Macrofouling induced localized corrosion of stainless steel in Singapore seawater

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Corrosion Science journal homepage: www.elsevier.com/locate/corsci

Macrofouling induced localized corrosion of stainless steel in Singapore seawater ⁎

Daniel J. Blackwooda, , Chin Sing Limb, Serena L.M. Teob, Hu Xiaopinga,c, Pang Jianjuna,d a

Department of Materials Science & Engineering, National University of Singapore, Singapore 117574, Singapore Tropical Marine Science Institute, National University of Singapore, Singapore 119227, Singapore c Advanced Technology & Materials, 12 Yongcheng Beilu, Beijing, China d School of Mechanical and Automotive Engineering, Zhejiang University of Water Resources and Electric Power, Zhejiang 310018, China b

A R T I C L E I N F O

A B S T R A C T

Keywords: A. Stainless steel B. Weight loss C. Crevice corrosion C. Microbiological corrosion

Biofouling induced corrosion of stainless steels grades UNS S31603 and UNS S31254 was investigated at three sites off Singapore for 30 months. No corrosion was observed on grade UNS S31254, while the propensity for corrosion by shellfish of UNS S31603 is ranked as oyster > > barnacles > > green mussels. Oysters caused extensive localised corrosion, penetrating 2 mm thick plates with 12 months with tracks of corrosion several centimetres long. Shallow crevice corrosion was observed under dead barnacles, with an explanation presented for why corrosion is more severe under dead barnacles than live ones. Green mussels did not cause any corrosion.

1. Introduction The corrosion of metals in natural seawater is the result of interactions between the metal, living organisms and seawater chemistry [1]. When a metal is immersed into seawater, corrosion and biofouling occur on approximately the same time scales [2]. This is typically followed by the formation of biofilm within days after immersion [3,4], and after a longer exposure, macrobiota like invertebrate larvae may interact with the biofilm and colonize on the surface [5,6]. The resultant growth of macrofoulers is a major industrial problem causing both corrosion of offshore structures [7] and blockages of cooling water circulation systems [8]. For example, the baseplate of shellfish, such as barnacles, grows it can plough into organic protective coatings and expose the underlying metal allowing subsequent corrosion to occur [9]. Most of the previous research on biocorrosion focused on the early micro-fouling stages, with less attention being placed on the macrofouling stage, due to the difficulty of monitoring the effects of macrofoulers [10,11]. Although macro-fouling has been reported to cause some localized corrosion [12], the processes leading to this are poorly understood beyond the likely involvement of differential aeration cells [13]. The rates at which materials corrode and foul in seawater are very much location specific and laboratory studies frequently fail to reproduce the actual rates found in the field [14]. The tropical marine environment thus is very different to temperate

⁎

waters where the majority of early biocorrosion research has been conducted. In this present work exposure experiments, along with insitu open-circuit potential (OCP) and linear polarization resistance (LPR) monitoring, were used to examine the influence of tropical marine fouling on metallic corrosion, with an emphasis on the extent of corrosion caused by different marine species. Two grades of stainless steel were investigated, the common marine grade 316L (UNS S31603) and the superaustenitic 254SMO (UNS S31254) that has a pitting resistance number greater than 40 and is thus expected to resistant to pitting in seawater [15]. The corrosion behaviours of both these grades in seawater having been previously extensively studied in the absence of fouling [16,17]. Three coastal test locations selected around Singapore were selected that were known to have different fouling pressures, due to local current flows, such that different marine species tend dominate the macrofouling [18,19]. 2. Materials and methods 2.1. Test locations The marine environment around Singapore is typical of Southeast Asia coastal seas: temperatures vary only slightly throughout the year, 29 ± 1 °C, and the seas support a high biodiversity, which contributes to a high settlement rate of fouling organisms [18]. Singapore lies at the southern tip of the Malay Peninsula between Malaysia and Indonesia

Corresponding author. E-mail address: [email protected] (D.J. Blackwood).

http://dx.doi.org/10.1016/j.corsci.2017.10.008 Received 2 May 2017; Received in revised form 17 October 2017; Accepted 17 October 2017 0010-938X/ © 2017 Published by Elsevier Ltd.

