Why does this happen on certain posts

[olrog]

Wondering why I am getting this attack on, in this case, a post, but also a railing and some deck boards. The construction was done in June this year with materials directly from the building market.

 

[Oldboy]

Is the post base sealed at the bottom creating a basin of water for the post to stand in?
Even if it isn't completely sealed at the bottom, it will likely be very damp most of the time since the metal doesn't allow for significant ventilation/drying out after each time it rains and gets wet.
I don't know what the black stuff is but would be surprised if it's not moisture-related.