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Corrosion investigation of stainless steel water pump components
EFA-02967; No of Pages 8 Engineering Failure Analysis xxx (2016) xxx–xxx
Contents lists available at ScienceDirect
Engineering Failure Analysis journal homepage: www.elsevier.com/locate/engfailanal
Corrosion investigation of stainless steel water pump components A. Vazdirvanidis, G. Pantazopoulos ⁎, A. Rikos ELKEME Hellenic Research Centre for Metals S.A., 56th km Athens-Lamia Highway, 32011 Oinofyta-Viotia, Greece
a r t i c l e
i n f o
Article history: Received 8 July 2016 Received in revised form 14 September 2016 Accepted 24 September 2016 Available online xxxx Keywords: Pitting corrosion Stainless steel Corrosion products
a b s t r a c t Two components of a water pump installed in a casting shop for recirculation of cooling water experienced severe and accelerated corrosion after two months in service. The received pieces of the water pump assembly were a shaft and a conical tube, which was used as connector with the impeller. The shaft exhibited circumferential pitting corrosion behavior in specific areas where it was in contact with another pump component. Light optical microscopy and Scanning Electron Microscopy coupled with Energy Dispersive X-ray Spectroscopy were mainly used as analytical techniques for corrosion process evaluation and for the identification of the morphology and chemical composition of corrosion products, in order to draw safe conclusions concerning the type of the corrosion and the respective root-source. The main findings of the investigation indicated that pitting corrosion was the dominant failure mechanism for both water pump components influenced by the presence of aggressive environmental conditions, characterized by the presence of chlorides and sulfates that accelerate corrosion process above a certain temperature range (T N 50–55 °C). © 2016 Elsevier Ltd. All rights reserved.
1. Introduction and background information The mostly known eight forms of corrosion were referred in a classical corrosion literature [1]. A summary of corrosion mechanisms occurred in chemical process industry is presented in Ref. [2]. Stress corrosion cracking (SCC) and pitting corrosion of stainless steels constitute a critical research field due to the extensive use of such components in special applications in manufacturing and chemical industry under harsh operating conditions [3]. The damaging role of chloride ions and their influence on stimulation of SCC mechanism in stainless steel components in process industry are addressed in various research works, see for instance Refs [3–6]. Transgranular and intergranular SCC were detected in case of naphtha hydrotreater furnace tubes [7]. The detrimental effects of corrosive species, such as Cl and S is underlined also, in case of localized corrosion of steel piping elbow in oil-gas separation system started from inorganic compounds contained in the crude oil [8]. The influence of corrosion environment in combination to fatigue related processes is reviewed in Ref [9]. Fatigue failure in rotating equipment is assisted by local stress concentration which is a crucial factor enhancing the risk of crack initiation [10]. Two components of a water pump installed in a casting shop for recirculation of cooling water experienced severe and accelerated corrosion after two months in service. The received components of the water pump assembly constituted a shaft and a conical tube, which worked as a connector with the impeller (Fig. 1). The shaft exhibited severe pitting corrosion in specific areas in connect with another pump component (Fig. 2).
⁎ Corresponding author. E-mail address: [email protected] (G. Pantazopoulos).
http://dx.doi.org/10.1016/j.engfailanal.2016.09.009 1350-6307/© 2016 Elsevier Ltd. All rights reserved.
Please cite this article as: A. Vazdirvanidis, et al., Corrosion investigation of stainless steel water pump components, Engineering Failure Analysis (2016), http://dx.doi.org/10.1016/j.engfailanal.2016.09.009
2
A. Vazdirvanidis et al. / Engineering Failure Analysis xxx (2016) xxx–xxx
Fig. 1. Images illustrating the components' assembly of the pump.
2. Experimental procedure Macroscopic observations of the failed components were performed with a high definition digital camera and a stereomicroscope. In order to reveal the main failure mechanisms, microscopic observations were carried out using scanning electron microscopy (SEM) equipped with an energy dispersive x-ray spectrometry (EDS) detector for elemental chemical analysis of selected areas. Cross-sections of the failed components were prepared using hot-mounting, wet grinding up to 1200 grit SiC paper and polishing with diamond and silica suspensions. Metallographic examination was conducted using an inverted optical microscope.
