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Effects of Cr, Ni and Cu on the Corrosion Behavior of Low Carbon Microalloying Steel in a Cl− Containing Environment
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J. Mater. Sci. Technol., 2013, 29(2), 168e174
Effects of Cr, Ni and Cu on the Corrosion Behavior of Low Carbon Microalloying Steel in a ClL Containing Environment Yanlei Zhou, Jun Chen, Yang Xu, Zhenyu Liu* State Key Laboratory of Rolling and Automation, Northeastern University, Shenyang 110004, China [Manuscript received March 30, 2012, in revised form August 22, 2012, Available online 27 December 2012]
The effects of Cr, Ni and Cu on the corrosion behavior of low carbon microalloying steel in a Cl containing environment were investigated. The results revealed that the corrosion process could be divided into the initial stage in which the corrosion rate increased with accumulation of corrosion products and the later stage in which homogeneous and compact inner rust layers started to protect steel substrate out of corrosion mediums. The results of X-ray diffraction (XRD) indicated that the rust layers of the three-group steels (Cr, CreNi and CreNieCu steels) were composed of a-FeOOH, b-FeOOH, g-FeOOH, Fe3O4 and large amounts of amorphous compounds. The content of amorphous compounds of CreNieCu steel was about 2%e3% more than that of CreNi steel. The results of electron probe microanalysis (EPMA) showed that Cr concentrated mainly in the inner region of the rust of CreNieCu steel, inner/outer interface especially, whereas Ni was uniformly distributed all over the rust and Cu was noticed rarely after 73 wet/dry cycles. The addition of Cr and Ni was beneficial to the formation of dense and compact inner rust layer, which was the most important reason for the improvement of corrosion resistance of experimental steel. KEY WORDS: Low carbon microalloying steel; Alloying elements; Corrosion resistance; Rust layer
1. Introduction Chemical elements, such as Cr, Ni and Cu play an important role in the corrosion process of structural steel, and the corrosion resistance of such steel is always due to the enrichment of these elements in the inner rust layers[1,2]. These elements can promote the formation of compact rust layers, which gradually become more dense over time and isolate the steels from various corrosion agents, resulting in substantial reduction of the corrosion rate[3e6]. The development of adherent and protective layers formed on the steels was the noticed phenomenon directly[7e10]. Choi and Kim[11] and Bousselmi et al.[12] studied that the Cr and Cu compounds can promote more or less protective rust layer on weathering steel in an aqueous condition, as they did under an atmospheric condition. Most researchers assume that microstructures only affect the corrosion behavior of bare steel and thus can be neglected after the surface of steel is covered by corrosion products. Currently, low carbon bainite steel is gradually replacing the ferrite and pearlite steels as the offshore * Corresponding author. Prof., Ph.D.; Tel.: þ86 24 83680571; Fax: þ86 24 23906472; E-mail address: [email protected] (Z. Liu). 1005-0302/$ e see front matter Copyright Ó 2013, The editorial office of Journal of Materials Science & Technology. Published by Elsevier Limited. All rights reserved. http://dx.doi.org/10.1016/j.jmst.2012.12.013
platform steel with high strength and toughness. Therefore, low carbon bainitic steels were chosen for experimental materials, and the effect of original microstructure was neglected in this study. Generally, the corrosion products on the surface of weathering steel are mainly a-FeOOH, b-FeOOH, g-FeOOH, Fe(OH)3, and Fe3O4. These products can coexist partly as crystalline and partly as amorphous structures[4,13]. It is also mentioned that when the steel contacts moisture and corrodes, the initial corrosion product is mainly g-FeOOH, but gradually this transforms to a-FeOOH as a result of the intermittently wet/dry conditions[4,13]. Chen et al.[6] pointed out that the alloying additions in weathering steel result in corrosion products which are more dense and more stable than those on carbon steel. Although a-FeOOH and g-FeOOH can be noticed in the corrosion products on carbon steel, they are porous since they are not enhanced by alloying elements, and in addition, the adhesion is poor between the rust layer and the substrate[6]. Because of the evaporation of seawater, offshore platform steel is always used in atmospheric environment containing Cl, and the enhancement of corrosion resistance is required. The aim of this investigation was to determine the effects of Cr, Ni and Cu on the corrosion behavior of low carbon microalloying steel in a Cl containing environment, by means of scanning electron microscopy (SEM), X-ray diffraction (XRD) and electron probe microanalysis (EPMA) especially.
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Table 1 Chemical compositions of the experimental steels (wt%)
Cr steel CreNi steel CreNieCu steel
C
Si
Mn
P
S
Mo
Nb
V
Ti
Cr
Ni
Cu
Fe
0.06 0.07 0.05
0.44 0.22 0.35
1.71 1.45 1.56
0.009 0.007 0.006
0.005 0.004 0.002
0.15 0.25 0.20
0.034 0.044 0.060
0.010 0.053 0.040
0.014 0.014 0.020
0.49 0.30 0.24
e 0.39 0.50
0.21
Bal. Bal. Bal.
