In System Prefs -> Network
From the pull-down menu choose 'New Location'. You'll notice I have several to keep all my different connections sorted.
Choose your newly-created location in the same pull-down menu, and configure the Airport and/or Ethernet settings to your liking. Once it's all set up, connect to the wi-fi network at your work. You'll need to enter all of the login information this first time. Afterwards, you can manage your locations by going into the Apple menu in the toolbar:
This is one of my favorite OS X features.
Even if you are entering a webpage portal, I think that this will help your keychain to be properly preserved. If not, let me know and I'll have a look more closely at keychain manager.