3. Results Two components of a water pump which experienced severe and accelerated corrosion after two months in operation. The chemical composition analysis by Optical Emission Spectroscopy showed that both materials match approximately to AISI 403 (UNS No. S40300) heat treatable stainless steel (Table 1). No internal defects, such as inclusions or other discontinuities that could be considered as preferential sites for corrosion initiation were found. The shaft exhibited significant circumferential pitting corrosion in specific areas that were in contact with another pump component (Fig. 3). Sections transverse to the axis of symmetry of the shaft were prepared in order to reveal the pits' morphology and investigate corrosion products elemental composition.
Fig. 2. Macrographs showing the corroded coupling and shaft parts of the pump.
Please cite this article as: A. Vazdirvanidis, et al., Corrosion investigation of stainless steel water pump components, Engineering Failure Analysis (2016), http://dx.doi.org/10.1016/j.engfailanal.2016.09.009
A. Vazdirvanidis et al. / Engineering Failure Analysis xxx (2016) xxx–xxx
3
Table 1 Chemical composition of shaft and coupling components (% wt.). Element/sample
C
Si
Mn
Cr
Ni
Mo
Shaft Conical tube (coupling)
0.18 0.19
0.37 0.37
0.55 0.55
12.46 12.55
0.17 0.27
0.05 0.06
Pitting corrosion extended around the entire examined circumference of the stainless steel shaft and maximum pit depth did not exceed 1 mm (Fig. 4). The main corrosion products consisted of Cr, Fe, Si oxides while S and Cl have been also detected (Figs. 5, 6). Similar findings were obtained concerning the pit floor (close to the metal interface), as well as the internal pits and the smaller pits (Figs. 7, 8). The conical tube (coupling) was sectioned on the rim parallel to the axis of symmetry. Optical microscopy observations showed that the rim of the tube was severely corroded with pits exceeding 5 mm in depth (Fig. 9). SEM/EDS analysis indicated that the corrosion products consisted mainly of Cr, Fe, Ca, Si and Mg (hydro)oxides (Fig. 10). Brittle scale deposits demonstrate severe cracking and high tendency of breaking off, causing large fragments that could cause clogging and erosion in circuit components leading to damage and unit function interruption (Fig. 10). No additional metallographic study was performed concerning stainless steel microstructure, at this stage of investigation, since the scale deposits analysis and degradation topology provided a strong indication of the influence of circulating water quality and operation conditions on the evolved corrosion processes. 4. Discussion and recommendations Hydrolysis of inorganic chloride salts produces HCl which results in pH reduction, creating aggressive corrosion conditions and leading to steel chemical attack and successive metal dissolution, see also Ref. [8]: CaCl2 þ H2 O→2HCl þ CaO
ð1Þ
The detrimental role of H2S aqueous solution was also highlighted in a case of steel absorber corrosion in a desulfurization plant [11]. The presence of sulfate reducing bacteria (SRB) led to the reduction of sulfates towards sulfides in an acidic environment [12]. Under low pH conditions (pH ~4), hydrolyses of FeS and sulfides take place and lead to the formation of H2S which promotes the generation of hydrogen, [Eqs. (2)–(4)], see also Ref. [12]. 2−
S
2þ
Fe
þ
−
þ H2 O→ HS þ OH −
−
þ
þ HS → H þ FeS −
H þ e → Had
ð2Þ ð3Þ ð4Þ
[where “ad” denotes adsorbed atom.] The presence of H2S in gas and oil industries produces atomic hydrogen which diffused-in and trapped in atomic scale imperfections, dislocations and non-metallic inclusions leading to hydrogen-induced-cracking (HIC) incidents in pipelines, see also [13]. In Ref. [14] steam coil corrosion was provoked by sulfuric and hydrochloric acid attack.
Fig. 3. Macrographs of the shaft and the circumferential pitting corrosion formation (images from the plant).
Please cite this article as: A. Vazdirvanidis, et al., Corrosion investigation of stainless steel water pump components, Engineering Failure Analysis (2016), http://dx.doi.org/10.1016/j.engfailanal.2016.09.009
4
A. Vazdirvanidis et al. / Engineering Failure Analysis xxx (2016) xxx–xxx
Fig. 4. Optical micrographs, transverse section showing pitting formation and depth on the shaft.