2. Experimental 2.1. Materials
sample weight after corrosion and S is the corrosion area on the sample. 2.3. Microstructure and surface analysis
Experimental steels were melted with a 50 kg vacuum induction furnace. Chemical composition (wt%) of the threegroup tested steels (Cr, CreNi and CreNieCu steels) used in the experiments are given in Table 1. Ingots were heated and forged into 120 mm 150 mm 200 mm slabs, and then the slabs were homogenized at 1200 C for 2 h in soaking pit for rolling process. Cr steel was rolled in non-recrystallization and recrystallization austenite region to a thickness of 15 mm, and then cooled to 500 C by ultra fast cooling (UFC) process. CreNi steel was treated by conventional quenchingetempering (QeT) process and CreNieCu steel was treated by lamellarizingetempering (LeT) process, respectively, after controlled rolling and cooling. Specimens for corrosion test were cut to 5 mm 40 mm 40 mm. All specimens were milled to 4 mm thickness and ground with 600e 1000 grit silicon carbide paper, then cleaned with acetone after they were rinsed with distilled water. Each specimen was mounted in a cured epoxy resin, an area of 25 mm 25 mm was left on the metal surface to add corrosive liquid periodically, and the epoxyespecimen interface was also painted to prevent the initiation of crevice corrosion between the epoxy and the specimen. 2.2. Constant temperature and humidity accelerated corrosion test ESPEC constant temperature and constant humidity chamber (PR-2KT) was used for the cyclic wet/dry test (30 C, 60% RH). 3.0 mass% aqueous NaCl solution was used as corrosive liquid in this experiment. Each wet/dry cycle (24 h) consisted of a wetting period and a dry period. Corrosive solution was dropped to working face of specimen (40 ml/cm2) during every corrosion cycle (12 h). After rinsing with distilled water and drying in air, the samples were weighted by using a Sartorius electronic analytical balance (d ¼ 0.01 mg). Weight gain of a unit area was calculated by (W1W0)/S in which W1 is the
To investigate the relationship between the alloying elements, microstructure and surface composition of the rust, the surface was examined by SEM, XRD and EPMA after corroding for different cycles. The rusted steel samples were cut to 4 mm 4 mm 10 mm in size, and then mounted using a SimpliMet 3000 automatic mounting press (AMP) for surface analysis. The surface and cross-section of the rust layers were analyzed using an FEI Quanta 600 scanning electron microscope. Structural analysis of rusted samples was carried out using a D/MAX-3A Xray diffractometer with a Cu target. The scan speed was 2 /min and the 2q angle ranges from 10 to 50 . Elements distribution of specimens on cross-section direction was determined using an electron probe microanalyzer, which was performed using the JXA-8530F, with an acceleration voltage of 20 kV, an irradiation current of 8 107 A, and a beam diameter of 1 mm. 3. Results and Discussion 3.1. Microstructures Microstructures of the three-group experimental steels are shown in Fig. 1. It can be seen that the microstructure of Cr steel consists mainly of lath-shape bainite (Fig. 1(a)); the main structure of CreNi steel is tempered martensite (Fig. 1(b)); microstructures of CreNieCu steel consist of polygonal ferrite (PF) and carbon-rich phases (retained MeA islands after tempering) which are distributed randomly (Fig. 1(c)). 3.2. Weight gain and corrosion rate curves Weight gain data were expressed as the weight gain per unit surface area in this study. Fig. 2(a) shows the weight gain curve of specimens in the cyclic wet/dry test. In general, the final weight gain of CreNieCu steel is obviously the lowest in the
Fig. 1 OM micrographs of the three-group experimental steels: (a) Cr steel, (b) CreNi steel, (c) CreNieCu steel.
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Fig. 2 Weight gain and corrosion rate of specimens corroded in testing chamber for different time: (a) weight gain curve, (b) corrosion rate curve.
three-group data, and that of Cr steel is the highest. Therefore, the steels can be arranged in the order of weight gain as follows: Cr steel (lath-shape bainite) > CreNi steel (tempered martensite) > CreNieCu steel (PF þ carbon-rich phases). It is also found that the three-group specimens need a long time to reveal their corrosion resistance, and the corrosion rate of steels decreases evidently after a certain number of corrosion cycles
(the certain number of cycles are 54, 50 and 45 of Cr steel, Cre Ni steel and CreNieCu steel, respectively). The weight gain curves can be divided into the initial stage and the later stage by the certain inflection points. Another phenomenon that CreNie Cu steel had the shortest initial stage was observed. This may be explained by the fact that for CreNieCu steel a shorter time is needed for the inner rust to adhere tightly to the substrate.
Fig. 3 Rust surface micrographs of the three-group steels after corrosion with 3.0 mass% aqueous NaCl solution for different cycles, respectively: (a) Cr steel, 10 cycles; (b) CreNi steel, 10 cycles; (c) CreNieCu steel, 10 cycles; (d) Cr steel, 30 cycles; (e) CreNi steel, 30 cycles; (f) CreNieCu steel, 30 cycles; (g) Cr steel, 73 cycles; (h) CreNi steel, 73 cycles; (i) CreNieCu steel, 73 cycles.
Y. Zhou et al.: J. Mater. Sci. Technol., 2013, 29(2), 168e174
Because of selective oxidation, the alloy elements such as Cu, Cr and Ni enriched in the inner rust layer, and these elements promoted the formation of compact rust layers, which contacted the steel substrate and obstructed the corrosion mediums in atmosphere environment from penetrating to achieve further corrosion resistance of steel. The relationship between corrosion rate and corrosion time is listed in Fig. 2(b). The corrosion rate of tested steels increases with accumulation of corrosion products in the initial stage, and a decline is apparent from the certain inflection points, which indicates that rust layers start to protect the steels from this time. 3.3. Analysis of the rust layers The surfaces of each steel after 10, 30 and 73 wet/dry cycles of accelerated corrosion test were observed by SEM, as shown in Fig. 3(a)e(i). It is found that the rust layer structure of each specimen changes from loose to compact with increasing corrosion time. Corrosion products which look like floc and sphericcluster (Fig. 3(a)e(c)) are noticed on rust surfaces at the initial stage. Pores and cracks existing among these corrosion products can visibly provide transmission channels for corrosion medium such as moisture and oxygen, which is the main reason for increasing corrosion rate of the first corrosion stage. Rust layers become homogeneous and compact after 73 cycles (Fig. 3(g)e(i)) which is in the later stage of corrosion process. At this time, reducing of holes and cracks is noticed, and this compact structure is beneficial to protecting steel substrate out of corrosion medium such as chloride. According to contrasting with Fig. 3(g)e(i), it is apparent that the surface of CreNieCu steel is the most homogeneous and compact of the three-group tested steels, and granules of corrosion products are much smaller. The substrate of steel is protected by the rust layer structure out of corrosion medium, thereby avoiding continuous corrosion which is our purpose.
