the World 'Round. Driving a new rod into the ground a few yards away from where your house's bonding rod is doesn't change much. If there are Stray Currents in the vicinity, they will be felt on your new grid as well.
Any electricians out there who want to contribute here, I'd appreciate it. I think my logic is right on this, but I invite everyone's input.
Having said all that; the fact that your second heater failed faster than your first heater AFTER you cleaned up the bonding indicates that the better the bonding for the appliance, the more susceptible it will be to Stray Current Corrosion. IF there is any Stray Current on the grid, it will be amplified by the highly conductive salt water and the cleaned up bonding grid will present even less resistance and a more conductive path to the equipment, in this case your heater.
So, the issue becomes one of either getting rid of the amplifier (salty water) or troubleshooting the grid to isolate and eliminate the sources of the Stray Currents. Here's a link to a story from a pool builder, Rod Ogilve, who had a similar problem with Stray Currents. It's a thread over at the APSP website about salt pools and what the builders think of them. Scroll down to his post that starts, "at the risk of sounding like another salt blast..." It is very applicable to your problem. After that, scroll through the whole thread while you're there. There's lots of great Horror Show pictures of salt damage to pools.
Also, take a look at this post; The Trouble With Heater. Scroll down until you get to the part in italics. it's the latest update to the Never Ending Saga of trying to find a pool heater that plays well with salt. So far, none do.
My best advice is take the salt system off your spa, use a little Twenty Mule Team Borax to soften the water and forget all your problems with Stray Current Corrosion. Or spend hundreds of dollars on electricians to find out that your neighbor two blocks away has a bad sump pump.
Good luck with your spa!
