View Article Online View Journal

Faraday Discussions Accepted Manuscript

This manuscript will be presented and discussed at a forthcoming Faraday Discussion meeting. All delegates can contribute to the discussion which will be included in the final volume. Register now to attend! Full details of all upcoming meetings: http://rsc.li/fd-upcoming-meetings This is an Accepted Manuscript, which has been through the Royal Society of Chemistry peer review process and has been accepted for publication. Accepted Manuscripts are published online shortly after acceptance, before technical editing, formatting and proof reading. Using this free service, authors can make their results available to the community, in citable form, before we publish the edited article. We will replace this Accepted Manuscript with the edited and formatted Advance Article as soon as it is available.

Faraday Discussions Royal Society of Chemistry

You can find more information about Accepted Manuscripts in the Information for Authors. Please note that technical editing may introduce minor changes to the text and/or graphics, which may alter content. The journal’s standard Terms & Conditions and the Ethical guidelines still apply. In no event shall the Royal Society of Chemistry be held responsible for any errors or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains.

This article can be cited before page numbers have been issued, to do this please use: T. Rayment, S. R. Street, N. Mi, A. J. M. C. Cook, H. B. Mohammed-Ali, L. Guo and A. Davenport, Faraday Discuss., 2015, DOI: 10.1039/C4FD00246F.

www.rsc.org/faraday_d

Page 1 of 16

Faraday Discussions View Article Online

Published on 16 January 2015. Downloaded by Seton Hall University on 30/03/2015 08:57:40.

Atmospheric pitting corrosion of 304L stainless steel: the role of highly concentrated chloride solutions Steven R Street, Na Mi, Angus J M C Cook, Haval B Mohammed-Ali, Liya Guo, Trevor Rayment and Alison J Davenport.

Abstract The morphology of atmospheric pitting corrosion in 304L stainless steel plate was analysed using MgCl2 droplets in relation to changes in relative humidity (RH) and chloride deposition density (CDD). It was found that highly reproducible morphologies occur that are distinct at different RH. Pitting at higher concentrations, i.e. lower RH, resulted in satellite pits forming around the perimeter of wide shallow dish regions. At higher RH, these satellite pits did not form and instead spiral attack into the shallow region was observed. Increasing CDD at saturation resulted in very broad-mouthed pitting attack within the shallow dish region. Large data sets were used to find trends in pit size and morphology in what is essentially a heterogeneous alloy. Electrochemical experiments on 304 stainless steel wires in highly saturated solutions showed that passive current density increased significantly above 3 M MgCl2 and breakdown pitting potential dropped as concentration increased. It is proposed that the shallow dish regions grow via enhanced dissolution of the passive film, whereas satellite pits and spiral attack take place with active dissolution of bare metal surfaces.

Introduction Atmospheric pitting corrosion occurs stochastically on stainless steels with key factors influencing initiation and propagation not obvious on the scale that the phenomenon is typically observed (1). As such, individual instances of pitting can only be considered anecdotal not archetypal. To draw conclusions from observing pitting on industrial alloys, large data sets have become increasingly used to get a sense of the significant factors influencing atmospheric pitting corrosion (2-4). Key to the concept of atmospheric corrosion is the presence of small volumes of solution which are highly concentrated, with Cl-rich solutions being of particular interest (5-7). The concentration of these solutions is governed by the relative humidity (RH) to which the solutions are exposed (8). A range of deposition methods have been used to study atmospheric corrosion including inkjet printing (9-11), manually-deposited salt crystals (12), and dissolving in alcohol and drying (13, 14). Droplets have been widely used to analyse atmospheric corrosion phenomena in steels (15-20) and other alloys (21-29) as their well-defined geometry allows claims to be made about mechanisms of corrosion such as available cathodic current (30) and chloride deposition density (CDD). The nuanced nature of droplet deposition allows control of solution volumes and concentrations with relative ease. Pit morphologies have been explored both analytically and schematically (31, 32). Vera Cruz et al. (33) described “colonies” of pits occurring in stainless steel with a single large pit surrounded by 1

Faraday Discussions Accepted Manuscript

DOI: 10.1039/C4FD00246F

Faraday Discussions

Page 2 of 16 View Article Online

DOI: 10.1039/C4FD00246F

Published on 16 January 2015. Downloaded by Seton Hall University on 30/03/2015 08:57:40.

This work will explore the influence of concentrated MgCl2 on the morphology of atmospheric pitting corrosion and full immersion current characteristics of 304L stainless steel.

