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Microstructural characterisation and corrosion performance of old railway girder bridge steel and modern weathering structural steel
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Microstructural characterisation and corrosion performance of old railway girder bridge steel and modern weathering structural steel N.K. Tewary a , A. Kundu a , R. Nandi a , J.K. Saha b , S.K. Ghosh a,∗ a b
Department of Metallurgy and Materials Engineering, Indian Institute of Engineering Science and Technology, Shibpur, Howrah – 711 103, India Institute for Steel Development & Growth, Kolkata – 700 019, India
a r t i c l e
i n f o
Article history: Received 11 May 2016 Received in revised form 21 September 2016 Accepted 13 October 2016 Available online xxx Keywords: A. Low alloy steel B. SEM B. X-ray diffraction C. Acid corrosion C. Rust
a b s t r a c t A comparison on microstructure and corrosion performance has been made between the two structural steels used in old railway girder bridge (Sample A) and modern grades of weathering structural steel (Sample B). The microstructures, viewed under optical microscope and scanning electron microscope (SEM), show mainly ferrite-pearlite phase constituents in both the steels, A and B. The phase fraction analysis shows higher amount of pearlite in steel A compared to that of steel B. The grain size of steel A is larger than that of steel B under identical processing condition. The immersion corrosion test in 3.5% NaCl shows that the corrosion rate of steel A increases with time, while the same for steel B decreases with time. On the other hand, corrosion test in 1% HCl shows that the corrosion rate of both steel A and B is higher as compared to that of NaCl which always decreases with time. The XRD analysis of corrosion products show the presence of many oxides, hydroxide and oxy-hydroxide like Lepidocrocite (␥-FeOOH), Goethite (␣FeOOH), Akaganeite (-FeOOH), Magnetite (Fe3 O4 ) and Maghemite (␥-Fe2 O3 ) in both the steels. The SEM images of corroded surfaces reveal different morphologies like flowery, cotton balls and rosette etc. which indicate that the corrosion products primarily contain Lepidocrocite (␥-FeOOH), Goethite (␣-FeOOH) and Akaganeite (-FeOOH). © 2016 Elsevier Ltd. All rights reserved.
1. Introduction Weathering steel is a known material that has been used for different construction purposes which are generally exposed in atmosphere [1]. But the main difficulty of this steel is the atmospheric corrosion [2]. For these reasons, researchers have paid much attention on prevention of corrosion and extend the life of these steels by changing the chemical composition or some other ways. In recent years, the corrosion studies of structural steels mainly used in Railway Bridges have great concern due to increase in its life span. The old structural steels possess a good performance background but the history of their production is missing. On the other hand, the modern steels used for structural purposes are much cleaner compared to old steels, but require heavy maintenance to increase their longevity. The important problem for the structural steel is corrosion that is the greatest drawback of steel used in construction line up, mainly in bridges. The steel chemistry is important for increasing the life span of steel used in structural areas [3]. However, the steel life is also affected by environmen-
∗ Corresponding author. E-mail address: [email protected] (S.K. Ghosh).
tal conditions, because in different environmental conditions of corrosion products as well as corrosion rate vary due to the different environmental factors [2]. To differentiate the corrosion rates of different steels in laboratory conditions there are several techniques for accelerating these rates [4]. These experiments are very important to evaluate the corrosion behaviours of steels exposed in different environmental condition. In order to understand the corrosion behaviour of steel as a function of type of steel, environmental conditions and exposure time, the corrosion products have been studied on many occasions. Weathering steel is a high-strength, low-alloy steel which provides a significantly higher corrosion resistance than the regular carbon steel [5]. Use of high strength steels has been increased in recent decades in structural purpose due to its light weight combined with high strength [6]. In general, the chemical composition of weathering steel contains little amount of Cu, Cr, Ni and P which are known to restrict the corrosion rate of these steels after exposure in the atmosphere by formation of a stable and protective oxide layer [7]. In view of the above the corrosion behaviour of weathering steels used in construction purposes is very much concern to enhance the life of these steels. To understand the comparison of corrosion behaviour between old structural steel and modern structural steel, two steels have been taken. Between the
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2 Table 1 Chemical composition of the steels A & B. Sample Id
C
Mn
Si
S
P
Al
Cu
Cr
Ni
Equivalent Grade
Old steel: Steel A Modern steel: Steel B
0.14 0.05
0.90 0.24
0.08 0.34
0.03 0.01
0.03 0.09
0.04 0.01
0.01 0.39
0.01 0.41
0.01 0.24
ASTM A 36 ASTM A 242
Table 2 Results of bulk hardness, micro-hardness, average grain size and phase fraction measurements. Sample identification
Bulk hardness (HV)
Average grain size
Steel A Steel B
182 ± 4 194 ± 3
28 ± 2 12 ± 1
Phase fraction
Micro-hardness (HV)
Ferrite (%)
Pearlite (%)
Ferrite
Pearlite
81 ± 3 92 ± 2
19 ± 3 8±1
170–180
245–255
two steels, one has been taken from railway Girder Bridge and the other one is obtained from the modern structural steel (i.e. weathering steel). Earlier, a lot of studies [8–13] have reported the corrosion behaviour of weathering steel. However, the present study is dealing with the corrosion behaviour in two very important solutions of NaCl and HCl. The present study has been aimed to examine the corrosion performance of these steels in marine atmosphere [14] (3.5% NaCl solution) as well as in acidic atmosphere [15] (1% HCl solution) in industrial areas. A few previous studies [16,17] are available on the corrosion study of old structural steel. However, the present study focuses with the comparison of microstructural and corrosion behaviour of modern and old structural steels in two useful aqueous solutions rarely found in literature. In this regard, this study can be considered as novel and very much important for design and calculation of life cycle cost of above structures. In the first part of this investigation, the microstructure and hardness have been evaluated to understand the general behaviour of these steels and in second part, the emphasis is given specially on the corrosion performance as well as on the characterization and understanding the mechanism of the formation of corrosion products during the corrosion test. Finally a comparison has been made to understand the variation of performance of the old and modern structural steels. 2. Experimental procedure The experiments were done with the two grades of steels among them one had been used in old railway girder bridge (Sample A) and the other one is modern grades of weathering structural steel (Sample B). Weathering steels like ASTM A588, A36, A242, A606-4 and Cor-Ten exhibit superior corrosion resistance compared to regular carbon steel due to formation of a protective oxide film on the metals surface which slows down the further corrosion rate. These steels were designed primarily to be used in unpainted applications such as bridges, rail cars, transmission towers, highway poles and shipbuilding [18]. The present investigated steels like Girder Bridge and weathering steel are comparable to the above mentioned weathering steels. These experimental steels were taken in hot rolled and air-cooled plate form. In the present investigation, experiment was done on two to three samples of each grade. But due to repeatability of the results, the average values were chosen. The specimens were prepared by conventional metallographic techniques and etched with 2% nital solution to reveal microstructures in optical microscope (Carl Zesis Axiovert 40 Mat). By optical microscopy, the grain size and the phase fraction of different phases were calculated. Phase fraction analysis and grain size analysis were done by using Axiovision (version-4.8) software. To study in greater detail, the samples were studied under scanning electron microscope (SEM: HITACHI: Model No. S-3400N). The bulk hardness measurement was done by the Vickers hardness tester (BV-250(SPL)) of the two steels A and B.
