Titanium is IMPERVIOUS to corrosion, which means it will never corrode or wear out.
Because Titanium is so expensive, heat pump manufacturers have looked at various alternative heat exchanger tubing materials and designs to try to lower their manufacturing costs. Some use only one wrap of titanium tubing in their heat exchanger which drastically reduces efficiency (COP).
Another potential solution is to use less expensive copper tubing just like the gas heaters do. After all, copper is a very good heat conductor.
The problem with bare copper is that it will rapidly corrode in high chlorine pool water and a green copper oxide coating will form which is a very poor heat conductor. If corrosion continues then the tube wall will leak and ruin the heat pump system.
To protect the copper some manufacturers have looked at various plastic coatings, however, plastic coatings are just as bad as copper oxide at heat conduction. They insolate the hot refrigerant gas inside the tube from the water outside, lower heat transfer efficiency. This increases the compressor watts and lowers your COP.
In fact, all plastic materials have heat transfer coefficient less than 1% of copper or titanium metal, so even a thin coating of any type of plastic will significantly decrease heat transfer and efficiency. Only the thinnest coating can even hope to come close to bare Titanium in heat conduction and even then the thinner it is the more likely it is to have a small pinhole or incomplete coverage of the vulnerable copper tube underneath.