Experimental Method Atmospheric Corrosion Samples were studied using two protocols: in the first samples were prepared and examined after 24 hours and in the second, the evolution of pitting was monitored at regular shorter intervals for 24 hours. Cold-rolled and solution heat-treated stainless steel 304L plate (Table 1) of 3 mm thickness was used (Aperam, France). This plate is known to have up to 3% ferrite distributed along the rolling direction. Plate was cut into 75 mm x 25 mm samples and ground using 800 grit SiC paper then cleaned in deionised (DI) water (18 MΩ.cm) in an ultrasonic bath for 15 minutes. Samples were then rinsed with DI water, then dried by blowing air from an empty DI water bottle. Samples were then left for 24 hours in covered, ambient conditions before droplet deposition. Arrays of 2 μl droplets of 0.27 M MgCl2 were deposited, with a final CDD of approx. 750 μg/cm2. Droplets were deposited using a Multiprobe II liquid handling system (Perkin-Elmer Life Sciences) at 5 mm intervals, in arrays of 4 x 14 or 4 x 15 (56-60 droplets). Droplets were deposited with the pipette tip at 0.91 mm from plate surface at a rate of 10 μl/s. The full array was deposited within 8 minutes. The ambient temperature was between 20-23 °C and the humidity 25-44 %RH during deposition. The experimental conditions used are reported in Table 2. Table 1 Foundry specifications of 304L plate used in atmospheric corrosion experiments.

Element (wt %)

Fe

Ni

Cr

Mn

S

Foundry spec.

Balance

8-10.5

18-19.5

2

0.015

2

C

Si

0.03 0.75

N

P

0.1

0.045

Faraday Discussions Accepted Manuscript

several smaller ones when under thin solute layers. Morphology is also influenced by microstructure (9, 34), solution concentration at the pit mouth (32, 35, 36) and pit potential (37). Instances have been recorded where these morphologies have had implications in failure under SCC and fatigue regimes (38).

Page 3 of 16

Faraday Discussions View Article Online

DOI: 10.1039/C4FD00246F

(

Figure 1). This equates to droplet solution concentrations between 3.7 M and 5 M MgCl2. After 24 hours each droplet was photographed with a microscope and then plate was washed with DI and Methanol and dried using hot hairdryer.

3

Faraday Discussions Accepted Manuscript

Published on 16 January 2015. Downloaded by Seton Hall University on 30/03/2015 08:57:40.

Once deposited, plate was held in an ECO135 atmospheric chamber (TAS Ltd) at 30±1 °C and at a controlled humidity between 33-56% RH for 24 hours (15)

Faraday Discussions

Page 4 of 16 View Article Online

DOI: 10.1039/C4FD00246F

Humidity ± S.D. (%RH)

Average Droplet CDD ± S.D. (μg/cm2)

33±2

80±3

670±40

38±2

-

43±2

“80”

“750”

“5400”

5400±300

0.048±0.001

0.057±0.003

0.051±0.001

820±20

-

-

0.047±0.001

-

-

790±40

-

-

0.048±0.002

-

48±2

-

670±40

-

-

0.057±0.003

-

56±2

-

730±40

-

-

0.052±0.002

-

4

“80”

“750”

Droplet area on deposition (cm2)

“5400”

Faraday Discussions Accepted Manuscript

Published on 16 January 2015. Downloaded by Seton Hall University on 30/03/2015 08:57:40.

Table 2 Chloride deposition density (CDD) for as-deposited droplets of MgCl2 deposited on 304L stainless steel plate. Samples sizes are 56-60, with standard deviation given for data set in each case. Droplets deposited at 20-23 °C in ambient relative humidity (25-44 %RH). Samples were put into controlled experimental humidity within 8 minutes of first deposition.

Page 5 of 16

Faraday Discussions View Article Online

Figure 1 Relationship between solution concentration (M) of MgCl2 and (a) relative humidity (%) , (b) electrical 2+ 2 conductivity (mho/m), and (c) self-diffusivity of Fe (m /s) in MgCl2 solutions at 30 °C, calculated using OLI Analyzer 9 (39). Humidities investigated in this work are marked on (a).

Samples of 304L stainless steel for time-lapse experiments were prepared as above. 2 μl droplets of 0.27 M MgCl2 were deposited on the sample and kept in a sealed transparent container in which humidity was controlled by saturated salt solutions (40). The container was kept in environmental chamber at 30±1 °C. Once an hour, it was removed from the chamber and the droplet was photographed using optical microscope. The time out of environmental chamber was approx. 3 minutes per hour. Electrochemical sweeps Wires of 304 stainless steel (250 μm diameter) were abraded along their length using 4000 grit SiC paper, and then rinsed with DI water. Each wire was then immersed in a MgCl2 solution with concentration ranging from 0.05 – 5 M. The wire electrode was bent into a J-shape so the cut ends were not left in solution. The surface area of the wire in solution was controlled to be 0.5 cm2. Tape was placed 5 mm above where the waterline was on the wire to avoid this being a crevice corrosion site. A SCE reference electrode and platinum auxiliary electrode were used in approx. 200 ml of solution. The solutions used were deaerated by bubbling argon for 20 minutes and argon before wire electrode was put into solution. Once the wire electrode was put into the solution, the flow of argon into the solution was stopped and a flow of argon into the air above the solution was started to prevent oxygen diffusion into the solution during the experiment. A new wire electrode was prepared and used in each experiment.