The hardness of the two main phase constituents were determined using 100 gf (gram-force) load and 20 s dwelling time in a Vickers micro-hardness tester (Leica-VMHT). The study of the corrosion behaviour of these samples was done by immersion test in 3.5% NaCl solution and 1.0% HCl solutions for a considerable duration of 4 and 8 weeks in both cases. The sample sizes used for immersion tests were 25 mm × 25 mm × 15 mm which was measured with a digital vernier caliper (Mitutoyo – AOS Digimatic Caliper, accuracy is ±0.001inch) and immersion test was carried out at room temperature. Before immersion test, the samples were ground with SiC paper with grit size of 120 to 1200 followed by diamond (6 − 1 m) polishing and washed with distilled water. Then the samples were rinsed with ethanol and finally dried with a hair drier and weight was taken in an analytic weighing balance (±0.0001 g). After immersion test, samples were taken out from the solutions, dried in air, visually inspected and lightly scrubbed with a brush to remove brittle corrosion products. After that these steels were immersed in 1000 mL Clark’s solution (ASTM G1 of 1981) (11.42 mol/dm3 of HCl (with specific gravity of 1.19 & concentration of 35%) + 0.069 mol/dm3 of antimony trioxide (Sb2 O3 ) + 0.264 mol/dm3 of stannous chloride (SnCl2 )) for 30 min followed by cleaning with a soft cloth to remove remaining corroded products. Then the samples were dried and these were again weighed. This change in weight was recorded and used to calculate the corrosion rate (mg cm−2 h−1 ) using the following equation (1) [19,20], Corrosionrate =
W b − Wa AT
(1)
Where Wb − Wa is the weight loss measured (mg) after immersion test and A, T are the total area of the specimens (cm2 ) and exposure time (h), respectively. The surfaces (cross section) corroded by 3.5% NaCl as well as 1% HCl solutions were observed under SEM (HITACHI, S-3400N) operated at 20 kV in secondary electron mode for evaluating the morphology of the corrosion products. Characterisation of the corrosion products was done by X-ray diffraction (XRD) which was usually considered as the established method for the same [21]. The corroded surface of TMT rebars were characterised by ‘Bruker-Advance D8 XRD machine integrated with a copper tube with an operating voltage and current of 40 kV and 30 mA, respectively. XRD data was collected over a 2 range of 20◦ to 85◦ with a step of 0.01◦ /s. XRD patterns were further analysed by Panalytical X-Pert High-score software comparing with the standard PDF cards and results were reported. 3. Results and discussion 3.1. Characterisation of microstructure Optical Emission Spectrometer was used to get the chemical composition of the steels. Carbon and sulphur were also analysed by
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Fig. 1. Optical and SEM micrographs of (a, b): steel A & (c, d): steel B.
the quantitative phase fraction as well as average grain size in both the steels. Steel A contains higher pearlite (19%) but lower amount of ferrite as compared to those of steel B (Table 2). It may be mentioned that the higher content of pearlite in steel A is related to the higher content of carbon content in steel A (Table 1). In this context, it is important to note that grain size of A is larger than that of B (Table 2). 3.2. Hardness evaluation Table 2 shows the bulk hardness analysis results for steels A and B which clearly reveals that steel B shows higher hardness values. Though steel A has higher percentages of carbon (Table 1) as compared to steel B, higher hardness of steel B is related to (Table 2) the fine ferrite grain structure. The micro-hardness of the two major phase constituents i.e., ferrite and pearlite have been given in Table 2, where the hardness of ferrite lies in the range of 170 to 180 HV and hardness of pearlite is in the range of 245 to 255. Fig. 2. XRD profiles of corrosion products formed on steel A and B in 3.5% NaCl solution after 4 and 8 weeks.
Combustion-Infrared detection method using LECO Instruments. The chemical composition of the two steels is shown in Table 1. In this context, the most comparable steel grades with the investigated steels are also mentioned in Table 1. Fig. 1 shows the optical and SEM micrographs of steels A and B in as received condition which represents mixture of ferrite and pearlite. The white regions are ferrite and the dark regions are the pearlite (denoted by arrow). Ferrites are generally appeared as polygonal morphology and pearlite are resolved as fine lamellar structure (Fig. 1(b)). A comparison among the micrographs indicates that the steel B shows lower amount of pearlite than the steel A. SEM micrographs show the fine distribution of pearlite lamellae and also the network structure of pearlite is evident in steels A & B. Table 2 shows
3.3. Corrosion behaviour 3.3.1. Corrosion rate Table 3 summarises the corrosion behaviour of steel A and B by immersion process in various concentrations of NaCl and HCl solutions for different durations. Individual values of two measurements of each type of sample (A1, A2, B1 and B2) have been added in Table 3. It is found that the corrosion rate of steel A is slightly lower compared to steel B after 4 weeks (i.e. 672 h) in both the solutions. But after 8 weeks (i.e. 1344 h) the corrosion rate of A is much higher than that of steel B in 3.5% NaCl solution but lower in HCl solution. Moreover, the corrosion rate of all the steel samples are more in 1.0% HCl solution than in 3.5% NaCl solution indicating that HCl environment is more corrosive than NaCl environment. Here, in NaCl environment the corrosion rate of steel A increases with time, whereas the same for steel B decreases. As chromium content in
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Table 3 Corrosion rates (mg cm−2 h−1 ) shown by steel A and B in different solutions. Sample identification
Solution
A1 A2 B1 B2 A1 A2 B1 B2
3.5% NaCl
1% HCl
Corrosion rate (mg cm−2 h−1 ) 4 weeks (672 h)
8 weeks (1344 h)
0.009 0.011 0.013 0.012 0.070 0.073 0.110 0.120
0.010 0.013 0.010 0.011 0.046 0.049 0.060 0.058
Fig. 3. XRD profiles of corrosion products formed on steel A and B in 1% HCl solution after 4 and 8 weeks.
