Hi can you tell me whether the hinge is Grade 316 or 304 - I have used other stainless stell hinges for external window use and they have rusted within 1 year!

These are grade 201 stainless steel. 
If you are using satin stainless hinges then I imagine what you are seeing is a surface reaction with the finish. Polished Stainless would perform better as there are no grooves for the surface water to sit in.