Electrochemical (Corrosion) Behavior of Amorphous/Microcrystalline Ion Plated Fe-Cr Alloy Films

Abstract

Abstract Amorphous and microcrystalline thin films of chromium-bearing iron alloys produced by ion plating exhibit high resistance to localized (pitting) corrosion in neutral NaCl aqueous solutions. The films are produced by thermal evaporation of the alloys through an argon glow discharge onto Ebrite (Fe-Cr) substrates maintained at low temperatures. Potentiodynamic tests indicate that the ion plated films exhibit a Tafel-like behavior in the “passive” potential range, whereas Ebrite and melt-spun amorphous alloys of composition similar to the films have a constant current passive region. Transient current data obtained from tests at successively higher potentiostatic voltages (V) show that such currents decrease exponentially with time. The current relaxation time constant (τ) of the films are five to ten times larger than those of Ebrite, and increases rapidly with V up to about 0.3 V, decreasing slowly thereafter. For the Ebrite substrate, τ shows only increasing values, but with a sharp discontinuity in the V versus τ curve at about 0.4 V. Transient data obtained for bulk amorphous Metglas 2826A above 0.5 V shows that the variation of τ with V is similar to that of the ion plated films. The results indicate that it takes longer to complete the passive layer on the ion plated films than on the crystalline substrate; the decrease in τ with increasing V in the middle potential region may be related to the amorphous or amorphous/microcrystalline structure of Metglas 2826A and the ion plated films. The results are consistent with a change in the character of the passive layer at 0.3 to 0.4 V.