Effect of coal slurry on the corrosion of coal-mine equipment

Mining Science and Technology(China)21(2011)413-417Contents lists available at Science Direct氵Mining Science and Technology( China)ELSEVIERjournalhomepagewww.elseviercom/locate/mstcEffect of coal slurry on the corrosion of coal-mine equipmentZhang Qi, Xie Jingxuan*. Zhao Wei, Bai Shasha, Zhong Shiteng, Chu ZhenfengKey laboratory of Coal Processing and Eficient Utilization, School of Chemical Engineering and Technology, China University of Mining and Technology, Xuzhou 221008, ChinaARTICLE INFOABSTRACTThe corrosion of coal mine equipment immersed in coal slurry is addressed. The corrosion of low carbonReceived 22 october 2010Received in revised form 18 November 2010steel samples immersed in coal slurries of different concentrations(80. 130, and 180 g/L)preparedAccepted 15 December 2010rom coals of different rank(long-flame coal, meager lean coal, and anthracite)and different granularityAvailable online 12 June 2011(0.25-0.5 mm. 0.074-0. 25 mm, and less than 0.074 mm particle size) was studied by the electrochemicalmethod of polarization curve measurement controlled potential sweeping and continuous scanning. Theresults show that the corrosion rate in an anthracite slurry where the coal has high coalification, is fargreater than corrosion in a long-flame or a meager lean coal slurry. Furthermore the corrosion current.polarization current, and corrosion rate of low carbon steel become larger, and the polarizability becomessmaller. as the coal particle size decreases. The same trend is seen as the concentration of the coal slurryo 2011 Published by Elsevier B V, on behalf of China University of Mining Technology1 Introduction2. ExperimentalEquipment corrosion has always been a key factor related2.1. Instruments and reagentsservice life in coal mining. Some equipment is immersed in slimewater for a long time and any low-carbon steel structure isThe electrochemical workstation(IM6e, Zahnereletrik Gmbhdestroyed by this corrosive medium. in addition, running equip- Co., KG), potentiometer(EL20, Shanghai). and conductivity meterment also suffers serious erosion due to abrasion between media (DDS]-308A, Shanghai)were used in this study a low carbon steeland equipment. For example, the flotation vanes of a coal washery (Q235)with 0. 14-0. 22 carbon content was used. The working elec-located in the Huaibei mining field suffered corrosion and erosion trode had an area of 7. 065 cm and a saturated calomel electrodeof a large area after only three months in operation. Furthermore, was used as the reference. Anthracite meager lean coal, and longthe coal slurry transportation pipelines were worn out. These prob- flame coal samples were from the Taixi, Xuzhou and Yulin mineslems created great difficulties in terms of maintenance and caused respectively. the chemical composition of the test solution comeshuge economic losses to the enterprisefrom water quality analysis of flotation feed and flotation tailingsFactors influencing electrochemical corrosion of metal are found in the Guobei coal washery. Its chemical components arediverse. Many publications address the characteristics of the metal given in Table 1and the corrosive medium including pH value, concentration ofanionic and cationic ions, oxygen content, and the distribution ofoxygen within the area suffering corrosion 1-4]. However, in the 22. Methodscoal industry corrosion of equipment is closely related to contactwith slime water containing complex ions and coal.First, the coal sample was crushed and screened into three dif-The effect of the coal on metal corrosion is the primary focus of ferent size ranges: 0. 25-0. 25 mm. 0.074-0 25 mm, and less thanthis paper. Preliminary studies on the corrosion of low carbon steel 0.074 mm. These granulated samples were used in making the coalimmersed in coal slurries of various concentrations prepared from slurry at concentrations of 80, 130, or 180 g/L using the test solucoals of different rank and particle size have been investigated by tion as the liquid medium.electrochemical methods. The goal is to provide useful informationThe low carbon steel was de-rusted, polished, and then cleanedconcerning the corrosion and protection of coal mining equipment. with distilled water, acetone, and then distilled water again. It wasthen put into the slime water for one hour while ensuring adequatecontact between the steel and