Your query is not finding what you are looking for. You are all companies from A and matching them with all companies in B, meaning it will return a record for Societe Generale <> Fortis
Start with this:
SELECT B.Corporation, B.C_Name
FROM data_excel B
LEFT OUTER JOIN corporation_name A
ON B.C_Name = A.Name
WHERE A.Name IS NULL
This will still not solve everything, you will still have to replace
ON B.C_Name = A.Name
With something like what John Doyle suggested, because Société will still not match Societe!