Please cite this article as: Blackwood, D.J., Corrosion Science (2017), http://dx.doi.org/10.1016/j.corsci.2017.10.008

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RYSC and SJI test sites, due difficulties accessing the Changi site. The SCE reference electrode was periodically inspected to determine if it still contained KCl crystals (an indication of saturation conditions being retained) as well as to remove fouling species from its external surfaces. Its potential was also checked against a second SCE electrode to ensure it was functioning correctly. All potentials quoted in this paper are versus SCE.

and her coastal waters are bounded by the (East and West) Johor Strait in the North, and the Singapore Strait in the South. The hydrodynamic currents around Singapore coastal waters have been described previously by Chan et al. [19] and are dominated by semi-diurnal tides in the eastern (Southwest Monsoon) and western (Northeast Monsoon) directions. Due to its geographical location, the salinity distribution of its waterways are influenced by the mixing between influx of freshwater supplied mainly by the Sungai Johor and seawater from monsoon driven currents and tidal fluctuations [20]. Tidal velocities vary spatially, from 0.5 to 1.0 m s-1 in the Singapore Strait to less than 0.5 m s-1 in the East Johor Strait [21]. The annual mean salinity in the East Johor Strait and the Singapore Strait is approximately 29 ppt and 31.5 ppt respectively [20]. The stainless steel test coupons were immersed at three sites around Singapore: off a floating raft at the Republic of Singapore Yacht Club (RSYC) located on the southwest coast of the main island of Singapore; off a floating fish farm in the Johore Straits located to the northeast (Changi); and on off a barge located in a channel between Lazarus Island and St John’s Island (SJI), which lies approximately 6 km south of the main island. The physical and chemical characteristics of the seawater at the three locations was recorded at set times.

2.4. Post retrieval analysis After retrieval of the specimens, the fouling and corrosion products were removed by water-jet cleaning at 138, 276, 827 and 1655 kPa with photographs taking after each stage. Finally, to remove the basal plates (predominately calcite) and any corrosion products the samples were immersed in 10% HNO3 at 60 °C for ten minutes followed by light scrubbing with a soft sponge. Corrosion rates were evaluated from weight loss data, initial weight minus weight after the HNO3 etch, with a second acid etch showing that the cleaning procedure did not remove measurable weight-loss from either grade of stainless steel. The coupons were weighed on a balance of with a precision of 0.1 mg, but the corrosion rates are only reported to the nearest 0.5 μm yr−1, which corresponds to a weight change of about 0.02 g for the shortest 3 month exposures. However, it should be born in mind that corrosion rates determined from both the LPR and weight loss measurements assume general uniform corrosion, which is rarely the case with stainless steel. The corrosion morphology was determined by visual inspections and SEM observations. The microstructure of the stainless steel plates used for weight loss measurements was revealed by metallographic etching using Carpenter 300 series stainless steel etchant (FeCl3 8.5 g; CuCl2 2.4 g; ethanol 122 ml; HCl 122 ml; HNO3 6 ml) for about 5 s. Before etching, small pieces were cut off from the plates, mounted with epoxy resin and then polished to a glossy surface with 1 μm alumina suspension.

2.2. Specimen preparation The two grades of stainless steel investigated were an austenitic UNS S31603 grade (316L: 17.21 wt% Cr, 12.16 wt% Ni, 2.23 wt% Mo, 1.05 wt% Mn, 0.013 wt% C, 0.73 wt% Si, 0.033 wt% P, 0.001 wt% S, bal. Fe) and the super-austenitic UNS S31254 grade (SMO: 20.03 wt% Cr, 18.12 wt% Ni, 6.07 wt% Mo, 0.69 wt% Cu, 0.55 wt% Mn, 0.26 wt% Co, 0.182 wt% N, 0.010 wt% C, 0.31 wt% Si, 0.022 wt% P, 0.001 wt% S, bal. Fe), with pitting resistance numbers (PREN) of 24.5 and 43 respectively. For the exposure experiments, pre-weighed sample coupons measuring 100 mm × 100 mm × 2 mm were ground to a 1200 grit finish. The coupons were mounted across a centre hole on a 10 mm diameter 316L stainless steel rod encased in silicone tubing to provide electrical isolation and prevent direct contact between the mounting rod and the test specimens. The distance between coupons was 100 mm, and the depth of immersion in the seawater was 0.5 m. The experimental setup was designed to allow samples to be retrieved for examinations such as weight loss evaluation and visual inspection after 3, 6, 12, 24 and 30 months.

3. Results 3.1. Fouling species The dominant fouling species was different at each test site, but independent of the grade of stainless steel. At SJI after 3 months, the fouling consisted mainly of algae settlement. After 12 months, oysters and barnacles were the dominant foulers present. For samples immersed at Changi site, barnacles, mussels and sponges were initially the main colonisers. After 24 months, green mussels dominated the community. At RSYC, the early fouling community consisted of calcareous polychaetes but after 6 months, these were overgrown by oysters and barnacles (Fig. 1). The diversity of fouling species materials increased with time, with hard foulers dominating in the long-term. The extent and composition of the biofouling did not vary significantly between the two grades of stainless steel. Table 1 shows the physical and chemical characteristics of the seawater at the three locations at the time of launch and retrieval of the test coupons (data collected at other time periods were similar to that displayed in Table 1). Apart from the salinity being marginally higher at SJI, there were no consistent differences between the seawater characteristics at the three locations; as these are coastal locations salinity drops after heavy tropical down pores. Although at the time of launch, the total organic carbon contents at the SJI and Changi sites were unusually high, these returned to levels that are more normal within 3 months.