Results Effect of chloride deposition density Figure 2 shows typical droplets at 33% RH for CDDs of 80 μg/cm2, 750 μg/cm2, and 5400 μg/cm2. Droplets usually contained only one main pit, but in a few cases two were observed. At a CDD of 80 μg/cm2, droplets that contained pits often showed needle-like crystal growth along the surface of the droplet. Droplets with an initial CDD of 5400 μg/cm2 (Figure 2c) increased in surface area during 24 hours with an average value of 0.078±0.004 cm2, an increase of 53%. Droplets at lower CDD did not change perimeter noticeably.

5

Faraday Discussions Accepted Manuscript

Published on 16 January 2015. Downloaded by Seton Hall University on 30/03/2015 08:57:40.

DOI: 10.1039/C4FD00246F

Faraday Discussions

Page 6 of 16 View Article Online

2

2

Figure 2 Optical microscope images of droplets after 24 hours at 33% RH and 30 °C. a) 80 μg/cm , b)750 μg/cm CDD and 2 c) 5400 μg/cm CDD. Dark spots in droplets are pits.

Most pits that occurred at a CDD of 80 μg/cm2 showed a shallow dish-shaped region, with one side significantly more attacked, and with several satellite pits occurring at the perimeter of the dish or nearby (Figure 3a). Crystallographic etching can be seen in the shallow dish region. Out of 56 droplets, 43 pitted (77%) and of these, 36 showed this morphology. 18 of the 43 pitted droplets had needle-like crystals growing across the droplet surface, a phenomenon that did not occur in nonpitting droplets. The same etched shallow dish region with satellite pits was observed for droplets with a CDD of 750 μg/cm2, usually with one side showing more corrosion (Figure 3b). The shallow dish region usually had a smoother circumference than at a CDD of 80 μg/cm2. 52 out of 56 droplets pitted in this condition, with 49 of those that pitted showed this morphology. All 56 droplets with a CDD of 5400 μg/cm2 showed a single pit. 33 out of 56 droplets showed shallow dish regions with less pronounced etching than that observed at lower CDDs, with a deeper pit in part of the shallow dish (Figure 3c). Most of the remaining pits had very large pit mouth areas ≥100 μm in diameter that had probably consumed an initial shallow dish that was no longer visible. One pit showed the satellite morphology seen at lower CDD.

6

Faraday Discussions Accepted Manuscript

Published on 16 January 2015. Downloaded by Seton Hall University on 30/03/2015 08:57:40.

DOI: 10.1039/C4FD00246F

Page 7 of 16

Faraday Discussions View Article Online

2

2

2

Figure 3 SEM images of pit grown in a MgCl2 droplet with a) 80 μg/cm CDD, b) 750 μg/cm CDD and c) 5400 μg/cm CDD at 33% RH and 30 °C for 24 hours.

Time-lapse measurement of pit growth Figure 4a shows images of a growing pit as a function of time under a droplet at a RH of 33% with a CDD of ~750 µg/cm2. After 1 hour, the shallow dish region was still growing radially, but by 2 hours the diameter of the dish had stopped increasing, but further etching can be seen. In this same time period, small dark spots that developed into satellite pits had begun to appear around the shallow dish, becoming more prominent by 3 hours. These satellite pits grew concurrently with continued etching behaviour at the bottom of the shallow dish over the next 3 hours. By 24 hours, the shallow dish region had ceased to develop but the satellite pit mouths had each grown significantly in size. At 43% RH (Figure 4b), the initial pit growth was similar to that at 33% RH: after 1 hour, radial growth of the shallow dish region continued, but by 3 hours, radial growth had ceased radially but etching continued. No satellite pits formed; instead growth into the shallow region continued in a spiral fashion. These time lapse images are on a smaller scale than those shown at 33% RH, and a smaller pit was formed in this instance. However, in both 33% and 43% RH high-throughput experiments, the shallow dish regions formed showed significant variation in size.

7

Faraday Discussions Accepted Manuscript

Published on 16 January 2015. Downloaded by Seton Hall University on 30/03/2015 08:57:40.

DOI: 10.1039/C4FD00246F

Faraday Discussions

Page 8 of 16 View Article Online

DOI: 10.1039/C4FD00246F

2

Figure 4 Time-lapsed images of pitting of 304L stainless steel under 750 μg/cm MgCl2 at 30 °C and a) 33% RH, b) 43% RH, and c) 56% RH. All times from deposition.

