Yea it seems like including Ni and Co as ‘ferrous’ materials is only done in certain industries/applications. From a pure chemistry point of view, only iron (Fe) is ferrous. [source: got bachelors in chem]. Also, as mentioned elsewhere, some people/industries [incorrectly] use the term ‘ferrous’ to mean magnetic, but the actual term is ‘ferromagnetic’. Lots of the old terminology (lots of latin) is still used even though we now know they are technically incorrect.