steel B is higher than that of steel A, steel B can form very stable, thin resistant surface which results less oxidising conditions. But the corrosion rate of both the steels A and B decreases with time in HCl solution. This behaviour is attributed to the formation of a thick oxide film (rust) on the metal surface, which serves as a protective barrier to further corrosion [22]. 3.3.2. X-Ray diffraction study of corrosion products Figs. 2 and 3 are showing the X-ray diffraction profiles of corrosion products formed on both the steel surfaces after immersion in the 3.5% NaCl and 1% HCl solutions for a period of 4 and 8 weeks. XRD patterns are analysed by X-pert High-score software comparing with the standard PDF cards and due to significant scattering effect, low angle has not been included. The broad peaks in some X-ray profiles are attributed to scattering effects of X-rays. These scatterings usually occur when the sample, target and detector are kept at low angle and some of the diffracted X-rays may not be detected. In the present study, two XRD plots (Figs. 2 and 3) reveal a large numbers of peaks i.e., almost 5–6 peaks within 1–2◦ (2 angle) interval and therefore, some phases overlap with each other. Under these circumstances, one peak representing more than one phase is labelled. Primarily all the X-ray profiles in Figs. 2 and 3 reveal many oxides, hydroxide and oxy-hydroxide like Lepidocrocite (␥-FeOOH) (JCPDS Ref. Code: 00-044-1415), Goethite (␣-FeOOH) (JCPDS Ref. Code: 00-029-0713), Akaganeite (-FeOOH) (JCPDS Ref. Code: 00008-0093), agnetite (Fe3 O4 ) (JCPDS Ref. Code: 01-075-0033) and Maghemite (␥-Fe2 O3 ) (JCPDS Ref. Code: 00-025-1402). Chromite (FeCr2 O4 ) (JCPDS Ref. Code: 00-004-0759) and Guyanaite (CrOOH) (JCPDS Ref. Code: 00-020-0312) were also observed in steel B as it has higher amount of Cr (Table 1). Similar kind of result was reported earlier for the weathering steel [23]. In some of the earlier
studies of weathering steels [24,25], lepidocrocite and akaganeite were detected at higher angles. During corrosion test in both the solutions, high concentration of oxygen in air promotes the formation of oxide, hydroxide and oxy-hydroxide. Presence of Cr oxide in the rust layer in steel B was detected by XRD, but due to its low content XRD results does not reveal all the peaks of chromium oxide. Cr is a strong oxide filmforming element and less soluble than Fe oxide in the inner rust layer which reduces the further formation of corrosion products. In this regard, it is important to note that in all the profiles, one BCC peak has been obtained. As the XRD has taken from the corroded surface of the steel specimens, one peak of BCC cannot be overruled. The outer layer of the corroded surface of the specimen consisting of brittle/fragile particles are normally non adherent and the inner layer composed of ␣-FeOOH acts as a protective layer and reduces the corrosion rate [26]. On the other hand, -FeOOH formation on the surface of steel further improves the corrosion resistance. 3.3.3. Corrosion micrographs Figs. 4 and 5 show the corrosion micrographs of steel A and B after immersion test in NaCl and HCl solutions for the duration 4 and 8 weeks. SEM micrographs in Fig. 4(a) and (d) shows the flowery structure which indicates the presence of lepidocrocite (␥-FeOOH) [27], whereas, Fig. 4(b) and (c) reveals cotton balls (globular) and rosette morphologies signifying the formation of goethite (␣-FeOOH) and akaganeite (-FeOOH) [28]. In this context, it is important to note that Fig. 5(a) and (b) shows the flowery type morphology of lepidocrocite (␥-FeOOH) and Fig. 5(c) reveals globular morphologies of goethite (␣-FeOOH). Fig. 5(d) reveals globular and flowery structures of goethite (␣-FeOOH) and lepidocrocite (␥-FeOOH). In Fig. 4, the formation of corrosion products of different morphologies in NaCl solution after 4 weeks is quite less and the sizes are very tiny. However, these products increase in size and volume, after 8 weeks for both the samples A and B which are in good agreement with the obtained corrosion rates. In this regard, it is very important to note that the size and volume of globule and flowery type morphologies decrease in HCl solution after 8 weeks as the corrosion rates drops down. Micrographs shown in Figs. 4 and 5 reveal cracking and flaking of corrosion layers formed in NaCl and HCl solutions as well. In this context, it is important to note that thin films of FeClx compounds could be formed on the steel surface subjected to HCl or NaCl solutions. These hydrated films of iron-chloride remain stable in completely dry air, but are transformed to -FeOOH when exposed to high humid condition [29]. Iron-chloride may be present as yellow crystals (FeCl2 ·2H2 O) below about 20% RH (relative humidity), as green crystals (FeCl2 ·4H2 O) between 20 and 55% RH. When the relative humidity is high, these compounds absorb water, dissolve and form wet droplets of orange coloured liquid. Therefore, in the aqueous solutions of HCl and NaCl, the stability of this FeClx is questionable. 3.3.4. Correlations between corrosion study and its mechanism The corrosion of iron in the atmosphere proceeds by the formation of hydrated oxides. The half-cell reactions can be expressed as: ½O2 + H2 O + 2e = 2(OH)− (Cathodicreaction) Fe = Fe2+ + 2e(Anodicreaction) Further reactions can then occur, such as: Fe2+ + 2OH− = Fe(OH)2
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Fig. 4. SEM images of corroded surface formed after 4 weeks in 3.5% NaCl solution of (a) steel A & (b) steel B and in 1% HCl solution of (c) steel A & (d) steel B.
Fig. 5. SEM images of corroded surface formed after 8 weeks in 3.5% NaCl solution of (a) steel A & (b) steel B and in 1% HCl solution of (c) steel A & (d) steel B.
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After oxidation of Fe(OH)2 with water:
O2
O2
Cleaned Surface
2Fe(OH)2 + H2 O + ½O2 = 2Fe(OH)3 [30] According to Tamura [31], Fe generally oxidizes at the anode to dissolve Fe2+ ions and this dissolved oxygen is reduced at the cathode to form OH− ions, and then these two, jointly deposit as hydroxide solid Fe(OH)2 . This corrosion is enhanced by the presence of electrolytes like NaCl and HCl solutions. At low concentrations of oxidising ion, the metal surface gets oxidised more and the active region is formed. As the concentration of the oxidising ions increase, the corrosion rate decreases. Now in 1.0% HCl solution, the concentration of H+ ions is less than that in 3.5% NaCl solution. As a consequence, the corrosion rate is less (Table 3) in 3.5% NaCl solution than in 1% HCl solution. Ferrous (Fe2+ ) and ferric (Fe3+ ) ions are formed due to corrosion of steel. The spontaneous chemical oxidation of ferrous to ferric by O2 is a complex process involving a variety of partially oxidised meta-stable ferrous–ferric intermediate species (e.g. green rusts) which are difficult to characterise or predict. These Fe-intermediates ultimately transform into a variety of end-products such as Lepidocrocite (␥FeOOH), Goethite (␣-FeOOH), Magnetite (Fe3 O4 ) and Maghemite (␥-Fe2 O3 ). The exact end-product(s) formed in corroded surfaces depends upon the environmental conditions, among which the important factors are pH, temperature, solution composition and oxidation rate. HCl being strongly acidic (i.e. pH very low), corrosion rate is very fast. Since NaCl is having neutral pH, the corrosion rate is comparatively low. In aqueous solution of hydrochloric acid, the H+ adds a water molecule and form a hydronium ion, H3 O+ [15]. HCl + H2 O → H3 O + + Cl− 2(OH)− produces a stable film which generally formed on iron in alkaline media and the stability of this film depends on availability of oxygen and pH of the solution [32,33]. The presence of iron oxy-hydroxides and oxides are very important constituents in determining the protective ability of the rust layer. ␥-FeOOH is a semi-conducting and electrochemically active whereas, ␣-FeOOH is non-conducting and non-active [34]. ␥-FeOOH is formed by oxidation of FeOH+ by dissolved oxygen. ␥-FeOOH is also formed on the outer surface and transformed to ferric oxy-hydroxide by dissolution, which may transform to ␣-FeOOH or Fe3 O4 followed by Fe3 O4 to ␥-Fe2 O3 transformation [26]. Cathodicreaction : -FeOOH + e → Fe3 O4 [26] Anodicreaction : Fe3 O4 → ␥-Fe2 O3 + e[26] It is reported earlier that ␥-FeOOH promotes the corrosion in the initial stage of rusting. However, after a long period of exposure, ␥-FeOOH reduces the corrosion rate [35]. ␣-FeOOH in the thin rust part has a protective nature because of the stabilized ␣FeOOH and it is difficult for chloride ions to enter into the rust layer. ␣-FeOOH is formed through amorphous ferric oxy-hydroxide and makes the rust layer thick and the green rust is formed and oxygen supply is very slow [36]. Fig. 6 is showing how the oxygen supply is obstructed by the thick layer of the rust after some period of the corrosion. At the initial stage, corrosion rate is high due to the clean surface which helps to form rust very quickly. Therefore, the formation of the corrosion products reduce in the absence of oxidising element, thereby the rate of the corrosion decreases. At the initial stage, the corrosion rate is high because of the formation ␥-FeOOH which is electrochemically active and works as a cathode site [37]. As the duration of exposure increases, the corrosion rate slows down because of the thick and dense rust formation. At this stage the formation rate of the rust is slow by formation of ␥- FeOOH to ␣-FeOOH via amorphous ferric oxyhydroxide [38]. According to
O2
Cleaned Surface
O2 O2 After a few days of corrosion
O2
O2
Corroded Surface
Fig. 6. Schematic illustration of the oxygen deficiency on metal surface after a few days of corrosion and formation of protective rust layer.