Pit morphologies The characteristic pit morphologies of pits grown at a CDD of 750 µg/cm2 are shown in Figure 5. At 33% RH, a shallow dish region that showed crystallographic etching with deeper attack on one side and satellite pits at or near the dish circumference was found for 49 out of 52 pits (Figure 5a). Pit caps were seen on the top of the satellite pits (41, 42), and showed grinding marks that lined up with those on the bulk metal surface. EDX analysis of the pit caps gave typical compositions of ~40% Cr, ~45% O, ~5% S, and ~10% Cl (Figure 6). At 38% RH, all 56 droplets pitted, with two dominant pit morphologies occurring. 37 pits showed the “satellite” morphology described above (Figure 5b) and 16 pits showed deeper “spiral” attack in the shallow dish region (Figure 5c). At 43% RH, 56 out of 60 droplets pitted, and 54 of these showed “spiral” morphology, with an etched shallow dish region more strongly attacked at one side, with the attack continuing deeper into the metal (Figure 5d). One pit showed the “satellite” pit morphology, and one pit had grown to propagate outside the droplet region and had developed an irregular morphology. Pits with the “spiral” morphology were also found for 53/56 droplets at 48% RH (Figure 5e). Two droplets contained pits that showed growth along the rolling direction of the steel, indicating a susceptibility to microstructural features and one droplet did not contain a pit. All 56 droplets at 56% RH had pits that did not have a shallow dish region, but instead showed narrow circular mouths (Figure 5f). 24 droplets (43%) showed at least two pits that were more than 500 μm apart. Analysis of the rust deposits around the pits suggested that these pits occurred consecutively not concurrently in each case.

8

Faraday Discussions Accepted Manuscript

Published on 16 January 2015. Downloaded by Seton Hall University on 30/03/2015 08:57:40.

At an RH of 56%, only a small amount of radial growth occurred within the first hour, with a greater extent of attack into the metal (Figure 4c). The radial growth of the pit mouth was completed by 4 hours, but the pit appeared to continue to grow deeper.

Page 9 of 16

Faraday Discussions View Article Online

Figure 5 Typical pit morphologies of 750 μg/cm2 CDD MgCl2 droplets at a) 33% RH (b) 38% RH near droplet edge (c) 38% RH near droplet centre, (d) 43% RH, (e) 48% RH and (f) 56% RH. Pits were grown for 24 hours at 30 °C.

Figure 6 Satellite pit from 33% RH (a) before and (b) after ultrasonic cleaning in DI water for 30 seconds. Pit caps of the type seen in (a) showed typical EDX compositions of ~40% Cr, ~45% O, ~5% S, and ~10% Cl.

Effect of pit location on the size of the shallow dish region Pits were observed to occur at all locations in droplets in all conditions, with no clear trend in initiation site towards either edge or centre. However, close inspection suggested that the size of shallow dish regions was affected by the location of the pit within the droplet. The diameter was determined by drawing a circle around the dish region that had remained after continued pitting (Figure 7). This technique was based on the assumption that dish regions were circular; any pits that did not show this feature were not included. Plots of the diameter of the shallow dish as a function of distance from the droplet edge are shown in Figure 8.

9

Faraday Discussions Accepted Manuscript

Published on 16 January 2015. Downloaded by Seton Hall University on 30/03/2015 08:57:40.

DOI: 10.1039/C4FD00246F

Faraday Discussions

Page 10 of 16 View Article Online

Figure 7 Technique for measuring shallow dish region seen during pitting.

At 33% RH, the dish diameters showed a high degree of scatter, but despite the wide variation, a trend of a larger diameter dish towards the centre of the droplet can be observed in Figure 8a. A similar trend existed at higher humidities, with less scatter at 38% RH and 48% RH. From 38% RH to 48% RH, droplets that occurred near the edge were not only smaller but had less variation in diameter compared with droplets formed towards the centre. At 38% RH, a transition occurred in morphology with pit location: pits with satellites predominately formed near the edge of droplets, and spiral pits were found towards the centre. At 43% and 48% RH, the majority of pits were spiral, and there were fewer dish regions smaller than 80 μm. There are no data for pits formed at 56% RH as they showed no shallow dish region after 24 hours.

10

Faraday Discussions Accepted Manuscript

Published on 16 January 2015. Downloaded by Seton Hall University on 30/03/2015 08:57:40.

DOI: 10.1039/C4FD00246F

Page 11 of 16

Faraday Discussions View Article Online

Figure 8 Diameter of shallow dish regions against distance from droplet edge in MgCl2 droplets with a CDD of 750 μg/cm2 after 24 hours at 30 °C. Satellite pits are shown as blue diamonds and spiral pits are shown as red circles. 2

Table 3 Average diameters and standard deviations of pits formed in MgCl2 droplets with a CDD of 750 μg/cm after 24 hours at 30 °C.

