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Received: August 30, 2016 Accepted: September 29, 2016 Published: December 27, 2016

The Electrochemical Society of Japan http://dx.doi.org/10.5796/electrochemistry.85.7 Article Electrochemistry, 85(1),7–12 (2017) Performance of High Strength Low Alloy AISI 4135 in the Marine Splash Zone Yanliang HUANG,a,* Xiuming YU,a,b Qichao ZHANG,a,b and Roland DE MARCOc,d,e a Key Laboratory of Marine Environmental Corrosion and Bio-fouling, Institute of Oceanology, Chinese Academy of Sciences, Qingdao 266071, P. R. China b University of Chinese Academy of Sciences, Beijing 100049, P. R. China c Faculty of Science, Health, Education and Engineering, University of the Sunshine Coast, 90 Sippy Downs Drive, Queensland 4556, Australia d Department of Chemistry, Curtin University, GPO Box U1987, Perth, Western Australia 6109, Australia e School of Chemical and Molecular Biosciences, University of Queensland, Brisbane, Queensland 4072, Australia * Corresponding author: [email protected]

ABSTRACT The corrosion behavior of high strength low alloy AISI 4135 steel was studied following extended exposure to a Qingdao field environment. When electrochemically active γ-FeOOH was formed in conjunction with easily reducible β-FeOOH, these corrosion products acted to accelerate corrosion attack in the marine splash zone (i.e., steel structure above the line that is splashed by sea spray). It had also been observed that the pH beneath the rust layer exhibited a minimum during the non-splash part of wet and non-splash corrosion cycling. These acidic conditions contributed significantly to an increase in the rate of localized corrosion, both beneath the rust deposits and within rust cracks and voids within pockets of rust deposits over the steel surface. © The Electrochemical Society of Japan, All rights reserved.

Keywords : High Strength Low , , Rust, Marine Splash Zone

1. Introduction Table 1. Chemical composition of AISI 4135 (UNS G41350) steel (wt%). High strength, low-alloy (HSLA) steel comprising low amounts of alloying elements can reduce environmental burdens by using CSiMnP S NiCrMoFe less material to meet the same requirements in varied structures, and 0.399 0.293 0.509 0.0146 0.0144 0.0804 0.903 0.204 Bal. therefore it has potential applications in far and deep sea develop- ment. Accordingly, an understanding of the corrosion behavior of HSLA steel is beneficial for safety in real-world applications in the 2. Experimental marine environment, especially in the marine splash zone where structures experience the most severe corrosion.1–3 It is well known 2.1 Material and field exposure site that localized corrosion is potentially catastrophic compared to The chemical composition of the HSLA steel AISI 4135 utilized general corrosion. Pitting corrosion is one form of localized in this study is listed in Table 1. The steel was produced by the corrosion, with previous research showing that pitting occurs after Special Steel Department of Beijing Shougang New Steel Co., Ltd., prolonged exposure to the marine environment.4 Consequently, it is noting that Ni was also present in of the same grades used by important to evaluate the corrosion behavior of HSLA in the marine other researchers,18 but was also absent in the studies of others.19 splash zone and to elucidate the mechanism of pitting corrosion Specimens of steel plates (100 mm © 50 mm © 5 mm) were exposed under these environmental conditions. in the splash zone at a marine corrosion field site in Qingdao, Many researchers studied the rust layers of corroded steels.5–17 China, which is a typical semidiurnal tidal area. The preparation Rust layers often form on the surface of steel structures experiencing and treatment before and after experiments follow the procedures of a corrosion process, and the associated corrosion performance is ASTM standard G52.20 The cleaned specimens were exposed in dependent on a number of factors including rust formation, rust triplicate on open-air racks at an angle of 90° from the horizontal evolution and the inherent properties of the rust layers such as plane, with the open-air exposure racks fixed to rocks at the seaside. composition and structures. If pitting corrosion of HSLA steel Test specimens were retrieved from the field marine test site for occurs after prolonged exposure in the marine splash zone, as analysis after 1–3 years of continuous exposure. The meteorological observed in R. Jeffery et al.’s exposure tests,4 it can be speculated data and characteristics at the marine exposure site are that the rust layer may contribute to the origins of pitting corrosion. presented in Table 2.21 Accordingly, research on the exact chemical nature of the corrosion products formed through the corrosion of and its alloys under 2.2 Characterization of rust deposits field conditions in the marine splash zone, referred to as “rust”, will Infrared (IR) analysis was used to characterize the field rust assist with understanding the effect of rust layers on the pitting deposits. A Magna-IR 560 infrared spectrophotometer was used to corrosion of HSLA. This is the motivation of the present study on measure the IR spectra of the rust layers in the range of 400 cm¹1 to the corrosion behavior of HSLA steel, so as to elucidate the 4000 cm¹1 at a spectral resolution of 4 cm¹1. Both the morphologies mechanism of corrosion in a field environment that is representative on the surfaces of the rust deposits and the cross-sections of of the authentic field conditions of the marine splash zone. specimens with rust were observed by SEM (Hitachi S-3400N) and light microscopy.