Tanaka et al. [39], ␣-FeOOH produced from -FeOOH and ␥-FeOOH and as it is a stable rust particle, which generally prevents further corrosion. -FeOOH is generally dispersed on the corroded layer and formed probably due to the reaction between iron deposited Cl− ions [4]. However, -FeOOH can also be seen in the inner layer due to water containing chloride ions penetrates through the cracks formed in the rust layer. Fe3 O4 shows quite high conductivity and formed by the reduction of ␥-FeOOH by oxidised back to ␥-Fe2 O3 and accelerates the corrosion [1]. In weathering steels, alloying elements like chromium, copper, phosphorus etc. have significant effect on densification and adhesion of the rust layers to get better protection against corrosion [40–42]. On the other hand, lowering the carbon content of the weathering steel helps to decrease in strain energy and the amount of carbide formation [43]. This can be beneficial for enhancing the corrosion resistance of these steels. In this context, it can be noted that copper generally promotes the formation of rust in the early stage of exposure, but in later stage it will enrich the rust layers resulting in enhancement of the protection layer by densification of the rust layers [22]. Nishimura [44] showed that grain refinement improves corrosion resistance for Si- and Al-bearing ultrafine grained weathering steel. In a previous study [45], it has shown that corrosion resistance is not deteriorated in fine grain ultralow carbon bainitic steel. Panda et al. [46] reported that pearlite decreases the corrosion resistance of steels because high carbon content leads to the formation of higher amount of cementite which is unfavourable for corrosion resistance [47]. Steel B with the aforesaid alloying effects and fine grain structure along with lower carbon content may be more effective than steel A for lowering the corrosion rate. In view of the above, the modern weathering steel is more corrosion resistant than old railway girder bridge steel in both the NaCl and HCl solutions for a longer period of time. This suggests that structures made from this modern weathering steel experience higher life than those made from old railway Girder Bridge steel. 4. Conclusions 1. The microstructures of old railway Girder Bridge steel A and modern structural steel B show the presence of polygonal ferrite and lamellar pearlite. However, greater amount of pearlite in steel A than that of steel B corroborates with its higher carbon content. 2. Higher hardness values (194 HV) of steel B as compared to the steel A can be related with the fine grain size along with the effects of alloying elements in that steel. 3. The immersion corrosion tests in 3.5% NaCl solution have shown that the corrosion rate of Steel A increases with time, while that of Steel B decreases with time which may be related to the finer grain size and the chemical composition of the steel. 4. Corrosion results in 1% HCl solution shows that the corrosion rates of both the steels A and B are higher as compared to those in NaCl solution. The decrease in corrosion rate in 1% HCl solution
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with time may be associated with the thickening of corroded layer. 5. The XRD analysis of the corroded surface of Steels A & B reveal the presence of Lepidocrocite (␥-FeOOH), Goethite (␣-FeOOH), Akaganeite (-FeOOH) and Fe2 O3 /Fe3 O4 /␥-Fe2 O3 . 6. The SEM images of corroded products show different morphologies like flowery, cotton balls and rosette etc. which indicates that the corrosion products contain primarily Lepidocrocite (␥-FeOOH), Goethite (␣-FeOOH), Akaganeite (-FeOOH) and Fe2 O3 /Fe3 O4 /␥-Fe2 O3 . 7. Steel B has better corrosion resistance during the prolong exposure in both 3.5% NaCl and 1% HCl solutions. References [1] T. Kamimura, S. Hara, H. Miyuki, M. Yamashita, H. Uchida, Composition and protective ability of rust layer formed on weathering steel exposed to various environments, Corros. Sci. 48 (2006) 2799–2812. [2] Yuhai Qian, Chaohui Ma, Dun Niu, Jingjun Xu, Meishuan Li, Influence of alloyed chromium on the atmospheric corrosion resistance of weathering steels, Corros. Sci. 74 (2013) 424–429. [3] Sei J. Oh, D.C. Cook, H.E. Townsend, Atmospheric corrosion of different steels in marine, rural and industrial environments, Corros. Sci. 41 (1999) 1687–1702. [4] J. Wang, Z.Y. Wang, W. Ke, Corrosion behaviour of weathering steel in diluted Qinghai salt lake water in a laboratory accelerated test that involved cyclic wet/dry conditions, Mater. Chem. Phys. 124 (2010) 952–958. [5] N. Damgaard, S. Walbridge, C. Hansson, J. Yeung, Corrosion protection and assessment of weathering steel highway structures, J. Construct. Steel Res. 66 (2010) 1174–1185. [6] Akhtar S. Khan, Muneer Baig, Shi-Hoon Choi, Hoe-Seok Yang, Xin Sun, Quasi-static and dynamic responses of advanced high strength steels: experiments and Modelling, Int. J. Plast. 30–31 (2012) 1–17. [7] M. Morcillo, B. Chico, I. Díaz, H. Cano, D. De la Fuente, Atmospheric corrosion data of weathering steels. A review, Corros. Sci. 77 (2013) 6–24. [8] P. Montoya, I. Díaz, N. Granizo, D. De la Fuente, M. Morcillo, A study on accelerated corrosion testing of weathering steel, Mater. Chem. Phys. 142 (2013) 220–228. [9] I. Diaz, H. Cano, D. De la Fuente, B. Chico, J.M. Vega, M. Morcillo, Atmospheric corrosion of Ni-advanced weathering steels in marine atmospheres of moderate salinity, Corros. Sci. 76 (2013) 348–360. [10] M. Morcillo, I. Díaz, B. Chico, H. Cano, D. De la Fuente, Weathering steels: from empirical development to scientific design a review, Corros. Sci. 83 (2014) 6–31. [11] Atsushi Nishikata, Qingjun Zhu, Eiji Tada, Long-term monitoring of atmospheric corrosion at weathering steel bridges by an electrochemical impedance method, Corros. Sci. 87 (2014) 80–88. [12] T. Nishimura, Corrosion resistance of Si?Al-bearing ultrafine-grained weathering steel, Sci. Technol. Adv. Mater. 9 (1) (2008) 1–7. [13] Xiao-ming Xiao, Yun Peng, Cheng-yong Ma, Zhi-ling Tian, Effects of alloy element and microstructure on corrosion resistant property of deposited metals of weathering stee, J. Iron Steel Res. Int. 23 (2) (2016) 171–177. [14] D. Rhodes, P. Felker, Mass screening of Prosopis (mesquite) seedlings for growth at seawater salinity concentrations, For. Ecol. Manage. 24 (3) (1988) 169–176. [15] C.A. Loto, R.T. Loto, Electrochemical corrosion resistance evaluation of ferritic stainless steel in HCl, Int. J. Electrochem. Sci. 7 (2012) 11011–11022. [16] J.R. Kayser, A.S. Nowak, Capacity loss due to corrosion in steel-girder bridges, J. Struct. Eng. 115 (6) (1989) 1525–1537. [17] M.W. Gewertz, Causes and repair of deterioration to a california bridge due to corrosion of reinforcing steel in a marine environment. Part I: method of repair, Highway Res. Board Bull. 182 (1958) 1–17. [18] URL: http://www.corten.com. [19] María V. Fiori-Bimbi, Patricia E. Alvarez, Hugo Vaca, Claudio A. Gervasi, Corrosion inhibition of mild steel in HCL solution by pectin, Corros. Sci. 92 (2015) 192–199. [20] H. Zarrok, A. Zarrouk, B. Hammouti, R. Salghi, C. Jama, F. Bentiss, Corrosion control of carbon steel in phosphoric acid by purpald – weight loss, electrochemical and XPS studies, Corros. Sci. 64 (2012) 243–252. [21] D. de la Fuente, J. Alcántara, B. Chico, I. Díaz, J.A. Jiménez, M. Morcillo, Characterisation of rust surfaces formed on mild steel exposed to marine
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Microstructural characterisation and corrosion performance of old railway girder bridge steel and modern weathering structural steel N.K. Tewary a , A. Kundu a , R. Nandi a , J.K. Saha b , S.K. Ghosh a,∗ a b
Department of Metallurgy and Materials Engineering, Indian Institute of Engineering Science and Technology, Shibpur, Howrah – 711 103, India Institute for Steel Development & Growth, Kolkata – 700 019, India
a r t i c l e
i n f o
Article history: Received 11 May 2016 Received in revised form 21 September 2016 Accepted 13 October 2016 Available online xxx Keywords: A. Low alloy steel B. SEM B. X-ray diffraction C. Acid corrosion C. Rust
a b s t r a c t A comparison on microstructure and corrosion performance has been made between the two structural steels used in old railway girder bridge (Sample A) and modern grades of weathering structural steel (Sample B). The microstructures, viewed under optical microscope and scanning electron microscope (SEM), show mainly ferrite-pearlite phase constituents in both the steels, A and B. The phase fraction analysis shows higher amount of pearlite in steel A compared to that of steel B. The grain size of steel A is larger than that of steel B under identical processing condition. The immersion corrosion test in 3.5% NaCl shows that the corrosion rate of steel A increases with time, while the same for steel B decreases with time. On the other hand, corrosion test in 1% HCl shows that the corrosion rate of both steel A and B is higher as compared to that of NaCl which always decreases with time. The XRD analysis of corrosion products show the presence of many oxides, hydroxide and oxy-hydroxide like Lepidocrocite (␥-FeOOH), Goethite (␣FeOOH), Akaganeite (-FeOOH), Magnetite (Fe3 O4 ) and Maghemite (␥-Fe2 O3 ) in both the steels. The SEM images of corroded surfaces reveal different morphologies like flowery, cotton balls and rosette etc. which indicate that the corrosion products primarily contain Lepidocrocite (␥-FeOOH), Goethite (␣-FeOOH) and Akaganeite (-FeOOH). © 2016 Elsevier Ltd. All rights reserved.
