A typical hot concentrated alkaline corrosion environment exists in alumina metallurgical industry, so that steel materials with outstanding alkaline corrosion resistance are strongly demanded for its processing equipment. In this paper, the corrosion resistance of two kinds of martensitic cast steels containing chromium in static 303g/L NaOH alkaline solution at 85℃ was studied through polarization and potential-time curves, corrosion weight loss and corrosion morphology analysis. Experimental results showed that protection effect by passive film of cast steel containing Cr was temporary. The low carbon steel without Cr content also exhibited chemical passivity in the same solution. The corrosion mode of the tested Cr-containing cast steel was composed of active dissolving corrosion and caustic embrittlement cracking. Dissolving corrosion was the primary mechanism for the induced weight loss, while severe caustic embrittlement cracking was secondary. With the increase of chromium content in the cast steel, the tendency of the caustic embrittlement cracking decreased, while the active dissolving corrosion increased

LI Wei

LIU Jun-quan

TU Xiao-hui3

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Research & DevelopmentFebruary 2007001Cast steels containing Cr, a passive element, are commonlyused as corrosion-resistant materials in acid, neutral anddiluted alkaline solutions. The function of the Cr element in thehot concentrated alkaline medium is interesting, but reports onthe corrosion behavior of Cr-containing cast steels in suchconditions were limited. Studies showed that anode dissolvingcorrosion could easily occur on iron and steel materials in hotconcentrated alkaline at a temperature above 80ņ [1-3], andcorrosion rates accelerated with the increasing of temperature and/or concentration of alkali [4, 5]. Contrary opinions existed over theeffects of Cr content on the weight loss of cast iron and steel in thehot concentrated alkaline corrosion. Some research [6] reported thepotential of Cr in the hot concentrated alkaline corrosion was morepositive than that of Fe, while other studies [7] reported the stabilityof Cr in alkaline solution is worse than that of Fe. Causticembrittlement cracking is a typical phenomenon in the hotconcentrated alkaline irons and steels for corrosion. Althoughsome achievements have been obtained on the causticembrittlement cracking of ferritic and austenitic steels, the effectsof Cr on the caustic embrittlement cracking need furtherCorrosion of two kinds of cast steelscontaining chromium in hotconcentrated alkaline*LI Wei1, LIU Jun-quan2, TU Xiao-hui3(1.Department of Materials Science and Engineering, Jinan University, Guangzhou 510632, China; 2.College of Industrial Equipment andControl Engineering, South China University of Technology, Guangzhou 510640, China; 3.Guangzhou Research Institute of Non-ferrousMetals, Guangzhou 510650, Guangdong, China)Abstract:  A typical hot concentrated alkaline corrosion environment exists in alumina metallurgical industry, sothat steel materials with outstanding alkaline corrosion resistance are strongly demanded for its processing equipment.In this paper, the corrosion resistance of two kinds of martensitic cast steels containing chromium in static 303g/LNaOH alkaline solution at 85ņ was studied through polarization and potential-time curves, corrosion weight lossand corrosion morphology analysis. Experimental results showed that protection effect by passive film of cast steelcontaining Cr was temporary. The low carbon steel without Cr content also exhibited chemical passivity in the samesolution. The corrosion mode of the tested Cr-containing cast steel was composed of active dissolving corrosion andcaustic embrittlement cracking. Dissolving corrosion was the primary mechanism for the induced weight loss, whilesevere caustic embrittlement cracking was secondary. With the increase of chromium content in the cast steel, thetendency of the caustic embrittlement cracking decreased, while the active dissolving corrosion increased.Key words: cast steel containing Cr; caustic embrittlement cracking; alkaline corrosionCLC number: TG142.71                        Document Code: A                        Article ID: 1672-6421(2007)01-001-04investigation. It has been reported that the Cr addition reducesthe