Investigation into composition and deposition of artificially produced marine aerosols on austenitic stainless steels

Sample preparation

Solution annealed type 304L austenitic SS obtained from Goodfellow Cambridge Ltd. with composition C ∼0·03%, Cr ∼18-20%, and Ni ∼8-12% was used in this study. Before use, several specimens were etched in oxalic acid to confirm there was no preferential grain orientation.

For interfacial and cyanoacrylate fuming studies flat plates with dimensions of 30×20×0·9 mm were employed. Before use, these were progressively polished to a 1 μm finish using 800, 1200, 2400, 4000 grade SiC papers and the appropriate diamond paste.

AISCC tests were performed using U-bend specimens with a bend radius of ∼9 mm formed from planar strips with dimensions of 120×15×0·9 mm. These strips were polished using 800 and 1200 grade SiC paper before bending on a standard screw press.

All specimens were ultrasonically cleaned before use with ethanol, acetone and deionised water.

Experimental solutions

For this study, artificial sea-water (SW) – with no organic content – was prepared using analytical grade salts following the ‘Kester’ recipe. ⁹ Each organically-enriched SW solution contained one of two bioexudates grown from nanoplankton and diatom cultures. The organic carbon concentration is attributed to colloidal nanogels and is of the same order as found in oceanic surface waters (between 92 and 451 μM). ¹⁰ Verdugo et al. have studied the complex nature and composition of the bioexudate, which typically consists of an evolving culture of embedded biopolymers in water with dissolved neutral sugars and proteins. ¹¹ Phaeocystis cf. globosa (Phaeo) and Thalassiosira Rotula (T. Rotula) were the two algal species employed in the production of bioexudate used in these experiments. The solutions were produced as detailed in prior literature. ¹⁰

CAM tensiometer studies

The KSV Instruments CAM 100 tensiometer suite analyses the contact angle (θ_C) by exploiting the evaporating sessile drop method.

A droplet (5·0±0·1 μL) was deposited onto a 304L plate and imaged using a Firewire camera during evaporation. Once the density of the test solution has been defined, the software will overlay a sessile drop model shape (with characteristic surface tension values) over the droplet image to generate a contact angle, according to the Young equation. Measurements were made at 70 s intervals until the drop had evaporated.

Pure water, artificial SW and Phaeo-enriched SW were analysed for comparison. Tests were conducted at room temperature within a closed humidity chamber at 70±1% relative humidity (RH).

Cyanoacrylate fuming

Cyanoacrylate fuming was used to detect any residual surface organic content following droplet evaporation. The technique involves exposure of specimens to an acrylate vapour which reacts on contact with organic material.

Samples were prepared by depositing a 5 μL droplet (of an artificial SW, deionised water or organically-enriched sea-salt solution) onto a 304L SS plate from a height of <1 cm and subsequently dried for 24 h in a desiccator. Each specimen was then cleaned using deionised water and re-dried prior to placement in an airtight chamber containing a beaker holding 2 mL of deionised water and another holding 2 mL of cyanoacrylate. The chamber was subsequently exposed to a dull heat source (60 W) for three hours to ensure the acrylate had fully vaporised.

Following exposure, specimens were examined using a Zeiss Lab.A1 AX10 Stereomicroscope at 10× magnification. To increase the visibility of the organic, differential interference contrasting (DIC) was utilised; a quasi-3D optical technique, with image contrast enhanced via the optical path difference between two phases.

AISCC testing

To ascertain the effect of primary organic matter on AISCC development in 304L SS, U-bends laden with nominally dry salt deposits, produced via evaporation of droplets from the appropriate SW solution, were exposed to environmental conditions of 50·0±0·5°C and 30±1% RH for a period of 12 weeks. Deposition details are outlined in Table 1. Two 5 μL droplets (ca. 3 mm diameter with good reproducibility) were placed on to the apex of each U-bend. Environmental exposure was performed in a Binder KMF 720 climatic chamber. RH and T were monitored at 15 min intervals using a Lascar EL-USB-2-LCD probe.

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Table 1

Outline of droplet deposition procedure

Specimen	Deposition solution	Number of specimens	Number of deposits per specimen	MgCl2 deposition density per deposit
304L U-bend	Sea water	3	2	360 μg cm−2
	MgCl2.6H2O	3	2	360 μg cm−2
	Sea water enriched with Phaeocystis exudate	3	2	360 μg cm−2
	Sea water enriched with T. Rotula exudate	3	2	360 μg cm−2

Optical images were obtained on a weekly basis using an Olympus SZH Stereo Zoom Microscope and captured using Motic's Advanced Images software suite. During imaging, U-bends were removed from the chamber for a maximum period of 30 min.

