Influence of Nitrate on Pit Stability in Austenitic Stainless Steel

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In this paper we investigate the influence of nitrate (NO À 3 ) on the dissolution and repassivation kinetics of freshly bared stainless steel 304L surfaces generated during scratch tests. To differentiate anodic dissolution from film formation during early decay times, t < 20 ms, we have developed a mathematical expression for fitting this portion of the transient. The expression assumes the total current owes to dissolution plus film formation where the oxide coverage is derived from Avrami growth kinetics. In this manner the approach allows the investigator to separate the film formation and dissolution currents from the decay transient and analyze them individually. It is shown that film formation in 0.9 M NO À 3 occurs more rapidly as compared to 0.1 M chloride (Cl À ). In addition, NO À 3 was characterized by thinner films (< 0.6 nm) and higher electric field strengths (1.1 Â 10 7 V/cm) at early times. Correspondingly, increasing Cl À concentration from 0.1 to 0.9 M resulted in thicker passive films (0.6-2.2 nm) and lower electric field strengths (3.7 Â 10 6 V/cm).Nitrate inhibition of pitting in Fe-Cr-Ni alloys was reported by Schwenk and Uhlig and Gilman in the early 1960s. 1,2 In subsequent publications Leckie argued that nitrate (NO À 3 ) inhibition could be explained by a competitive adsorption model. 3 At low potentials it was proposed that chlorides (Cl À ) preferentially chemisorbed on the surface while at high potentials NO À 3 was preferentially chemisorbed on the surface thus displacing Cl À and preventing attack. An alternate mechanism for NO À 3 inhibition has been proposed by Newman, who used SS microelectrodes to simulate the occluded cell chemistry of a pit. 4 Newman reported that NO À 3 only passivated salt-covered surfaces. It was argued that, in a NO À 3 rich salt film, the reduction of NO À 3 consumes acid and thus promotes passivation as steel passivates more readily in neutral to alkaline pH.In a previous publication we investigated metastable pitting of SS 304L in NO À 3 . 5 The impetus for that investigation was the storage of high level radioactive waste (HLRW) at the Idaho Nuclear Technology and Engineering Center tank farm where high concentrations of sodium nitrate are present in the acid-chloride waste. In that work, two types of metastable pitting transients were identified, sharp-rise transients, having lifetimes on the order of milliseconds, and slow-rise transients which were stable up to 10's of seconds. It was found that sufficiently high concentrations of NO À 3 additions to Cl À solution resulted in the elimination of slow rise metastable pitting transients and, correspondingly, the inhibition of localized corrosion in SS 304L. It was also found that in solutions of NO À 3 sharprise metastable transients were associated with very high pit current densities; 50-200 A/cm 2 . Thus, it was conclude that NO À 3 does not inhibit pitting corrosion by reducing the current density at incipient pit surfaces.In this paper we investigate the influence of NO À 3 on the repassivation ...

Section: Resultsmentioning

confidence: 99%

The Kinetics of Anodic Dissolution and Repassivation on Stainless Steel 304L in Solutions Containing Nitrate

Lillard

Vasquez

Bahr

2011

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“…10,16 Nitrate has also been shown to eliminate slow rise electrochemical transients, associated with metastable pitting events which may form a salt layer. 9,40 Furthermore, nitrate has been shown to affect re-passivation kinetics on freshly scratched surfaces, with higher nitrate content leading to faster re-passivation. 40 In the current work, there was no gradual change in the amount of rust (and, by implication, pit volume) with increasing nitrate levels up to the point where complete inhibition occurred.…”

Section: 39mentioning

confidence: 99%

“…In particular, anions such as nitrate and sulfate, which are likely to be present in amounts comparable to chlorides in waste stores, 3 have been shown to inhibit corrosion processes in bulk solutions above specific critical ratios to the chloride ion. [7][8][9][10][11][12][13][14][15][16][17] The concentration of MgCl 2 in equilibrium with an ambient RH of 90% is ∼1.5 M, and increases with decreasing humidity. 18 Inhi- * Electrochemical Society Member.…”

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confidence: 99%

Effect of Nitrate and Sulfate on Atmospheric Corrosion of 304L and 316L Stainless Steels

