Observations on the Anodic Behavior off Nickel and Chromium: Surface Topography and Temperature Effect

Abstract

Metallographic examination of anodically polarized nickel and chromium after long-time exposure at selected active, passive and transpassive potentials in H2-saturated, IN H2SO4 at 25 C (77 F) revealed significant differences in surface topography. Etch figures, present only in the trans-passive state, supported the belief that “active patches” are generated in passive films of these metals at potentials more noble than the passive region. The difference in nature of nickel surface in the early trans-passive region and visible oxygen evolution region is discussed. Effect of temperature on passive nickel (i.e., nickel in passive potential region) over the range 25 to 95 C (77 to 203 F) was determined for H2-saturated 0.5, 1, 5 and ION H2SO4 (pH = 0.85 to −1.05). Apparent activation energies for dissolution of passive nickel were determined. Inflection temperature (Ti) above which “active patches” apparently are formed in passive film was dependent on acid concentration according to the expression 1/Ti × 103 = −0.11 pH+ 3.04 over the pH range investigated and was independent of the direction from which the temperature was approached. The change in apparent activation energy (ΔQ) was a function of direction from which the temperature was approached. Measurements in which temperature was increased stepwise showed that ΔQ is related to pH by the expression ΔQ (kg - cal/mole) = 15,4 pH - 4.7 for acid concentrations studied. Because Ti and ΔQ are dependent on acid concentration, Seeger's activation energy for crystallization of a very thin pseudomorphic film and Kramer's exo-electron emission temperature which are independent of concentration cannot be used to explain fully generation of “active patches” in a passive film as proposed by previous investigators.

The Thermodynamics and Electrode Kinetic Behavior of Nickel in Acid Solution in the Temperature Range 25° to 300°C

“…The limitations therein are well known. For example, it is well known (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11) that the region of thermodynamic stability of Ni + + at 25~ on a potential-pH diagram actually exhibits a wide range of effective passivity. The kinetic data reported in this study for the room temperature range are in agreement with these findings.…”

“…+ E(H+/H2, ealc.) [11] true corrosion potential on the SHE scale corrosion potential of the nickel sample in the autoclave as measured through the solution bridge E(H+/H2, meas.) _--hydrogen redox potential of the floating hydrogen electrode as measured through the solution bridge E(H+/H2, calc.)…”

“…Then E (H +/H2, calc), Ecorr (measured) and E (H +/H~_, measured) were substituted into Eq. [11] to determine the actual value of Ecorr on the SHE scale at the test temperature. Using this technique, the corrosion potential, and thus the potentials of the polarization curves were on the same thermodynamically meaningful scale, SHE at T, as the calculated potential-pH diagrams.…”

“…The temperatures above which passivity was no longer observed is shown below as a function of pH and scan direction Since the results from potentiokinetic experiments are not exact relative to potentiostatic measurements, the above values should be taken as approximate. It is clear however that a critical temperature for loss of passivity does exist and is in the range of 75~176 Myers et aL (11) have reported an inflection point in the Arrhenius plot of log minimum passive current density. This inflection point is a function of pH and is characterized by the equation 1/T x 10 ~ : --0.11 pH 4-3.04.…”

“…Nickel was chosen as the test material for three reasons: (i) Nickel serves as the base element for many alloy systems used industrially for corrosion resistance at high temperatures. (ii) The electrode kinetic behavior of nickel at 25~ is well documented (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11) and could serve as a useful basis for interpreting higher temperature results. (iii) Nickel shows an active to passive transition which is typical of many corrosion resistant industrial alloys.…”