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Table 2. Meteorological data and seawater characteristics at the marine exposure site.21

Site Northern latitude 36°03B East longitude 120°25B Mean air temperature (°C) 12.3 Mean air humidity (%)71 Mean annual rainfall (mm a¹1) 643 Mean seawater temperature (°C) 13.7 Mean seawater salinity (‰) 31.5 Mean dissolved in seawater 8.4 (mg L¹1) Mean seawater pH 8.3

Figure 2. (Color online) Surface appearance of AISI 4135 steel specimens exposed to the field marine splash zone: (a) Specimen with cracks in the rust (left) and specimen with the rust removed (right), 1 year’s exposure; (b) Specimen with outer rust partially Figure 1. (Color online) Schematic for the measurement of pH broken off (left) and specimen with the rust removed (right), 1 year’s beneath the rust layer and potential of the specimen during an exposure; (c) Specimen with rust and specimen with the rust alternation of wetting and drying in a simulated environment. The removed (right), 3 year’s exposure. and pH glass electrode are in contact with the inner rust layer, which is always wetted by that is rich in the inner rust layer at high RHs in the marine splash zone. that the salt bridge and pH glass electrode were in contact with the inner rust deposit. The Cl¹ activity in the rust layer pore solution was determined 2.3 pH, open circuit potential and Cl− activity measurements using a Cl¹ -selective electrode (ISE) during the wet period of associated with rust deposit the corrosion cycle. When measuring the Cl¹ activity in the solution A specimen comprising a thick rust deposit following field residing in the pores of the rust deposits, the Cl¹ ISE was inserted exposure to the marine splash zone for 1 year was recovered during into the rust deposit through the outer rust layer cracks. low tide. Subsequently, this field corroded specimen was kept in a chamber to measure the pH within the rust pores as well as the open 3. Results circuit potential of the coupon. When measure pH, the temperature of the chamber was kept at 25°C and the humidity was kept at 100% 3.1 Morphology of the specimen exposed in the marine splash and 60% respectively to simulate wetting and drying stage. The dry zone condition simulated the non-splashing stage. The seawater for splash Figure 2 presents the physical appearance of AISI 4135 steel was natural seawater collected at Huiquan Bay, Qingdao, China, after 1 and 3 years of exposure within the marine splash zone. Pits with salinity of 31.5‰ as listed in Table 2. The pH value beneath were formed beneath the rust deposits after 1 year’s exposure, with 3 the wet rust deposit was recorded by a pH meter and the cell years’ exposure leading to severe pitting. It can be seen in Fig. 3 that presented in Fig. 1 was used. For corrosion potentials measurement, micro-cracks (a) and macro-cracks (b) were formed on the surface cell similar to Fig. 1 was used but in open air. The corrosion of rust deposit. Cross-sections of specimens with rust deposits are potentials of the specimen during wetting and drying were measured presented in Fig. 4. Generally, the pits were under the rust layer using a PARSTAT 2273 potentiostat. A saturated calomel electrode cracks. Here, we pay special attention to the cracks of the rust (SCE) was used as the reference electrode, and the potentials were deposits. By contrast, the inner rust deposit was crack-free, adherent reported vs. the SCE. Humidity near the surface of specimen was and much thinner than the outer rust deposit. also recorded when measuring potential. Holes of 2.5 mm in The rust on the steel surface breaks down due to the following diameter were bored into the outer rust deposit to fit the capillary of processes:22 1) portions of the rust dissolve depending on the a salt bridge and the pH glass electrode beneath the rust layer. solubility as rain, splash and condensate contact the rust, without a The tips of the capillary and the glass electrode were both 1 mm in steady supply of iron and arising from corrosion; 2) diameter. The depths of these bored holes satisfied the dimensions of further dehydration and crystallization of rust takes place, with Fe3+ the capillary of the salt bridge, noting that the pH glass electrode oxyhydroxides in the rust undergoing transformation into non- contacted the inner rust deposit where the electrolyte was rich in the hydrated such as hematite or A-Fe2O3. The annual rainfall in marine splash zone. It was found that the rust deposit could absorb Qingdao is limited as shown in Table 2. There are not always rain electrolyte from the splash and water condensates in the marine and marine splash events. Consequently, dehydration occurs when splash zone since evaporation of electrolyte from the rust is impeded there is no rain and the tide is low. These processes are the ultimate underneath the thick outer rust deposit in the marine splash zone causes of the formation of cracks and voids within the rust, as where the relative humidity (RH) is very high. Cross-sections of observed in Figs. 2–4. These cracks and voids also develop into these specimens following the aforesaid measurements confirmed channels connecting the surface to the external environment,

