When on the Corporate profile page, that menu item has the following classes: current_page_item current-menu-ancestor current_page_ancestor. Any of these are attached to a menu item when you are on that page, or a child page.
Target these in your stylesheet, and set the .sub-container div to have display:block;. That should do the trick.
Also, your menu appears to be animated by some JS, I suggest perhaps doing the displaying of the submenu within that same JS file.