Type of the Paper (Article

corrosion and materials degradation

Article Corrosion of Stainless Steel by Urea at High Temperature

Anastasiia Galakhova 1 , Fabian Kadisch 1, Gregor Mori 1,*, Susanne Heyder 2, Helmut Wieser 2, Bernhard Sartory 3 and Simon Burger 4

1 Chair of General and Analytical Chemistry, Montanuniversität Leoben, 8700 Leoben, Austria; [email protected] (A.G.); [email protected] (F.K.) 2 Faculty of Mechanical and Process Engineering, Augsburg University of Applied Sciences, 86161 Augsburg, Germany; [email protected] (S.H.); [email protected] (H.W.) 3 Materials Center Leoben Forschung GmbH, 8700 Leoben, Austria; [email protected] 4 Faurecia Emissions Control Technologies, Germany GmbH, 86154 Augsburg, Germany; [email protected] * Correspondence: [email protected]; Tel.: +43-664-923-7315

Abstract: The corrosion mechanism of stainless steel caused by high temperature decomposition of aqueous urea solution has been investigated. The relationship between aqueous urea solution, its thermal decomposition products and the corrosion mechanism of stainless steel is studied by FTIR spectroscopy, SEM and stereo microscopy. The corroded steel samples, together with deposits, were obtained from the injection of aqueous urea solution on the steel plate at high temperatures. Uniform corrosion underneath the deposits was proposed as the main driver for corrosion of the steel samples. At the crevices, corrosion due to the used geometry and due to high temperature cycling could play an acceleration role as well. Keywords: stainless steel; urea; biuret; cyanuric acid; ammelide; melamine; uniform corrosion Citation: Galakhova, A.; Kadisch, F.; Mori, G.; Heyder, S.; Wieser, H.; Sartory, B.; Burger, S. Corrosion of Stainless Steel by Urea at High 1. Introduction Temperature. Corros. Mater. Degrad. The corrosion of stainless steel by hot urea solution has not been largely studied; on 2021, 2, 461–473. https://doi.org/ the one hand, this may be due to the limited industrial demand, on the other hand, urea 10.3390/cmd2030024 itself is considered as non-corrosive for stainless steel, while some of its decomposition products at high temperature may lead to corrosion [1–3]. Academic Editor: Geoffrey D. Will Research work on corrosion involving urea mainly belongs to urea production, the fertilizer industry, and selective catalytic reduction (SCR) technology [2,4]. For example, Received: 27 July 2021 during the manufacturing of urea under high pressure, the most critical intermediate Accepted: 26 August 2021 Published: 30 August 2021 step leading to corrosion is the formation of ammonium carbonate by-product [2,5]. In the fertilizer industry, urea is used as a source of nitrogen in the products, causing some

Publisher’s Note: MDPI stays neutral corrosion problems [2,6]. In SCR technology, urea is used as a source of ammonia to reduce with regard to jurisdictional claims in the amount of NOx exhaust gases in automotive systems. The corrosion mechanism in published maps and institutional afﬁl- urea-related technology was explained in several ways; one such way was cyclic oxidation iations. caused by thermal cycling [7], and another—external corrosion by road salts [8,9]. Addition- ally, the effect of acidic condensate with chlorides and active carbon at low temperatures was mentioned [2,10,11]. Besides this, there are studies on melamine, ammonia, nitrogen and g-C3N4 (all of them exist during urea decomposition process) reactions with metal oxides, which attract attention as nitrifying and carburization reagents [12–14]. However, Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. more generally, it is concluded that the cracking and fracturing of metal is related to urea This article is an open access article decomposition products and intergranular attack [2,4,14–18]. By many scientists, an inter- distributed under the terms and granular corrosion mechanism, sometimes attributed to chromium depletion, and nitride conditions of the Creative Commons precipitation at grain boundaries, and sometimes without any indication of decrease or Attribution (CC BY) license (https:// increase in the amount of chromium at grain boundaries, was described [4,19,20]. ◦ creativecommons.org/licenses/by/ During the desired decomposition of urea at temperatures above 130 C, some unde- 4.0/). sired parallel and equilibrium intermediates and by-products in liquid, solid and gaseous

Corros. Mater. Degrad. 2021, 2, 461–473. https://doi.org/10.3390/cmd2030024 https://www.mdpi.com/journal/cmd Corros. Mater. Degrad. 2021, 2, FOR PEER REVIEW 2

