We know that we have to solder fast, but how fast? This question can not be answered without knowing the base metal and the speed it dissolves in the solder material. Of all solder materials, tin based solders, in particular, are especially aggressive dissolving metals. The figure below illustrates the approximate dissolution rates of a few typical base metals in tin.
From the above graph it is clear that Ni has one of the lowest dissolution rates. So it is not surprising that nickel is frequently used as a barrier for soldering.