Fitted a load of decking July 2015. I used the cleverly designed SPAX screws which are threaded, blank and threaded, with the upper thread going into the decking board while the lower one tightens into the joist.
They seem coated - maybe zinc ? They're supposed to be corrosion resistant.
Just had to lift a board (SWMBO thought she smelt something from the drain) and the 8 screws I removed to do so were definitely a light rusty shade ...
Be fun when they're a few more years old ...