2.3. Electrochemical measurements The electrodes for the OCP and LPR experiments were cut from either 6 mm diameter 316L stainless steel rods or 3 mm thick × 9 mm wide SMO plates that were ground to a 1200 grit SiC finish. Copper wires were connected to the top of the electrodes, which were then insulated with polyvinylchloride (PVC) sleeves, and the joints sealed with silicon sealant, leaving an exposed surface areas of 25.7 cm2 and 31.5 cm2 for the 316L and SMO, respectively. A larger surface area than that which would normally be used in an electrochemical experiment was required in order to obtain representative fouling settlement [22]. The stainless steel electrodes, via the insulated copper wires, were connected to the instrumentation through PVC piping that allowed the electrodes to be immersed to a depth of 0.5 m. The open-circuit potentials of the electrodes were monitored against a standard calomel reference electrode (SCE) via a Grant Squirrel 1000® datalogger with readings recorded every 20 min. The data was downloaded from the logger to a laptop on a weekly basis. The LPR measurements were performed manually once a week via a two electrode ACM instruments pocket LPR meter, with an additional stainless steel rod being placed in the seawater to act as the counter electrode; prior laboratory based tests revealed that LPR readings were the same regardless of whether or not samples remained connected to the datalogger. The OCP and LPR measurements were only conducted at the

3.2. Electrochemical analysis The development of biofilms is known to cause ennoblement in the OCP of stainless steels [23]. The evolution of the OCP of the two grades of stainless steel at SJI and RSYC sites are shown in Fig. 2. Initially the 2

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Fig. 1. Photographs of 316L stainless steel coupons (10 cm × 10 cm) as retrieved from the three test sites after 30 months of exposure.

open-circuit potentials recorded for the 316L stainless steel at the SJI and RSYC sites were similar at around −0.1 V vs SCE. However, continuous slow ennoblement was observed at the SJI site with the OCP eventually reaching 0.3 V vs SCE after 30 months. In contrast, at the RSYC site the OCP remained close to −0.1 V vs SCE, with only occasional short periods (e.g. a week) of ennoblement to 0.1 V vs SCE. For the SMO stainless steel at the SJI site the ennoblement of the OCP reached 0.15 V vs SCE in less than a week, where it remained for almost 6 months at which point it again rapidly increased to 0.35 V vs SCE. It remained at this value for a further 8 months before slowly declining to between 0.15 V and 0.20 V vs SCE for the remaining time of the test. At the RYSC the ennoblement of the OCP of the SMO grade was less dramatic (in keeping with the observations from the 316L) hovering between 0.0 V and −0.5 V vs. SCE for about 17 months after which it rapidly (over a space of a week) to 0.2 V vs SCE followed by a gradual decline to about 0.0 V vs SCE for the remainder of the 30 month test duration. The previous works of de Messano et al. [24,25] and that of the present authors [26] showed that the presence of barnacles and other shellfish did not have an appreciable influence on OCP unless localized corrosion occurred. The LPR data for the 316L stainless steel indicated that there was a marginally higher general corrosion rate at RSYC than at SJI, with average values of about 2.0 and 1.2 μm yr−1 respectively (Fig. 3). There was also more variation in the corrosion rates recorded at RSYC with values ranging from 1.0 to 3.5 μm yr-1, whereas at SJI the fluctuations were less frequent and only between 1.0 and 2.0 μm yr-1, For the SMO stainless steel the measured corrosion rate was around 0.5 μm yr-1 or less at both test sites, which is the limit of resolution for the LPR instrument used.

(a) RSYC

316L SMO

0.2 0

0 0.6

4

8

12

16

20

Immersion Time (Months)

(b) SJI

24

316L SMO

0.4 0.2 0

-0.4

0

4

Fig. 2. Open-circuit potentials recorded at (a) RSYC and (b) SJI test sites. 316L dotted black line, SMO solid blue line. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Table 1 Physical and chemical characteristics of the seawater at the three test location. Time (months)

Site

pH

Salinity (ppt)

Dissolved Oxygen (% air saturation)

Temperature (°C)

Total Organic Carbon (mg dm−3)

Inorganic Carbon (mg dm−3)

Total Carbon (mg dm−3)

0

SJI Changi RSYC SJI Changi RSYC SJI Changi RSYC SJI Changi RSYC SJI Changi RSYC SJI Changi RSYC

7.9 7.9 7.8 8 8.4 8.4 8.4 8.2 8.3 7.5 8 7.9 8 7.9 8.2 8.1 8.2 7.9

29.8 27.7 27.9 26.8 25.4 26.8 35.5 32.2 33 32.5 31.2 31.9 31.2 29.1 31.1 31.5 30.3 28.8

