Contacts corroding - why?
I would place the problem here on the connector itself. Just looking at the picture and the poor quality with which the solder has failed to flow around the connector pin tails tells this story. 

In the past I have had some connectors that were sourced from Asia that had horrible plating and oil and other contaminants on the pins that made them almost impossible to solder. Even with copious amounts of flux the soldering job could not be done. 

I suggest that you try a different supplier and run some comparative solderability tests.