Corros. Mater. Degrad. 2021, 2 462 During the desired decomposition of urea at temperatures above 130 °C, some unde- sired parallel and equilibrium intermediates and by-products in liquid, solid and gaseous form are producedform [21]. are produced From the [literature21]. From data, the literature more than data, 23 morepossible than reactions, 23 possible includ- reactions, including ing urea and itsurea numerous and its numerousby-products, by-products, biuret, cyanuric biuret, acid, cyanuric ammelide, acid, ammelide, ammeline, ammeline, mel- melamine amine and others,and exist others, [22]. exist In the [22 whole]. In the reaction whole scheme, reaction isocyanic scheme, acid isocyanic has been acid found has been found to

to play a majorplay role a[22,23]. major roleNonetheless, [22,23]. Nonetheless,it evaporates itat evaporates23.5 °C, reactions at 23.5 occurring◦C, reactions at occurring at temperatures exceedingtemperatures 133 exceeding°C were fo 133rmulated◦C were in the formulated condensed in thephase condensed [24]. phase [24]. At room temperature,At room urea temperature, is a white urea crysta islline a white substance. crystalline At 133 substance. °C, urea At starts 133 ◦toC, urea starts to melt and, in themelt range and, of in140–180 the range °C, ofit grad 140–180ually◦C, decomposes it gradually to decomposes isocyanic acid to isocyanic(H-NCO acid (H-NCO 3 (l [23]/g [24]) and(l [ 23ammonia]/g [24])) (NH and ammoniag [23]). Obtained (NH3 g isocyanic [23]). Obtained acid slowly isocyanic decomposes acid slowly with decomposes with 3 2 a catalyst into NHa catalyst and CO into. NHIf heating3 and COis not2. If very heating intensive, is not verythe highly intensive, reactive the highly isocyanic reactive isocyanic acid may formacid with may undecomposed form with undecomposed urea biuret or ureatriuret biuret with orammonia triuret with [24]. ammoniaAccording [24 ]. According + − + − to another theory,to another biuret and theory, triuret biuret are andformed triuret from are ions formed NH4 from and OCN ions NH[24,25],4 and a so- OCN [24,25], a called self-recombiso-callednation self-recombination of urea. of urea. Some theories Someabout theoriesphase transformations about phase transformations of biuret in the of range biuret ofin 190 the and range 250 of°C 190 and 250 ◦C exist. Accordingexist. to [24], According urea and to biuret [24], urea form and an biureteutectic form mixture, an eutectic where mixture, biuret has where two biuret has two melting points.melting At 193 points.°C, biuret At 193starts◦C, to biuret melt and starts decompose. to melt and At decompose. around 210 At °C, around the 210 ◦C, the decompositiondecomposition slows down, where slows biuret down, beco wheremes biuret a solid becomes and, presumably, a solid and, there presumably, is no there is no longer liquid urealonger present. liquid At urea this present. stage, the At thissolid stage, triuret the is solidformed, triuret which is formed, then reacts which then reacts further to formfurther solid deposits to form of solid cyanuric deposits acid of and cyanuric ammelide. acid and Triuret ammelide. is known Triuret as a highly is known as a highly unstable substanceunstable (stable substance up to 192 (stable °C, but up toliterature 192 ◦C, data but literatureare insufficient). data are As insufﬁcient). seen for As seen for triuret, one cannottriuret, look one at the cannot decomposition look at the decompositionof one substance of independently. one substance independently. One always One always has to keep in mindhas to the keep thermodynamic in mind the thermodynamic ensemble consisting ensemble of isocya consistingnic acid, of isocyanicurea, biuret acid, urea, biuret and triuret [24].and At triuret230 °C, [24 the]. Atsecond 230 ◦ decompositionC, the second decomposition step of biuret takes step ofplace, biuret where takes place, where biuret becomesbiuret liquid becomes again [22–24]. liquid again [22–24]. There are also Theresome theories are also someabout theoriesthe decomposition about the decomposition of cyanuric acid. of cyanuricThus, solid acid. Thus, solid cyanuric acid maycyanuric transfer acid directly may transfer into the directlygaseous intophase the during gaseous evaporation phase during [24] or evaporation gas- [24] or eous cyanic acidgaseous [23]. cyanic acid [23]. The remainingThe solid remaining substances, solid ammelide substances,, ammeline, ammelide, and melamine, ammeline, gradually and melamine, de- gradually ◦ compose by numerousdecompose reactions by numerous at higher reactions temperatures, at higher around temperatures, 360–400 °C around [24]. For 360–400 ex- C[24]. For ample, melamineexample, forms during melamine heating forms the during following heating products: the following melam, melem, products: melon melam, and melem, melon graphic carbonand nitride graphic g-C3N4 carbon [12]. nitride g-C3N4 [12]. The overall reactionThe overall scheme reaction (Figure scheme1) is much (Figure simpler1) is than much described, simpler but than it is described, nec- but it is essary to note thatnecessary the reaction to note sequen that thece reactionis not completely sequence understood is not completely yet. understood yet.

Figure 1. SchemeFigure of 1.aqueousScheme urea of aqueous decomposition urea decomposition under heating under [22–24]. heating [22–24].