1. Introduction Weathering steel is a known material that has been used for different construction purposes which are generally exposed in atmosphere [1]. But the main difficulty of this steel is the atmospheric corrosion [2]. For these reasons, researchers have paid much attention on prevention of corrosion and extend the life of these steels by changing the chemical composition or some other ways. In recent years, the corrosion studies of structural steels mainly used in Railway Bridges have great concern due to increase in its life span. The old structural steels possess a good performance background but the history of their production is missing. On the other hand, the modern steels used for structural purposes are much cleaner compared to old steels, but require heavy maintenance to increase their longevity. The important problem for the structural steel is corrosion that is the greatest drawback of steel used in construction line up, mainly in bridges. The steel chemistry is important for increasing the life span of steel used in structural areas [3]. However, the steel life is also affected by environmen-
∗ Corresponding author. E-mail address: [email protected] (S.K. Ghosh).
tal conditions, because in different environmental conditions of corrosion products as well as corrosion rate vary due to the different environmental factors [2]. To differentiate the corrosion rates of different steels in laboratory conditions there are several techniques for accelerating these rates [4]. These experiments are very important to evaluate the corrosion behaviours of steels exposed in different environmental condition. In order to understand the corrosion behaviour of steel as a function of type of steel, environmental conditions and exposure time, the corrosion products have been studied on many occasions. Weathering steel is a high-strength, low-alloy steel which provides a significantly higher corrosion resistance than the regular carbon steel [5]. Use of high strength steels has been increased in recent decades in structural purpose due to its light weight combined with high strength [6]. In general, the chemical composition of weathering steel contains little amount of Cu, Cr, Ni and P which are known to restrict the corrosion rate of these steels after exposure in the atmosphere by formation of a stable and protective oxide layer [7]. In view of the above the corrosion behaviour of weathering steels used in construction purposes is very much concern to enhance the life of these steels. To understand the comparison of corrosion behaviour between old structural steel and modern structural steel, two steels have been taken. Between the
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2 Table 1 Chemical composition of the steels A & B. Sample Id
C
Mn
Si
S
P
Al
Cu
Cr
Ni
Equivalent Grade
Old steel: Steel A Modern steel: Steel B
0.14 0.05
0.90 0.24
0.08 0.34
0.03 0.01
0.03 0.09
0.04 0.01
0.01 0.39
0.01 0.41
0.01 0.24
ASTM A 36 ASTM A 242
Table 2 Results of bulk hardness, micro-hardness, average grain size and phase fraction measurements. Sample identification
Bulk hardness (HV)
Average grain size
Steel A Steel B
182 ± 4 194 ± 3
28 ± 2 12 ± 1
Phase fraction
Micro-hardness (HV)
Ferrite (%)
Pearlite (%)
Ferrite
Pearlite
81 ± 3 92 ± 2
19 ± 3 8±1
170–180
245–255
two steels, one has been taken from railway Girder Bridge and the other one is obtained from the modern structural steel (i.e. weathering steel). Earlier, a lot of studies [8–13] have reported the corrosion behaviour of weathering steel. However, the present study is dealing with the corrosion behaviour in two very important solutions of NaCl and HCl. The present study has been aimed to examine the corrosion performance of these steels in marine atmosphere [14] (3.5% NaCl solution) as well as in acidic atmosphere [15] (1% HCl solution) in industrial areas. A few previous studies [16,17] are available on the corrosion study of old structural steel. However, the present study focuses with the comparison of microstructural and corrosion behaviour of modern and old structural steels in two useful aqueous solutions rarely found in literature. In this regard, this study can be considered as novel and very much important for design and calculation of life cycle cost of above structures. In the first part of this investigation, the microstructure and hardness have been evaluated to understand the general behaviour of these steels and in second part, the emphasis is given specially on the corrosion performance as well as on the characterization and understanding the mechanism of the formation of corrosion products during the corrosion test. Finally a comparison has been made to understand the variation of performance of the old and modern structural steels. 2. Experimental procedure The experiments were done with the two grades of steels among them one had been used in old railway girder bridge (Sample A) and the other one is modern grades of weathering structural steel (Sample B). Weathering steels like ASTM A588, A36, A242, A606-4 and Cor-Ten exhibit superior corrosion resistance compared to regular carbon steel due to formation of a protective oxide film on the metals surface which slows down the further corrosion rate. These steels were designed primarily to be used in unpainted applications such as bridges, rail cars, transmission towers, highway poles and shipbuilding [18]. The present investigated steels like Girder Bridge and weathering steel are comparable to the above mentioned weathering steels. These experimental steels were taken in hot rolled and air-cooled plate form. In the present investigation, experiment was done on two to three samples of each grade. But due to repeatability of the results, the average values were chosen. The specimens were prepared by conventional metallographic techniques and etched with 2% nital solution to reveal microstructures in optical microscope (Carl Zesis Axiovert 40 Mat). By optical microscopy, the grain size and the phase fraction of different phases were calculated. Phase fraction analysis and grain size analysis were done by using Axiovision (version-4.8) software. To study in greater detail, the samples were studied under scanning electron microscope (SEM: HITACHI: Model No. S-3400N). The bulk hardness measurement was done by the Vickers hardness tester (BV-250(SPL)) of the two steels A and B.