Scale formation in piping and rotating shafts has a protective effect against corrosion. Scaling susceptibility in terms CaCO3 precipitation can be assessed by the Langelier (LSI) and Ryznar Saturation Indices (RSI). The involved chemical reactions and Saturation Index (S.I.) calculations with respect to the corrosiveness of the cooling water are presented according to the following Eqs. (5)–(13): þ
H2 CO3 →H þ HCO3 −
þ
HCO3 →H þ CO3
− 0
2−
K
1
0
K
ð5Þ
2
CaðHCO3 Þ2 →CaCO3 ðsÞ þ CO2 ðgÞ þ H2 O
ð6Þ ð7Þ
The Langelier and Ryznar indices depended mainly on temperature and total alkalinity. These indices are defined as follows: LSI ¼ Langelier Saturation Index ¼ pH– pHs
ð8Þ
Fig. 5. SEM micrograph of the corrosion products of the shaft and corresponding EDS spectra showing Cr, Si oxides and S, Cl presence.
Please cite this article as: A. Vazdirvanidis, et al., Corrosion investigation of stainless steel water pump components, Engineering Failure Analysis (2016), http://dx.doi.org/10.1016/j.engfailanal.2016.09.009
A. Vazdirvanidis et al. / Engineering Failure Analysis xxx (2016) xxx–xxx
5
Fig. 6. Another area of interest. SEM micrograph of the corrosion products of the shaft and EDS spectra showing the presence of Cr, Si oxides and S and Cl.
RSI ¼ Ryznar Saturation Index ¼ 2pHs –pH 0 pHs ¼ Saturation pH ¼ pK 2 –pK sp þ pCa þ pAlk
K
0
2
h i h i h i h i h i 2− − 2− þ 2þ = HCO3 ; K sp ¼ Ca CO3 ¼ H CO3
h i þ Alkalinity ¼ H
tit
h i h i h i 2− − − þ HCO3 þ OH ¼ 2 CO3
ð9Þ ð10Þ
ð11Þ
ð12Þ
Fig. 7. SEM micrograph illustrating pit growth mechanism and EDS spectrum corresponding to the pit floor.
Please cite this article as: A. Vazdirvanidis, et al., Corrosion investigation of stainless steel water pump components, Engineering Failure Analysis (2016), http://dx.doi.org/10.1016/j.engfailanal.2016.09.009
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A. Vazdirvanidis et al. / Engineering Failure Analysis xxx (2016) xxx–xxx
Fig. 8. SEM micrographs showing internal and isolated pits and respective EDS spectrum of the corrosion products.
The pHs (saturation pH) which is used mainly for index calculations can be calculated by the following equations: pHs ¼ ð9:3 þ A þ BÞ–ðC þ DÞ
ð13Þ
where: A. = [log(TDS) − 1] / 10 [TDS: Total Dissolved Solids (mg/L)]
Fig. 9. Optical micrographs, lower and higher magnification of the corrosion pits on the tube rim. Sectioning parallel to the tube axis of symmetry.
Please cite this article as: A. Vazdirvanidis, et al., Corrosion investigation of stainless steel water pump components, Engineering Failure Analysis (2016), http://dx.doi.org/10.1016/j.engfailanal.2016.09.009
A. Vazdirvanidis et al. / Engineering Failure Analysis xxx (2016) xxx–xxx
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Fig. 10. SEM micrographs of the corrosion products of the tube and respective EDS spectra showing Cr, Fe, Ca, Si and Mg (hydro)oxides.
B. = −13.12 log[T(°C) + 273] + 34.55 C. = log[Ca2+ as CaCO3] − 0.4 D. = log [alkalinity as CaCO3], [CaCO3] = 2.5[Ca2+] Scaling susceptibility and corrosion protection/aggressiveness with respect to S.I. levels (LSI and RSI) is presented in Table 2 [15,16]. Proper water monitoring comprises also the S.I. determination for continuous health monitoring and water corrosion susceptibility. The above findings suggest that the accelerated corrosion of the water pump parts was likely caused by: 1. Contaminated water characterized by the presence of sulfates/sulfides and chlorides that resulted in severe chemical attack of the various pump components. This condition may be aggravated at higher operating temperatures, i.e. T N 50–55 °C. 2. High concentration of total suspended solids (TSS), such as silica based particles, leading to erosion and mechanical damage phenomena, that could enhance pitting corrosion. 3. Galvanic corrosion is an additional synergistic mechanism especially on the dissimilar metal contact locations. This is a rather generic hypothesis which is based on the metal-to-metal contacts between different pump components, such as shaft, impeller, bearings and connectors. 4. Crevice corrosion can be also considered as another contributing corrosion mechanism, especially in restricted areas (e.g. shaftcoupling assembly areas), where concentration gradients together with the accumulation of hard solid deposits, might also aggravate localized material degradation. Crevices are formed in connection areas of pump assembly components, where stagnant water conditions could result in condensation and in the formation of localized concentration cells. 5. Low pitting resistance of the steel. The pitting corrosion resistance number (PREN) can be used to select a more corrosion resistant alloy: PREN ¼ %Cr þ 3:3ð%Mo þ 0:5%WÞ þ X%N
ð14Þ
where, X = 16 for duplex stainless steels and 30 for austenitic stainless steels, see also [17].