RH (%)

Diameter average (μm)

33

95±17

38

110±23

43

110±14

48

120±15

Electrochemistry Figure 9 shows polarisation curves of 304 wires in solutions of different concentrations of MgCl2. In solutions between 0.05 M and 3 M, passive current densities are of similar magnitude. Each of these curves show metastable pitting peaks as pitting potential is approached. In concentrations higher than 3 M (the solution in equilibrium with 68% RH) the passive current density increased with concentration (Figure 9b). A small amount of metastable pitting was observed at 3.2 M (equivalent 11

Faraday Discussions Accepted Manuscript

Published on 16 January 2015. Downloaded by Seton Hall University on 30/03/2015 08:57:40.

DOI: 10.1039/C4FD00246F

Faraday Discussions

Page 12 of 16 View Article Online

DOI: 10.1039/C4FD00246F

Figure 9 Current vs. potential polarisation curves of 304 stainless steel. 250 μm diameter wires were put into deaerated solutions immediately after abrasion using 4000 grit SiC papers. MgCl2 solutions with concentrations (a) 0.05-3 M and (b) 3-5 M. The potential sweep rate is 0.2 mV/s.

Discussion Atmospheric corrosion is characterised by a limited electrolyte volume, differential aeration, and highly concentrated solutions in which a further increase in concentration leads to a decrease in ionic conductivity and diffusion coefficient of ions. These effects must be considered to understand mechanisms of growth in droplet experiments. At RH values of 48% and below, localised corrosion starts with a shallow dish (19, 20, 33). The surface of the dish is etched, suggesting that no salt layer is present since dissolution under a salt layer leads to polished or roughened surfaces (43, 44), which can be seen in the deep pitting in Figure 3. Crystallographic attack could take place on a bare metal surface, or on a surface with a passive film. It is known that the passive film on both iron and stainless steel is crystalline and formed epitaxially on the metal (45, 46), so crystallographic etching of the passive film could lead to differential rates of oxidation on different grains. The typical current density for vertical growth of the shallow dish can be estimated from Faraday’s law using a density of 7.9 g/cm3 and an average charge on the dissolving ions of 2.2 for 304L. The shallow dish regions have a typical depth of 10 μm, with the majority of growth occurring in the first two hours. This yields a current density of ~4 µA/cm2, which is in the same order of magnitude as the passive current densities observed at high chloride concentration in Figure 9, indicating that dissolution of the shallow dish region may take place via concurrent passive oxide film growth and dissolution. Furthermore, it is clear from Figure 9 that the passive current density increases significantly with solution concentration, suggesting that the dish shape may be influenced by the higher concentration of the solution in the vicinity of the initial breakdown site at the centre of the dish.

12

Faraday Discussions Accepted Manuscript

Published on 16 January 2015. Downloaded by Seton Hall University on 30/03/2015 08:57:40.

to 64% RH), but significantly less than at lower solution concentrations. Passive current densities are similar for solutions of 3.2, 3.5, and 4 M MgCl2 (equivalent to 64% RH, 59% RH and 50% RH, respectively), but increase significantly at 5 M (33% RH).

Page 13 of 16

Faraday Discussions View Article Online

DOI: 10.1039/C4FD00246F

Time-lapse imaging shows that for 33% and 43% RH at a CDD of 750 μg/cm2 (Figure 4), the dishshaped region stops growing radially and deeper and more localised attack develops. At 33% RH, small satellite pits form around the shallow dish region that are protected by pit caps enriched in Cr, O, S, and Cl, suggesting that they form at the site of sulphide inclusions. Only limited attack continues into the shallow dish region. At 43% RH, these satellite pits are not observed and attack continues into the shallow dish, forming a spiral morphology. At 56% RH, no sign of the initial shallow dish region was seen as pits grew. A similar change in morphology is seen at 33% RH on increasing the CDD from 80 µg/cm2, where satellite pits are formed, to 5400 µg/cm2, where part of the dish is attacked more deeply. In some cases the entire shallow dish region at 5400 µg/cm2 CDD was attacked, leaving no trace of it after 24 hours. For a fixed RH, the solution concentration (and thus its resistivity) is constant, but the droplet height is greater for a higher CDD. This suggests that the increase in localised attack is associated with a lower resistance, and thus lower IR drop between the cathode and the anode, as well as a less constrained geometry that will allow faster diffusion of ions away from the pit. The role of IR drop and diffusion in controlling the extent of pitting attack is also valid for the transition from satellite to spiral pits between 33% and 43% RH, since for fixed CDD, the droplet height will be greater at 43% RH and the solution resistivity is lower, both leading to a lower IR drop, and the self-diffusion coefficient is higher. Furthermore, at 38% RH, satellite pits are seen near the edge of the droplet, where the droplet height is low, the resistance path is high to most of the droplet perimeter, and the diffusion rate of metal ions is lower, whereas spiral pits are observed when pits are located towards the centre of droplets. Where the IR drop is lower, the interfacial potential within the pit will be higher, increasing the likelihood that it will be greater than the “breakdown potential” observed in the polarisation curves (Figure 9). The lack of development of a shallow dish region in pitting at 56% RH may be linked to the lower passive current density at lower concentrations, but the higher solution conductivity and droplet height leads to a lower IR drop so that it is easier for the breakdown potential to be exceeded. For the concentrated solutions and thin layer of electrolytes found at low RH, satellite pitting sites that initiate outside the shallow dish are able to overcome the transition from metastability (47) because pit caps form, and the escape of ions from the pits is low since diffusion is limited. The satellite pits draw a continually increasing share of the limited cathodic current (30) as they grow. This might be expected to the shallow dish of necessary cathodic current to continue, passivating this region. At higher RH and with thicker solution layers, spiral growth is observed. Although IR drop is reduced in these conditions compared with those showing satellite morphology, there remains enough to allow only partial attack of this region. Only a small area of the shallow dish region can sustain a sufficient current density to propagate, the rest passivating. It is possible that the spiral geometry is favoured as it increases occlusion of the pit, but further investigations are necessary to confirm this. 13