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InIn thisthis study,study, wewe investigateinvestigate whichwhich step(s)step(s) of the urea decomposition decomposition process process plays plays a arole role in in the the corrosion corrosion of of stainless stainless steel. steel. For For this this purpose, purpose, we we analyse analyse all allcorresponding corresponding de- decompositioncomposition products, products, which which exist exist in the in solid, the solid, liquid liquid and gaseous and gaseous state and state correlate and corre- with latetemperature with temperature and pH value and at pH which value they at which may le theyad to mayuniform lead corrosion. to uniform To corrosion. test the chem- To testical thecomposition chemical compositionof solid deposits, of solid we use deposits, the spectroscopic we use the spectroscopicmethod FTIR (Fourier-trans- method FTIR (Fourier-transformform infrared spectroscopy). infrared spectroscopy). While for corrode Whiled steel for corrodedsamples, we steel do samples, a visual microscopic we do a vi- sualexamination microscopic of the examination attacked area of the by attacked SEM (s areacanning by SEM electron (scanning micros electroncopy). Additionally, microscopy). Additionally,the presence theof precipitates presence of on precipitates grain boundaries on grain is boundaries investigated is investigated to verify the to origin verify of the in- origintergranular of intergranular corrosion by corrosion nitridation by nitridation or carburization. or carburization.

2.2. MaterialsMaterials andand MethodsMethods 2.1. Materials 2.1. Materials The corrosion experiment was conducted with a sample made of the highly alloyed The corrosion experiment was conducted with a sample made of the highly alloyed ferritic grade 1.4509. The microstructure is purely ferritic with a ﬁne grain size between 10 ferritic grade 1.4509. The microstructure is purely ferritic with a fine grain size between and 30 µm. The chemical composition is shown in Table1. 10 and 30 µm. The chemical composition is shown in Table 1. Table 1. Chemical composition of steel sample. All data in mass percentage. Table 1. Chemical composition of steel sample. All data in mass percentage. EN AISI Fe C Si Mn Cr Ni Mo Nb Ti P S EN AISI Fe C Si Mn Cr Ni Mo Nb Ti P S 1.45091.4509 441 441 Bal. Bal. 0.0170.017 1.0 1.0 1.0 17.99017.990 0.233 0.233 0.075 0.075 0.391 0.391 0.115 0.115 0.040 0.0150.015

2.2.2.2. CorrosionCorrosion TestingTesting TheThe corroded corroded steel steel sample, sample, together together with with deposits, deposits, was obtainedwas obtained from anfrom experimental an experi- testmental bench test where bench the where injection the ofinjection aqueous of ureaaqueous solution urea (32.5% solution urea (32.5% and 67.5% urea deionizedand 67.5% water,deionized AdBlue, water, AVIA AdBlue, AG, AVIA München, AG, Germany)München, Germany) on the steel on plate the steel at high plate temperatures at high tem- forperatures different for times different took times place. took The place. test track The andtest track sample and are sample shown are in shown Figure 2in. TheFigure gas 2. (air)The massgas (air) ﬂow mass and temperature flow and temperature together with toge solutionther with injection solution rate injection and frequency rate and can fre- be varied.quency As can can be bevaried. seen inAs the can scheme, be seen thein the solution scheme, injector the solution is located injector right is in located front of right the sample.in front Theof the area sample. with aThe sample area iswith covered a sample with is a covered high-temperature with a high-temperature resistant glass windowresistant toglass allow window for the to visual allow observation for the visual of theobservation experiment. of the experiment.

FigureFigure 2. 2.Schematic Schematic illustration:illustration: 1—test1—test track, 2—test sample, 3—inj 3—injectionection system, system, 4—monitoring 4—monitoring window,window, 5—mass 5—mass ﬂow flow direction. direction.

InIn Figure Figure3 ,3, on on the the left left side side a a graphical graphical representation representation of of the the test test cycle cycle is is shown. shown. The The durationduration ofof oneone cyclecycle isis oneone hour.hour. After After estimation, estimation, a a total total test test time time of of 100 100 h h (meaning (meaning 100 100 cycles)cycles) waswas specifiedspecified forfor thethe corrosioncorrosion experiment.experiment. InIn FigureFigure3 ,3, on on the the right right side side the the test test specimenspecimen duringduring thethe experimentexperiment isis shown.shown. InIn thethered red highlighted highlightedarea, area, a a sample sample surface surface duringduring thethe highhigh temperaturetemperature phasephase (600 (600◦ °C)C) is is shown. shown. Afterwards, Afterwards, the the temperature temperature and and massmass flow flow of of injected injected urea urea solution solution is is changed, changed, where where the the formation formation and and decomposition decomposition of urea-relatedof urea-related deposits deposits take take place place (orange (orange marked marked image). image). On On both both images, images, one one may may see see a signa sign of formerof former electrolyte electrolyte flow flow (visible (visible in grey in grey colour). colour). The The difference difference between between the images, the im- isages, a formed is a formed deposit deposit shown shown inside inside the dashed the dashed green circle.green Duecircle. to Due cyclic to loading,cyclic loading, a more a harmfulmore harmful corrosive corrosive effect waseffect found was whenfound comparedwhen compared to isothermal to isothermal tests [4 tests]. [4].