88 87 77 – – – 75 68 68 90 93 84 85 66 111 91 112 64

30.2 30.6 30 29 28 28.2 29.1 29.5 30.2 29.6 27 29.6 29.3 29.4 30 30.1 31.1 30.5

20 12.3 3 3.3 3.6 2 1.8 2.5 1.8 3.4 4.3 5.9 4.8 3.6 3.3 4.2 3.8 4.8

1 1.6 0.6 3.7 3.3 1.1 0.4 0.3 7.4 0.4 0.2 4.2 0.3 0.1 0.2 6.9 9 5.9

21 13.9 3.6 7 6.9 3.1 2.2 2.8 9.2 3.8 4.5 10.1 5.1 3.7 3.5 11.1 12.8 10.7

3

6

12

18

24

3

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Fig. 5. Correlation between the weight of the biofouling (dashed line) and the corrosion rate (solid line) found by weight loss measurements for 316L stainless steel at the RSYC test sites; similar trends were observed at the other two test sites. The lines are only for guides to the eye.

cathodically limited is rejected because the large variations between the three test sites. Fig. 5 shows that during the first 3 months of immersion at all three locations, the weight loss from the 316L specimens was inversely proportional to the extent of biofouling, indicating that the fouling was protective. However, this correlation was lost in the 6 months data, suggesting that protection offered by the biofilm is only short-lived, possibly due to the onset localized corrosion resulting from the colonisation of macro-foulers that dominated in the long-term. Fig. 2 also shows that the weight-loss corrosion rates are higher at SJI and RSYC than at Changi. The very high corrosion rate observed for the third month sample retrieved from SJI resulted from crevice corrosion around the edge where the coupon was mounted. The corrosion rates measured at SJI and RSYC were higher than what has been reported in the literature for 316L stainless steel (< 5 μm yr−1), possibly due to the higher temperatures in the tropics [15]. Since only one coupon was used for each weight lost measurements, it is important not to over analyse the corrosion rates determined. Nevertheless, the longterm corrosion rates at each location were nearly independent of time, i.e. for coupons removed from 12 months onwards gave very similar corrosion rates, meaning that the values can be considered as being averaged over a number of samples. The average corrosion rates, with standard deviations in brackets, for samples removed between 12 and 30 months were: SJI 7.3 (1.8) μm yr−1; Changi 3.3 (0.6) μm yr−1; RSYC 6.7 (0.9) μm yr−1. At this point, it is worth re-emphasizing that because localized corrosion dominates the metal loss it is important to interpret weight loss and LPR data only in terms of relative aggressiveness of environments, rather than true general corrosion rates. For the SMO stainless steel only the specimen exposed at RSYC for 30 months showed any measurable weight loss and this converted to a negligible corrosion rate of 0.018 μm yr−1, with all the others showing weight gains between 0.001 g and 0.008 g. This suggest that SMO stainless steel is immune to corrosion in Singapore seawater at ambient temperatures.

Fig. 3. Corrosion rates determined from LPR measurements recorded at (a) RSYC and (b) SJI test sites. 316L dotted black line, SMO solid blue line. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

3.3. Weight loss measurements The general corrosion rates determined from the 316L stainless steel weight loss coupons were in the range of 3–10 μm yr−1 (Fig. 4), that is about a factor of three greater than that indicated by the weekly LPR measurements. Based on weight loss measurements of the 316L stainless steels the corrosion rate at Changi and SJI was generally higher during the first 3–6 months, after which it declined by about 50% and remained relatively stable for the remainder of the 30 months duration. Whereas at RSYC the corrosion rate was nearly constant throughout the entire exposure time. The observation that the corrosion rates shown in Fig. 4 come to a near constant value after 12 months, as opposed to continually increasing, implies that once compete biofouling coverage is established the initiation of new localization sites is rare. An alternative explanation that the total localized corrosion becomes

3.4. Types of corrosion After chemical cleaning, the coupons were inspected to determine the morphology of corrosion that had occurred. On the 316L stainless steel crevice corrosion sites were found under some, but not all, barnacle bases after as little as 3 months (Table 2 & Fig. 6). The average diameter of barnacles causing crevice corrosion was 11.5 mm, with a standard deviation of 4.1 mm. Although this size range is reasonably consistent with the findings of Relini et al. [27] and Neville and Hodgkiess [28] who concluded that barnacles have to a basal diameter

Fig. 4. Corrosion rates for 316L stainless steel determined from weight loss measurements; the lines are guides to the eye.