Faraday Discussions Accepted Manuscript

Published on 16 January 2015. Downloaded by Seton Hall University on 30/03/2015 08:57:40.

Maier and Frankel (19) have proposed the adsorption of S to the oxide layer around the pit, causing attack in a radial fashion. Hastuty et al. (20) have also proposed a mechanism that causes radial growth by hydrolysis after going through a thiosulphate intermediate stage. However, the variation in dish diameter with distance from the droplet edge (Figure 8) indicates that the controlling factor is not simply the presence of sulphur species.

Faraday Discussions

Page 14 of 16 View Article Online

DOI: 10.1039/C4FD00246F

The considerable scatter in measurements of shallow dish diameter and CDD show that there is a need for large data sets when dealing with commercial alloys and droplets as atmospheric corrosion models. Trends in these variables only make themselves apparent when high throughput techniques are used. It is also clear that the pitting behaviour is highly sensitive to local variations in conductivity and diffusivity with solution chemistry.

Conclusions Atmospheric pitting corrosion of 304L stainless steel was analysed using droplets of MgCl2 after 24 hours of pitting at 30 °C. It was found that the pit morphology is a sensitive function of RH and CDD. Pits grown at RH values ≤48% grow initially as crystallographically-etched shallow dishes with a current density (estimated from the typical depth as a function of time) that is consistent with the passive current density of stainless steel in concentrated MgCl2 solutions. At low RH (33%), the shallow dishes develop small satellite pits that grow under caps rich in Cr, S, O and Cl. At higher RH (43% and 48%, attack into the shallow region continues, but only in a small region, developing spiral attack with a roughened surface. At 56% RH, shallow dishes are not observed, and deeper and rougher pits are observed. The change in pit morphology is attributed to changes in the IR drop between anode and cathode, and changes in solution conductivity and diffusivity with RH. Deeper spiral attack is observed for higher droplets (higher CDD at fixed RH or higher RH at fixed CDD). For 38% RH, pits close to the droplet edge form satellites, whereas pits that form towards the centre of the droplet, where it is higher, show spiral attack. Higher RH values not only increase droplet height, but also increase the ionic conductivity and diffusivity of these highly concentrated solutions. High throughput methods are particularly important for providing statistically significant data on corrosion behaviour of commercial alloys.

References 1. Strehblow H. Mechanisms of Pitting Corrosion. In: Marcus P, editor. Corrosion Mechanisms in Theory and Practice. 2nd ed. New York, NY: Marcel Dekker, Inc.; 2002. 2. Azmat NS, Ralston KD, Muster TH, Muddle BC, Cole IS. A High-Throughput Test Methodology for Atmospheric Corrosion Studies. Electrochemical and Solid State Letters. 2011;14(6):C9-C11. 3. Schindelholz E, Risteen BE, Kelly RG. Effect of Relative Humidity on Corrosion of Steel under Sea Salt Aerosol Proxies. Journal of the Electrochemical Society. 2014;161(10):C460-C470. 4. Schindelholz E, Risteen BE, Kelly RG. Effect of Relative Humidity on Corrosion of Steel under Sea Salt Aerosol Proxies. Journal of the Electrochemical Society. 2014;161(10):C450-C459. 5. Tomashov ND. Development of Electrochemical Theory of Metallic Corrosion. Corrosion. 1964;20(1):7t-14t. 6. Tsuru T, Nishikata A, Wang J. Electrochemical Studies on Corrosion under a Water Film. Materials Science and Engineering a-Structural Materials Properties Microstructure and Processing. 1995;198(1-2):161-168. 14

Faraday Discussions Accepted Manuscript

Published on 16 January 2015. Downloaded by Seton Hall University on 30/03/2015 08:57:40.