The hardness of the two main phase constituents were determined using 100 gf (gram-force) load and 20 s dwelling time in a Vickers micro-hardness tester (Leica-VMHT). The study of the corrosion behaviour of these samples was done by immersion test in 3.5% NaCl solution and 1.0% HCl solutions for a considerable duration of 4 and 8 weeks in both cases. The sample sizes used for immersion tests were 25 mm × 25 mm × 15 mm which was measured with a digital vernier caliper (Mitutoyo – AOS Digimatic Caliper, accuracy is ±0.001inch) and immersion test was carried out at room temperature. Before immersion test, the samples were ground with SiC paper with grit size of 120 to 1200 followed by diamond (6 − 1 m) polishing and washed with distilled water. Then the samples were rinsed with ethanol and finally dried with a hair drier and weight was taken in an analytic weighing balance (±0.0001 g). After immersion test, samples were taken out from the solutions, dried in air, visually inspected and lightly scrubbed with a brush to remove brittle corrosion products. After that these steels were immersed in 1000 mL Clark’s solution (ASTM G1 of 1981) (11.42 mol/dm3 of HCl (with specific gravity of 1.19 & concentration of 35%) + 0.069 mol/dm3 of antimony trioxide (Sb2 O3 ) + 0.264 mol/dm3 of stannous chloride (SnCl2 )) for 30 min followed by cleaning with a soft cloth to remove remaining corroded products. Then the samples were dried and these were again weighed. This change in weight was recorded and used to calculate the corrosion rate (mg cm−2 h−1 ) using the following equation (1) [19,20], Corrosionrate =
W b − Wa AT
(1)
Where Wb − Wa is the weight loss measured (mg) after immersion test and A, T are the total area of the specimens (cm2 ) and exposure time (h), respectively. The surfaces (cross section) corroded by 3.5% NaCl as well as 1% HCl solutions were observed under SEM (HITACHI, S-3400N) operated at 20 kV in secondary electron mode for evaluating the morphology of the corrosion products. Characterisation of the corrosion products was done by X-ray diffraction (XRD) which was usually considered as the established method for the same [21]. The corroded surface of TMT rebars were characterised by ‘Bruker-Advance D8 XRD machine integrated with a copper tube with an operating voltage and current of 40 kV and 30 mA, respectively. XRD data was collected over a 2 range of 20◦ to 85◦ with a step of 0.01◦ /s. XRD patterns were further analysed by Panalytical X-Pert High-score software comparing with the standard PDF cards and results were reported. 3. Results and discussion 3.1. Characterisation of microstructure Optical Emission Spectrometer was used to get the chemical composition of the steels. Carbon and sulphur were also analysed by
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Fig. 1. Optical and SEM micrographs of (a, b): steel A & (c, d): steel B.
the quantitative phase fraction as well as average grain size in both the steels. Steel A contains higher pearlite (19%) but lower amount of ferrite as compared to those of steel B (Table 2). It may be mentioned that the higher content of pearlite in steel A is related to the higher content of carbon content in steel A (Table 1). In this context, it is important to note that grain size of A is larger than that of B (Table 2). 3.2. Hardness evaluation Table 2 shows the bulk hardness analysis results for steels A and B which clearly reveals that steel B shows higher hardness values. Though steel A has higher percentages of carbon (Table 1) as compared to steel B, higher hardness of steel B is related to (Table 2) the fine ferrite grain structure. The micro-hardness of the two major phase constituents i.e., ferrite and pearlite have been given in Table 2, where the hardness of ferrite lies in the range of 170 to 180 HV and hardness of pearlite is in the range of 245 to 255. Fig. 2. XRD profiles of corrosion products formed on steel A and B in 3.5% NaCl solution after 4 and 8 weeks.
Combustion-Infrared detection method using LECO Instruments. The chemical composition of the two steels is shown in Table 1. In this context, the most comparable steel grades with the investigated steels are also mentioned in Table 1. Fig. 1 shows the optical and SEM micrographs of steels A and B in as received condition which represents mixture of ferrite and pearlite. The white regions are ferrite and the dark regions are the pearlite (denoted by arrow). Ferrites are generally appeared as polygonal morphology and pearlite are resolved as fine lamellar structure (Fig. 1(b)). A comparison among the micrographs indicates that the steel B shows lower amount of pearlite than the steel A. SEM micrographs show the fine distribution of pearlite lamellae and also the network structure of pearlite is evident in steels A & B. Table 2 shows
3.3. Corrosion behaviour 3.3.1. Corrosion rate Table 3 summarises the corrosion behaviour of steel A and B by immersion process in various concentrations of NaCl and HCl solutions for different durations. Individual values of two measurements of each type of sample (A1, A2, B1 and B2) have been added in Table 3. It is found that the corrosion rate of steel A is slightly lower compared to steel B after 4 weeks (i.e. 672 h) in both the solutions. But after 8 weeks (i.e. 1344 h) the corrosion rate of A is much higher than that of steel B in 3.5% NaCl solution but lower in HCl solution. Moreover, the corrosion rate of all the steel samples are more in 1.0% HCl solution than in 3.5% NaCl solution indicating that HCl environment is more corrosive than NaCl environment. Here, in NaCl environment the corrosion rate of steel A increases with time, whereas the same for steel B decreases. As chromium content in
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Table 3 Corrosion rates (mg cm−2 h−1 ) shown by steel A and B in different solutions. Sample identification
Solution
A1 A2 B1 B2 A1 A2 B1 B2
3.5% NaCl
1% HCl
Corrosion rate (mg cm−2 h−1 ) 4 weeks (672 h)
8 weeks (1344 h)
0.009 0.011 0.013 0.012 0.070 0.073 0.110 0.120
0.010 0.013 0.010 0.011 0.046 0.049 0.060 0.058
Fig. 3. XRD profiles of corrosion products formed on steel A and B in 1% HCl solution after 4 and 8 weeks.