sensitivity of the caustic embrittlement cracking for lowcarbon ferritic steel [8], and enhances fracture resistance of causticembrittlement cracking for austenitic stainless steel[9]. However,no literature could be found on the study of caustic embrittlementcracking of martensitic cast steels in hot concentrated alkalinesolutions, especially the effect of Cr content on causticembrittlement cracking of high strength martensitic alloys. Thisis just the background of the present research.The typical hot concentrated alkaline environment can be foundin alumina metallurgical processing where metals encounter bothcorrosion and erosion. The key problem for investigation iswhether the martensitic cast steels containing Cr can resist theenvironmental attack, and can be used as corrosion resistantmaterials for contacting the solution. To obtain useful data,corrosive environment of the alumina production process underBayer method was referred in this paper. Several types of Cr-containing martensitic cast steels were immersed in the hotconcentrated alkaline solution to evaluate their corrosionbehaviors/performances. In particular, the effect of Cr contenton weight loss and caustic embrittlement cracking wasinvestigated and discussed.1 Experimental condition and methodThe chemical compositions of three kinds of steels used in thestudy are shown in Table 1. The specimens of cast steel 20Cr5Male, born in 1963, Doctor candidate, professor. Researchinterests:  wear resistant and corrosion-resistant materials and itsapplication.E-mail: lwxasn@sohu.comReceived date: 2006-07-03;  Accepted date: 2006-10-29*LI WeiCHINA FOUNDRYVol.4 No.1002and 20Cr11 were taken from Y-block castings and then heattreated to obtain martensitic matrix. The samples in steel 10 (or10# steel) were cut from a rolled plate. The same preparationprocess was used for the specimens containing Cr, so that theirstress condition could be controlled to the same or similar level.Microstructure and macro hardness of these specimens are listedin Table 2.The 303g/L NaOH solution made of chemical grade reagentand deionized water was used in measurements on corrosionrate and electrochemistry properties. The temperature of thesolution was maintained at 85ņ by a thermostat water tank.Specimens for the measurement of the static immersion corrosionrate were sized to 20 mmġ20 mmġ8 mm, with all the samplesurfaces polished. Three samples of each material were carried/tested, and the specimens from the same material were placed inthe same container. The contact between specimens and theirglass epoxy supporters was line-contact, and the ratio betweenthe volume of test solution and specimen surface area was 20mL/cm2. Test durations (or immersion periods) were set at 14,62 and 230 h, respectively. After corrosion test, specimens werewiped with rubber or soft cloth to remove the corrosion products,washed with water and placed into ethanol immediately. Afterhaving dried with warm air, the specimens were placed into adrier (storage), and their weights were measured after cooling.The corrosion rate was defined as the average weight loss rateof the three samples.Cylinder specimens with diameter of 10 mm were used for themeasurement of potential-time curve in static immersion. Oneend of the cylinder specimens was polished to mirror surface-finish and taken as a working electrode. The other end was weldedto a copper cable that would act as electrode line. All surfacesexcept the working (or test) surface were coated with epoxy,and after the resin setting, test specimens were covered with asilicon-rubber tube of diameter of 9 mm, to avoid these surfacescontacting with the alkaline solution. These samples wereimmersed continually in the testing medium at 85ņ. Theirpotentials were recorded everyday by using Hg/HgO/NaOH(1N)as the reference electrode. In this paper, all potentials are inreference to this electrode unless otherwise mentioned.The working and reference electrodes used for anodicpolarization curves were the same as that for the potential-timecurve test. Graphite counter electrode was used, and the positivesweep (rate=10 mV/min) from rest potential of working electrodestarted after 20 minutes immersion.Finally, corrosion morphology of cast steel containing Cr afterstatic immersing test was investigated with scanning electronicmicroscopy (SEM).2 Results and discussionFigure 1 shows anodic polarization curves of 20Cr5, 20Cr11 andsteel 10 in the testing solution. The difference in the shape/characteristics of the anodic polarization curves indicated a greatinfluence of Cr content on the corrosion process. The curve ofsteel 10 was a typical polarization curve of chemical passivesystem. The rest potential of the steel 10 was about -480 mV,and the current density increased gradually when the rest potentialincreased to -100 mV, and the current density kept unchangedin the range from -100 mV to 500 mV. This suggested that thecorrosion of the steel was controlled by anodic passivity. Thebehavior of steel 10 could be explained by potential-pHequilibrium diagram for the system of Fe-H2O [10]. The pH of thetesting solution was between 14 and 15. At this pH value, thestable materials in the system are Fe, HFeO2- and Fe2O3 in orderwhen the potential changes from negative to positive direction.With the potential was more positive, the system turned into over-passivation. In a wide range of the potential from -750 mV to250 mV (vs. standard hydrogen electrode), the stable materialwas Fe2O3. The RP of steel 10 was -480mV, which was locatedin the stable range of Fe2O3. Fe2O3 film was formed on the surfaceof steel 10 at this potential level. This film resulted in thecorrosion resistance of the steel in the testing solution.Table 2  Microstructure and hardness of test materialsMaterialsSteel 1020Cr520Cr11MicostructureFM+ArM+ArHRC-4444Note: M-martensite, Ar-retained austenite, F-ferriteTable 1  Chemical composition of test specimens, wt. %C0.100.220.22Cr-4.6310.69Ni-0.820.93Si0.130.730.75Mn0.470.920.78Mo-0.260.10Materials20Cr520Cr11P0.0200.0350.030S0.0100.0200.040Steel 10(10# Steel)The anodic polarization curves of 20Cr5,20Cr11 and steel 10Fig. 1By a comparison between the Fig. 1 and the previous work ofthe present author (Fig. 3 in reference [11]), it could be foundthat the polarization curves of 20Cr11 and Cr26 iron weresimilar. Thus, the polarization curve of the 20Cr11 may be thepolarization curve of film. The potential curve of 20Cr5 wassimilar to that of like naked metal. Similar to the Cr13 andCr26 irons [11], the two types of the cast steel containing Crexhibited active dissolution by their anodic polarization curves.Research & DevelopmentFebruary 2007003This fact showed again that the Cr addition enabled iron-basedalloys to appear electrochemical activity in concentratedalkaline solution. Referring to the potential-pH equilibriumdiagram for Cr-H2O [10], the element Cr actively dissolvedforming CrO33- ion in high pH area and when the potential waslower than -300 mV (vs. standard hydrogen electrode). Thisbehavior might be induced in the Fe based alloys when Cr wasadded. The curve shape, especially the characteristic of activedissolution of alloy observed in the experiment should beattributed by the Cr dissolution.Figure 2 shows the potential-time curves of steel 10, 20Cr5and 20Cr11. The potential of steel 10 was from -200 mV to-500 mV during the immersion, i.e. the electrode was always inpassivity and Fe formed Fe2O3. On the other hand, the potentialsof 20Cr5 and 20Cr11 varied greatly during the immersion, whichmeans that surfaces of the two electrodes also varied greatly. Itshould be noted that when potential was located at about -1 000mV, there existed not only Cr dissolution but also Fe dissolutionto form HFeO2- (referring to potential-pH equilibrium diagramfor Fe-H2O). The potential variation of the steel containing Crwith the immersion time was similar to those two types of thecast iron [11], and therefore, the corrosion process could also bedivided into three stages. The first stage was passive filmdissolving. Because of the high Cr content in the steels, passivefilms were formed in atmosphere and their thickness was relatedto the Cr content. When the passive film was complete, arelatively high self rest potential of -400mV to -200mV, closeto the equilibrium potential of partial cathode reaction, had beenreached for the specimens in the hot concentrated alkaline of thepresent test, leading to that the film dissolved at a low rate andwas un-repairable by itself. The steel 20Cr11 with high Cr contenthad a relatively thick film that required certain time for itsdissolution. In this case, the time from