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Figure 3.Figure Graphic 3. representationGraphic representation of the test of cycle the test for cyclethe corrosion for the corrosion experiment. experiment. For the understanding of the relationship between the chemical composition of de- For the understanding of the relationship between the chemical composition of deposits and heating temperature, a set of six experiments on isothermal heating of injected posits and heating temperature, a set of six experiments on isothermal heating of injected aqueous urea solution was performed. For this purpose, each experiment was carried out aqueous urea solution was performed. For this purpose, each experiment was carried out at a strongly defined temperature of the airflow in the chamber. Besides the temperature of at a strongly defined temperature of the airflow in the chamber. Besides the temperature the airflow, the temperature of the steel sample was measured by installed thermocouples, of the airflow, the temperature of the steel sample was measured by installed thermocou- both temperatures are listed in Table2. For understanding the chemical composition of the ples, both temperaturesproduct (s) are related listed to in corrosion Table 2. ofFor stainless understanding steel, the the cyclic chemical heating composition of injected urea solution of the productwas (s) performedrelated to corrosion (sample C,of Tablestainl2ess). Throughout steel, the cyclic the text,heating all depositof injected samples urea are labelled solution was performedwith the short (sample names, C, givenTable in2). Table Throughout2. For example, the text, in al thel deposit sample samples index, I-110,are the letter I labelled with meansthe short isothermal names, given heating, in theTable number 2. For 110example, means in the the temperature sample index, of the I-110, deposit collected the letter I meansduring isothermal heatingat heating, 200 ◦C airthe flownumber in the 110 closed means chamber. the temperature of the deposit collected during heating at 200 °C air flow in the closed chamber. Table 2. Summary of deposit samples. Table 2. Summary of deposit samples. ◦ ◦ Sample Index Tsample, CTair, C Test Duration Sample Index Tsample, °C Tair, °C Test Duration I-110 110 I-110 200110 20010 min 10 min I-181 181 I-181 250181 25010 min 10 min I-208 208 I-208 300208 30010 min 10 min I-258 258 I-258 350258 35010 min 10 min I-301 301 I-301 375301 37510 min 10 min I-322 322 I-322 400322 40010 min 10 min C cyclic heating cyclic heating 100 h C cyclic heating cyclic heating 100 h

2.3. Characterization Methods 2.3. Characterization Methods The cross-section morphology of the corroded sample was analysed by scanning elec- The cross-section morphology of the corroded sample was analysed by scanning electron microscopy (SEM/EVO MA 25, Carl Zeiss SMT, Oberkochen, Germany) coupled with tron microscopy (SEM/EVO MA 25, Carl Zeiss SMT, Oberkochen, Germany) coupled with energy dispersive X-ray spectrometry (EDX/X-ACT10, Oxford Instruments NanoAnaly- energy dispersive X-ray spectrometry (EDX/X-ACT10, Oxford Instruments NanoAnalysis, sis, Bucks, UK). The EDX point analysis was performed with the interaction volume of 1 Bucks, UK). The EDX point analysis was performed with the interaction volume of 1 µm. µm. After the experiment, two steel parts were cleaned from deposits and carefully sepa- After the experiment, two steel parts were cleaned from deposits and carefully separated rated from each other. Before cross-section analysis, the corroded specimen of defined from each other. Before cross-section analysis, the corroded specimen of deﬁned geometry geometry was cut out of the baseplate along the dashed line shown in Figure 4b. A pol- was cut out of the baseplate along the dashed line shown in Figure4b. A polished cross- ished cross-section was prepared and investigated in the direction shown with arrows in section was prepared and investigated in the direction shown with arrows in Figure4b . Figure 4b. An ion slicer (IM4000+, Hitachi High-Technologies Europe GmbH, Krefeld, An ion slicer (IM4000+, Hitachi High-Technologies Europe GmbH, Krefeld, Germany) Germany) was used for the final polishing of samples before SEM analysis (visible on a was used for the ﬁnal polishing of samples before SEM analysis (visible on a specimen in specimen in Figure 5a). Figure5a).

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Figure 4. Macro- and microscopic images of corroded specimen after 100 h, 1.4509 grade (cleaned Figure 4. Macro- and microscopicFigure 4. Macro- images and of corrodedmicroscopic specimen images of after corroded 100 h, specimen 1.4509 grade after (cleaned100 h, 1.4509 from grade deposits). (cleaned from deposits). (a) macroscopic image, (b) microscopic image. (a) macroscopic image, (b) microscopicfrom deposits). image. (a) macroscopic image, (b) microscopic image.

Figure 5. SEM image of corroded specimen after 100 h, 1.4509 grade (a,b,c,d,f (SE detector),e (BS detector): different mag- Figure 5. SEM image of corroded specimen after 100 h, 1.4509 grade (a,b,c,d,f (SE detector),e (BS detector): different mag- nification). Figure 5. SEM image of corroded specimen after 100 h, 1.4509 grade ((a–d,f) (SE detector), (e) (BS detector): different nification).magniﬁcation). The chemical composition of deposits and reference materials was characterized with The chemical composition of deposits and reference materials was characterized with an FTIR spectrometerThe (Bruker chemical VERTEX composition 70, Billerica, of deposits MA, USA) and reference equipped materials with a Diamond was characterized with an FTIR spectrometer (Bruker VERTEX 70, Billerica, MA, USA) equipped with a Diamond ATR (attenuatedan total FTIR reflection) spectrometer unit. (Bruker The prepared VERTEX 70,specimens Billerica, were MA, USA)measured equipped in the with a Diamond ATRATR (attenuated (attenuated total total reflection) reﬂection) unit. unit. The The prepared prepared specimens specimens were were measured measured in in the the transmission mode in the wavelength range 4000–400 cm−1, resolution: 0.4 cm−1. Urea, bi- transmission mode in the wavelength range 4000–400 cm−1, resolution: 0.4 cm−1. Urea, biuret, cyanuric acid, ammelide, ammeline and melamine were used as reference materials. uret, cyanuric acid, ammelide, ammeline and melamine were used as reference materials.