8 Electrochemistry, 85(1),7–12 (2017)

Figure 3. SEM micrographs revealing cracks in the surface of the rust layer formed on AISI 4135 steel exposed to the marine splash Figure 4. SEM micrographs of polished cross-sections of the rust zone for 1 year: (a) micro crack of rust; (b) macro crack of rust. layer after 1 year of exposure time in the marine splash zone. (a) and (b) represent different cross-sections of the one specimen. Table 3. IR absorption data for some standard forms of rust24,25 (these are used as the benchmarks in Fig. 5). of the corrosion cycle, the thickness of the electrolyte layer is Infrared absorption peak locations, diminished. On the one hand, this thick and dense rust deposit Rust phase in wavenumber, cm¹1 prevents moisture from diffusing out of the rust deposit, noting that (relative intensity) the rust deposit does not dry out. Accordingly, the specimen is kept moist virtually all of the time in the marine splash zone, where A-FeOOH 1667, 1399, 1260, 881, 793, 608, 463 the average humidity was 71% as reported in Table 2. The high C -FeOOH 1625, 1450, 1152, 1017, 737, 487, 342 corrosion rate has been maintained since the metal surface has been B-FeOOH 1615, 1400, 858, 670, 490, 390 (300–500) kept moist for long periods during the wet-to-non-splash cycling of 27 C-Fe2O3 1790, 690, 628, 550, 475, 437, 418, 386, 360 the marine splash zone. It can be seen in Fig. 5 that B-FeOOH is formed in the inner rust Fe3O4 566 (500–700), 404 (300–500) deposit. Nishimura et al.28 demonstrated that B-FeOOH was easily Note: The most intense peaks for each compound are underlined. reduced via electrochemical measurements on a steel specimen comprising rust with B-FeOOH, which was formed during wet- 29 allowing water and O2 to reach the metal surface. And clearly, this to-dry cycling in the presence of and at low pH. loose outer rust deposit may easily absorb seawater from the marine Furthermore, the formation of B-FeOOH was related to the presence splash and water condensates, thereby maintaining the moisture of a threshold concentration of Cl¹.30 Nishimura et al.15 suggested levels of the rust deposit. And it is probable that some nanoscaled that the existence of loose and low adherence B-FeOOH weakens the pores may never dry out during wet/dry corrosion cycling.23 protective properties of the rust deposit and accelerates the corrosion rate of the steel under wet-dry conditions containing chloride. 3.2 Phase analysis of the field rust deposit The literature IR absorption data for several standard forms of 3.3 pH, open circuit potential and Cl− activity associated with rust are presented in Table 3.24,25 As evident in Fig. 5(a), C-FeOOH rust deposit during the wetting and drying was formed in the outer surface of the rust deposit exposed to the The existing of rust on the surface of specimen prevents the external environment with no C-FeOOH observed in the middle and splashed seawater from attaching the metal surface at once. The inner rust deposit zones. It is significant to note that C-FeOOH splashed seawater will reach the metal surface by diffusion through (lepidocrocite) is a semi-conductor and electrochemically reactive the rust layer. Because of the hydrolysis of corrosion product, the pH species,26 which is expected to accelerate corrosion in the cracks of the electrolyte at the metal surface maybe different from the pH of of the rust deposit by acting as an oxidant in conjunction with O2. the splashed seawater. The lowering of pH and therefore the change Since A-FeOOH is thermodynamically stable, it is electrochemically in corrosion potential will accelerate the corrosion process. difficult to reduce. Whereas, C-FeOOH is thermodynamically easy Because of the non-uniform distribution of the rust thickness and to reduce, noting that C-FeOOH is reduced to C-Fe.OH.OH, with electrolyte beneath the rust, the measured values of the pH and the this new phase representing an intermediate.6 The C-Fe.OH.OH corrosion potential were localized, but they could represent the intermediate phase comprising Fe2+ is reoxidized to C-FeOOH difference in tread between outer splashed seawater and inner during the drying process. During the subsequent non-splash part electrolyte.