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Table 2 Numbers of live and dead barnacles associated with crevice corrosion on 316L stainless steel coupon observed after water-jet cleaning at 276 kPa. Location

Changi

RSYC

SJI

Exposure time (Months)

3 3 6 6 12 12 24 24 30 30 3 3 6 6 12 12 24 24 30 30 3 3 6 6 12 12 24 24 30 30

Coupon face

Front Back Front Back Front Back Front Back Front Back Front Back Front Back Front Back Front Back Front Back Front Back Front Back Front Back Front Back Front Back

No. of Live barnacles

No. of Dead barnacles

Total

Crevices (dia)

Total

Crevices

3 0 1 7 0 5 4 3 1 2 7 3 2 7 2 3 4 0 6 5 19 4 4 4 1 0 0 3 7 4

0 0 0 0 0 0 1 (12.9 mm) 0 0 0 0 0 0 0 0 0 0 0 1 (12.4 mm) 0 0 0 0 1 (7.0 mm) 0 0 0 0 0 0

7 21 7 12 10 10 17 18 7 2 0 4 5 10 26 14 23 13 11 8 13 25 4 5 21 32 3 5 9 6

4 5 1 0 2 5 8 4 1 0 0 0 1 0 0 2 4 5 2 2 1 0 1 0 1 5 0 0 0 0

No. crevices not clearly caused by barnacles

Total crevices

Barnacle diameter causing largest crevice (mm)

1 2 0 0 2 2 6 6 5 9 0 2 0 0 2 1 3 13 2 3 0 0 0 0 0 0 6 0 1 1

5 7 1 0 4 7 15 10 6 9 0 2 1 0 2 3 7 18 5 5 1 0 1 1 1 5 6 0 1 1

4.7 13.1 12.2 No crevice 15.5 15.4 15.0 15.8 17.3 Not under barnacle No crevice Not under barnacle 14.1 No crevice Not under barnacle 17.8 15.5 7.7 14.2 11.1 7.6 No crevices 4.5 7.0 9 5.1 Not under barnacle No crevices Not under barnacle Not under barnacle

the base plates of both live and dead barnacles (Table 2). However, in some cases, the live barnacles had settled on the base plates of their dead predecessors and it was not possible to determine at what stage the corrosion was initiated. Of the crevices that could be correlated to barnacles, 54 out of 57 were associated with dead barnacles (Table 3). In the three incidents where the crevice corrosion was observed under live barnacles, the depth of attack was very shallow (ca. 0.1 mm). In contrast, by the end of the 30th month of the test period, some of the crevices beneath dead barnacles had reached depths of 0.5 mm. No corrosion was detected on the SMO samples, regardless of the extent or type of fouling or test location. Likewise, there was no evidence of mechanical damage on the surface of specimens, meaning that the barnacle basal plate did not protrude into the stainless steel during their growth. Weight losses indicated corrosion rates of the SMO stainless steel is < 1 μm yr−1 (Fig. 8).

of at least 10 mm to influence the stainless steel corrosion progress, crevices were found under dead barnacles as small as 4.2 mm in diameter and as large as 19.5 mm. Furthermore, Fig. 7 shows a cumulative plot of the number of dead barnacles causing crevices against their size, from which it can be seen that the cumulative number increases almost linearly from the smallest barnacle to cause to a crevice (4.2 mm) to the typical size of a mature barnacle (ca. 16 mm). This implies that there is no critical size for a barnacle’s basal diameter to initiate crevice corrosion. This is more agreement with the findings of de Messano et al. [25] who found shallow crevices beneath juvenile barnacles less than 7 mm in diameter. The reason for the localized corrosion is likely due to crevices forming under the barnacle bases resulting in differential oxygen cells on the metal surface, which is the commonly accepted mechanism for crevice corrosion [15]. In this regard, it is interesting to note that the smallest size barnacle associated with a crevice (4.2 mm) is similar in size to the “teeth” often used in crevice former assemblies [29]. For 316L stainless steel samples crevice corrosion was found under

Fig. 6. (a) Photograph of a 316L stainless steel coupon retrieved from Changi test site after 30 months after chemical cleaning. (b) Close-up of the crevice corrosion found below a barnacle; circled in (a).

