Freeze plug problem in 7A-FE

I'm having problems with freeze plugs leaking. I'm replacing the third one now. They have been installed properly but less then two years later leaks occure. Not around the edges but in the center. It appears that the plugs have erroded (not corroded). I've been careful to keep the proper antifreeze levels.

Anyone know of issues with freeze plugs for this engine?
Are there better aftermarket plugs?