steel B is higher than that of steel A, steel B can form very stable, thin resistant surface which results less oxidising conditions. But the corrosion rate of both the steels A and B decreases with time in HCl solution. This behaviour is attributed to the formation of a thick oxide film (rust) on the metal surface, which serves as a protective barrier to further corrosion [22]. 3.3.2. X-Ray diffraction study of corrosion products Figs. 2 and 3 are showing the X-ray diffraction profiles of corrosion products formed on both the steel surfaces after immersion in the 3.5% NaCl and 1% HCl solutions for a period of 4 and 8 weeks. XRD patterns are analysed by X-pert High-score software comparing with the standard PDF cards and due to significant scattering effect, low angle has not been included. The broad peaks in some X-ray profiles are attributed to scattering effects of X-rays. These scatterings usually occur when the sample, target and detector are kept at low angle and some of the diffracted X-rays may not be detected. In the present study, two XRD plots (Figs. 2 and 3) reveal a large numbers of peaks i.e., almost 5–6 peaks within 1–2◦ (2 angle) interval and therefore, some phases overlap with each other. Under these circumstances, one peak representing more than one phase is labelled. Primarily all the X-ray profiles in Figs. 2 and 3 reveal many oxides, hydroxide and oxy-hydroxide like Lepidocrocite (␥-FeOOH) (JCPDS Ref. Code: 00-044-1415), Goethite (␣-FeOOH) (JCPDS Ref. Code: 00-029-0713), Akaganeite (-FeOOH) (JCPDS Ref. Code: 00008-0093), agnetite (Fe3 O4 ) (JCPDS Ref. Code: 01-075-0033) and Maghemite (␥-Fe2 O3 ) (JCPDS Ref. Code: 00-025-1402). Chromite (FeCr2 O4 ) (JCPDS Ref. Code: 00-004-0759) and Guyanaite (CrOOH) (JCPDS Ref. Code: 00-020-0312) were also observed in steel B as it has higher amount of Cr (Table 1). Similar kind of result was reported earlier for the weathering steel [23]. In some of the earlier
studies of weathering steels [24,25], lepidocrocite and akaganeite were detected at higher angles. During corrosion test in both the solutions, high concentration of oxygen in air promotes the formation of oxide, hydroxide and oxy-hydroxide. Presence of Cr oxide in the rust layer in steel B was detected by XRD, but due to its low content XRD results does not reveal all the peaks of chromium oxide. Cr is a strong oxide filmforming element and less soluble than Fe oxide in the inner rust layer which reduces the further formation of corrosion products. In this regard, it is important to note that in all the profiles, one BCC peak has been obtained. As the XRD has taken from the corroded surface of the steel specimens, one peak of BCC cannot be overruled. The outer layer of the corroded surface of the specimen consisting of brittle/fragile particles are normally non adherent and the inner layer composed of ␣-FeOOH acts as a protective layer and reduces the corrosion rate [26]. On the other hand, -FeOOH formation on the surface of steel further improves the corrosion resistance. 3.3.3. Corrosion micrographs Figs. 4 and 5 show the corrosion micrographs of steel A and B after immersion test in NaCl and HCl solutions for the duration 4 and 8 weeks. SEM micrographs in Fig. 4(a) and (d) shows the flowery structure which indicates the presence of lepidocrocite (␥-FeOOH) [27], whereas, Fig. 4(b) and (c) reveals cotton balls (globular) and rosette morphologies signifying the formation of goethite (␣-FeOOH) and akaganeite (-FeOOH) [28]. In this context, it is important to note that Fig. 5(a) and (b) shows the flowery type morphology of lepidocrocite (␥-FeOOH) and Fig. 5(c) reveals globular morphologies of goethite (␣-FeOOH). Fig. 5(d) reveals globular and flowery structures of goethite (␣-FeOOH) and lepidocrocite (␥-FeOOH). In Fig. 4, the formation of corrosion products of different morphologies in NaCl solution after 4 weeks is quite less and the sizes are very tiny. However, these products increase in size and volume, after 8 weeks for both the samples A and B which are in good agreement with the obtained corrosion rates. In this regard, it is very important to note that the size and volume of globule and flowery type morphologies decrease in HCl solution after 8 weeks as the corrosion rates drops down. Micrographs shown in Figs. 4 and 5 reveal cracking and flaking of corrosion layers formed in NaCl and HCl solutions as well. In this context, it is important to note that thin films of FeClx compounds could be formed on the steel surface subjected to HCl or NaCl solutions. These hydrated films of iron-chloride remain stable in completely dry air, but are transformed to -FeOOH when exposed to high humid condition [29]. Iron-chloride may be present as yellow crystals (FeCl2 ·2H2 O) below about 20% RH (relative humidity), as green crystals (FeCl2 ·4H2 O) between 20 and 55% RH. When the relative humidity is high, these compounds absorb water, dissolve and form wet droplets of orange coloured liquid. Therefore, in the aqueous solutions of HCl and NaCl, the stability of this FeClx is questionable. 3.3.4. Correlations between corrosion study and its mechanism The corrosion of iron in the atmosphere proceeds by the formation of hydrated oxides. The half-cell reactions can be expressed as: ½O2 + H2 O + 2e = 2(OH)− (Cathodicreaction) Fe = Fe2+ + 2e(Anodicreaction) Further reactions can then occur, such as: Fe2+ + 2OH− = Fe(OH)2
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Fig. 4. SEM images of corroded surface formed after 4 weeks in 3.5% NaCl solution of (a) steel A & (b) steel B and in 1% HCl solution of (c) steel A & (d) steel B.
Fig. 5. SEM images of corroded surface formed after 8 weeks in 3.5% NaCl solution of (a) steel A & (b) steel B and in 1% HCl solution of (c) steel A & (d) steel B.
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After oxidation of Fe(OH)2 with water:
O2
O2
Cleaned Surface
2Fe(OH)2 + H2 O + ½O2 = 2Fe(OH)3 [30] According to Tamura [31], Fe generally oxidizes at the anode to dissolve Fe2+ ions and this dissolved oxygen is reduced at the cathode to form OH− ions, and then these two, jointly deposit as hydroxide solid Fe(OH)2 . This corrosion is enhanced by the presence of electrolytes like NaCl and HCl solutions. At low concentrations of oxidising ion, the metal surface gets oxidised more and the active region is formed. As the concentration of the oxidising ions increase, the corrosion rate decreases. Now in 1.0% HCl solution, the concentration of H+ ions is less than that in 3.5% NaCl solution. As a consequence, the corrosion rate is less (Table 3) in 3.5% NaCl solution than in 1% HCl solution. Ferrous (Fe2+ ) and ferric (Fe3+ ) ions are formed due to corrosion of steel. The spontaneous chemical oxidation of ferrous to ferric by O2 is a complex process involving a variety of partially oxidised meta-stable ferrous–ferric intermediate species (e.g. green rusts) which are difficult to characterise or predict. These Fe-intermediates ultimately transform into a variety of end-products such as Lepidocrocite (␥FeOOH), Goethite (␣-FeOOH), Magnetite (Fe3 O4 ) and Maghemite (␥-Fe2 O3 ). The exact end-product(s) formed in corroded surfaces depends upon the environmental conditions, among which the important factors are pH, temperature, solution composition and oxidation rate. HCl being strongly acidic (i.e. pH very low), corrosion rate is very fast. Since NaCl is having neutral pH, the corrosion rate is comparatively low. In aqueous solution of hydrochloric acid, the H+ adds a water molecule and form a hydronium ion, H3 O+ [15]. HCl + H2 O → H3 O + + Cl− 2(OH)− produces a stable film which generally formed on iron in alkaline media and the stability of this film depends on availability of oxygen and pH of the solution [32,33]. The presence of iron oxy-hydroxides and oxides are very important constituents in determining the protective ability of the rust layer. ␥-FeOOH is a semi-conducting and electrochemically active whereas, ␣-FeOOH is non-conducting and non-active [34]. ␥-FeOOH is formed by oxidation of FeOH+ by dissolved oxygen. ␥-FeOOH is also formed on the outer surface and transformed to ferric oxy-hydroxide by dissolution, which may transform to ␣-FeOOH or Fe3 O4 followed by Fe3 O4 to ␥-Fe2 O3 transformation [26]. Cathodicreaction : -FeOOH + e → Fe3 O4 [26] Anodicreaction : Fe3 O4 → ␥-Fe2 O3 + e[26] It is reported earlier that ␥-FeOOH promotes the corrosion in the initial stage of rusting. However, after a long period of exposure, ␥-FeOOH reduces the corrosion rate [35]. ␣-FeOOH in the thin rust part has a protective nature because of the stabilized ␣FeOOH and it is difficult for chloride ions to enter into the rust layer. ␣-FeOOH is formed through amorphous ferric oxy-hydroxide and makes the rust layer thick and the green rust is formed and oxygen supply is very slow [36]. Fig. 6 is showing how the oxygen supply is obstructed by the thick layer of the rust after some period of the corrosion. At the initial stage, corrosion rate is high due to the clean surface which helps to form rust very quickly. Therefore, the formation of the corrosion products reduce in the absence of oxidising element, thereby the rate of the corrosion decreases. At the initial stage, the corrosion rate is high because of the formation ␥-FeOOH which is electrochemically active and works as a cathode site [37]. As the duration of exposure increases, the corrosion rate slows down because of the thick and dense rust formation. At this stage the formation rate of the rust is slow by formation of ␥- FeOOH to ␣-FeOOH via amorphous ferric oxyhydroxide [38]. According to
O2
Cleaned Surface
O2 O2 After a few days of corrosion
O2
O2
Corroded Surface
Fig. 6. Schematic illustration of the oxygen deficiency on metal surface after a few days of corrosion and formation of protective rust layer.
