Passivation of Austenitic Stainless Steel
The objective of this thesis is to study the corrosion resistance and passivation of high alloyed austenitic stainless steels exposed to hydrochloric acid. The introductory section gives a background to the subject; electrochemical measurements and surface analysis by the ESCA-technique (Electron Spectroscopy for Chemical Analysis).
Three high alloyed austenitic stainless steels (Fe20Cr20Ni, Fe20Cr20Ni6Mo and Fe20Cr20Ni6Mo0.2N) and a commercial austenitic stainless steel Avesta 254SMO (Fe20Cr18Ni6Mo0.2N) were polarised in deaerated 0.1 M HCl + 0.4 M NaCl solution. Potentiodynamic polarisations of the steels show that Mo lowers the current in the active region. At higher potentials pits are formed on the Mo-free alloy. During the virgin polarisation of the steel, N does not influence the current in neither the active region nor the passive region.
Angle dependent ESCA analysis shows that the passive film formed during passivation at -75 mV(SCE) is in a transient condition, with hemispheres of oxide in the inner part of the film surrounded by an outer hydroxide. At higher potentials the oxide have grown and form a more uniform layer. The thickness of the passive film formed on this type of alloy after 10 min exposure is independent of the Mo- and N-content. Chloride is uniformly distributed throughout the oxide products. The chloride content is about 50 at.% higher on the surface of the Mo-free alloy compared to the Mo-containing alloys. Using angle resolved ESCA, it was found that nitrogen is enriched at the oxide/metal interface during passivation of the N-bearing steels. It is suggested that this nitrogen plays an important role in the repassivation kinetics of the steels.
Because stainless steels are used as implant materials, dissolution of stainless steel type 304 in artificial saliva was studied by electrochemical measurements, ESCA and AAS (Atom Adsorption Spectroscopy). The samples were polarised in the range -400 mV(SCE) to -50 mV(SCE). The total thickness of the surface layer was found to be 25 +- 3 Å independent of the potential. The electrochemical and the ESCA measurements show that 3.5 µg/cm2 of the metal dissolve in the solution at -350 mV(SCE) (near the open circuit potential of the steel) during 18 hours. Extrapolation to 1 year shows that 18 µg/cm2 of the alloying elements is expected to dissolve.