The black heating rods on the Camco 4500W and 5500W ULWD models are made from the highest Grade Nickel and Chromium Incoloy which is high temperature and corrosion resistant. It is essentially a mix of premium grade nickel and stainless steel. Incoloy is about 8 times more expensive, lighter, and more corrosion resistant than standard 304/316 stainless steel.
These heating elements are principally installed in home water heaters used in potable water systems. Tens of thousands are most likely in use worldwide in hot water tanks built by American, A.O. Smith, Bradford White, Craftmaster, General Electric, GS Wood, Kenmore, Lochinvar, Maytag, Mor-Flo, Reliance, Rheem, Richmond, Ruud, Sears, State, and Whirlpool Heaters, to name a few.
The threaded silver base that you ask about is, however, not corrosion resistant. Some rust may form on this base if water is left in the kettle for a prolonged period of time. These heating elements are normally used in electric water heaters that also have a sacrificial anode installed to combat this issue. The anode's sole purpose is to oxidize first, thereby protecting the heating element base from rusting. Brewing kettles do not have an anode so make sure you do not leave water in the kettle for a prolonged period of time. Leaving water in during a typical brewing period is not an issue. In fact, a protective coating will form on the element base in the Boil Kettle after a few uses. It's more of an issue in the Hot Liquor Tank if you leave water standing for extended periods. This may be exasperated if you have hard water. Some brewers will install a magnesium anode in the side of their kettle to combat this issue. See this post
for some example installations and instructions.