Table 2 Tendency of water with respect to LSI and RSI values. Langelier saturation index (LSI)
Ryznar saturation index (RSI)
Water tendency
2.0 0.5 0 −0.5 −2
b4 5–6 6–6.5 6.5–7 N8
Heavy scale formation and non-aggressive Slight scale formation and mildly aggressive Balanced or at saturation of CaCO3 No scale formation and slightly aggressive Under-saturated and highly aggressive
Please cite this article as: A. Vazdirvanidis, et al., Corrosion investigation of stainless steel water pump components, Engineering Failure Analysis (2016), http://dx.doi.org/10.1016/j.engfailanal.2016.09.009
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A. Vazdirvanidis et al. / Engineering Failure Analysis xxx (2016) xxx–xxx
Higher pitting resistance stainless steels with higher Cr, Mo and/or W contents can be utilized to minimize corrosion susceptibility during service.
Recommendations 1. Water monitoring concerning physicochemical parameters, such as pH, conductivity, total suspended solids (TSS), total dissolved solids (TDS), Cl, SO4 and operating temperature. 2. Use of physical/chemical processes for water cleaning and stabilize water quality, such as filtration, softening. 3. Implementation of regular “wash-off” cycles to clean and remove solid residues from pump components. 4. Selection and use of stainless steels with higher Cr and Mo content, offering higher Pitting Resistance Equivalence Number (PREN) could enhance the lifetime of stainless steel components rendering higher resistance against pitting corrosion. The UNS Nos. S42200 and/or S43400 stainless steel grades could stand as typical alternative suggestions of component material selections.
Acknowledgements The authors wish to express special thanks to Plant Production and Maintenance team for their assistance in background information and fruitful discussions.
References [1] M.G. Fontana, Corrosion Engineering, 3rd edition McGraw Hill, N.Y, 1986. [2] G.A. Pantazopoulos, P.P. Psyllaki, Progressive failures of components in chemical process industry: case history investigation and root cause analysis, in: A.S.H. Makhlouf, M. Aliofkhazraei (Eds.), Handbook of Materials Failure Analysis with Case Studies from Chemical, Concrete and Power Industries, Butterworth Heinemann – Elsevier 2016, pp. 1–23. [3] P.P. Psyllaki, G. Pantazopoulos, A. Pistoli, Degradation of stainless steel grids in chemically aggressive environment, Eng. Fail. Anal. 35 (2013) 418–426. [4] T.D. Traubert, T.A. Jur, Metallurgical analysis to evaluate cracking in a 316 L grade stainless steel spiral heat exchanger, J. Fail. Anal. Prev. 12 (2012) 198–203. [5] M.S.A. Asghar, F. Tariq, A. Ali, Failure analysis of AISI-304 stainless steel styrene storage tank, J. Fail. Anal. Prev. 10 (2010) 303–311. [6] G.X. Jin, Y.P. Xie, A.H. Tian, Corrosion failure analysis of the condensate collector vessel head of evaporation system in ethylene glycol device, Eng. Fail. Anal. 22 (2012) 113–120. [7] M.M. Jan, A. Jalil, Z. Ahmad, M.A. Khan, Application of fracture mechanics and non-destructive techniques for the condition assessment of naphtha hydrotreater furnace tubes in oil refinery, Eng. Fail. Anal. 40 (2014) 15–24. [8] H.M. Tawancy, L.M. Al-Hadhrami, F.K. Al-Yousef, Analysis of corroded elbow section of carbon steel piping system of an oil-gas separation vessel, Case Stu. Eng. Failure Anal. 1 (2013) 6–14. [9] N.I.I. Mansor, S. Abdullah, A.K. Ariffin, J. Syarif, A review of the fatigue failure mechanism of metallic materials under a corroded environment, Eng. Fail. Anal. 42 (2014) 353–365. [10] G. Pantazopoulos, S. Papaefthymiou, Failure and fracture analysis of austenitic stainless steel marine propeller shaft, J. Fail. Anal. Prev. 15 (2015) 762–767. [11] Z. Wang, C. Yang, L. Zhu, Corrosion analysis and inhibition studies in the process of natural gas wet desulfurization, Eng. Fail. Anal. 44 (2014) 66–73. [12] Z.Y. Liu, H. Li, Z.J. Jia, C.W. Du, X.G. Li, Failure analysis of P110 steel tubing in low-temperature annular environment of CO2 flooding wells, Eng. Fail. Anal. 60 (2016) 296–306. [13] M.A. Mohtabi-Bonab, J.A. Szpunar, S.S. Razavi-Tusi, A comparative study of hydrogen induced cracking behavior in API 5 L ×60 and ×70 pipeline steels, Eng. Fail. Anal. 33 (2013) 163–175. [14] S. Almaaesab, A. Alrefaie, Failure analysis of severely corroded steam coil in water seal vessel, J. Fail. Anal. Prev. 15 (6) (2015) 782–788. [15] ASM Handbook, Vol. 13: Corrosion, 1987. [16] D.A. Jones, “Principles and Prevention of Corrosion” 2nd EdPrentice Hall 1996. [17] M. Javidi, M.R. Nematollahi, M.M. Lalehparvar, A. Ghassemi, Failure analysis of AISI 321 austenitic stainless steel water piping in a power plant, J. Fail. Anal. Prev. 16 (2016) 209–215.