While satellite pitting sites are available around the shallow dish region, the diluted solution means these are not able to be sustained allowing growth into the shallow region to continue.

Page 15 of 16

Faraday Discussions View Article Online

7. Nishikata A, Yamashita Y, Katayama H, Tsuru T, Usami A, Tanabe K, et al. An Electrochemical Impedance Study on Atmospheric Corrosion of Steels in a Cyclic Wet-Dry Condition. Corrosion Science. 1995;37(12):2059-2069. 8. Tsutsumi Y, Nishikata A, Tsuru T. Initial stage of pitting corrosion of type 304 stainless steel under thin electrolyte layers containing chloride ions. J Electrochem Soc. 2005;152(9):B358-B363. 9. Mi N, Ghahari M, Rayment T, Davenport AJ. Use of inkjet printing to deposit magnesium chloride salt patterns for investigation of atmospheric corrosion of 304 stainless steel. Corrosion Science. 2011;53(10):3114-3121. 10. Schindelholz E, Kelly RG. Application of Inkjet Printing for Depositing Salt Prior to Atmospheric Corrosion Testing. Electrochemical and Solid State Letters. 2010;13(10):C29-C31. 11. Azmat NS, Ralston KD, Muddle BC, Cole IS. Corrosion of Zn under fine size aerosols and droplets using inkjet printer deposition and optical profilometry quantification. Corrosion Science. 2011;53(11):3534-3541. 12. Hihara LH, Li S. Atmospheric corrosion initiation on steel from predeposited NaCl salt particles in high humidity atmospheres. Corrosion Engineering, Science and Technology. 2010;45(1):49-56. 13. Chen ZY, Persson D, Nazarov A, Zakipour S, Thierry D, Leygraf C. In situ studies of the effect of CO2 on the initial NaCl-induced atmospheric corrosion of copper. Journal of the Electrochemical Society. 2005;152(9):B342-B351. 14. Chen ZY, Zakipour S, Persson D, Leygraf C. Effect of sodium chloride particles on the atmospheric corrosion of pure copper. Corrosion. 2004;60(5):479-491. 15. Tsutsumi Y, Nishikata A, Tsuru T. Pitting corrosion mechanism of Type 304 stainless steel under a droplet of chloride solutions. Corrosion Science. 2007;49(3):1394-1407. 16. Tsuru T, Tamiya KI, Nishikata A. Formation and growth of micro-droplets during the initial stage of atmospheric corrosion. Electrochimica Acta. 2004;49(17-18):2709-2715. 17. Li S, Hihara LH. In situ Raman spectroscopic identification of rust formation in Evans' droplet experiments. Electrochemistry Communications. 2012;18:48-50. 18. Wang YH, Liu YY, Wang W, Zhong L, Wang J. Influences of the three-phase boundary on the electrochemical corrosion characteristics of carbon steel under droplets. Materials and CorrosionWerkstoffe Und Korrosion. 2013;64(4):309-313. 19. Maier B, Frankel GS. Pitting Corrosion of Bare Stainless Steel 304 under Chloride Solution Droplets. Journal of the Electrochemical Society. 2010;157(10):C302-C312. 20. Hastuty S, Nishikata A, Tsuru T. Pitting corrosion of Type 430 stainless steel under chloride solution droplet. Corrosion Science. 2010;52(6):2035-2043. 21. Thomas S, Cole IS, Birbilis N. Compact Oxides Formed on Zinc during Exposure to a Single Sea-Water Droplet. Journal of the Electrochemical Society. 2013;160(2):C59-C63. 22. King PC, Cole IS, Corrigan PA, Hughes AE, Muster TH, Thomas S. FIB/SEM study of AA2024 corrosion under a seawater drop, part II. Corrosion Science. 2012;55:116-125. 23. Azmat NS, Ralston KD, Muddle BC, Cole IS. Corrosion of Zn under acidified marine droplets. Corrosion Science. 2011;53(4):1604-1615. 24. Muster TH, Bradbury A, Trinchi A, Cole IS, Markley T, Lau D, et al. The atmospheric corrosion of zinc: The effects of salt concentration, droplet size and droplet shape. Electrochimica Acta. 2011;56(4):1866-1873. 25. Li JF, Maier B, Frankel GS. Corrosion of an Al-Mg-Si alloy under MgCl2 solution droplets. Corrosion Science. 2011;53(6):2142-2151. 26. Cole IS, Muster TH, Lau D, Wright N, Azmat NS. Products Formed during the Interaction of Seawater Droplets with Zinc Surfaces II. Results from Short Exposures. Journal of the Electrochemical Society. 2010;157(6):C213-C222. 27. Venkatraman MS, Cole IS, Gunasegaram DR, Emmanuel B. Modeling Corrosion of a Metal under an Aerosol Droplet. In: Nie JF, Morton A, editors. Pricm 7, Pts 1-32010. p. 1650-1653.

