The effects of two methods for the surface addition of yttrium on the oxidation behaviour of 304 stainless steel have been investigated. Isothermal oxidation tests were performed at 1000°C in air for 100 h. The results show that ion implantation and sol-gel coating have similar effects, reducing signi cantly the scale growth rate. In situ X-ray diffraction analyses clearly demonstrate that surface additions of yttrium allow the oxidation rate of type 304 stainless steel to be reduced by limiting the growth of non-adherent iron rich oxide. In situ analyses were also carried out during cooling in order to observe the structural evolution of oxides formed at high temperature. CEST/1983 The authors are atIt is well established that oxygen active elements such as yttrium or the rare earths allow improvement of the oxidation resistance of chromia forming alloys at high temperature. 1 These elements added to the bulk alloy or to the surface are able to improve the scale adherence 1 -1 6 and decrease the oxidation rate. 2 -7 ,1 1 ,1 2 ,1 5 The exact mechanisms of the reactive element effect are still under discussion. Several mechanisms have been proposed to explain the bene cial rare earth in uence on the high temperature oxidation resistance of chromia forming alloys. Under high temperature oxidising conditions, the oxide scale is thinner and more protective than that of untreated steels. Moreover, the oxide scale adherence is generally improved under thermal cycling conditions. The scale growth mechanism changes from predominantly outward cation diffusion to inward anion diffusion. 2 ,4 -6 ,9 ,1 3 ,1 7 This phenomenon is generally attributed to the blocking of short circuit diffusion paths by yttrium segregation to the oxide scale grain boundaries. 2 ,5 ,6 ,9 ,1 3 Consequently, chromia forming alloys generally develop dense and protective layers of Cr 2 O 3 with low growth rates. 2 -7 ,1 1 ,1 2 ,1 5 The incorporation of yttrium into the matrix improves this protective effect not only by enhancing the selective oxidation of chromium 2 ,3 ,9 but also by limiting void coalescence at the metal/oxide interface. 2 Moreover, lower alloy chromium contents are required in the case of yttrium incorporated chromia forming alloys to develop continuous Cr 2 O 3 external oxides and to suppress the formation of the iron oxides. 4 ,9 ,1 5 The main bene cial effects of reactive elements are physical blocking by reactive element ions or second phase particles at grain boundaries; 6 pegging of the oxide to the alloy; 9 the annihilation of vacancies; 6 ,7 ,1 3 ,1 4 the reduction of oxide stresses; 9 the enhanced selective oxidation of chromium; 2 ,3 ,9 and the formation of a ne grained oxide layer. 2 ,4 -1 0 ,1 2 ,1 5Sol-gel coatings have the advantage of simple surface application by dipping. Such coatings offer potential commercial advantages related to low cost and a relatively easy application process, and avoids alloy manufacturing dif culties. 1 1 On the other hand, ion implantation appears to be one of the best methods to int...
