We have recently being going down to coast with our border collie, and she has really enjoyed playing in the sea.  She had a half-check collar, and after a couple of days we noticed that the chain had all gone rusty, and it has left rust stains in the white fur around her neck.  I have tried ordinary dog shampoo and also white coat shampoo, but nothing will shift them.  I wondered if anyone knows how to get these marks out? 
Many thanks for any advice.