9 Electrochemistry, 85(1),7–12 (2017)

Figure 6. (Color online) The pH change beneath the rust layer of a steel specimen during an alternation of wetting and drying. Seawater sprayed on the surface of the rust layer could alter the pH beneath rust layer. 1. Stage 1; 2. Stage 2; 3. Stage 3.

thereby making the rust weakly acidic. When the rust surface is sprayed with seawater, the pH quickly rises to a higher value, which is due to a wetting of the rust layer by fresh seawater that has a pH of 8.2. Stage 3: drying of the rust Anodic reaction: £-Fe:OH:OH ! £-FeOOH þ Hþ þ e ð6Þ Figure 5. (Color online) Infrared absorption spectra of the rust Reaction (1) above layer: (a) Integrated rust; (b) Rust collected at the section of the rust 2þ layer comprising cracks and the outer section of rust in the rust Fe þ 2Cl ! FeCl2 ð7Þ crack. The symbols of A, B and C represent A-FeOOH, B-FeOOH and Cathodic reaction: C-FeOOH, respectively. Reaction (5) above During the non-splashing stage, the rate of the diffusion limited O2 Figure 6 presents the pH within the rust deposit also comprising reduction reaction is hastened significantly due to a thinning of the cracks during an alternation of splashing and non-splashing electrolyte film in the inner surface of the rust deposit. The reduction simulated in the laboratory using a specimen that was recovered of O2 couples with the main cathodic reaction, with O2 able to re- from the field study. oxidize the reduced Fe2+ formed during the wetting stage. The pH Stage 1: wetting of the dry rust falls to as low as 2.75 in the rust (see Fig. 6) and, during this stage, the Anodic reaction: steel experiences acidic corrosion within the rust possessing cracks. Fe ! Fe2þ þ 2e ð1Þ A specimen exposed to the field marine splash zone for 1 year Fe2þ þ 2H2O ! FeðOHÞ2 þ 2Hþ ð2Þ was also used in a measurement of corrosion potentials following cycling between wet and non-splash condition simulated in the 2Fe2þ þ 1=2O þ 3H O ! 2FeOOH þ 4Hþ ð3Þ 2 2 laboratory. After a moistening of the specimen by seawater, the Cathodic reaction: rusted steel was stored in a dry environment and subsequently a wet £-FeOOH þ Hþ þ e ! £-Fe:OH:OH ðmajorÞð4Þ environment, respectively. Figure 7 presents the change in corrosion potential during wet and non-splash cycling. Due to a variation in O þ 2H O þ 4e ! 4OH ð5Þ 2 2 the thickness of the electrolyte layer during drying and wetting, the During this stage, the cathodic reaction process involves the rate of O2 reduction varies periodically giving rise to a systematic 31,32 reduction of dissolved O2 and C-FeOOH, with the O2 reduction variation in the corrosion potential. During the initial stage of reaction occurring at a very slow rate relative to the anodic iron drying, the corrosion potential decreases due to electrolyte in the rust dissolution reaction, thus the requirement for another oxidant C- evaporating slowly as the localized RH is maintained at relatively FeOOH in the rust.6 As the corrosion process proceeds in parallel high values; however, as the localized RH declines on extended with intensification in splashing, the pH beneath the rust decreases drying, the rate of water evaporation and thinning of the water layer gradually. is facilitated. Hence, during extended drying, the O2 cathodic Stage 2: wet rust reaction is facilitated as the O2 diffusion current increases with the Anodic reaction: decreasing thickness of the electrolyte layer. At the end of drying, Reaction (1) above this elevated cathodic current polarizes the rust layer to positive Cathodic reaction: potentials, thereby allowing oxidation of the species. Reaction (5) above Moreover, Fig. 6 reveals a low pH during the drying stage. On Once the reducible C-FeOOH has been consumed, the O2 the basis of these results, it is plausible to suggest that the corrosion reduction process becomes the dominant cathodic reaction. In this of steel in the marine splash zone also occurs during non-splashing reaction zone, the pH is relatively constant at about 4.2 (see Fig. 6), when there is no seawater splash at low tide.

