Heating element corrosion

[Cusser]

I understand that too-low pH can corrode heating elements in a spa. 

I have a 31-year-old 240 VAC CalSpa, and it takes screw-in heating elements, look the same as on a home water heater. The first two times I replaced the heating element, I bought from CalSpa by mail, and they were somewhat expensive, and back then I removed the stainless steel manifold which housed the element, a big hassle.  Then my daughter figured out that if I cut a recess into the inner plastic shield/liner, then I could unscrew the bad element and guide that into that recess, then simply remove it and install a new element, genius !!!  Since then, I've simply purchased 5500W (and occasionally 4500W) elements at the hardware store and used those, either straight or curved-back types, of these configurations.

 

Over the past 25 years I've had the elements go out and sometimes the element threads corrode and will leak, indicating that I need to replace it.  Yesterday, when I went to vacuum the spa (see a new different thread), I noticed that all the water had leaked out, the element had corroded and backed out. The stainless steel housing itself is not corroded or damaged, so easy R&R of the element.

So I have a question about heating element corrosion, of the threads themselves: Are spa heating elements metallurgically different than home water heater elements, so worth buying specific spa elements themselves?

[Cusser]

My records show the previous element lasted like 13 months.  Water pH is just under 8.0.