the coal. After the system reached astable potential its free corrosion potential was measured using aCorresponding author. Tel +86 15996915259potentiometer. Theum and the stea-E-mailaddress:zhangqi@126.comUxie).dy-state polarizat中国煤化工nd anode wer1674-5264/s-see front matter o 2011 Published by Elsevier B.V. on behalf of China University of Mining & TeCNMHGdoi:10.1016mstc201105015414Q Zhang et aL/ Mining Science and Technology(China)21(2011)413-417Table 1Chemical components of the test solution.ComponentO Mg* Ca2+c(mg/L)3202501.16measured with a conductivity meter and an electrochemical work-station, separately. a curve showing corrosion current density versus scanning onog coordinate scale was automaticallyplotted. This curve, E vs log i, along with the slope of the polariza-tion curve and some electrochemical parameters, including corrosion potential and corrosion current, were automatically obtainedThe corrosion rate was derived from the corrosion current accordEscE(ing to Faraday's law. The tests were done at ambient temperature. fig 2. Polarization curves of low carbon steel in coal slurries: diferent ranks ofhe media will be indicated in this paper as: A: blank solutionBlong flame coal slurry: C: meager lean coal slurry: D: anthracitecoalification.slurry: E: slurry prepared with 0.25-0.5 mm anthracite: F: slurryprepared with 0.074-0.25 mm anthracite: G: slurry prepared with the anthracite slurry is 480 us cm, which corresponds to the big-<0.074 mm anthracite: L: 80 g/L anthracite slurry: M: 130 g/ gest corrosion rate of low carbon steel in it. Whereas the conduanthracite slurry: N: 180 g/L anthracite slurrytivity is about 0. 245 us/ cm in the other media, correspondingly.the steel corrosion rate is lower. Previous work showed that the3. Results and discussionconductivity of the coal changes with the metamorphic grade ofthe coal [5-7]. Moisture and more polar groups. in the low-rank3. 1. Eject of coalification on corrosion of low carbon steelcoal, as well as high porosity, result in the high conductivity. Asthe degree of coal metamorphism increases to the stage of fat coalCorrosion currents through low carbon steel immersed in ten or coking coal, the conductivity declines, because the number ofsteel corrosion is most serious in D, which has a higher degree of Anthracite, because of increased graphitization, has more aromaticcoalification. The corrosion current is 1.5-1.9 times greater comnature so the motion of free electrons expands leading to a sharppared to the other media. The corrosion currents in the C B, and rise in the conductivity of anthracite slurry. the relationship be-A media are about 0.16 mA/cm?' with small fluctuations. As illus- tween steel corrosion in a coal slurry and the conductivity of thetrated in Fig. 2. the steady-state polarization curves also display medium can be understood by thissimilar results. Compared to long flame coal or meager lean coal,The shapes of the polarization curves in Fig. 2 are approximatelythe anthracite has the greatest effect on low carbon steel. The polar- the same Polarization curves without a passive region, trans-pas-ization current is obviously higher for anthracite than for the oth- sive region, sudden changes of current, and obvious Tafel regioners. This is especially true for the cathodic branch, which shows indicate that the corrosion process is controlled by the diffusionhat the cathodic reaction in this medium is more reactive. More- of the depolarizing agents to the electrode surface. Study of corro-over, the small polarizability also indicates that low carbon steel sion in neutral aqueous solutions shows that the polarization prois easily corroded. The corrosion current density in the C medium cess is controlled by diffusion of oxygen [8-10is smaller than in the B medium, as shown by anodic polarizationFig. 2 also shows that the polarization process of low carbonThis demonstrates that the least corrosive of these media steel in anthracite slurry d is relatively slow and that the polariza-is C. Similarly, the corrosion rate in anthracite slurry D is more than tion current increases rapidly. However, in other media, polariza-1. 