Tanaka et al. [39], ␣-FeOOH produced from -FeOOH and ␥-FeOOH and as it is a stable rust particle, which generally prevents further corrosion. -FeOOH is generally dispersed on the corroded layer and formed probably due to the reaction between iron deposited Cl− ions [4]. However, -FeOOH can also be seen in the inner layer due to water containing chloride ions penetrates through the cracks formed in the rust layer. Fe3 O4 shows quite high conductivity and formed by the reduction of ␥-FeOOH by oxidised back to ␥-Fe2 O3 and accelerates the corrosion [1]. In weathering steels, alloying elements like chromium, copper, phosphorus etc. have significant effect on densification and adhesion of the rust layers to get better protection against corrosion [40–42]. On the other hand, lowering the carbon content of the weathering steel helps to decrease in strain energy and the amount of carbide formation [43]. This can be beneficial for enhancing the corrosion resistance of these steels. In this context, it can be noted that copper generally promotes the formation of rust in the early stage of exposure, but in later stage it will enrich the rust layers resulting in enhancement of the protection layer by densification of the rust layers [22]. Nishimura [44] showed that grain refinement improves corrosion resistance for Si- and Al-bearing ultrafine grained weathering steel. In a previous study [45], it has shown that corrosion resistance is not deteriorated in fine grain ultralow carbon bainitic steel. Panda et al. [46] reported that pearlite decreases the corrosion resistance of steels because high carbon content leads to the formation of higher amount of cementite which is unfavourable for corrosion resistance [47]. Steel B with the aforesaid alloying effects and fine grain structure along with lower carbon content may be more effective than steel A for lowering the corrosion rate. In view of the above, the modern weathering steel is more corrosion resistant than old railway girder bridge steel in both the NaCl and HCl solutions for a longer period of time. This suggests that structures made from this modern weathering steel experience higher life than those made from old railway Girder Bridge steel. 4. Conclusions 1. The microstructures of old railway Girder Bridge steel A and modern structural steel B show the presence of polygonal ferrite and lamellar pearlite. However, greater amount of pearlite in steel A than that of steel B corroborates with its higher carbon content. 2. Higher hardness values (194 HV) of steel B as compared to the steel A can be related with the fine grain size along with the effects of alloying elements in that steel. 3. The immersion corrosion tests in 3.5% NaCl solution have shown that the corrosion rate of Steel A increases with time, while that of Steel B decreases with time which may be related to the finer grain size and the chemical composition of the steel. 4. Corrosion results in 1% HCl solution shows that the corrosion rates of both the steels A and B are higher as compared to those in NaCl solution. The decrease in corrosion rate in 1% HCl solution
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with time may be associated with the thickening of corroded layer. 5. The XRD analysis of the corroded surface of Steels A & B reveal the presence of Lepidocrocite (␥-FeOOH), Goethite (␣-FeOOH), Akaganeite (-FeOOH) and Fe2 O3 /Fe3 O4 /␥-Fe2 O3 . 6. The SEM images of corroded products show different morphologies like flowery, cotton balls and rosette etc. which indicates that the corrosion products contain primarily Lepidocrocite (␥-FeOOH), Goethite (␣-FeOOH), Akaganeite (-FeOOH) and Fe2 O3 /Fe3 O4 /␥-Fe2 O3 . 7. Steel B has better corrosion resistance during the prolong exposure in both 3.5% NaCl and 1% HCl solutions. References [1] T. Kamimura, S. Hara, H. Miyuki, M. Yamashita, H. Uchida, Composition and protective ability of rust layer formed on weathering steel exposed to various environments, Corros. Sci. 48 (2006) 2799–2812. [2] Yuhai Qian, Chaohui Ma, Dun Niu, Jingjun Xu, Meishuan Li, Influence of alloyed chromium on the atmospheric corrosion resistance of weathering steels, Corros. Sci. 74 (2013) 424–429. [3] Sei J. Oh, D.C. Cook, H.E. Townsend, Atmospheric corrosion of different steels in marine, rural and industrial environments, Corros. Sci. 41 (1999) 1687–1702. [4] J. Wang, Z.Y. Wang, W. Ke, Corrosion behaviour of weathering steel in diluted Qinghai salt lake water in a laboratory accelerated test that involved cyclic wet/dry conditions, Mater. Chem. Phys. 124 (2010) 952–958. [5] N. Damgaard, S. Walbridge, C. Hansson, J. Yeung, Corrosion protection and assessment of weathering steel highway structures, J. Construct. Steel Res. 66 (2010) 1174–1185. [6] Akhtar S. Khan, Muneer Baig, Shi-Hoon Choi, Hoe-Seok Yang, Xin Sun, Quasi-static and dynamic responses of advanced high strength steels: experiments and Modelling, Int. J. Plast. 30–31 (2012) 1–17. [7] M. Morcillo, B. Chico, I. Díaz, H. Cano, D. De la Fuente, Atmospheric corrosion data of weathering steels. A review, Corros. Sci. 77 (2013) 6–24. [8] P. Montoya, I. Díaz, N. Granizo, D. De la Fuente, M. Morcillo, A study on accelerated corrosion testing of weathering steel, Mater. Chem. Phys. 142 (2013) 220–228. [9] I. Diaz, H. Cano, D. De la Fuente, B. Chico, J.M. Vega, M. Morcillo, Atmospheric corrosion of Ni-advanced weathering steels in marine atmospheres of moderate salinity, Corros. Sci. 76 (2013) 348–360. [10] M. Morcillo, I. Díaz, B. Chico, H. Cano, D. De la Fuente, Weathering steels: from empirical development to scientific design a review, Corros. Sci. 83 (2014) 6–31. [11] Atsushi Nishikata, Qingjun Zhu, Eiji Tada, Long-term monitoring of atmospheric corrosion at weathering steel bridges by an electrochemical impedance method, Corros. Sci. 87 (2014) 80–88. [12] T. Nishimura, Corrosion resistance of Si?Al-bearing ultrafine-grained weathering steel, Sci. Technol. Adv. Mater. 9 (1) (2008) 1–7. [13] Xiao-ming Xiao, Yun Peng, Cheng-yong Ma, Zhi-ling Tian, Effects of alloy element and microstructure on corrosion resistant property of deposited metals of weathering stee, J. Iron Steel Res. Int. 23 (2) (2016) 171–177. [14] D. Rhodes, P. Felker, Mass screening of Prosopis (mesquite) seedlings for growth at seawater salinity concentrations, For. Ecol. Manage. 24 (3) (1988) 169–176. [15] C.A. Loto, R.T. Loto, Electrochemical corrosion resistance evaluation of ferritic stainless steel in HCl, Int. J. Electrochem. Sci. 7 (2012) 11011–11022. [16] J.R. Kayser, A.S. Nowak, Capacity loss due to corrosion in steel-girder bridges, J. Struct. Eng. 115 (6) (1989) 1525–1537. [17] M.W. Gewertz, Causes and repair of deterioration to a california bridge due to corrosion of reinforcing steel in a marine environment. Part I: method of repair, Highway Res. Board Bull. 182 (1958) 1–17. [18] URL: http://www.corten.com. [19] María V. Fiori-Bimbi, Patricia E. Alvarez, Hugo Vaca, Claudio A. Gervasi, Corrosion inhibition of mild steel in HCL solution by pectin, Corros. Sci. 92 (2015) 192–199. [20] H. Zarrok, A. Zarrouk, B. Hammouti, R. Salghi, C. Jama, F. Bentiss, Corrosion control of carbon steel in phosphoric acid by purpald – weight loss, electrochemical and XPS studies, Corros. Sci. 64 (2012) 243–252. [21] D. de la Fuente, J. Alcántara, B. Chico, I. Díaz, J.A. Jiménez, M. Morcillo, Characterisation of rust surfaces formed on mild steel exposed to marine
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