Most technical Ni-base alloys are so-called borderline materials from an oxidation point of view, i. e. their Al contents of 3-5 wt.-% are not sufficient to form a continuous dense and protective aluminium oxide layer on the surface. For this reason usually coatings are applied with Al contents of 8 wt.-% and higher. Unprotected Ni-base borderline alloys form a fast growing mixed oxide scale. Underneath the fast growing oxide scale internal oxidation takes place leading to discrete aluminium oxide particles which do not form a continuous diffusion barrier. Dense continuous and slow growing protective alumina scales can, however, be achieved even on borderline Ni-base metals by applying the halogen effect. Following Wagner`s theory of oxidation the formation of a dense alumina scale can be obtained, if the Al-activity on the surface is increased. The halogen effect was used as a method to increase the  Al-activity. Thermodynamical calculations for the alloys IN738  and IN939 show the existence of a region for a positive fluorine effect. Fluorine ion implantation was used to transform the thermodynamical predictions into F-concentrations on the metal surface. After oxidation (60h/1050°C) a protective alumina scale was found for  IN738. The oxidation protection still works after 1000 hours. However no protective scale was obtained for IN939. The PIGE was applied to study the F-loss during heating. The data reveal the limits of the thermodynamical predictions and explain the experimental results. The implantation of reactive elements (e. g. Y) improves the scale adherence.