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transmission mode in the wavelength range 4000–400 cm−1, resolution: 0.4 cm−1. Urea, biuret, cyanuric acid, ammelide, ammeline and melamine were used as reference materials. The structure of the deposits was examined by stereomicroscope (Olympus SZX12, Vienna, Austria).

3. Results 3.1. Characterization of Corroded Specimen 1.4509 Grade 100 h After 100 h of cyclic heating and injection of aqueous urea solution on the steel plate, the material shows a differently coloured, oxidized surface. The material loss is localized in the crevice where two design parts were in contact (Figure4a,b). There are also areas between two design parts, where no material loss was found, probably due to a narrower crevice between them. The cross-section morphology of the attacked zone is shown in detail in Figure5. In Figure5a, the yellow dashed line shows the boundary of the sample surface, which was prepared by argon ion for the SEM investigation, and the white dashed line shows the sample area selected for the detailed SEM analysis under higher magniﬁcation. SEM images taken with the SE-detector show attack along grain boundaries as well as uniformly. In some case, the grains are almost completely exposed. The chemical element analysis of the grain boundaries’ area near the occurred cracking was investigated by EDX-technique (area of analysis is shown by black dashed line in Figure5d). Neither signs of nitrogen nor any precipitation of chromium-rich phases at the grain boundaries were observed. In contrast, numerous large precipitates visible not just on the grain boundaries, but also in the grains, were identiﬁed by EDX-technique as primary precipitates Ti(C, N) and Nb(C, N) originating from the steel manufacturing process. Besides the intergranular cracking, the exposed grains indicate a dissolving type of attack, which is typical for uniform corrosion.

3.2. Deposit Analysis The identiﬁcation of obtained deposits was done using the FTIR technique. FTIR Corros. Mater. Degrad. 2021, 2, FOR PEERspectra REVIEW were recorded with emphasis on primary and secondary amide, primary amine7

and triazine absorption bands (Figures6–10).

◦ ◦ FigureFigure 6.6. FTIRFTIR spectra spectra of ofdeposit deposit I-110 I-110 and andI-181 I-181obtained obtained at Tair at= 200 Tair °C= (T 200sample C= 110 (Tsample °C) and= T 110air = 250C) and(Tsample Tair = 181= 250°C), ◦ (Tcorrespondingly.sample = 181 C), correspondingly.

Comparison of FTIR Spectra of Deposits I-208 and I-258 Spectra I-208 and I-258 in Figure 7 look relatively similar, but their comparison with spectrum I-181 revealed some differences. Firstly, a signal at 3254 cm−1 (N-H band) of deposit I-181 is not present in samples I-208 and I-258; instead, the new signal at 3175 cm−1 appeared nearby (N–H or C=O band). This is explained by the transformation of urea towards biuret and cyanuric acid, which is why sample I-181 contains the signal from urea, while samples I-208 and I-258 contain the signal from biuret and cyanuric acid. Secondly, the weak signal appears at 1778 cm−1 (C=O band) in spectrum I-258. Thirdly, the signal at 1587 cm−1 (NH2 band) of sample I-181 has changed its shape when looking at samples I- 208 and I-258, which both have two clear signals at 1615 cm−1 (C=O band) and 1556 cm−1 (NH2 band), indicative for the formation of biuret and ammelide, respectively. Besides this, the weak absorbance band found at 1360 cm−1 in sample I-258, is probably responsible for the ammelide product.

Figure 7. FTIR spectra of deposits I-181, I-208 and I-258 obtained at Tair = 250 °C (Tsample = 181 °C), Tair = 300 °C (Tsample = 208 °C) and Tair = 350 °C (Tsample = 258 °C), respectively.

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Figure 6. FTIR spectra of deposit I-110 and I-181 obtained at Tair = 200 °C (Tsample = 110 °C) and Tair = 250 (Tsample = 181 °C), correspondingly.

