Is it forbidden to wear the niqab only at work, then take it off afterward?

The scholars have agreed on the obligation for a woman to cover her face when there is a fear of fitna (temptation/discord) through her or for her. Today, the absence of fitna is hardly achievable, so covering the face is obligatory in places where there are non-mahram men who can see her. This obligation remains constant for a Muslim woman whether she covers her face or not. Wearing the niqab in some places is better than perpetually uncovering the face, less sinful than permanent sufoor (uncovering of the face), and fulfills some of the obligation.