On the Initiation process of pitting corrosion on austenitic stainless steel in chloride solutions

Abstract

Seys, van Haute and Brabers: Initiation process of pitting corrosion on austenitic stainless steel in chloride solutions 663 a) Auch bei Temperaturen < 0 ' C erfolgt Korrosion, ihre Geschwindigkeit ist aber vom S02-Zutritt und der Oberflachenwassermenge unabhangig. menge und die Temperatur die Korrosionsgeschwindigkeit besonders intensiv bei niedrigen SO2-Zutritten; die relativ hochste Geschwindigkeitssteigerung erfol t bei S02-Zutritten zwischen 10-30 mg . m-' . Tag-. In Anwesenheit niedriger Wassermengen (bis zu etwa 10 g . m-2) kontrollieren die Korrosionsgeschwindigkeit diejenigen Teilvorgange, welche zur Bildung von Sulfatnestern fuhren und die hydrolytische Rostentstehung bewirken. c) Bei gleichzeitiger Einwirkung hoher Oberflachenwassermengen und hoher Temperaturen begrenzt der gehemmte Sauerstofftransport die Korrosionsgeschwindigkeit. d) Die im Labor gefundenen ,,stationaren" Korrosionsgeschwindigkeiten stimmen mit Ergebnissen von Naturversuchen gut uberein. Eingegangen: 10. 4, 1974 b) Oberhalb von 0 O C beschleunigen Oberflachenwasser-P Schrifttum On the initiation process of pitting corrosion on austenitic stainless steel in chloride solutions

Stochastic Models of Pitting Corrosion of Stainless Steels: I . Modeling of the Initiation and Growth of Pits at Constant Potential

“…= ~ (Cv1~/2) ' P(v,) dr, where the subscript, un, denotes unstable pits. In the limit at long enough time <I> .... = aXCItx" [15] If the birth of stable pits is sufficiently rare in comparison with the birth of unstable pits, then the ensemble mean current for the complete model can be obtained as the sum of Eq. [11] and [14].…”

“…We further note that, with the ensemble mean current given by Eq. [15], Eq. [32] gives for the white noise level G~(0) -10(<I> .... )2/ak [33] Thus, in the case of unstable pits, the spectral density affords a convenient method for determination of both and tz.…”

“…As far as the nucleation of pits is concerned, authors have variously emphasized, among other phenomena, inhomogeneity in the metal, cracking and slow healing of the passive film (8,9), development of critical acidity levels in microscopic flaws (7), defect transport in passive films (10), and chloride adsorption or incorporation into localized areas of passive film (11,12), including the idea of adsorption of a number of halide ions into a "transitional complex" (12) or of local thinning of the oxide under chloride 'lislands" (13). Local acidification (14) has been related to local adsorption of chloride (15).…”

“…A further feature of pitting corrosion of stainless steels is the observation of an induction time, z, before pit initiation (12,15), either following potential change from the inert into the pitting range in a halide solution, or upon the addition of halide into an inert solution. Hoar and Jacob (12), using the second approach, in which the specimen was prepassivated in a slowly flowing solution, with subsequent addition of halide ions, showed that the induction time for initiation of stable pitting on 18Cr 8Ni stainless steel varied as [X-]'" with 2.5 < n < 4.5 for X = C1and 4 < n < 4.5 for X = Br-; the exponent n was different at different electrode potentials E, but did not show an obvious systematic variation with E. It was also shown that the induction time has a very strong temperature de-*Electrochemical Society Active Member.…”

“…It has been noted above, however, that localized corrosion is properly considered as a rare event, so that statistical treatments are appropriate for its description (2). A consequence of these rare events is the generation of current fluctuations at constant potential (3,5,6,15,17), and these fluctuations during the induction period make a definite induction time difficult to define experimentally. Shibata and Takeyama (18,19) determined z as the time for the current to exceed an arbitrarily determined (and rather high) value.…”