Please cite this article as: A. Vazdirvanidis, et al., Corrosion investigation of stainless steel water pump components, Engineering Failure Analysis (2016), http://dx.doi.org/10.1016/j.engfailanal.2016.09.009
Contents lists available at ScienceDirect
Engineering Failure Analysis journal homepage: www.elsevier.com/locate/engfailanal
Corrosion investigation of stainless steel water pump components A. Vazdirvanidis, G. Pantazopoulos ⁎, A. Rikos ELKEME Hellenic Research Centre for Metals S.A., 56th km Athens-Lamia Highway, 32011 Oinofyta-Viotia, Greece
a r t i c l e
i n f o
Article history: Received 8 July 2016 Received in revised form 14 September 2016 Accepted 24 September 2016 Available online xxxx Keywords: Pitting corrosion Stainless steel Corrosion products
a b s t r a c t Two components of a water pump installed in a casting shop for recirculation of cooling water experienced severe and accelerated corrosion after two months in service. The received pieces of the water pump assembly were a shaft and a conical tube, which was used as connector with the impeller. The shaft exhibited circumferential pitting corrosion behavior in specific areas where it was in contact with another pump component. Light optical microscopy and Scanning Electron Microscopy coupled with Energy Dispersive X-ray Spectroscopy were mainly used as analytical techniques for corrosion process evaluation and for the identification of the morphology and chemical composition of corrosion products, in order to draw safe conclusions concerning the type of the corrosion and the respective root-source. The main findings of the investigation indicated that pitting corrosion was the dominant failure mechanism for both water pump components influenced by the presence of aggressive environmental conditions, characterized by the presence of chlorides and sulfates that accelerate corrosion process above a certain temperature range (T N 50–55 °C). © 2016 Elsevier Ltd. All rights reserved.
1. Introduction and background information The mostly known eight forms of corrosion were referred in a classical corrosion literature [1]. A summary of corrosion mechanisms occurred in chemical process industry is presented in Ref. [2]. Stress corrosion cracking (SCC) and pitting corrosion of stainless steels constitute a critical research field due to the extensive use of such components in special applications in manufacturing and chemical industry under harsh operating conditions [3]. The damaging role of chloride ions and their influence on stimulation of SCC mechanism in stainless steel components in process industry are addressed in various research works, see for instance Refs [3–6]. Transgranular and intergranular SCC were detected in case of naphtha hydrotreater furnace tubes [7]. The detrimental effects of corrosive species, such as Cl and S is underlined also, in case of localized corrosion of steel piping elbow in oil-gas separation system started from inorganic compounds contained in the crude oil [8]. The influence of corrosion environment in combination to fatigue related processes is reviewed in Ref [9]. Fatigue failure in rotating equipment is assisted by local stress concentration which is a crucial factor enhancing the risk of crack initiation [10]. Two components of a water pump installed in a casting shop for recirculation of cooling water experienced severe and accelerated corrosion after two months in service. The received components of the water pump assembly constituted a shaft and a conical tube, which worked as a connector with the impeller (Fig. 1). The shaft exhibited severe pitting corrosion in specific areas in connect with another pump component (Fig. 2).