5 times greater than the rate in other media see Table 2tion current increases slowly with the change of potential. TheAs in Fig. 3a and b show the electrical conductivity of the difference between anodic and cathodic polarization currents incoal slurry has an obvious correlation with corrosion rate of anthracite slurry d is less than that seen in the other media, whichhe low carbon steel in the different media. The conductivity of illustrates that the reaction in this medium is much stronger. Sincecathodic depolarization of carbon steel in the coal slurry is mainly0.36r wZZ Different rank of coalificationcontrolled by diffusion of oxygen, and since the reduction ofs Different granularityoxygen is the major cathodic reaction, in the anthracite slurry0.32 D Different concentrationthe control of oxygen diffusion into the corrosion process is muchweaker than that seen in the other media. perhaps there are moreithways for oxygen reduction on the cathode or perhaps there aremore diffusion channels compared to other media.Coals with a high degree of coalification have a certain conductiv-024ty and relative electrode potential 11-13]. When metals contact婴with this kind of coal a potential difference can forms between themthat causes the metal to lose electrons and for an oxidation processto occur Electrons are transferred to the conductive coal and when0.16they meet oxygen there the oxygen depolarization reaction occursThese oxidation and reduction processes produce electrochemicalcells between the coal and the metal surface. hence it is no longerG L M N中国煤化工 emical cell on a metalpaths such as small rustholes. TheCNMH Ge for the serious corro-Flg. 1. Corrosion current density of low carbon steel in various coal slurries. sion is the nature or Coar conduction ratner than the migration andQ Zhang et aL/Mining Science and Technology( China)21(2011)413-41715Table 2Electrochemical parameters of low carbon steel immersed in different coal slurries.Ecor(mv)St(mv/decv(g/mh022315063395606480-243.30269-8097952.80.2575320.3083.220.2571150Note: K is the conductivity: Ecor the free corrosion potential; ioor the corrosion current: Sk the slope of cathodic polarization curve;; S, thelope of anodic polarization curve: and y the corrosion rate.30omal dll会System(a)Conductivity(b) Corrosion rate of low carbon steelFlg. 3. Different coal slurries.Ediffusion process of oxygen. Anthracite, owing to its high conductiv- Fig 5. Corrosion rates of low carbon steel: anthracite slurries of different particleity, can greatly accelerate the corrosion of low carbon steel im-mersed in a coal slurry.3. 2. Efects of coal particle size on the corrosion of low carbon steelFrom the point of view of electrochemical corrosion, oxygenreduction as a cathodic reaction during the metal corrosion hasthe diffusion and transmission process of oxygen as the electro-The electrochemical behavior of low carbon steel in coal slurries chemical controlling steps (9, 10). The corrosion rate is faster inprepared from Taixi anthracite of three different particle sizeranges,0.25-0.5 mm. 0.074-0.25 mm, and less than 0.074 mm is the medium with excellent oxygen access, thereforeHence, slurry from larger sized particles is expected to exhibitshown in Figs. 1. 4 and 5 as well as in Table 2. The conclusions more severe corrosion because the large coal particles provideare that the corrosion becomes more severe when the size of the more oxygen diffusion channels, which results in easier transmis-coal particle is smaller. The corrosion currents and corrosion ratession of oxygen. This obviously contradicts the results of the presentin the fine coal slurry are 1.2 times those in a coal slurry prepared study. therefore, there are more important factors leading to thefrom medium size particles. The current and corrosion rate are 1.4accelerated corrosion that have more influence than the depolatimes those in coal slurry prepared from the largest sized fractionThe anodic polarization curves in Fig. 4 show that the polarizationization of oxygen in this system.current rises as the coal particle size decreases. this indicates thatWhen immersed in the coal slurry the low carbon steel contactsthe coal at various points [10]. The potential difference betweenit is easier for low carbon steel to dissolve in the fine coal slurry. the coal and the steel causes the corrosion to first occur at the con-tact points and adjacent to them the base iron metal suffers ano-dic oxidation to produce Fe, which then causes corrosion pits. Apart of the Fe2- continues to be oxidized to give Fe304.porousFe0-OH and a small amount of Fe203. Some of the Fe 2*migratesthrough the conductive anthracite to other sites on the metasurface