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water-jet cleaning had removed these. The issue of whether live or dead barnacles initiate crevice corrosion has been the subject of debate. Eashwar et al. [13] only found corrosion under dead barnacles where as a number of other authors claim to have demonstrated attack under live barnacles [24,30,31]. de Messano et al. [25] used potentiodynmic polarization measurements to show that the presence of live juvenile barnacles reduced the breakdown potential (Eb) on 316L stainless steel by about 80 mV, compared with sterile controls. However, the magnitude of this negative shift in Eb is in line with that which one would be expected to occur in the presence of any surface crevice. On its own, it does not represent evidence to support the notion that the biochemistry associated with living barnacles influences the Eb. Furthermore, the reported Eb remained above 300 mV vs SCE, which is positive of any OCP recorded in either this work or that of de Messano et al. [24,25], so in itself cannot explain the subsequently observed localized corrosion. Overall, the observation that corrosion was found under only 3 out of 111 live barnacles and that the penetration depths of these were far shallower than under their dead counterparts, along with relevant data presented in the literature [13], it is evident that the extent of the corrosion beneath dead barnacles is more severe than below living ones. This suggests that even if the chemicals produced by the decay of the barnacles did not necessarily initiate localized corrosion, they accelerated the rate of corrosion. The decomposition of the shellfish is known to result in the production of amines, e.g. histamine that can act as oxygen scavengers, especially in the presence of enzymes, such as diamine oxidase [32]. Similar processes would result in the environment under dead barnacles being more anaerobic, creating a larger differential oxygen cells across the shell edges that would favour the onset of crevice corrosion, than would be the case under live barnacles. Once initiated the propagation rate of the crevice will depend on the development of acidic conditions in the occluded environment [33], although it is not obvious why this process should be more efficient under a dead barnacle than a live one, the decayed organisms will likely to result in the production of sulphides, which in a shielded environment can be detrimental to the stability of passive films or if oxidized to thiosulphates can have a synergic effect with chloride in accelerating localized corrosion [34,35]. Zhang and Dexter [36] also reported that biofilms can accelerate the propagation rates of already initiated localized corrosion by increasing the cathodic reaction rate.

Fig. 7. Cumulative plot of the total number of dead barnacles causing crevice corrosion against their size.

4. Discussion 4.1. Corrosion associated with Barnacles Table 3 reveals that for test coupons the percentage of dead barnacles associated with crevice corrosion varied from 27% at Changi to 14% at RSYC to 6.5% at SJI. In contrast, the percentage of live barnacles associated with crevices never exceeded 4% at any site, averaging less than 3%; that is crevices were found under only 3 out of 111 barnacles. A possible reason for the smaller percentage of live barnacles associated with crevices is that their average size is smaller, as many are still juveniles, than their dead counterparts. However, Table 2 shows that crevices were found under dead barnacles that were smaller in diameter than 5 mm, much smaller than the typical size of a mature barnacle (ca. 15 mm) and the linearity seen in Fig. 7 suggests that there is not a critical size for onset of crevice corrosion. Likewise, it could be argued that there was insufficient time for crevices to initiate under most of the live barnacles, but on its own this is insufficient to explain why crevices were associated with only 3 out of 111 barnacles. Nevertheless, the shorter time factor may explain why the crevices under the live barnacles were shallower (ca. 0.1) than those found under their dead counterparts (up to 0.5 mm). Given the very low percentage of live barnacles that caused crevice corrosion, it is unlikely that many of the crevices found under dead barnacles were initiated while the barnacle was still alive and given their greater depth it is certain that the crevice corrosion continued after the barnacles’ death. However, it should be noted that about 50% of the crevices could neither be assigned to dead nor live barnacles, because after removal of detritus by water jet cleaning at 276 kPa no species could be seen directly above the crevice site (Tables 2 and 3). It is likely that most of these crevices were originally associated with barnacles but that the

4.2. Corrosion associated with oysters The most damaging localized corrosion appeared to be associated with oyster shells (Fig. 9a). In some cases, tracks of crevice corrosion extended several centimetres inside the 316L stainless steel specimens (Fig. 9b). This concurs with the findings of Palanichamy and Subramanian [37] who immersed high strength low alloy steel samples at various locations of the Gulf of Mannar, India for 24 months and found what they described as “crevice tunnelling” on samples where oysters were the dominant hard-fouler. Almost complete penetration of the 2 mm 316L stainless was observed on a specimen retrieved from Changi after just 6 months, with the pit initiation site found under an oyster (Fig. 10). On the surface this

Table 3 Breakdown of percentages of live and dead barnacles causing crevice corrosion on 316L stainless steel test coupons retrieved from the three test locations. Test site

Number of live barnacles

Crevices under live barnacles

Percentage of live barnacles causing crevices

Number of dead barnacles

Crevices under dead barnacles

Percentage of dead barnacles causing crevices

Number of crevices that could not be assigned to any speciesa

Changi RSYC SJI Total

26 39 46 111

1 1 1 3

3.8% 2.6% 2.2% 2.7%

111 114 123 348

30 16 8 54

27.0% 14.0% 6.5% 15.5%

33 26 8 67

a

No species immediately found over crevice after water jet cleaning at 276 kPa.