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This excellent rust structure can also give a reason why CreNie Cu steel exhibits the lowest weight gain and corrosion rate as shown in Fig. 2(a) and (b). The cross-sections of specimens after 10, 30 and 73 wet/dry cycles of corrosion test are shown in Fig. 4(a)e(e). It illustrates that the rust layers in these tested steels all contain voids and microcracks after 10 cycles (Fig. 4(a)), which facilitate the penetration of the chloride solution to the substrate, and promote the continuous corrosion process in turn. With increasing corrosion time, the rust layers accumulate step by step, and the thickness of rust layers also increases obviously. After 30 cycles (Fig. 4(b)), the rust layer on Cr steel is composed of two parts: the loose outer rust layer and the dense inner rust layer. The formation of compact inner rust layer can protect substrate effectively out of corrosion mediums, and then slow down the increase of the corrosion rate. This phenomenon can explain the performance of corrosion rate curve very well (Fig. 2(b)). At the beginning of corrosion process (w30 cycles), the corrosion rate increases rapidly, and then slows down in spite of increasing performance after 30 cycles (as shown in Fig. 2(b), variation zone), as far as the certain inflection point (54 cycle), after which the corrosion rate starts to decrease with corrosion time, because of the enough accumulation of dense inner rust layer. It can be also noticed that the corrosion rate performance of CreNi steel and CreNieCu steel is similar to that of Cr steel. By comparing with the cross-section micrographs of Cr steel, CreNi steel and CreNieCu steel after 73 cycles (Fig. 4(c)e(e)), it can be seen that the three-group steels all have outer and inner rust layers. The outer rust layer is loose and porous and differs from the inner rust layer structure visibly. The structure of Cre NieCu inner rust layer is the most homogeneous and compact one, and there are few voids in it. The excellent rust structure is the most important reason why CreNieCu steel has the best corrosion resistance in the three-group steels, as shown in Fig. 2(a).
Fig. 4 SEM micrographs of cross-section of experimental steels after being subjected to the corrosion test for different cycles, respectively: (a) Cr steel, 10 cycles; (b) Cr steel, 30 cycles; (c) Cr steel, 73 cycles; (d) CreNi steel, 73 cycles; (e) CreNieCu steel, 73 cycles.
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Fig. 5 Formation process of rust layer in the environment containing Cl for offshore platform steel.
3.4. Composition and structure of the rust layers Corrosion products of the three-group steels that had been corroded for 30 and 73 cycles were analyzed by XRD, respectively. Fig. 5 shows that the rust layers are composed of goethite (a-FeOOH), akaganeite (b-FeOOH), lepidocrocite (g-FeOOH), magnetite (Fe3O4) and large amounts of amorphous compounds. These compounds are mainly microcrystalline oxides or hydroxides and could not be identified by XRD[14]. By comparing with rust phases of CreNi steel and CreNieCu steel corroded for 30 or 73 cycles, it can be seen that the quantity of a-FeOOH increases, the g-FeOOH also increases obviously while the b-FeOOH reduces slightly and Fe3O4 changes little. According to the analyzing result of rust phase of CreNieCu steel, it indicates that the content of a-FeOOH increases from 0.96% to 1.80%, g-FeOOH increases from 1.98% to 4.50%, b-FeOOH reduces from 2.82% to 2.43% slightly, Fe3O4 changes from 0.24% to 0.27%, and amorphous compounds reduces from 94% to 91% approximately. This observation can be explained by acknowledging that the g-FeOOH was transformed into other kinds of corrosion products[6]. Misawa et al.[15] and Asami and Kikuchi[16] proposed that dissolved Cr ions enhance the formation of uniform amorphous ferric oxyhydroxide, which protects the steel substrate, and that this amorphous ferric oxyhydroxide will further transform into more stable and protective structure of a-FeOOH. At the initial corrosion stage, corrosion medium, such as moisture and dissolved oxygen, could diffuse to steel substrate surface easily across loose rust which resulted in acceleration of corrosion process. In etchant solution containing dissolved oxygen, the following anodic and cathodic reactions occur:
Anodic:
Fe / Fe2þ þ 2e
(1)
Fe2þ þ 2Cl þ 4H2O / FeCl2$4H2O
(2)
FeCl2$4H2O / Fe(OH)2 þ 2Cl þ 2Hþ þ 2H2O (3)
4Fe(OH)þ þ 4OH þ O2/4FeOOH þ 2H2O
(4)
Cathodic:
O2 þ2H2O þ 4e / 4OH
(5)
Fe2þ þ 8FeOOH þ 2e / 3Fe3O4 þ 4H2O
(6)
The irreversible formation of b-FeOOH and g-FeOOH under oxidizing conditions are as follows:
2Fe(OH)þ þ 2OH þ 1/2O2 / 2b-FeOOH þ H2O (Cl containing environment)
Fig. 6 XRD analysis of rust layers from tested steels after exposure to Cl containing environment for 30 or 73 wet/dry cycles.
(7)
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Fig. 7 EPMA mapping of alloying elements in the rust formed on the CreNieCu steel after different corrosion cycles, respectively: (a) 10 cycles, (b) 30 cycles, (c) 73 cycles.