where it encounters oh and forms Fe0-oH. the cathodicprocess, on one hand, adsorbs electrons and induces oxygen depo-arization depending upon the diffusion and transmission of oxygen. On the other hand, electrons migrate to locations away fromthe metal surface by means of the conductive coal and magnetite.This initiates the reduction of Feo-OH3Fe0-OH +e- Fe3 O4+H20+OH-1.6Exc(vwhere there is go中国煤化工d to feo- oH byFlg 4. Polarization curves of low carbon steel in coal slurries: anthracite of4Fe3O4+02+6HYHCNMHG16Q Zhang et al/ Mining Science and Technology ( China)21(2011)413-417are typically unevenly distributed and the closer a contact point is,relative to the point of interest, the greater corrosion currents areand, thus, the more the metal loss will be. Current always tendsto flow along the path of least resistance so solution resistance doescause an"effective distance "effect 16). The greater the resistanceis, then the shorter the"effective distance"becomes. Thereforeincreased concentrations of coal in the slurry cause a blockade atthe interface so that current flow becomes more difficult and theeffective distance"shortens. Consequently, corrosion currentsbecome concentrated near the contact sites and form deep corro-sion trenches. This"localized corrosion"may lead to very seriousdamage.EScE (V)Moreover, there is a stagnant layer next to the electrode surfaceand mass transfer through this layer occurs by diffusion process.Fig 6. Polarization curves of low carbon steel: coal slurries having different The electro-migration of charged particles provides a motive forcefor mass transfer through the layer. a highly concentrated slurryhas a thick stagnant layer as a result of coal sorption so that oxygendiffusion from the outside to the electrode surface becomes morein summary, the controlling steps of theinclude electron transfer and Fe 2*transmission inreaction difficult[17]. This results in the formation of an oxygen concentra-gen diffusion. This explains the phenomenon ofto oxy- tion gradient, especially in the grooves near the contact points. It isalso easy for fine coal to deposit within the trenches these thingsexhibited by the steady-state polarization curves of the present facilitate the formation of an oxygen concentration cell that leadsto accelerated metal corrosion on the anodeThe smaller coal particle sizes provide more contact points andWhen the metal electrode surface is completely covered by fineso more corrosion pits will form. During the reaction process cor- coal particles the exchange current density from the electroderosion pits fill with a large number of fine coal particles. This reaction may be increased owing to changed electrode propertiesallows the base metal to directly contact the coal again leading thus enhancing the corrosion rate [18]to deepening of the corrosion pits. In addition, the coal in the pitsand the adjacent steel suffer contact corrosion again, resulting inthe dissolution of the surrounding metal. Finally, the corrosion 4 Conclusionslayers peel off, accelerating the dissolution of the steel. This isthe reason that anodic polarization currents in medium G are(1) Anthracite can greatly promote the corrosion of low carbonhigher than those in medium F, and much higher than those insteel because of its high conductivity. the corrosion rate ismedium E.about 1.7 times that seen in the other media. the slurriesMoreover, the contact between coal and steel becomes moreprepared from meager lean coal or long flame coal have littleintimate when the coal particles are smaller. this leads to a smallereffect on steel corrosion. there is little difference in corro.mpedance between them and, hence a smaller ohmic drop Corrosion current between the coal slurry and the blank solutionsion currents on the anode will then become greater and as a resultin these casesthe low carbon steel will suffer more severe corrosion(2)The slurry contacts the low carbon steel through points. TheSubstances that lead to coal formation may also lead to metalformation and development of corrosion pits result in accelcorrosion. The study of trace elements in coal has found that hav-erated steel corrosion. When the coal particle size decreasesing extremely fine coal particles leads to these elements being in ahe corrosion rate increases. The polarization current in astate of dispersion [14, 15]. when interacting with other compofine coal slurry is much greater