which the electrodepotential was -400 to -200 mV was near 70 h. The film of steel20Cr5 was very thin, so that it dissolved in a very short time,with exposition of the metal and decreasing the electrode potentialto about -1 000 mV. The second stage was the exposed surfacecorrosion. In this stage, metal matrix dissolved. The cathodicprocess was a depolarization of oxygen and water with a highrate, as a result of decreased potential to about -1 000 mV. Thedifference of time duration in the second stage existed betweenthe two types of cast steels 20Cr11 and 20Cr5. After the sampleswere covered with the corrosion products to a certain degree,the potential rose from about -1 000 mV to about -400 mV andthe corrosion went into the third stage, in which the corrosionoccurred under the corrosion products. In this stage, the restpotential of the cast steel fluctuated in a range from -450 mV to-300 mV and the corrosion rate was obviously lower than thatof the second stage.Table 3 shows the weight loss of steel 10, 20Cr5 and 20Cr11 inthe testing solution. The weight loss increased with the increaseof the Cr content in the three time durations, which was consistentwith the results of polarization curve. From Table 3 it can beseen that the weight loss was different in the three time durations,corresponding to the location of the potential curve. If a materialin the immersion corrosion testing had a long time when itspotential was located at about -1 000 mV, its weight loss valuewas large and vice-versa (see Fig. 2). The phenomenon wassimilar to that for the Cr13 and Cr26 cast irons [11].Figure 3 and 4 are appearances of the cast steel 20Cr5 and20Cr11 after the immersion respectively. Two main corrosioncharacteristics may be seen on the corroded specimens. First,the surfaces of the sample were rough after the corrosion andthere were several etching pits on them. Especially on the surfaceof 20Cr11, etching pits appeared in a fairly large area (see Fig.4). The corrosion pit damages were resulted from active corrosionduring the immersion. Second, caustic embrittlement crackinginduced cracks/fracture obviously existed in both steels,indicating that stress corrosion cracking occurred. The crackingThe electrode potential - time relationshipof steel 10, 20Cr5 and 20Cr11Fig. 2Table 3 The weight loss of steel 10, 20Cr5 and20Cr11 in the testing solution, g/m2Ą Ą Ą Ą ĄhMaterialsSteel 1020 Cr520Cr1114 h0.07300.08430.129062 h0.03910.09070.1400230 h0.03160.07850.1205Fig. 3 Morphology of the 20Cr5Fig. 4 Morphology of the 20Cr11CHINA FOUNDRYVol.4 No.1004tendency of 20Cr11 with higher Cr content was weaker thanthat of 20Cr5. After immersion corrosion, crack and small etchingpits appeared on 20Cr11 surface, while coarse cracks resultedon 20Cr5 surface (see Fig. 3).Although the tendency of caustic embrittlement cracking ofthe 20Cr11 was lower than that of the 20Cr5, the corrosion rateof 20Cr11 was high, indicating that the weight loss caused bydissolving corrosion was higher than that by the causticembrittlement cracking. The findings also indicated that anodicdissolving corrosion was a main cause for weight loss of thesteels and the caustic embrittlement cracking extensivelyfractured the material matrix. With increase of Cr content, weightloss increased but the caustic embrittlement cracking tendencydecreased.3 Conclusions(1) The protection effect by passive film of martensitic caststeel containing Cr when immersed statically in 85ņ, 303g/LNaOH solution was temporary. The low carbon steel without Crexhibited chemical passivity in the same solution.(2) The corrosion of the martensitic cast steel in the hotconcentrated alkaline included active dissolving corrosion andcaustic embrittlement cracking. The active dissolving corrosionwas the main cause for the weight loss, and the serious causticembrittlement cracking fractured the metal matrix.(3) The effect of Cr content on corrosion of the cast steel in thesolution was remarkable. 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(in Chinese)[1][2][3][4][5][6][7][8][9][10][11]ReferencesFoundation item: The item is sponsored by the Foundation of China National Nonferrous Metals Industry Corporation (96-124).

Corrosion of two kinds of cast steels containing chromium in hot concentrated alkaline

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