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Fig. 8. Photographs of an SMO stainless steel coupon as retrieved from the St John’s Island test site after 30 months of exposure (a) before and (b) after cleaning revealing no evidence of corrosion.

contrast, Hansen et al. [40] have demonstrated that a catecholic mussel adhesive protein can be used as a corrosion inhibitor for UNS S30403 stainless steel in ferric chloride. An additional explanation as to why the oysters caused far more aggressive corrosion than the barnacles and mussels could be variations in the biofilm beneath the different fouling species, that is microbial influence corrosion could be involved in the initiation and propagation of the crevices. Unfortunately, to prove this would be extremely difficult (impossible?) as how would one ensure that the biofilm beneath an oyster can be characterized without massive contamination from the surrounding organisms when the macrofouling layer is centimetres thick? Even if it were possible to show that the biofilm beneath an oyster is different to that beneath a mussel or barnacle, it would still be a leap of faith to claim that this was the major source of the different corrosion behaviours. Nevertheless, it is worth pointing out that Beech et al. [41] reported that the sulphate reducing bacteria that they recovered from beneath oysters that were associated with deep crevice corrosion to a ship’s hull was the most aggressive they had ever encountered. By grinding down part of the coupons, it was revealed that the pinholes did not exceed more than 3 mm in diameter at any point throughout the depth of the stainless steel. Where pits appeared as clusters, as in Fig. 9a, these were isolated from one another, that is, the pin-holes did not belong to a single large subsurface crevice. Likewise, the pinholes along the long corrosion tracks are only connected by the shallow (∼0.3 mm deep) surface track itself, there being no continuous subsurface feature (Fig. 11). Metallographic etching did not reveal any preferential direction in the microstructure of the 316L stainless steel (Fig. 12). As such, there is no evidence to suggest that the corrosion tracks follow the rolling direction in a manner akin to end-grain corrosion. The acidified occluded crevice solutions are denser than bulk seawater, so the growth direction of the corrosion tracks could be influenced by gravity. The photographs presented by Palanichamy and Subramanian [37] of oyster initiated crevice tunnels in low alloy steel, although not mentioned by the authors, suggest that the corrosion proceeded vertically down the exposed specimens. However, this does not appear to have been the case for the 316L stainless steel samples exposed to Singapore seawater, as there was no consistent correlation between the corrosion tracks and the

pit appeared as a small pin-hole (Fig. 10c) but cross sectional analysis shows that acidification of the solution in the occluded cell resulted in extensive corrosion below the surface (Fig. 10d). Specimens retrieved from the same Changi site without oysters present did not display similar signs, and showed only the broad hemi-spherical 0.5 mm crevices associated with barnacles. Only one complete pinhole penetration was observed on a specimen removed from the Changi site after 30 months. Unfortunately, no shellfish was found directly over the site of the corrosion, with the nearest species being a vermetid but it is possible that the real culprit had become detached during the 30 months of exposure. Deep pit penetration of the 316L stainless, similar to that on the 6th month Changi sample, was observed on all specimens retrieved after 12 months or more exposure from both RSYC and SJI. In each case, the penetration was restricted to one or two clusters of pinholes no more than 10 mm apart or long corrosion tracks (up to 5 cm) with a few pinholes located along the tracks (Fig. 9). In most cases, the initiation sites appear to be associated with oysters, which are far more prevalent at these two sites than at Changi. In a few cases, it was impossible to identify the initiating the corrosion attack since it was spread over several centimetres; but oysters, vermetids and barnacles were observed in the vicinity. A possible reason for the more extensive corrosion under oysters is that the shell is strongly attached over a wider area, whereas mussels attach to the substrate by a byssus thread. The adhesive from the oysters is also different from barnacles and mussels. Burkett et al. [38] and Alberts et al. [39] have reported the presence of organic radicals and evidence for oxidative cross-linking in the curing of oyster adhesive, respectively. Both of these would accelerate the removal of dissolved oxygen, an excellent radical scavenger, underneath the oyster, favouring the onset of crevice corrosion. Once initiated the development of the acidic conditions within the crevice that are responsible for its propagation are known to be a function of the tightness of the crevice [33], the strong attachment of oysters likely means a tighter crevice and thus more aggressive corrosion. In addition, Alberts et al. [39], using to energy dispersive x-ray spectroscopy, found that oysters had increased levels of chlorine in their adhesive compared to their shells, presumably this is in the form of chloride so it may have contributed to the initiation of localized corrosion. Crevices were not found under all oysters, as they sometimes occurred as secondary fouling and were not attached directly to the stainless steel substrate. In

Fig. 9. (a) Cluster of localized corrosion pits (circled) and (b) a long corrosion track on 316L stainless steel coupons retrieved from RSYC after 24 and 12 months respectively; both of which were found beneath oysters.