than in a coal slurry preparednents of the coal these elements not only directly accelerate metalfrom medium or large sized particlecorrosion but also lower coal resistivity and enhance coal conduc(3)The effect of corrosion currents depends on an"effective dis-tivity. This can facilitate corrosion of low carbon steeltance". Highly concentrated coal slurries can shorten theeffective distance"which will greatly increase the localized3.3. Corrosion of low carbon steel in coal slurries of differentcorrosion rate resulting in an early failure of the equipment.concentrationIncreased concentration of the coal slurry causes corrosiof low carbon steel to become more severeCorrosion of low carbon steel in anthracite slurries of three dif.ferent concentrations(80, 130, and 180 g/ L) is shown in Figs. 1, 5and 6 and in Table 2. Fig. 1 shows that the corrosion current in a Acknowledgments180g/L slurry is larger than that in a 80 or 130 g/L slurry. Fig.5and Table 2 show that the corrosion rate of low carbon steel isThis work was subsidized by the National Natural Science Foun-about 2.75 g/(m h)in the 80, or 130 g/L slurry, but increases to dation of China for Innovative Research Group(No 50921002)and3.38g/(m?h)in the 180 g/L slurry. The steady-state polarization the Priority Academic Program Development of Jiangsu Highercurves displayed in Fig. 6 show that the polarization current inEducation Institutionssample N is much higher than in sample L or M.Therefore, highconcentration has a greater promotion effect on the corrosion. ReferencesThe cathodic polarization curves given in Fig. 6 show that cathodicpolarizability decreases as the slurry concentration increases. this [11 Melchers RE The effects of water pollution on the immersion corrosion of mildand low alloy steels Corros Sci 2007: 49(8indicates that a higher concentration slurry can result in easier [21 Soares CG, caelectrode reaction and more severe corrosion中国煤化工 nvironmental factors onThe contact corrosion produced between slurry and steel is sim- [3]Li FM.Yuan00951(9ilar to galvanic corrosion. The corrosion currents at the contact sitesCNMHGof steel bars embedded iniv Min Technol 2008: 37(4): 565-9 lin Chinese).Q Zhang et aL/Mining Science and Technology( China)21 (2011)413-417417cmmm别比配 dies s tee in expuencm ntal sted of s ice12m乙m妞 hen T Corrosion behavior ofcarbon steel in two-ph201052(10):3235-42ot2001;13(2)116-8I5] Kang JN. Research on relationship between coal conductivity and ground 113) Sun z wang wY. Chen HL Action of coal on metal corrosion in coalstress. J Jiaozuo Inst Technol 2005: 24(6): 430-3 [ in ChineChina Univ Min Technol 1998: 9(1): 27.[61 Zhang XM. Physical properties of coal. Inner Mongolia Coal Econ 2006(4): 70-1[14] Potgieter JH, Olubambi PA, Cornish L Machio CN, Sherif EM. Influence[7 Rimmer SM, Yoksoulian LE, Hower JC. Arof an intruded coal. I: Effectstainless steels. Corros Sci 2008: 50(9): 2572-9of contact metamorphism onSpringfield(no 5) [15]Maslennikov A, Bessonow A Peretroukhine V. Budanova N. Gedgovd K, Bulatownnsylvanian)coal. IllInois Basin. Int JCeG. TsIvadze A, Delegard C Uranium and U-Zr and U-Ru alloy corrosion rates in[8] Soosalprakasam IR, Veawab A. Corrosion and polarization behavior of carbonthe transpassive state. J Alloy Compd 2007: 444-445: 345-51steel in MEA-based COz capture process. Int J Greenhouse Gas Control [16] Lin YZ, Wang D). Corrosion and pnnciples of corrosion control. Beijing: China008:2(4):553-62etrochemical Press: 2007 in Chinesel[9] Yamashita M, Nagano H, Oriani RA. Dependence of corrosion potential and [171 Velasco JGOn the dependence of the Nemst diffusion layer thickness oncorrosion rate of a low-alloy steel upon depth of aqueous solution. Corros Scipotential and sweep rate for reversible and of the thickness of the chargeansfer layer for irreversible processes studied by application of the lino] Hussain RR, Ishida T Influence of connectivity of concrete pores and associatedpotential sweep method. Electrochim Acta 2006: 51(14): 2971-6diffusion of oxygen on corrosion of steel under high humidity. Co[18] Harinipriya S. Sangaranarayanan MV. Estimation of exchange currentater2010246):1014-9density for ferricreaction at electrode surfaces--influence of ionic111] Sun Z, Sun L Ou XM, Fu RL Corrosion of metals in coal with electricaldsorption. J Colloid Interface Sci 2004: 273(1):conductivity. J China Coal Soc 1994; 19(6): 605-10 (in Chinese].247-55中国煤化工CNMHG