Since the 6V supply that you are switching is greater than the 5V Arduino pin output, you should also connect a resistor between the base and emitter of the transistor. Make it the same value as the base resistor to get a margin of 1.2v, or half the value of the base resistor to get a margin of 1.8V. Get rid of the resistor in series with the emitter [EDIT: looks like you have already done that bit].