15

Faraday Discussions Accepted Manuscript

Published on 16 January 2015. Downloaded by Seton Hall University on 30/03/2015 08:57:40.

DOI: 10.1039/C4FD00246F

Faraday Discussions

Page 16 of 16 View Article Online

28. Chen ZY, Persson D, Leygraf C. In situ studies of the effect of SO2 on the initial NaCl-induced atmospheric corrosion of copper. Journal of the Electrochemical Society. 2005;152(12):B526-B533. 29. Cole IS, Lau D, Paterson DA. Holistic model for atmospheric corrosion - Part 6 - From wet aerosol to salt deposit. Corrosion Engineering Science and Technology. 2004;39(3):209-218. 30. Chen ZY, Kelly RG. Computational Modeling of Bounding Conditions for Pit Size on Stainless Steel in Atmospheric Environments. Journal of the Electrochemical Society. 2010;157(2):C69-C78. 31. Standard Guide for Examination and Evaluation of Pitting Corrosion. ASTM Standard G46-94: ASTM International; 2013. 32. Laycock NJ, White SP. Computer simulation of single pit propagation in stainless steel under potentiostatic control. Journal of the Electrochemical Society. 2001;148(7):B264-B275. 33. Cruz RPV, Nishikata A, Tsuru T. Pitting corrosion mechanism of stainless steels under wet-dry exposure in chloride-containing environments. Corrosion Science. 1998;40(1):125-139. 34. Davenport AJ, Guo L, Mi N, Mohammed-Ali H, Ghahari SM, Street SR, et al. Mechanistic studies of atmospheric pitting corrosion of stainless steel for ILW containers. Corrosion Engineering, Science and Technology. 2014;49(6):514-520. 35. Ghahari SM, Krouse DP, Laycock NJ, Rayment T, Padovani C, Suter T, et al. Pitting corrosion of stainless steel: measuring and modelling pit propagation in support of damage prediction for radioactive waste containers. Corrosion Engineering Science and Technology. 2011;46(2):205-211. 36. Ernst P, Laycock NJ, Moayed MH, Newman RC. The mechanism of lacy cover formation in pitting. Corrosion Science. 1997;39(6):1133-1136. 37. Frankel GS. The growth of 2-D pits in thin-film aluminum. Corrosion Science. 1990;30(12):1203-1218. 38. Turnbull A, Mingard K, Lord JD, Roebuck B, Tice DR, Mottershead KJ, et al. Sensitivity of stress corrosion cracking of stainless steel to surface machining and grinding procedure. Corrosion Science. 2011;53(10):3398-3415. 39. OLI Systems I. OLIAnalyzer. 2013. 40. E104-02 Standard Practice for Maintaining Constant Relative Humidity by Means of Aqueous Solutions. West Conshohocken, PA: ASTM International; 2007. 41. Wranglen G. PITTING AND SULFIDE INCLUSIONS IN STEEL. Corrosion Science. 1974;14(5):331-349. 42. Ryan MP, Laycock NJ, Newman RC, Isaacs HS. The pitting behavior of iron-chromium thin film alloys in hydrochloric acid. Journal of the Electrochemical Society. 1998;145(5):1566-1571. 43. Xu W. Synchrotron X-ray and Electrochemical Studies of Pitting Corrosion of Iron [PhD]. Birmingham, UK: University of Birmingham; 2014. 44. Beck TR. PITTING OF TITANIUM .2. ONE-DIMENSIONAL PIT EXPERIMENTS. Journal of the Electrochemical Society. 1973;120(10):1317-1324. 45. Davenport AJ, Oblonsky LJ, Ryan MP, Toney MF. The structure of the passive film that forms on iron in aqueous environments. Journal of the Electrochemical Society. 2000;147(6):2162-2173. 46. Maurice V, Yang WP, Marcus P. X-ray photoelectron spectroscopy and scanning tunneling microscopy study of passive films formed on (100) Fe-18Cr-13Ni single-crystal surfaces. Journal of the Electrochemical Society. 1998;145(3):909-920. 47. Pistorius PC, Burstein GT. Metastable pitting corrosion of stainless-steel and the transition to stability. Philosophical Transactions of the Royal Society of London Series a-Mathematical Physical and Engineering Sciences. 1992;341(1662):531-559.

16

Faraday Discussions Accepted Manuscript

Published on 16 January 2015. Downloaded by Seton Hall University on 30/03/2015 08:57:40.

DOI: 10.1039/C4FD00246F

Atmospheric pitting corrosion of 304L stainless steel: the role of highly concentrated chloride solutions.

The morphology of atmospheric pitting corrosion in 304L stainless steel plate was analysed using MgCl(2) droplets in relation to changes in relative h...
6MB Sizes 0 Downloads 12 Views