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thicker rust deposit as a consequence of surface rust film dissolution via the washing effect of fresh seawater together with film shrinkage from dehydration and crystallization of the rust reaction products induces deterioration in the protective nature of the rust film. During low tide, seawater may not splash on the surface of the specimen giving rise to rust dehydration. Optical micrographs when the specimen rust layer was removed (see Fig. 2) showed that large pits existed under the continuous rust layer. The macro-cracks in the rust deposit resulted from the physical stresses associated with a growth of the rust layer. These cracks provide a series of channels for the ingression of electrolyte and O2 whereby, in the presence of NaCl, worsening the corrosive condition below the spot of rust layer crack, a new stage of corrosion is initiated at the buried steel/rust interface at this spot with water and O2 undergoing transportation allowing the formation of FeCl2 in the anodic channels and NaOH in the cathodic regions, giving rise of the possibility of higher corrosion rate at this spot and resulting in macro corrosion pit after long exposure. 3+ This aerial oxidation of FeCl2 generates HCl, Fe , Fe(OH)3(s) as well as B-FeOOH. The strongly acidic conditions in the rust deposit diminish the rate of aerial oxidation of Fe2+, thereby increasing the solubility and concomitant surface protection via Fe(OH)3(s) and B- FeOOH reaction species. Chloride can easily penetrate the thin film and accumulate at the buried steel/rust interface. Nishimura et al.28 studied steels covered with a rust deposit of B-FeOOH demonstrat- ing that B-FeOOH may also be easily reduced promoting corrosion under environmental conditions involving . In this case, the surface positive charge from dissolved Fe2+ is counter balanced by electrolyte anions such as Cl¹ from the NaCl electrolyte forming ¹ FeCl2 in the anodic channel, while the negative charge of OH is counter balanced by the electrolyte cation Na+ to form NaOH in the Figure 7. (Color online) The corrosion potential of a rusted cathodic channels. The FeCl2 solution is only weakly acidic because specimen during drying (a) and wetting (b) cycles. 2+ Fe hydrolyzes weakly. Hence, FeCl2 is oxidized by O2 transported through the anodic channels as proposed below: Table 4. Cl¹ activities in outer and inner rust pore solutions at FeCl þð1=4ÞO þð5=2ÞH O ! FeðOHÞ ðsÞþ2HCl ð8Þ four different positions. The Cl¹ ion activity of seawater acts as a 2 2 2 3 ¹ reference to show the enrichment of Cl in the rust layer. The aerial oxidation of FeCl2 generates HCl as well as Fe(OH) (s), FeCl , and FeOHCl as the predominant species.13 Surface rust (mol L¹1) 0.603 0.706 0.639 0.846 3 3 2 These compounds are either soluble or are intermediates, and ¹1 Inner rust (mol L ) 1.340 1.882 1.773 1.959 therefore avoid detection by infrared . The most easily ¹ Seawater (mol L 1) 0.520 detected compounds are obviously the most insoluble, and have been detected at high concentrations. The Fe(OH)3(s) species precipitated in the anodic channels may transform into B-FeOOH The chloride accumulation in the rust was verified through as a yellow precipitate,33 which is incorporated in the rust deposit, chloride activity measurement as shown in Table 4. Increased as has been shown via the formation of B-FeOOH that is facilitated chloride concentration facilitates the formation of B-FeOOH30 and by strongly acidic solutions containing high concentrations of 34 then increases the corrosion rate. chloride. Reaction (8) suggests that the aerial oxidation of FeCl2 generates , as inferred in Fig. 8. Acid is formed 4. Discussion because the NaOH in the cathodic channels and the FeCl2 in the anodic channels do not react directly, and because the O2-based The corrosion processes underneath the thick deposits of rust oxidation of FeCl2 in the anodic channels produces protons due to formed in the marine splash zone have been hypothesized to take place the subsequent hydrolysis of the electrogenerated Fe3+ species. In according to the following physicochemical reaction mechanism. the narrow anodic channels, the pH decreases as acid is formed, with The corrosive environment is marine splash zone with seawater the subsequent deposition of Fe3+ hydroxide noting that this anodic foam splashing onto the steel surface randomly. This condition process represents a thermodynamically unfavoured reaction. Due to determines that the steel surface can’t be uniformly wetted and the a conglomeration of these factors, the rate of aerial oxidation of Fe2+ corrosion can’t be uniformly initiated with rust spots appearing on hydroxide diminishes. Also, a decrease in pH results in an increase the surface at initial stage. Although the inner rust is formed on the in corrosion rate, with Tamura et al.35 reporting that the reaction overall surface later, it is reasonable to imagine that the thickness of product of the aerial oxidation of Fe3+ hydroxide having an the rust layer is not evenly distributed with the early initiated rust electrocatalytic effect, thereby facilitating the rate of aerial oxidation spot thicker. Since the seawater foam (electrolyte) is randomly of Fe2+ via autocatalysis. The oxyhydroxides of Fe3+ together with splashing onto the steel surface, it will take time for electrolyte at A-, B-, and C-FeOOH, also act as catalysts,36 with the aerial wetted spot to spread at the steel/rust interface. And the distance oxidation of Fe2+ in the anodic channels at acidic proceeding as between wetted spots maybe quite long, reaching a uniform a consequence of the heterogeneous of the rust compounds. electrolyte distribution at the steel/rust interface is difficult with The acidic HCl formed in the anodic channel in contact with the seawater foam splashing on the surface in a random manner. With the steel surface is subjected to acidic corrosion as well as Fe3+ development of corrosion, cracks and voids formed in some area of from FeCl3 in solution according to the following reactions:

