Capacitive Soil Moisture corrosion (SKU:SEN0193)

Hi all,
I'm working (like many others) on a greenhouse project.
Like many others I've tried to used resistive sensors without major success : this kind of sensor work well but they have a short Time To dfrobot (1 to 4 weeks with my tests). After that corrosion is stronger than the sensor.

I've decided to test the Capacitive version of this sensor ... that work really great ... I've run it during 7 weeks with a measure every 5 min ... but now the older sensor give some sign of corrosion 


We can see corrosion near the DFRobot logo for the sensor on top.

I've think that this sensor was much more resistive to corrosion ... but not so much.

Any hint regarding the problem ? 
I've bought 5 units recently .... I don't want to put them on trash in 10 weeks.