2Fe(OH)þ þ 2OH þ 1/2O2 / 2g-FeOOH þ H2O (8) At the later corrosion stage, the meta-stable phases, such as
b-FeOOH and g-FeOOH, transformed into a thermodynamically stable phase of a-FeOOH[17e19]. The relationship of
corrosion products can be expressed simply as follows in this stage: Fe / Fe2þ / FeOHþ / Fe3O4/b-FeOOH / gFeOOH / a-FeOOH. The detailed transformation process is shown in Fig. 5. Corrosion resistance is improved obviously with the formation of continuous and compact rust layers whose structures are mainly composed of large amounts of amorphous compounds, g-FeOOH and a-FeOOH. Fig. 6 shows that the content of amorphous compounds of CreNieCu steel is about 2%e3% more than that of CreNi steel. Inouye et al.[20] in their study considered that Cu element had inhibition effect on the formation of crystallinity products, such as g-FeOOH and a-FeOOH. Because the crystalline process was difficult and the formation of micro holes and cracks was not easy, therefore, the process that dissolved oxygen transferred from the solution to the reduction rust Fe(OH)2 at the bottom of rust layer occurred, as a result, the oxidationereduction reaction was inhibited. The dense inner rust layer with little pores and cracks of CreNieCu steel was mainly composed of large amounts of amorphous compounds and a tiny amount of a-FeOOH. Such an excellent structure was the most important reason for protecting the steel substrate out of continuous and rapid corrosion process. 3.5. Distribution of alloy elements of the rust layers EPMA mapping of alloying elements in the rust layers of Cre NieCu steel corroded for different cycles is shown in Fig. 7. The result indicates that Cr concentrates mainly in the inner region of the rust, inner/outer interface especially, whereas Ni is uniformly distributed all over the rust and Cu is noticed rarely after 73 cycles. It is demonstrated that the increase of Cr concentration in the a-(Fe1ex,Crx)OOH, Cr-goethite, resulted in dense aggregation of small crystals, which provided high protective ability of the rust layer for atmospheric corrosives[21]. It was reported that
a rust containing Cr forms CrO2 in the rust structure and this promotes high corrosion resistance[22,23]. It was also pointed out that Cr-goethite possesses cation selectivity[24]. Therefore, Cr enrichment can promote the formation of compact inner rust layers, which impedes the transmission of corrosion mediums and slows down the electrochemical reaction process. Little Cu element is observed in rust layers, as shown in Fig. 7, while the content of Cu and Cr is equivalent approximately in CreNieCu steel (0.24% Cr and 0.21% Cu). Chen et al.[6] pointed out that Cu only concentrated obviously at the interface, which had little effect on promoting the protectiveness of the rust layers. In this study, there was no enrichment of Cu in rust layers, and it had less effect on corrosion resistance of experimental steels than Cr. Kimura et al.[25] mentioned that Ni atoms can substitute Fe-sites of Fe3O4 to form Fe3exNixO4. The chemical valence of Ni (þ2) was lower than that of Fe (average þ8/3) in Fe3exNixO4, which resulted in positive electricity. Therefore, Cl with negative electricity could be attracted instead of diffusing into inner rust layers, in other words, substrate was protected. The rust tended to prevent the migration of Cl from the rust/steel interface due to the preservation of electrostatic charge with the rust layer, and protected steel substrate for a long period. In other words, addition of Cr and Ni elements was another important reason for protective ability of rust layers. 4. Conclusions The results obtained from the study of effects of Cr, Ni and Cu on the corrosion behavior of low carbon microalloying steel in a Cl containing environment were presented. It was indicated that the addition of Cr and Ni elements was beneficial to improvement of corrosion resistance. CreNieCu steel had the most excellent corrosion resistance among the three-group tested steels. The following conclusions can be summarized from this investigation: (1) The corrosion process was divided into initial stage and later stage by the certain inflection points, and the shortest initial stage was observed for CreNieCu steel. The
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corrosion rate of tested steels increased with accumulation of corrosion products in initial stage, and a decline of corrosion rate was apparent from the certain inflection points. (2) The three-group steels all had outer and inner rust layers, and the outer rust layer was loose and porous, but the inner rust structure was different. CreNieCu inner rust layer was the most homogeneous and compact one with little pores and cracks. (3) The rust layers of the three-group steels were found to be composed of a-FeOOH, b-FeOOH, g-FeOOH, Fe3O4 and large amounts of amorphous compounds. The content of amorphous compounds of CreNieCu steel was about 2%e3% more than that of CreNi steel. (4) Cr concentrated mainly in the inner region of the rust of CreNieCu steel, inner/outer interface especially, whereas Ni was uniformly distributed all over the rust and Cu was noticed rarely after 73 cycles. Cr enrichment could promote the formation of compact rust layer, which could slow down the electrochemical reaction process and protect the substrate of the tested steel. Acknowledgments This work was supported by the High Technology Research and Development Program of China (No. 2007AA03Z504) and the Fundamental Research Funds for the Central Universities (No. N100507002).