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Figure 8. (Color online) Schematic of acidic corrosion in the rust under layer.

Acknowledgments Fe þ 2HCl ! FeCl2 þ H2 ð9Þ Fe þ 2FeCl ! 3FeCl ð10Þ 3 2 This work was supported by the National Natural Science 2+ where HCl and FeCl3 are provided by the aerial oxidation of Fe . Foundation of China under Grant number 41276087. The sites where hydrogen evolves at new lead to the creation of new cathodic and anodic reaction products that are References inseparable. The consumption of acid via the reaction depicted in Eq. (9) results in an increase in pH, with this increase in pH 1. M. Smith, C. Bowley, and L. Williams, Mater. Perform., 41, 30 (2002). 2. A. Aghajani, Mater. Perform., 47, 38 (2008). accelerating the aerial oxidation of FeCl2 by Eq. (8), resulting in a concomitant diminution in pH via the regeneration of HCl. These 3. X. R. Zhu, G. Q. Huang, L. Y. Lin, and D. Y. Liu, Corros. Eng. Sci. Technol., 43, 328 (2008). reactions balance each other and the apparent result is a steady-state 4. R. Jeffrey and R. E. Melchers, Corros. Sci., 49, 2270 (2007). with corrosion proceeding at a condition of a constant acidity or pH. 5. L. Hao, S. X. Zhang, J. H. Dong, and W. Ke, Corros. Sci., 58, 175 (2012). The acidic corrosion occurring beneath the rust deposit spreads 6. M. Stratmann, K. Bohnenkamp, and H. J. Engell, Corros. Sci., 23, 969 (1983). progressively, invading neighboring corrosion reaction sites, and the 7. M. Yamashita, H. Miyuki, Y. Matsuda, H. Nagano, and T. Misawa, Corros. Sci., 36, 283 (1994). cracks and voids observed along layer planes may also promote 8. E. Burger, M. Fénart, S. Perrin, D. Neff, and P. Dillmann, Corros. Sci., 53, 2122 acidic corrosion. As demonstrated via pH measurements (see (2011). Fig. 6), the lowest measured pH is 2.75, although the pH is 9. U. R. Evans and C. A. J. Taylor, Corros. Sci., 12, 227 (1972). considerably higher during the bulk of a wetting and non-splashing 10. M. Cohen and K. Hashimoto, J. Electrochem. Soc., 121, 42 (1974). cycle. These comparatively mild acidic conditions determine that pit 11. J. T. Keiser, C. W. Brown, and R. H. Heidersbach, J. Electrochem. Soc., 129, 2686 (1982). formation is not drastic, so as to eat away the base metal quickly 12. U. Evans, Corros. Sci., 9, 813 (1969). resulting in holes over a short period (i.e., 1 or 2 years), but holes 13. T. Kamimura, S. Hara, H. Miyuki, M. Yamashita, and H. Uchida, Corros. Sci., 48, appeared after 3 years because of the uneven local corrosive 2799 (2006). environment beneath the rust deposit, as illustrated in Fig. 8. Our 14. I. M. Allam, J. S. Arlow, and H. Saricimen, Corros. Sci., 32, 417 (1991). 15. T. Nishimura, H. Katayama, K. Noda, and T. Kodama, Corros., 56, 935 (2000). earlier work demonstrated that hydrogen permeation through the 16. M. Yamashita, H. Miyuki, and H. Nagano, J. Jpn. Inst. Met., 7, 448 (1997). steel can be detected, verifying that acidic conditions beneath the 17. M. Yamashita, A. Maeda, H. Uchida, T. Kamimura, and H. Miyuki, J. Jpn. Inst. rust deposit are formed during the corrosion process.37 Met., 65, 967 (2001). 