Cleaning the chain on my commuter today I noticed a broken link ... this is my 5th this winter! Anyone have any idea on why so many chain links are breaking?

I live in BC where the roads are usually wet and this winter saw an exceptional amount of salt on the road due to snow. I grease my chain frequently (1/wk), but maybe it's not enough. Do you think that conditions are causing rusting or making the chain more fragile?

It's a 9sp shimano chain that is around 6 months old. If this keeps up I'll have more powerlinks on my chain than regular links Thanks.