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[4] T. Misawa, K. Asami, K. Hashimoto, S. Shimodaira, Corros. Sci. 14 (1974) 279e289. [5] H.C. Shih, J.C. Oung, J.T. Hsu, J.Y. Wu, F.I. Wei, Mater. Chem. Phys. 37 (1994) 230e236. [6] Y.Y. Chen, H.J. Tzeng, L.I. Wei, L.H. Wang, J.C. Oung, H.C. Shih, Corros. Sci. 47 (2005) 1001e1021. [7] H.E. Townsend, Corrosion 57 (2001) 497e501. [8] H.E. Townsend, T.C. Simpson, G.L. Johnson, Corrosion 50 (1994) 546e554. [9] M. Yamashita, H. Nagano, T. Misawa, H.E. Townsend, ISIJ Int. 38 (1998) 285e290. [10] Q.C. Zhang, J.S. Wu, J.J. Wang, W.L. Zheng, J.G. Chen, A.B. Li, Mater. Chem. Phys. 77 (2002) 603e608. [11] Y.S. Choi, J.G. Kim, Corrosion 56 (2000) 1202e1210. [12] L. Bousselmi, C. Fiaud, B. Tribollet, E. Triki, Corros. Sci. 39 (1997) 1711e1724. [13] T. Misawa, K. Hashimoto, S. Shimodaira, Corros. Sci. 14 (1974) 131e149. [14] J. Guo, S.W. Yang, C.J. Shang, Y. Wang, X.L. He, Corros. Sci. 51 (2009) 242e251. [15] T. Misawa, T. Kyuno, W. Suetaka, S. Shimodaira, Corros. Sci. 11 (1971) 35e48. [16] K. Asami, M. Kikuchi, Corros. Sci. 45 (2003) 2671e2688. [17] S.J. Oh, D.C. Cook, H.E. Townsend, Corros. Sci. 41 (1999) 1687e 1702. [18] J.E. Hiller, Mater. Corros. 17 (1966) 943e951. [19] P. Keller, Mater. Corros. 18 (1967) 865e878. [20] K. Inouye, K. Ichimura, K. Kaneko, T. Ishikawa, Corros. Sci. 16 (1976) 507e517. [21] M. Yamashita, H. Miyuki, H. Nagano, T. Misawa, in: Proceedings of 13th International Corrosion Congress, Melbourne, 1996, p. 258. [22] M. Yamashita, T. Shimizu, H. Konishi, J. Mizuki, H. Uchida, Corros. Sci. 45 (2003) 381e394. [23] M. Yamashita, H. Konishi, J. Mizuki, H. Uchida, Mater. Trans. 45 (2004) 1920e1924. [24] H. Miyuki, M. Yamashita, M. Fujiwara, T. Misawa, Zairyo to Kankyo 47 (1998) 186e192 (in Japanese). [25] M. Kimura, H. Kihira, M. Nomura, Y. Kitajima, in: Joint International Meeting, Honolulu, 2004.
J. Mater. Sci. Technol., 2013, 29(2), 168e174
Effects of Cr, Ni and Cu on the Corrosion Behavior of Low Carbon Microalloying Steel in a ClL Containing Environment Yanlei Zhou, Jun Chen, Yang Xu, Zhenyu Liu* State Key Laboratory of Rolling and Automation, Northeastern University, Shenyang 110004, China [Manuscript received March 30, 2012, in revised form August 22, 2012, Available online 27 December 2012]
The effects of Cr, Ni and Cu on the corrosion behavior of low carbon microalloying steel in a Cl containing environment were investigated. The results revealed that the corrosion process could be divided into the initial stage in which the corrosion rate increased with accumulation of corrosion products and the later stage in which homogeneous and compact inner rust layers started to protect steel substrate out of corrosion mediums. The results of X-ray diffraction (XRD) indicated that the rust layers of the three-group steels (Cr, CreNi and CreNieCu steels) were composed of a-FeOOH, b-FeOOH, g-FeOOH, Fe3O4 and large amounts of amorphous compounds. The content of amorphous compounds of CreNieCu steel was about 2%e3% more than that of CreNi steel. The results of electron probe microanalysis (EPMA) showed that Cr concentrated mainly in the inner region of the rust of CreNieCu steel, inner/outer interface especially, whereas Ni was uniformly distributed all over the rust and Cu was noticed rarely after 73 wet/dry cycles. The addition of Cr and Ni was beneficial to the formation of dense and compact inner rust layer, which was the most important reason for the improvement of corrosion resistance of experimental steel. KEY WORDS: Low carbon microalloying steel; Alloying elements; Corrosion resistance; Rust layer
1. Introduction Chemical elements, such as Cr, Ni and Cu play an important role in the corrosion process of structural steel, and the corrosion resistance of such steel is always due to the enrichment of these elements in the inner rust layers[1,2]. These elements can promote the formation of compact rust layers, which gradually become more dense over time and isolate the steels from various corrosion agents, resulting in substantial reduction of the corrosion rate[3e6]. The development of adherent and protective layers formed on the steels was the noticed phenomenon directly[7e10]. Choi and Kim[11] and Bousselmi et al.[12] studied that the Cr and Cu compounds can promote more or less protective rust layer on weathering steel in an aqueous condition, as they did under an atmospheric condition. Most researchers assume that microstructures only affect the corrosion behavior of bare steel and thus can be neglected after the surface of steel is covered by corrosion products. Currently, low carbon bainite steel is gradually replacing the ferrite and pearlite steels as the offshore * Corresponding author. Prof., Ph.D.; Tel.: þ86 24 83680571; Fax: þ86 24 23906472; E-mail address: [email protected] (Z. Liu). 1005-0302/$ e see front matter Copyright Ó 2013, The editorial office of Journal of Materials Science & Technology. Published by Elsevier Limited. All rights reserved. http://dx.doi.org/10.1016/j.jmst.2012.12.013
platform steel with high strength and toughness. Therefore, low carbon bainitic steels were chosen for experimental materials, and the effect of original microstructure was neglected in this study. Generally, the corrosion products on the surface of weathering steel are mainly a-FeOOH, b-FeOOH, g-FeOOH, Fe(OH)3, and Fe3O4. These products can coexist partly as crystalline and partly as amorphous structures[4,13]. It is also mentioned that when the steel contacts moisture and corrodes, the initial corrosion product is mainly g-FeOOH, but gradually this transforms to a-FeOOH as a result of the intermittently wet/dry conditions[4,13]. Chen et al.[6] pointed out that the alloying additions in weathering steel result in corrosion products which are more dense and more stable than those on carbon steel. Although a-FeOOH and g-FeOOH can be noticed in the corrosion products on carbon steel, they are porous since they are not enhanced by alloying elements, and in addition, the adhesion is poor between the rust layer and the substrate[6]. Because of the evaporation of seawater, offshore platform steel is always used in atmospheric environment containing Cl, and the enhancement of corrosion resistance is required. The aim of this investigation was to determine the effects of Cr, Ni and Cu on the corrosion behavior of low carbon microalloying steel in a Cl containing environment, by means of scanning electron microscopy (SEM), X-ray diffraction (XRD) and electron probe microanalysis (EPMA) especially.