Comparison of FTIR Spectra of Deposits I-208 and I-258 Spectra I-208 and I-258 in Figure 7 look relatively similar, but their comparison with spectrum I-181 revealed some differences. Firstly, a signal at 3254 cm−1 (N-H band) of deposit I-181 is not present in samples I-208 and I-258; instead, the new signal at 3175 cm−1 appeared nearby (N–H or C=O band). This is explained by the transformation of urea towards biuret and cyanuric acid, which is why sample I-181 contains the signal from urea, while samples I-208 and I-258 contain the signal from biuret and cyanuric acid. Secondly, the weak signal appears at 1778 cm−1 (C=O band) in spectrum I-258. Thirdly, the signal at 1587 cm−1 (NH2 band) of sample I-181 has changed its shape when looking at samples I- 208 and I-258, which both have two clear signals at 1615 cm−1 (C=O band) and 1556 cm−1 (NH2 band), indicative for the formation of biuret and ammelide, respectively. Besides Corros. Mater. Degrad. 2021, 2 467 this, the weak absorbance band found at 1360 cm−1 in sample I-258, is probably responsible Corros. Mater. Degrad. 2021, 2, FOR PEERfor theREVIEW ammelide product. 8

Comparison of FTIR Spectra of Deposits I-301 and I-322 The FTIR spectrum of deposit I-301 in Figure 8 shows the strong absorbance at higher wavenumbers 3448, 3320 and 3184 cm−1 to be indicative for N-H vibration. The strong absorbance bands in the fingerprint region at 1778, 1732 and 1667 cm−1 originate from the C=O band. Two overlapped signals of strong intensity detected at 1593 and 1563 cm−1 and two medium signals at 763 and 699 cm−1 can be attributed to the NH2 band. Five strong signals in the region 1456–1061 cm−1 are assigned to the C–N band. After the comparison of deposit I-301 with spectra from reference materials (Figure 10), a clear presence of biuret, ammelide and cyanuric acid in the sample can be observed. The spectrum, however, contained many overlapping signals, suggesting a lower content of biuret in comparison to ammelide and cyanuric acid. The FTIR spectrum of deposit I-322 in Figure 8 looks similar to sample I-301, but a slight difference was still observed. The high wavenumber region 3500–3100 cm−1 results in minor differences in signal shape. Additionally, the fingerprint region at 1732–1333 cm−1 shows that sample I-301 has better separated, straight signals in comparison to overlapped signals of sample I-322. Additionally, unlike sample I-322, the deposit I-301 contains a −1 ◦ ◦ ◦ Figure Figure7. FTIR 7. spectraFTIR spectra of depositssignal of deposits I-181, at 830 I-181,I-208 cm I-208and (cyanuric I-258 and I-258obtained acid). obtained atThus, Tair at = Tthe air250= comparison °C 250 (TsampleC (Tsample = 181 of =°C),both 181 T samples,airC), = 300 Tair °C= 300shows (TsampleC that= 208 sam- ◦ ◦ ◦ °C) and(T Tsampleair = 350= 208 °C C)(Tsample andT =pleair 258= I-301 350 °C),C respectively.contains (Tsample = more 258 C), cyanuric respectively. acid by-product than deposit I-322.

◦ ◦ ◦ FigureFigure 8. FTIR 8. spectraFTIR spectra of deposits of deposits I-301 I-301and I-322 and I-322obtained obtained at Tair at = T 375air = °C 375 (TsampleC (T sample= 301 =°C) 301 andC) Tair and = 400 Tair °C= 400(TsampleC = 322 ◦ °C) correspondingly.(Tsample = 322 C) correspondingly.

3.2.2. One Hundred Hours Cyclic Heating and Injected Aqueous Urea Solution (Deposit Sample C) The FTIR spectrum in Figure 9 shows a group of sharp strong overlapped signals in the region 1720–1644 cm−1 belonging to C=O stretching band. Two strong signals observed at 1455 and 1415 cm−1 are assigned to ring str band from triazine. Another strong sharp signal at 1179 cm−1 originated from a symmetric N–C–N stretching band. The signal with a shoulder detected at 773 cm−1 could belong to N–H or C=O band. The analysis of the spectrum revealed that deposit C contains mostly cyanuric acid and ammelide in its composition.

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Figure 9. FTIR spectra of deposit C obtained after 100100h h cyclic cyclic heating heating and and urea urea injection. injection. Figure 9. FTIR spectra of deposit C obtained after 100h cyclic heating and urea injection.

Figure 10. FTIR spectra of reference materials. FigureFigure 10. 10.FTIR FTIR spectra spectra of of reference reference materials. materials.