18. S. Li, E. Akiyama, N. Uno, K. Hirai, K. Tsuzaki, and B. Zhang, Corros. Sci., 52, 5. Conclusions 3198 (2010). 19. M. Wang, E. Akiyama, and K. Tsuzaki, Corros. Sci., 49, 4081 (2007). 20. ASTM G52, Standard Practice for Exposing and Evaluating and Alloys in (1) Rust cracks and voids that absorb seawater from the splash Surface Seawater, ASTM International, West Conshohocken, PA, 2011. and water condensates were formed when the AISI 4135 steel was 21. G. Q. Huang, J. Chin. Soc. Corros. Prot., 22, 211 (2002). exposed to the field marine splash zone. Significantly, these cracks 22. H. Tamura, Corros. Sci., 50, 1872 (2008). 23. D. Fyfe, The atmosphere, in: L. L. Shreir, R. A. Jarman, G. T. Burstein (Eds.), provided channels for ingression of electrolyte and O2 to the inner Corrosion, vol. 1, third ed., Butterworth–Heinemann, Oxford, 1994. rust deposit. 24. A. Raman, B. Kuban, and A. Razvan, Corros. Sci., 32, 1295 (1991). (2) C-FeOOH was present in the rust deposit formed on the AISI 25. L. Y. Song, Z. Y. Chen, and B. R. Hou, Corros. Sci., 93, 191 (2015). 4135 steel exposed in the marine splash zone. C-FeOOH is an 26. K. Kaneko and K. Inouye, Bull. Chem. Soc. Jpn., 47, 1139 (1974). electrochemically active species that accelerates corrosion in the rust 27. M. Stratmann, Ber. Bunsen-Ges. Phys. Chem., 94, 626 (1990). 28. T. Nishimura, I. Tanaka, and Y. Shimizu, J. Jpn. Inst. Met., 81, 1079 (1995). deposit cracks. Notably, the AISI 4135 steel exposed to the marine 29. K. Nomura, M. Tasaka, and Y. Ujihira, Corros., 44, 131 (1988). splash zone for 1 year contained high concentrations of chloride in 30. T. Ishikawa, R. Katoh, A. Yasukawa, K. Kandori, T. Nakayama, and F. Yuse, the inner rust deposition zone, promoting the formation of B-FeOOH Corros. Sci., 43, 1727 (2001). that is easily reduced under the environmental conditions of the 31. Q. C. Zhang, J. S. Wu, J. J. Wang, W. L. Zheng, and A. B. Li, Mater. Chem. Phys., 77, 603 (2003). marine splash zone. 32. J. G. Castano, C. A. Botero, A. H. Restrepo, E. A. Agudelo, E. Correa, and F. (3) The pH during wet and non-splash corrosion cycles showed Echeverría, Corros. Sci., 52, 216 (2010). a minimum during the non-splash period. In the narrow anodic 33. H. Katsuki and S. Komarneni, J. Am. Ceram. Soc., 84, 2313 (2001). channels, the acidic conditions in contact with the steel surface is 34. R. M. Cornell and U. Schwertmann, ‘The iron oxides: structure, properties, ’ subjected to accelerated and localized corrosion with pits forming reactions, occurrence, and uses , VCH, Weinheim, 1996. 35. H. Tamura, K. Goto, and M. Nagayama, Corros. Sci., 16, 197 (1976). under a rust deposit comprising cracks and voids after prolonged 36. H. Tamura, S. Kawamura, and M. Nagayama, Corros. Sci., 20, 963 (1980). exposure of specimens. 37. Y. L. Huang and Y. Y. Zhu, Corros. Sci., 47, 1545 (2005).

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