Y. Zhou et al.: J. Mater. Sci. Technol., 2013, 29(2), 168e174
169
Table 1 Chemical compositions of the experimental steels (wt%)
Cr steel CreNi steel CreNieCu steel
C
Si
Mn
P
S
Mo
Nb
V
Ti
Cr
Ni
Cu
Fe
0.06 0.07 0.05
0.44 0.22 0.35
1.71 1.45 1.56
0.009 0.007 0.006
0.005 0.004 0.002
0.15 0.25 0.20
0.034 0.044 0.060
0.010 0.053 0.040
0.014 0.014 0.020
0.49 0.30 0.24
e 0.39 0.50
0.21
Bal. Bal. Bal.
2. Experimental 2.1. Materials
sample weight after corrosion and S is the corrosion area on the sample. 2.3. Microstructure and surface analysis
Experimental steels were melted with a 50 kg vacuum induction furnace. Chemical composition (wt%) of the threegroup tested steels (Cr, CreNi and CreNieCu steels) used in the experiments are given in Table 1. Ingots were heated and forged into 120 mm 150 mm 200 mm slabs, and then the slabs were homogenized at 1200 C for 2 h in soaking pit for rolling process. Cr steel was rolled in non-recrystallization and recrystallization austenite region to a thickness of 15 mm, and then cooled to 500 C by ultra fast cooling (UFC) process. CreNi steel was treated by conventional quenchingetempering (QeT) process and CreNieCu steel was treated by lamellarizingetempering (LeT) process, respectively, after controlled rolling and cooling. Specimens for corrosion test were cut to 5 mm 40 mm 40 mm. All specimens were milled to 4 mm thickness and ground with 600e 1000 grit silicon carbide paper, then cleaned with acetone after they were rinsed with distilled water. Each specimen was mounted in a cured epoxy resin, an area of 25 mm 25 mm was left on the metal surface to add corrosive liquid periodically, and the epoxyespecimen interface was also painted to prevent the initiation of crevice corrosion between the epoxy and the specimen. 2.2. Constant temperature and humidity accelerated corrosion test ESPEC constant temperature and constant humidity chamber (PR-2KT) was used for the cyclic wet/dry test (30 C, 60% RH). 3.0 mass% aqueous NaCl solution was used as corrosive liquid in this experiment. Each wet/dry cycle (24 h) consisted of a wetting period and a dry period. Corrosive solution was dropped to working face of specimen (40 ml/cm2) during every corrosion cycle (12 h). After rinsing with distilled water and drying in air, the samples were weighted by using a Sartorius electronic analytical balance (d ¼ 0.01 mg). Weight gain of a unit area was calculated by (W1W0)/S in which W1 is the
To investigate the relationship between the alloying elements, microstructure and surface composition of the rust, the surface was examined by SEM, XRD and EPMA after corroding for different cycles. The rusted steel samples were cut to 4 mm 4 mm 10 mm in size, and then mounted using a SimpliMet 3000 automatic mounting press (AMP) for surface analysis. The surface and cross-section of the rust layers were analyzed using an FEI Quanta 600 scanning electron microscope. Structural analysis of rusted samples was carried out using a D/MAX-3A Xray diffractometer with a Cu target. The scan speed was 2 /min and the 2q angle ranges from 10 to 50 . Elements distribution of specimens on cross-section direction was determined using an electron probe microanalyzer, which was performed using the JXA-8530F, with an acceleration voltage of 20 kV, an irradiation current of 8 107 A, and a beam diameter of 1 mm. 3. Results and Discussion 3.1. Microstructures Microstructures of the three-group experimental steels are shown in Fig. 1. It can be seen that the microstructure of Cr steel consists mainly of lath-shape bainite (Fig. 1(a)); the main structure of CreNi steel is tempered martensite (Fig. 1(b)); microstructures of CreNieCu steel consist of polygonal ferrite (PF) and carbon-rich phases (retained MeA islands after tempering) which are distributed randomly (Fig. 1(c)). 3.2. Weight gain and corrosion rate curves Weight gain data were expressed as the weight gain per unit surface area in this study. Fig. 2(a) shows the weight gain curve of specimens in the cyclic wet/dry test. In general, the final weight gain of CreNieCu steel is obviously the lowest in the
Fig. 1 OM micrographs of the three-group experimental steels: (a) Cr steel, (b) CreNi steel, (c) CreNieCu steel.
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Fig. 2 Weight gain and corrosion rate of specimens corroded in testing chamber for different time: (a) weight gain curve, (b) corrosion rate curve.
three-group data, and that of Cr steel is the highest. Therefore, the steels can be arranged in the order of weight gain as follows: Cr steel (lath-shape bainite) > CreNi steel (tempered martensite) > CreNieCu steel (PF þ carbon-rich phases). It is also found that the three-group specimens need a long time to reveal their corrosion resistance, and the corrosion rate of steels decreases evidently after a certain number of corrosion cycles
(the certain number of cycles are 54, 50 and 45 of Cr steel, Cre Ni steel and CreNieCu steel, respectively). The weight gain curves can be divided into the initial stage and the later stage by the certain inflection points. Another phenomenon that CreNie Cu steel had the shortest initial stage was observed. This may be explained by the fact that for CreNieCu steel a shorter time is needed for the inner rust to adhere tightly to the substrate.
Fig. 3 Rust surface micrographs of the three-group steels after corrosion with 3.0 mass% aqueous NaCl solution for different cycles, respectively: (a) Cr steel, 10 cycles; (b) CreNi steel, 10 cycles; (c) CreNieCu steel, 10 cycles; (d) Cr steel, 30 cycles; (e) CreNi steel, 30 cycles; (f) CreNieCu steel, 30 cycles; (g) Cr steel, 73 cycles; (h) CreNi steel, 73 cycles; (i) CreNieCu steel, 73 cycles.