4.3.2.1.4. Discussion Discussion Ten-Minute Isothermal Heating of Injected Aqueous Urea Solution ComparisonTwoTwo types types of FTIRof of experiments experiments Spectra of Depositswere were performed performed I-110 and in in I-181this this research research work. work. The The main main experi- experi- mentmentThe was was FTIRcyclic cyclic spectrum heating heating of ofof a depositastainless stainless I-110 steel steel shows sample sample a under group under continued of continued strong signalsinjection injection in of theof aqueous aqueous higher −1 ureawavenumberurea solution. solution. One region One hundred hundred 3440–3200 cycles cycles cm (1 (1 cycle cycleassigned lasted lasted one to one the hour) hour) N-H in in stretchingtotal total were were performed. band performed. (Figure Dur- Dur-6). −1 ingTheing this detectedthis experiment experiment strong corrosive corrosive peak with attack attack shoulders and and depo depo atsits 1664sits were were and obtained. 1588obtained. cm The Thebelongs deposits deposits to were thewere C=O col- col- lected for identification of chemical composition by help of FTIR analysis. According−1 to stretchlected vibrationfor identification and NH of2 band;chemical the composition last one was by also help found of FTIR at 785 analysis. and 713 According cm . The to −1 −1 thisobservedthis analysis, analysis, strong the the deposit peak deposit at obtained 1454 obtained cm after afterand 100h 100h weak experiment experiment peak at 1057 mainly mainly cm consists consistsare responsible of of cyanuric cyanuric for acid theacid andanti-symmetricaland ammelide ammelide products, products, stretch the C–Nthe sample sample band, resembles whileresembles a signal a awhite white of mediumpowder powder with intensitywith beige beige atcolour colour 1153 inclu- could inclu- sionsoriginatesions (Figure (Figure from 9). 9). the N–C–N band. Thus, all signals presented on spectrum I-110 belong to ToTo understand understand the the urea-decomposition urea-decomposition re reactionsactions happening happening during during different different tem- tem- peraturesperatures within within one one heating heating cycle, cycle, six six shor short tisothermal isothermal experiments experiments were were performed performed in in

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characteristic groups of urea product, and this spectrum corresponds closely to that of the reference pure urea material (Figure 10). Besides all signals presented in sample I-110, the spectrum I-181 contains some new signals that correspond to the reference pure biuret material. For example, two observed bands of medium intensity at 1731 and 1327 cm−1 correspondingly belong to C=O and C–N bands of secondary amides. Small peaks at 945, 785 and 713 cm−1 could be assigned to the NH2 band. Hence, the composition of sample I-181 consists of urea and biuret as the main constituents.

Comparison of FTIR Spectra of Deposits I-208 and I-258 Spectra I-208 and I-258 in Figure7 look relatively similar, but their comparison with spectrum I-181 revealed some differences. Firstly, a signal at 3254 cm−1 (N-H band) of deposit I-181 is not present in samples I-208 and I-258; instead, the new signal at 3175 cm−1 appeared nearby (N–H or C=O band). This is explained by the transformation of urea towards biuret and cyanuric acid, which is why sample I-181 contains the signal from urea, while samples I-208 and I-258 contain the signal from biuret and cyanuric acid. Secondly, the weak signal appears at 1778 cm−1 (C=O band) in spectrum I-258. Thirdly, the signal −1 at 1587 cm (NH2 band) of sample I-181 has changed its shape when looking at samples I-208 and I-258, which both have two clear signals at 1615 cm−1 (C=O band) and 1556 cm−1 (NH2 band), indicative for the formation of biuret and ammelide, respectively. Besides this, the weak absorbance band found at 1360 cm−1 in sample I-258, is probably responsible for the ammelide product.

Comparison of FTIR Spectra of Deposits I-301 and I-322 The FTIR spectrum of deposit I-301 in Figure8 shows the strong absorbance at higher wavenumbers 3448, 3320 and 3184 cm−1 to be indicative for N-H vibration. The strong absorbance bands in the ﬁngerprint region at 1778, 1732 and 1667 cm−1 originate from the C=O band. Two overlapped signals of strong intensity detected at 1593 and 1563 cm−1 and −1 two medium signals at 763 and 699 cm can be attributed to the NH2 band. Five strong signals in the region 1456–1061 cm−1 are assigned to the C–N band. After the comparison of deposit I-301 with spectra from reference materials (Figure 10), a clear presence of biuret, ammelide and cyanuric acid in the sample can be observed. The spectrum, however, contained many overlapping signals, suggesting a lower content of biuret in comparison to ammelide and cyanuric acid. The FTIR spectrum of deposit I-322 in Figure8 looks similar to sample I-301, but a slight difference was still observed. The high wavenumber region 3500–3100 cm−1 results in minor differences in signal shape. Additionally, the ﬁngerprint region at 1732–1333 cm−1 shows that sample I-301 has better separated, straight signals in comparison to overlapped signals of sample I-322. Additionally, unlike sample I-322, the deposit I-301 contains a signal at 830 cm−1 (cyanuric acid). Thus, the comparison of both samples, shows that sample I-301 contains more cyanuric acid by-product than deposit I-322.

3.2.2. One Hundred Hours Cyclic Heating and Injected Aqueous Urea Solution (Deposit Sample C) The FTIR spectrum in Figure9 shows a group of sharp strong overlapped signals in the region 1720–1644 cm−1 belonging to C=O stretching band. Two strong signals observed at 1455 and 1415 cm−1 are assigned to ring str band from triazine. Another strong sharp signal at 1179 cm−1 originated from a symmetric N–C–N stretching band. The signal with a shoulder detected at 773 cm−1 could belong to N–H or C=O band. The analysis of the spectrum revealed that deposit C contains mostly cyanuric acid and ammelide in its composition.