⁎ Corresponding author. E-mail address: [email protected] (G. Pantazopoulos).
http://dx.doi.org/10.1016/j.engfailanal.2016.09.009 1350-6307/© 2016 Elsevier Ltd. All rights reserved.
Please cite this article as: A. Vazdirvanidis, et al., Corrosion investigation of stainless steel water pump components, Engineering Failure Analysis (2016), http://dx.doi.org/10.1016/j.engfailanal.2016.09.009
2
A. Vazdirvanidis et al. / Engineering Failure Analysis xxx (2016) xxx–xxx
Fig. 1. Images illustrating the components' assembly of the pump.
2. Experimental procedure Macroscopic observations of the failed components were performed with a high definition digital camera and a stereomicroscope. In order to reveal the main failure mechanisms, microscopic observations were carried out using scanning electron microscopy (SEM) equipped with an energy dispersive x-ray spectrometry (EDS) detector for elemental chemical analysis of selected areas. Cross-sections of the failed components were prepared using hot-mounting, wet grinding up to 1200 grit SiC paper and polishing with diamond and silica suspensions. Metallographic examination was conducted using an inverted optical microscope.
3. Results Two components of a water pump which experienced severe and accelerated corrosion after two months in operation. The chemical composition analysis by Optical Emission Spectroscopy showed that both materials match approximately to AISI 403 (UNS No. S40300) heat treatable stainless steel (Table 1). No internal defects, such as inclusions or other discontinuities that could be considered as preferential sites for corrosion initiation were found. The shaft exhibited significant circumferential pitting corrosion in specific areas that were in contact with another pump component (Fig. 3). Sections transverse to the axis of symmetry of the shaft were prepared in order to reveal the pits' morphology and investigate corrosion products elemental composition.
Fig. 2. Macrographs showing the corroded coupling and shaft parts of the pump.
Please cite this article as: A. Vazdirvanidis, et al., Corrosion investigation of stainless steel water pump components, Engineering Failure Analysis (2016), http://dx.doi.org/10.1016/j.engfailanal.2016.09.009
A. Vazdirvanidis et al. / Engineering Failure Analysis xxx (2016) xxx–xxx
3
Table 1 Chemical composition of shaft and coupling components (% wt.). Element/sample
C
Si
Mn
Cr
Ni
Mo
Shaft Conical tube (coupling)
0.18 0.19
0.37 0.37
0.55 0.55
12.46 12.55
0.17 0.27
0.05 0.06
Pitting corrosion extended around the entire examined circumference of the stainless steel shaft and maximum pit depth did not exceed 1 mm (Fig. 4). The main corrosion products consisted of Cr, Fe, Si oxides while S and Cl have been also detected (Figs. 5, 6). Similar findings were obtained concerning the pit floor (close to the metal interface), as well as the internal pits and the smaller pits (Figs. 7, 8). The conical tube (coupling) was sectioned on the rim parallel to the axis of symmetry. Optical microscopy observations showed that the rim of the tube was severely corroded with pits exceeding 5 mm in depth (Fig. 9). SEM/EDS analysis indicated that the corrosion products consisted mainly of Cr, Fe, Ca, Si and Mg (hydro)oxides (Fig. 10). Brittle scale deposits demonstrate severe cracking and high tendency of breaking off, causing large fragments that could cause clogging and erosion in circuit components leading to damage and unit function interruption (Fig. 10). No additional metallographic study was performed concerning stainless steel microstructure, at this stage of investigation, since the scale deposits analysis and degradation topology provided a strong indication of the influence of circulating water quality and operation conditions on the evolved corrosion processes. 4. Discussion and recommendations Hydrolysis of inorganic chloride salts produces HCl which results in pH reduction, creating aggressive corrosion conditions and leading to steel chemical attack and successive metal dissolution, see also Ref. [8]: CaCl2 þ H2 O→2HCl þ CaO
ð1Þ
The detrimental role of H2S aqueous solution was also highlighted in a case of steel absorber corrosion in a desulfurization plant [11]. The presence of sulfate reducing bacteria (SRB) led to the reduction of sulfates towards sulfides in an acidic environment [12]. Under low pH conditions (pH ~4), hydrolyses of FeS and sulfides take place and lead to the formation of H2S which promotes the generation of hydrogen, [Eqs. (2)–(4)], see also Ref. [12]. 2−
S
2þ
Fe
þ
−
þ H2 O→ HS þ OH −
−
þ
þ HS → H þ FeS −
H þ e → Had
ð2Þ ð3Þ ð4Þ
[where “ad” denotes adsorbed atom.] The presence of H2S in gas and oil industries produces atomic hydrogen which diffused-in and trapped in atomic scale imperfections, dislocations and non-metallic inclusions leading to hydrogen-induced-cracking (HIC) incidents in pipelines, see also [13]. In Ref. [14] steam coil corrosion was provoked by sulfuric and hydrochloric acid attack.