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Fig. 10. Photographs of 316L stainless steel coupon removed from Changi test site after 6 months immersion. (a) Prior to cleaning green mussels are the dominant species; (b) After water jet cleaning at 0.827 MPa an oyster can be seen (circled); (c) chemical cleaning revealed a pit opening beneath this oyster; (d) cross-section of the pit revealing extensive subsurface corrosion.

biofouling varied with location, but was independent of the grade of stainless steel. General corrosion rates were about twice that reported in temperate waters. No localized corrosion was observed on SMO stainless steel, while the propensity for localized corrosion of grade 316L by shellfish is ranked as oysters > > barnacles > > green mussels. Oysters caused extensive localised corrosion, penetrating 2 mm into 316L stainless steel in less than 12 months. Multiple tracks of corrosion several centimetres long (up to 5 cm) developed inside the stainless steel coupons at the same time. There was no evidence suggesting that these corrosion tracks were following grain boundaries, as there was no specific orientation observed in the microstructure of the coupons. The corrosion associated with barnacles was predominately shallow (< 0.5 mm) crevice corrosion and was mainly found under dead specimens with an average diameter 11.5 mm and a standard deviation of 4.1 mm. The fact that crevices were mostly found under dead barnacles suggests that their decay helps initiate the corrosion process. It is

orientation of the panels. Visual inspection of the 316L stainless rods used for the LPR measurements revealed much less localized corrosion than had been seen on the flat plate weight loss coupons, consistent with the lower corrosion rates predicted by the former technique. In particular, although shallow crevices were observed under dead barnacles, none of the deep crevice corrosion associated with oysters were found on the rods. It was found to be easier to detach oysters from the rods than the plates. It is possible that the weaker attachment to the rounded surface of the rods is the reason for the lack severe crevice attack below the oysters.

5. Conclusions Biofouling induced corrosion of 316L and SMO stainless steels has been investigated at three different test sites off the coasts of tropical Singapore over a period of 30 months. The extent and nature of the

Fig. 11. Photographs of the corrosion track on a 316L stainless steel coupon retrieved from RYC after 12 months of exposure that has been ground to depths of (a) 0.2 mm, (b) 0.6 mm, (c) 1.0 mm and (d) 1.8 mm. It can be seen the pinholes are only connected by the shallow surface track shown in Fig. 8b.

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Fig. 12. Optical Image 316L stainless steel after metallographic etching, revealing no preferential grain orientation.

postulated that this involves removal of dissolved oxygen underneath the barnacles, possibly by the oxidation of amines, thus creating a larger differential oxygen cells across the shell edges that would favour the onset of crevice corrosion. No evidence of corrosion was observed to be associated with green mussels; indeed, their presence may be desirable as they can prevent the attachment of more aggressive species. The difference in aggressiveness of the localized corrosion beneath the shellfish is likely associated with the strength of attachment, area of cover and chemical properties of the adhesive. Acknowledgments The authors acknowledge support from the US Office for Naval Research Award No. N00014-04-1-0789, the Singapore Defence Research & Technology Office Grant POD0411835 and partial funding under A*STAR SERC grant No. 1123004034. The authors thank Mr Razali from the Tropical Marine Science Institute for technical assistance in the project. References [1] M. Gerchakov, B. Sallman, Biofouling and effects of organic compounds and microorganisms on corrosion processes, in: R.M. Gerhold (Ed.), Proceedings of Microbiology of Power Plant Thermal Effluents Symposium, University of Iowa, September 1977. University of Iowa Press, Iowa City, 1977, pp. 67–72 Reproduced 1979 as: Effects of marine organisms, in: M. Schumacher (Ed.), Seawater Corrosion Handbook, Noyes Data Corporation, Park Ridge, New Jersey 1977, pp. 366–384. [2] A. Mollica, Biofilm and corrosion on active-passive alloys in seawater, Int. Biodeterior. Biodegrad. 29 (1992) 213–229. [3] C.E. Zobell, E.C. Allen, The significance of marine bacteria in the fouling of submerged surfaces, J. Bacteriol. 29 (1935) 239–251. [4] M.J. Dempsey, Marine bacterial fouling: a scanning electron microscope study, Mar. Biol. 61 (1981) 305–315. [5] M.E. Callow, R.L. Fletcher, The influence of low surface energy materials on bioadhesion – a review, Int. Biodeterior. Biodegrad. 34 (1994) 333–334. [6] P.Y. Qian, S.C.K. Lau, H.-U. Dahms, S. Dobretsov, T. Harder, Marine biofilms as mediators of colonization by marine macroorganisms: implications for antifouling and aquaculture, Mar. Biol. 9 (2007) 399–410. [7] G. Relini, E. Tixi, M. Relini, G. Torchia, The macrofouling on offshore platforms at Ravenna, Int. Biodeterior. Biodegrad. 41 (1998) 41–55. [8] T.E. Cloete, V.S. Brözel, A. von Holy, Practical aspects of biofouling control in industrial water systems, Int. Biodeterior. Biodegrad. 29 (1992) 299–341. [9] E.C. Haderlie, A brief overview of the effects of macrofouling, in: J.D. Costlow, R.C. Tipper (Eds.), Marine Biodeterioration: An Interdisciplinary Study, Proceedings of Symposium on Marine Biodeterioration, 20th–23rd April 1984, US Naval Institute, Annapolis, 1984, pp. 163–166. [10] S.C. Dexter, Role of microfouling organisms in marine corrosion, Biofouling 7 (1993) 97–127.

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