Crack lengths were quantified using the public domain Java program ImageJ. The average maximum crack length across all deposits in each deposition regime was calculated according to equation (1) (1)

Tensiometer analysis

Contact angle versus time measurements for artificial SW (no added organic) show a two-stage evaporation process (see Fig. 1a). During the first stage the droplet evaporates with a reduction in height, a progressive decrease in θ_C and no significant change in diameter. During the second stage, the droplet contracts radially inwards with a progressive reduction in diameter and an approximately constant θ_C.

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1

Contact angle against time for evaporating sessile drop of a artificial seawater and b phaeo-enriched seawater. Experiment conducted at room temperature (70±1% RH)

By contrast, evaporating organically-enriched SW droplets do not reach a constant θ_C, as confirmed by the data shown in Fig. 1b. It would appear, therefore, that organic matter aggregates towards the triple-phase boundary and effectively ‘pins’ the droplets’ contact diameters, as evidenced by the data displayed in Fig. 2.

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2

Contact diameter against time for four sessile drops of differing composition. Experiment conducted at room temperature (70±1% RH)

Cyanoacrylate fuming

The DIC observations shown in Fig. 3b clearly demonstrate that an organic film adheres to the SS surface after superficial cleaning with deionised water. The film appears to be relatively homogeneous, retaining the dimensions of the initial droplet. The presence of the organic could have a significant impact upon the onset of localised corrosion in the initial droplet, as induction times are inversely proportional to the area of the electrolyte covered region. It is also possible that if the organic forms a continuous layer that lies between the SS surface and any inorganic salt solution it may function as a corrosion inhibitor.

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3

Epi-DIC micrographs of 304L SS surface exposed to a seawater and b Phaeocystis-enriched seawater – which have been cleaned and developed using acrylate vapour

Fuming of specimens contacted with artificial SW and subsequently cleaned with de-ionised water gives no visible result, demonstrating the absence of any detectable organic content, see Fig. 3a. This shows the inorganic salt has been effectively removed during cleaning and nothing remains to react with the acrylate. Comparison of Fig. 3a and b confirms that acrylate fuming is only sensitive to organic species.

These results highlight that deposits formed from droplets of pure inorganic salt or SW solutions on 304L SS may behave significantly differently from organically-enriched deposits. This may have implications for ILW storage, as the wetting characteristics and morphology of surface deposits are key factors influencing localised corrosion behaviour.

AISCC testing

After 12 weeks of exposure to conditions of 50·0±0·5°C and 30±1% RH, specimens with deposits produced via evaporation of both artificial SW and organically-enriched SW solutions exhibit similar AISCC characteristics; see micrographs in Fig. 4b and f respectively. The aforementioned specimens display similar average maximum crack lengths and initial growth rates (3·50±0·35 mm and approximately 0·06 mm per day respectively). These were extracted from Fig. 5. ¹²

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4

Salt deposits residing on 304L U-bends after a, c and e four weeks exposure and b, d and f 12 weeks exposure (T = 50±0·5°C; 30±1% RH). Salt deposition results from the evaporation of either a, b a droplet of organically-enriched seawater, c, d 360 μg cm⁻² of MgCl₂.6H₂O or e, f artificial seawater

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5

Average crack length against time for 304L samples exposed to several solution types

Secondary spreading, as evident in the micrographs presented in Fig. 4b and f, acts to expand the wetted area beyond that observed following initial drying. Discrete NaCl crystals (0·30±0·02 mm in size) are visible following spreading. This is most likely due to wet-dry cycles that deposits experience as a result of temperature and RH changes that accompany the repeated removal of specimens from the chamber.

Specimens contacted with MgCl₂.6H₂O deposits exhibit different wetting behaviour from those with artificial sea-salt and exudate-enriched sea-salt deposits, see Fig. 4. On average, SW droplets only contracted to 90% of their original contact area, MgCl₂.6H₂O droplets evaporated to ∼25%. After 12 weeks of exposure, samples contacted with MgCl₂.6H₂O deposits, showed an average maximum crack length of 0·89±0·10 mm, which is much lower than for all other specimens.

The average crack lengths and growth rates in specimens contacted with T. Rotula-enriched SW solutions (2·28±0·19 mm and approximately 0·02 mm per day respectively) are lower than the Phaeo-enriched or SW solutions, as indicated in Fig. 5. This raises the possibility that different organic exudates may behave differently, either in their surfactant or colligative properties and their subsequent effects on corrosion. Further work will need to be undertaken in order to assess the variation in properties of such algal exudates.