Fig. 3. Macrographs of the shaft and the circumferential pitting corrosion formation (images from the plant).
Please cite this article as: A. Vazdirvanidis, et al., Corrosion investigation of stainless steel water pump components, Engineering Failure Analysis (2016), http://dx.doi.org/10.1016/j.engfailanal.2016.09.009
4
A. Vazdirvanidis et al. / Engineering Failure Analysis xxx (2016) xxx–xxx
Fig. 4. Optical micrographs, transverse section showing pitting formation and depth on the shaft.
Scale formation in piping and rotating shafts has a protective effect against corrosion. Scaling susceptibility in terms CaCO3 precipitation can be assessed by the Langelier (LSI) and Ryznar Saturation Indices (RSI). The involved chemical reactions and Saturation Index (S.I.) calculations with respect to the corrosiveness of the cooling water are presented according to the following Eqs. (5)–(13): þ
H2 CO3 →H þ HCO3 −
þ
HCO3 →H þ CO3
− 0
2−
K
1
0
K
ð5Þ
2
CaðHCO3 Þ2 →CaCO3 ðsÞ þ CO2 ðgÞ þ H2 O
ð6Þ ð7Þ
The Langelier and Ryznar indices depended mainly on temperature and total alkalinity. These indices are defined as follows: LSI ¼ Langelier Saturation Index ¼ pH– pHs
ð8Þ
Fig. 5. SEM micrograph of the corrosion products of the shaft and corresponding EDS spectra showing Cr, Si oxides and S, Cl presence.
Please cite this article as: A. Vazdirvanidis, et al., Corrosion investigation of stainless steel water pump components, Engineering Failure Analysis (2016), http://dx.doi.org/10.1016/j.engfailanal.2016.09.009
A. Vazdirvanidis et al. / Engineering Failure Analysis xxx (2016) xxx–xxx
5
Fig. 6. Another area of interest. SEM micrograph of the corrosion products of the shaft and EDS spectra showing the presence of Cr, Si oxides and S and Cl.
RSI ¼ Ryznar Saturation Index ¼ 2pHs –pH 0 pHs ¼ Saturation pH ¼ pK 2 –pK sp þ pCa þ pAlk
K
0
2
h i h i h i h i h i 2− − 2− þ 2þ = HCO3 ; K sp ¼ Ca CO3 ¼ H CO3
h i þ Alkalinity ¼ H
tit
h i h i h i 2− − − þ HCO3 þ OH ¼ 2 CO3
ð9Þ ð10Þ
ð11Þ
ð12Þ
Fig. 7. SEM micrograph illustrating pit growth mechanism and EDS spectrum corresponding to the pit floor.
Please cite this article as: A. Vazdirvanidis, et al., Corrosion investigation of stainless steel water pump components, Engineering Failure Analysis (2016), http://dx.doi.org/10.1016/j.engfailanal.2016.09.009
6
A. Vazdirvanidis et al. / Engineering Failure Analysis xxx (2016) xxx–xxx
Fig. 8. SEM micrographs showing internal and isolated pits and respective EDS spectrum of the corrosion products.