Cook

Padovani

Davenport

2017

The effects of nitrate and sulfate salts on the chloride-induced atmospheric pitting corrosion of 304L and 316L stainless steel was investigated through automated deposition of droplets of magnesium and calcium salts. Nitrate was found to inhibit pitting under magnesium salt droplets when the ratio between the deposition density of nitrate anions and chloride anions was above a critical value, which was the same for both 304L and 316L. This critical ratio was found to decrease with increasing humidity. Sulfate was also observed to inhibit pitting for MgCl 2 + MgSO 4 mixtures, but only at higher humidities. Sulfate did not show any inhibition for CaCl 2 + CaSO 4 mixtures, an effect attributed to the low solubility of CaSO 4 . At low relative humidities, precipitation of the inhibiting salt was observed, leading in some cases to crevice-like corrosion under salt crystals. The pitting behavior was explained in terms of the thermodynamic behavior of concentrated solutions. In the UK, stainless steel is used to package intermediate level radioactive waste, ILW, which is characterized by relatively large volumes and variable levels of radioactivity.1 Whatever the strategic approach to its management c , most ILW has been packaged in thinwalled containers (2.3-6 mm thick, typically grades 304L or 316L, UNS S304003 and UNS S316003, respectively) and will undergo long periods of exposure to atmospheric conditions, either in surface or underground facilities prior to permanent disposal.During these periods, waste containers will be exposed to regimes of varying temperature and relative humidity (RH), as well as to chloride-containing salts arising from aerosol deposition. As a result, it is important to identify suitable storage conditions to ensure durability of waste containers, in particular to avoid conditions associated with the development of pitting and, even more importantly, atmospherically-induced stress corrosion cracking (AISCC). 2,3Monitoring of ILW storage facilities and other indoor locations considered broadly representative of ILW stores suggests that temperature and relative humidity, which are key parameters in the development of atmospheric corrosion, are expected to vary between ∼0-30• C and ∼30-100% RH, respectively. 3 Ionic chemical species deposited on surfaces after relatively long periods of indoor storage found in swab tests in a variety of real storage facilities include calcium, magnesium, sodium, potassium, chloride, nitrate and sulfate ions. Inside the storage buildings surveyed, chloride deposition densities were found to be below ∼20 μg/cm 2 , with deposition rates of the order of 1 μg/cm 2 per year estimated. 3 With such a deposition rate, the chloride deposition density could increase to ∼100 μg/cm 2 over the next century.In atmospheric conditions relevant to this work, a number of tests have been carried out to evaluate the atmospheric pitting corrosion of stainless steel in the presence of chloride deposits (e.g., Refs. 2, 4-6) but none of these have been carried out in the presence of...

“…The presence of a "nitrate-rich salt layer" has been proposed, 8 which may be a molten salt as suggested for dissolution of Fe in concentrated NaNO 3 solutions. 12 The correlation observed between the passivation potential and nitrate/chloride ratio 4,11,13,14 has been attributed to competitive adsorption of nitrate and chloride ions. 4 Nitrate reduction reactions have also been suggested as the cause of passivation since they may lead to increasing pH.…”

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confidence: 99%

“…[8][9][10] Nitrate additions to chloride solutions can also affect metastable pitting events in austenitic stainless steels, inhibiting slow-rise transients and causing localized inhibition. 11 The exact mechanism of nitrate-induced passivation remains elusive. The presence of a "nitrate-rich salt layer" has been proposed, 8 which may be a molten salt as suggested for dissolution of Fe in concentrated NaNO 3 solutions.…”

mentioning

confidence: 99%

The Effect of Nitrate on Salt Layers in Pitting Corrosion of 304L Stainless Steel

Street

Amri

et al. 2015

Current oscillations were observed during one-dimensional pitting corrosion of 304 L stainless steel in neutral 1 M NaCl solutions with varying NaNO 3 concentrations. Synchrotron X-ray diffraction was used to identify the salt layer at the corrosion front. It was found that, although current oscillations were induced in solutions with higher concentrations of NaNO 3 , the salt species in the pit did not change and a nitrate-free salt was present in all solutions. At higher NaNO 3 concentrations, a change of salt crystal morphology was detected. Electrochemical oscillations were seen to coincide with secondary pitting on the pit surface indicating that two corrosion regimes were operating in parallel. Synchrotron radiography was used on artificial pits to measure the change in corrosion front and material loss in situ. Before nitrate was added, the corrosion front showed non-uniform material loss across the interface when beneath the salt layer. Nitrate addition induced a local region of passivation that propagated across the pit surface. Surface roughness was quantified using R-values and seen to vary without a clear trend until passivation, after which it stayed constant. A mechanism is suggested in which partial passivation occurs in these systems, where passivated areas are undercut as the corrosion front moves, generating surges in current.