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Because of selective oxidation, the alloy elements such as Cu, Cr and Ni enriched in the inner rust layer, and these elements promoted the formation of compact rust layers, which contacted the steel substrate and obstructed the corrosion mediums in atmosphere environment from penetrating to achieve further corrosion resistance of steel. The relationship between corrosion rate and corrosion time is listed in Fig. 2(b). The corrosion rate of tested steels increases with accumulation of corrosion products in the initial stage, and a decline is apparent from the certain inflection points, which indicates that rust layers start to protect the steels from this time. 3.3. Analysis of the rust layers The surfaces of each steel after 10, 30 and 73 wet/dry cycles of accelerated corrosion test were observed by SEM, as shown in Fig. 3(a)e(i). It is found that the rust layer structure of each specimen changes from loose to compact with increasing corrosion time. Corrosion products which look like floc and sphericcluster (Fig. 3(a)e(c)) are noticed on rust surfaces at the initial stage. Pores and cracks existing among these corrosion products can visibly provide transmission channels for corrosion medium such as moisture and oxygen, which is the main reason for increasing corrosion rate of the first corrosion stage. Rust layers become homogeneous and compact after 73 cycles (Fig. 3(g)e(i)) which is in the later stage of corrosion process. At this time, reducing of holes and cracks is noticed, and this compact structure is beneficial to protecting steel substrate out of corrosion medium such as chloride. According to contrasting with Fig. 3(g)e(i), it is apparent that the surface of CreNieCu steel is the most homogeneous and compact of the three-group tested steels, and granules of corrosion products are much smaller. The substrate of steel is protected by the rust layer structure out of corrosion medium, thereby avoiding continuous corrosion which is our purpose.
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This excellent rust structure can also give a reason why CreNie Cu steel exhibits the lowest weight gain and corrosion rate as shown in Fig. 2(a) and (b). The cross-sections of specimens after 10, 30 and 73 wet/dry cycles of corrosion test are shown in Fig. 4(a)e(e). It illustrates that the rust layers in these tested steels all contain voids and microcracks after 10 cycles (Fig. 4(a)), which facilitate the penetration of the chloride solution to the substrate, and promote the continuous corrosion process in turn. With increasing corrosion time, the rust layers accumulate step by step, and the thickness of rust layers also increases obviously. After 30 cycles (Fig. 4(b)), the rust layer on Cr steel is composed of two parts: the loose outer rust layer and the dense inner rust layer. The formation of compact inner rust layer can protect substrate effectively out of corrosion mediums, and then slow down the increase of the corrosion rate. This phenomenon can explain the performance of corrosion rate curve very well (Fig. 2(b)). At the beginning of corrosion process (w30 cycles), the corrosion rate increases rapidly, and then slows down in spite of increasing performance after 30 cycles (as shown in Fig. 2(b), variation zone), as far as the certain inflection point (54 cycle), after which the corrosion rate starts to decrease with corrosion time, because of the enough accumulation of dense inner rust layer. It can be also noticed that the corrosion rate performance of CreNi steel and CreNieCu steel is similar to that of Cr steel. By comparing with the cross-section micrographs of Cr steel, CreNi steel and CreNieCu steel after 73 cycles (Fig. 4(c)e(e)), it can be seen that the three-group steels all have outer and inner rust layers. The outer rust layer is loose and porous and differs from the inner rust layer structure visibly. The structure of Cre NieCu inner rust layer is the most homogeneous and compact one, and there are few voids in it. The excellent rust structure is the most important reason why CreNieCu steel has the best corrosion resistance in the three-group steels, as shown in Fig. 2(a).
Fig. 4 SEM micrographs of cross-section of experimental steels after being subjected to the corrosion test for different cycles, respectively: (a) Cr steel, 10 cycles; (b) Cr steel, 30 cycles; (c) Cr steel, 73 cycles; (d) CreNi steel, 73 cycles; (e) CreNieCu steel, 73 cycles.
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Fig. 5 Formation process of rust layer in the environment containing Cl for offshore platform steel.
3.4. Composition and structure of the rust layers Corrosion products of the three-group steels that had been corroded for 30 and 73 cycles were analyzed by XRD, respectively. Fig. 5 shows that the rust layers are composed of goethite (a-FeOOH), akaganeite (b-FeOOH), lepidocrocite (g-FeOOH), magnetite (Fe3O4) and large amounts of amorphous compounds. These compounds are mainly microcrystalline oxides or hydroxides and could not be identified by XRD[14]. By comparing with rust phases of CreNi steel and CreNieCu steel corroded for 30 or 73 cycles, it can be seen that the quantity of a-FeOOH increases, the g-FeOOH also increases obviously while the b-FeOOH reduces slightly and Fe3O4 changes little. According to the analyzing result of rust phase of CreNieCu steel, it indicates that the content of a-FeOOH increases from 0.96% to 1.80%, g-FeOOH increases from 1.98% to 4.50%, b-FeOOH reduces from 2.82% to 2.43% slightly, Fe3O4 changes from 0.24% to 0.27%, and amorphous compounds reduces from 94% to 91% approximately. This observation can be explained by acknowledging that the g-FeOOH was transformed into other kinds of corrosion products[6]. Misawa et al.[15] and Asami and Kikuchi[16] proposed that dissolved Cr ions enhance the formation of uniform amorphous ferric oxyhydroxide, which protects the steel substrate, and that this amorphous ferric oxyhydroxide will further transform into more stable and protective structure of a-FeOOH. At the initial corrosion stage, corrosion medium, such as moisture and dissolved oxygen, could diffuse to steel substrate surface easily across loose rust which resulted in acceleration of corrosion process. In etchant solution containing dissolved oxygen, the following anodic and cathodic reactions occur:
Anodic:
Fe / Fe2þ þ 2e
(1)
Fe2þ þ 2Cl þ 4H2O / FeCl2$4H2O
(2)
FeCl2$4H2O / Fe(OH)2 þ 2Cl þ 2Hþ þ 2H2O (3)
4Fe(OH)þ þ 4OH þ O2/4FeOOH þ 2H2O
(4)
Cathodic:
O2 þ2H2O þ 4e / 4OH
(5)
Fe2þ þ 8FeOOH þ 2e / 3Fe3O4 þ 4H2O
(6)
The irreversible formation of b-FeOOH and g-FeOOH under oxidizing conditions are as follows:
2Fe(OH)þ þ 2OH þ 1/2O2 / 2b-FeOOH þ H2O (Cl containing environment)
Fig. 6 XRD analysis of rust layers from tested steels after exposure to Cl containing environment for 30 or 73 wet/dry cycles.