The pHs (saturation pH) which is used mainly for index calculations can be calculated by the following equations: pHs ¼ ð9:3 þ A þ BÞ–ðC þ DÞ
ð13Þ
where: A. = [log(TDS) − 1] / 10 [TDS: Total Dissolved Solids (mg/L)]
Fig. 9. Optical micrographs, lower and higher magnification of the corrosion pits on the tube rim. Sectioning parallel to the tube axis of symmetry.
Please cite this article as: A. Vazdirvanidis, et al., Corrosion investigation of stainless steel water pump components, Engineering Failure Analysis (2016), http://dx.doi.org/10.1016/j.engfailanal.2016.09.009
A. Vazdirvanidis et al. / Engineering Failure Analysis xxx (2016) xxx–xxx
7
Fig. 10. SEM micrographs of the corrosion products of the tube and respective EDS spectra showing Cr, Fe, Ca, Si and Mg (hydro)oxides.
B. = −13.12 log[T(°C) + 273] + 34.55 C. = log[Ca2+ as CaCO3] − 0.4 D. = log [alkalinity as CaCO3], [CaCO3] = 2.5[Ca2+] Scaling susceptibility and corrosion protection/aggressiveness with respect to S.I. levels (LSI and RSI) is presented in Table 2 [15,16]. Proper water monitoring comprises also the S.I. determination for continuous health monitoring and water corrosion susceptibility. The above findings suggest that the accelerated corrosion of the water pump parts was likely caused by: 1. Contaminated water characterized by the presence of sulfates/sulfides and chlorides that resulted in severe chemical attack of the various pump components. This condition may be aggravated at higher operating temperatures, i.e. T N 50–55 °C. 2. High concentration of total suspended solids (TSS), such as silica based particles, leading to erosion and mechanical damage phenomena, that could enhance pitting corrosion. 3. Galvanic corrosion is an additional synergistic mechanism especially on the dissimilar metal contact locations. This is a rather generic hypothesis which is based on the metal-to-metal contacts between different pump components, such as shaft, impeller, bearings and connectors. 4. Crevice corrosion can be also considered as another contributing corrosion mechanism, especially in restricted areas (e.g. shaftcoupling assembly areas), where concentration gradients together with the accumulation of hard solid deposits, might also aggravate localized material degradation. Crevices are formed in connection areas of pump assembly components, where stagnant water conditions could result in condensation and in the formation of localized concentration cells. 5. Low pitting resistance of the steel. The pitting corrosion resistance number (PREN) can be used to select a more corrosion resistant alloy: PREN ¼ %Cr þ 3:3ð%Mo þ 0:5%WÞ þ X%N
ð14Þ
where, X = 16 for duplex stainless steels and 30 for austenitic stainless steels, see also [17].
Table 2 Tendency of water with respect to LSI and RSI values. Langelier saturation index (LSI)
Ryznar saturation index (RSI)
Water tendency
2.0 0.5 0 −0.5 −2
b4 5–6 6–6.5 6.5–7 N8
Heavy scale formation and non-aggressive Slight scale formation and mildly aggressive Balanced or at saturation of CaCO3 No scale formation and slightly aggressive Under-saturated and highly aggressive
Please cite this article as: A. Vazdirvanidis, et al., Corrosion investigation of stainless steel water pump components, Engineering Failure Analysis (2016), http://dx.doi.org/10.1016/j.engfailanal.2016.09.009
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Higher pitting resistance stainless steels with higher Cr, Mo and/or W contents can be utilized to minimize corrosion susceptibility during service.
Recommendations 1. Water monitoring concerning physicochemical parameters, such as pH, conductivity, total suspended solids (TSS), total dissolved solids (TDS), Cl, SO4 and operating temperature. 2. Use of physical/chemical processes for water cleaning and stabilize water quality, such as filtration, softening. 3. Implementation of regular “wash-off” cycles to clean and remove solid residues from pump components. 4. Selection and use of stainless steels with higher Cr and Mo content, offering higher Pitting Resistance Equivalence Number (PREN) could enhance the lifetime of stainless steel components rendering higher resistance against pitting corrosion. The UNS Nos. S42200 and/or S43400 stainless steel grades could stand as typical alternative suggestions of component material selections.
Acknowledgements The authors wish to express special thanks to Plant Production and Maintenance team for their assistance in background information and fruitful discussions.
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Please cite this article as: A. Vazdirvanidis, et al., Corrosion investigation of stainless steel water pump components, Engineering Failure Analysis (2016), http://dx.doi.org/10.1016/j.engfailanal.2016.09.009