Localized Corrosion on Slip Steps of Aluminum Straining

Abstract

The initial steps of localized corrosion of aluminum strained in aggressive solutions were studied by electron microscopy of oxide replicas. Aluminum samples were strained at a constant potential in the following deaerated solutions: 1M NaC1 (pH 7) ; 4M NaC1 (pH 11) ; and 1M NaNO8 (pH 7). No pitting was found on the aluminum strained in those solutions below the pitting potential; the slip bands showed the same substructure as that found on aluminum strained in air. At, and above, the pitting potential, pitting was found on the surface of the strained aluminum. Pitting nucleated on the whole surface in neutral NaC1 solutions. In 4M NaC1 solution (pH 11) and in 1M NaNO~ solution pitting nucleated preferentially on the slip bands, leading to semicylindrical pits. It was also concluded that mechanical film breakdown is not a sufficient condition for pit initiation. Another process, such as localized aci.dification, must be assumed to explain pit initiation at pitting potential.

Mechanism of Passivity Breakdown of High Purity Cadmium

“…If reducible anions are absent, there are only two processes responsible for proton consumption in the acidified zone. One will be hydrogen evolution, which is the only cathodic reaction possible in the locally acidified zone 2H + + 2e =/-/2 [5] and the other will be proton diffusion from the acidified zone to the bulk of the solution dif.…”

“…The present work is part of a research program in which the passivity breakdown mechanism of several high purity metals and binary alloys was studied in the presence of different electrolytes. The results obtained in the case of high purity aluminum (1)(2)(3)(4)(5)., high purity zinc (6), high purity iron (7,8), and binary A1-Cu (2, 9), A1-Mg (10), and A1-Zn (10) alloys have been published. These results suggested that in all the systems so far studied the pitting potential was the potential above which localized acidification could be maintained on the metal-solution interface (11)(12)(13).…”

Pitting Corrosion of Metals

“…One factor that may have influenced Galvele in developing his pitting model was his experience of the role of plastic strain in stress corrosion cracking, which led him to study the pitting of plastically straining metals. [7][8][9] The reasoning is simple-if pits prefer to nucleate at sites of passive-film rupture, yet still respect the pitting potential, the role of Newman (continued from previous page) the passive film in setting E pit has to be questioned. The passive film, according to this argument, merely affects the pit nucleation rate.…”

2‐hydroxyethilammonium oleate protic ionic liquid as corrosion inhibitor for aluminum in neutral medium

“…During this first contact between the aluminum 1100 electrode and the electrolytes of only NaCl and NaCl with PIL at lower concentration, OCP instability could have been caused by the competing reactions of passive layer rupture by the chloride anions, with pit initiation, and the pit repassivation. [ 4,33,34 ] Because the same OCP variation was not observed for NaCl with 5 × 10 −4 mol/L 2HEAOl, which could be attributed to the formation of a uniform adsorbed layer of the organic compound that could hinder the formation of incipient pits and promote the stability observed in Figure 2a.…”

“…During this first contact between the aluminum 1100 electrode and the electrolytes of only NaCl and NaCl with PIL at lower concentration, OCP instability could have been caused by the competing reactions of passive layer rupture by the chloride anions, with pit initiation, and the pit repassivation. [4,33,34] Because the same OCP variation was not observed for NaCl with 5 × 10 −4 mol/L 2HEAOl, which could be attributed to the formation of a uniform adsorbed layer of the organic compound that could hinder the formation of incipient pits and promote the stability observed in Figure 2a. On the contrary, the addition of 5 × 10 −4 mol/L 2HEAOl (Figure 2a) shifted the potential toward more negative values during the first minutes of the experiment, possibly due to the modification of the structure of the passive layer, [35] or due to a more accentuated reduction of the kinetics of the cathodic processes, than of the anodic ones.…”