(7)
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Fig. 7 EPMA mapping of alloying elements in the rust formed on the CreNieCu steel after different corrosion cycles, respectively: (a) 10 cycles, (b) 30 cycles, (c) 73 cycles.
2Fe(OH)þ þ 2OH þ 1/2O2 / 2g-FeOOH þ H2O (8) At the later corrosion stage, the meta-stable phases, such as
b-FeOOH and g-FeOOH, transformed into a thermodynamically stable phase of a-FeOOH[17e19]. The relationship of
corrosion products can be expressed simply as follows in this stage: Fe / Fe2þ / FeOHþ / Fe3O4/b-FeOOH / gFeOOH / a-FeOOH. The detailed transformation process is shown in Fig. 5. Corrosion resistance is improved obviously with the formation of continuous and compact rust layers whose structures are mainly composed of large amounts of amorphous compounds, g-FeOOH and a-FeOOH. Fig. 6 shows that the content of amorphous compounds of CreNieCu steel is about 2%e3% more than that of CreNi steel. Inouye et al.[20] in their study considered that Cu element had inhibition effect on the formation of crystallinity products, such as g-FeOOH and a-FeOOH. Because the crystalline process was difficult and the formation of micro holes and cracks was not easy, therefore, the process that dissolved oxygen transferred from the solution to the reduction rust Fe(OH)2 at the bottom of rust layer occurred, as a result, the oxidationereduction reaction was inhibited. The dense inner rust layer with little pores and cracks of CreNieCu steel was mainly composed of large amounts of amorphous compounds and a tiny amount of a-FeOOH. Such an excellent structure was the most important reason for protecting the steel substrate out of continuous and rapid corrosion process. 3.5. Distribution of alloy elements of the rust layers EPMA mapping of alloying elements in the rust layers of Cre NieCu steel corroded for different cycles is shown in Fig. 7. The result indicates that Cr concentrates mainly in the inner region of the rust, inner/outer interface especially, whereas Ni is uniformly distributed all over the rust and Cu is noticed rarely after 73 cycles. It is demonstrated that the increase of Cr concentration in the a-(Fe1ex,Crx)OOH, Cr-goethite, resulted in dense aggregation of small crystals, which provided high protective ability of the rust layer for atmospheric corrosives[21]. It was reported that
a rust containing Cr forms CrO2 in the rust structure and this promotes high corrosion resistance[22,23]. It was also pointed out that Cr-goethite possesses cation selectivity[24]. Therefore, Cr enrichment can promote the formation of compact inner rust layers, which impedes the transmission of corrosion mediums and slows down the electrochemical reaction process. Little Cu element is observed in rust layers, as shown in Fig. 7, while the content of Cu and Cr is equivalent approximately in CreNieCu steel (0.24% Cr and 0.21% Cu). Chen et al.[6] pointed out that Cu only concentrated obviously at the interface, which had little effect on promoting the protectiveness of the rust layers. In this study, there was no enrichment of Cu in rust layers, and it had less effect on corrosion resistance of experimental steels than Cr. Kimura et al.[25] mentioned that Ni atoms can substitute Fe-sites of Fe3O4 to form Fe3exNixO4. The chemical valence of Ni (þ2) was lower than that of Fe (average þ8/3) in Fe3exNixO4, which resulted in positive electricity. Therefore, Cl with negative electricity could be attracted instead of diffusing into inner rust layers, in other words, substrate was protected. The rust tended to prevent the migration of Cl from the rust/steel interface due to the preservation of electrostatic charge with the rust layer, and protected steel substrate for a long period. In other words, addition of Cr and Ni elements was another important reason for protective ability of rust layers. 4. Conclusions The results obtained from the study of effects of Cr, Ni and Cu on the corrosion behavior of low carbon microalloying steel in a Cl containing environment were presented. It was indicated that the addition of Cr and Ni elements was beneficial to improvement of corrosion resistance. CreNieCu steel had the most excellent corrosion resistance among the three-group tested steels. The following conclusions can be summarized from this investigation: (1) The corrosion process was divided into initial stage and later stage by the certain inflection points, and the shortest initial stage was observed for CreNieCu steel. The
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corrosion rate of tested steels increased with accumulation of corrosion products in initial stage, and a decline of corrosion rate was apparent from the certain inflection points. (2) The three-group steels all had outer and inner rust layers, and the outer rust layer was loose and porous, but the inner rust structure was different. CreNieCu inner rust layer was the most homogeneous and compact one with little pores and cracks. (3) The rust layers of the three-group steels were found to be composed of a-FeOOH, b-FeOOH, g-FeOOH, Fe3O4 and large amounts of amorphous compounds. The content of amorphous compounds of CreNieCu steel was about 2%e3% more than that of CreNi steel. (4) Cr concentrated mainly in the inner region of the rust of CreNieCu steel, inner/outer interface especially, whereas Ni was uniformly distributed all over the rust and Cu was noticed rarely after 73 cycles. Cr enrichment could promote the formation of compact rust layer, which could slow down the electrochemical reaction process and protect the substrate of the tested steel. Acknowledgments This work was supported by the High Technology Research and Development Program of China (No. 2007AA03Z504) and the Fundamental Research Funds for the Central Universities (No. N100507002).
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