JW,
Sorry for my late reply.
I think you already understand the difference from the diaphragm and impeller forms of pumps.
So I don't need to go there to describe the issues.
I think you already nailed the problem when the company started replacing your item.
Actually I don think they've done you or others any good service but I'll leave that for now. No problem actually can't because that's the root problem.
I suspect it is the inferior materials in construction.
They have found another manufacturer and they have produced or used inferior grades of metals.
Just because the stamp says ss 316 or similar,the supplier of those metals will be the problem.
The corrosion is the fact that the material compound is low grade.
Hence you are getting high levels of corrosion in moving parts.
And to boot you may even have a lower quality of seal, o rings, and the list goes on.
So disappointing to here it's rule.
I have taken the approach myself to disassemble any brand. And replace seals and lubricate with a high quality purpose made grease.
The consumer in me that pays for a quality product puts it squarely back on the manufacturer via the supplier or retail outlet.
After two times.
They refund me all my money and supply me with the product they sold me with further costs for them with any replacements.
We all have to stand up and demand better products at the manufacturers expense until they sort out there problem.
Not much help I know, but you already figured the problem.
It's how you deal with it.
Just cruising in my now sweetas pimped out Southern 755 HT0!