Lecture 20 Passivity Definitions and Influencing Parameters Keywords: Definition of Passivity, Flade Potential, Anodic Polarization, Critical Anodic Current Density. In the Eh ph diagrams, resistance to metallic corrosion is indicated at stability regions where either the metal remains thermodynamically stable (immunity) or the metal surface is covered with an oxide / hydroxide layer (passivity). Passivity is due to the formation of thin, impermeable and adherent surface films under oxidizing conditions often associated with anodic polarization. Only certain metals and alloys exhibit active-passive behavior, which is essentially an acquired property. Faraday in the 1840 s showed that iron reacted rapidly in dilute nitric acid, but was visibly unattacked in concentrated (fuming) HNO 3. An invisible surface oxide film formed in concentrated acid was found to be unstable in dilute acid and through scratching, the surface oxide could be removed. Definitions of passivity as proposed by Uhlig are given below: 1. A metal active in the EMF series or an alloy composed of such metals is considered passive when its electrochemical behavior becomes that of an appreciably less active or noble metal. 2. A metal or alloy is passive if it substantially resists corrosion in an environment where thermodynamically there is a large free energy change associated with its passage from the metallic state to appropriate corrosion products. 1
Examples for definition 1 are Cr, Ni, Ti, Zr and stainless steels. Examples for definition 2 are lead in sulfuric acid, magnesium in water and iron in inhibited pickling acid. Two types of passivity thus exist. a) A metal is passive if it resists corrosion under anodic polarization (noble potential, low corrosion rate). b) A metal is passive if it resists corrosion in spite of thermodynamic amenability to react (active potential, low corrosion rate). The Eh ph diagram for the Fe H 2 O O 2 system can be superimposed on that for chromium to understand the role of chromium as an alloying addition in steel for enhanced corrosion resistance (Fig. 20.1). Chromium forms very stable, thin and resistant surface films in less oxidizing conditions. Chromium addition is the basis for stainless steels and other corrosion resistant alloys. Fig 20.1 Eh ph diagram for iron superimposed on the chromium diagram (enhanced passivity range due to stable Cr 2O 3) 2
Since chromium is capable of forming a very stable oxide at much lower potentials, alloying with chromium (minimum 12%) leads to development of corrosion resistant stainless steels and cast irons. Other metals that can form passive surface films include aluminium, silicon, titanium, tantalum and niobium. Electrochemical basis of active-passive behavior is illustrated in Fig. 20.2 Fig 20.2 Potentiostatic Anodic polarization curve E pp Primary passive potential, above which passive film becomes stable. i crit = Critical passivating anodic current density, at which passivity is induced. i pass Passive current density. 3
On increasing the potential beyond the passive region, the passive film breaks down and anodic corrosion current further increases in the transpassive state. Oxygen evolution at the anode occurs at higher potentials. Based on the above, it is possible to establish a) Passive potential region. b) Passive corrosion rate and c) Necessary conditions to achieve and maintain passivity. Decay of passivity on interruption of anodic current is characterized by Flade potential. If the potential as a function of time is monitored after interrupting the applied current, the potential value first changes to a value more noble on the hydrogen scale, then slowly changes and finally rapidly decays towards the normal active value. The noble potential reached just before rapid decay was found by Flade to be more noble, the more acid the solution in which passivity decayed (Fig. 20.3). Fig 20.3 Decay of passivity showing Flade potential 4
E F = E 0 F 0.059 ph (for Fe, Ni, Cr and alloys of Fe). Stability of passivity is related to E F. The lower the E 0 F, the easier it becomes for passivation and higher film stability. For Cr Fe alloys, the value ranges from 0.63 V to -0.10V with 25% chromium addition. 5