4. Discussion Two types of experiments were performed in this research work. The main experiment was cyclic heating of a stainless steel sample under continued injection of aqueous urea Corros. Mater. Degrad. 2021, 2 470

solution. One hundred cycles (1 cycle lasted one hour) in total were performed. During this experiment corrosive attack and deposits were obtained. The deposits were collected for identification of chemical composition by help of FTIR analysis. According to this analysis, the deposit obtained after 100h experiment mainly consists of cyanuric acid and ammelide products, the sample resembles a white powder with beige colour inclusions (Figure9). To understand the urea-decomposition reactions happening during different temperatures within one heating cycle, six short isothermal experiments were performed in addition to the main 100 h corrosion experiment. The IR results of 10 min heating experi- ◦ ◦ ment (Tair = 200 C), at which the collected deposit had a temperature of 110 C, showed that the product consists of urea, this is in agreement with the colourless gel state of the ◦ ◦ sample I-110 (Table3). At higher heating temperature (T air = 250 C; Tsample = 181 C), the collected deposit I-181 has slightly changed its aggregate state to solidified gel and its colour became denser white (Table3), which can be explained by the beginning of the ◦ urea decomposition process (Turea dec. = 133 C[22,26,27]) and formation of biuret product ◦ ◦ ◦ (Tbiuret form. = 150 C[22,27]). Sample I-208 obtained at 208 C (Tair = 300 C) as a white powder (Table3) consisted mostly of biuret with a small amount of urea and cyanuric acid. ◦ At this stage, no liquid urea was present in the sample (Turea instable = 210 C[24]) and biuret ◦ reached its first decomposition step (Tbiuret 1st dec. = 193 C[22,24,27]), which is explained ◦ by the appearance of cyanuric acid. Sample I-258, heated at Tair = 350 C, has a milky colour and crystallized form (Table3), which can happen when biuret has passed the second ◦ decomposition step (Tbiuret 2nd dec = 230 C[24]) from solid into the liquid phase. The IR analysis confirmed this and showed the presence of thermally stable at this temperature ◦ ◦ ammelide (Tammelide form. = 250 C[22,27], Tammelide dec. = 360–410 C[22,24,26,27]) as the ◦ main product with a sign of ammeline (Tammeline form. = 250 C[22,27], Tammeline dec. = 360– ◦ ◦ ◦ 430 C[22,24,27]). Deposit I-301, collected at 301 C (Tair = 375 C), was the first sample of beige colour (powder-crystalline) (Table3), which is confirmed by the detection of cyanuric acid (it has a characteristic beige colour) besides ammelide. The last sample, I-322, from ◦ ◦ the isothermal set obtained at Tsample = 322 C (Tair = 400 C), has a milkier colour than the beige sample I-301 (Table3) because the cyanuric acid was decomposed until gaseous ◦ products (Tcyanuric acid dec. = 320–330 C[22,27]) and ammelide could be directly obtained from biuret or cyanuric acid. Thus, sample I-322 consists of ammelide and contains less cyanuric acid in comparison to sample I-301. Metallographic analysis of the stainless steel sample corroded during 100 h of cyclic heating and injection of aqueous urea solution revealed a mixed type of corrosion. Inter- granular corrosion favoured at the crevice area could not be confirmed due to the absence of chromium depletion or enrichment and any nitride or carbide precipitates. Partially and completely exposed grains could indicate a dissolving character of attack typical for uniform corrosion. It could be assumed that urea-related products were likely to accu- mulate at the crevice supported by its cavity, where several chemical reactions could be responsible for the dissolution of the oxide layer and base metal. Thus, uniform corrosion underneath the deposits is assumed to be the main cause of the attack. Generally, to obtain uniform corrosion at these temperatures, two types of conditions are required, a low pH value and the presence of an electrolyte. After reviewing all chemical transitions happening during urea decomposition in the heating cycle, the liquid phase of biuret ◦ (Tbiuret 2nd dec = 230 C[24]) and constant humidity were found to play a role of the electrolyte necessary for uniform corrosion. The regular injection of aqueous urea solution and release of water molecule during most of the chemical transitions are assumed to be the source of high constant humidity in the experiment. The dissolved cyanuric acid deposit (pH = 3.8) and/or gaseous isocyanic acid (pH = 3.7) may continuously decrease the pH value in the cavity. The cyanuric acid is one of the main products of the deposit mixture obtained after 100 h cycle experiment; the cyanic acid is consumed and released during one pathway of urea thermal decomposition. Besides uniform corrosion, found here on a large scale, a certain preferential attack along grain boundaries was observed. On the one side, it could be a notice of not confirmed Corros. Mater. Degrad. 2021, 2, FOR PEER REVIEW 10 Corros. Mater. Degrad. 2021, 2, FOR PEER REVIEW 10 Corros. Mater. Degrad. 2021, 2, FOR PEER REVIEW 10

addition to the main 100 h corrosion experiment. The IR results of 10 min heating experi- addition to the main 100 h corrosion experiment. The IR results of 10 min heating experi- mentaddition (Tair to = the200 main°C), at 100 which h corrosion the collected experime depont.sit The had IR a results temperature of 10 min of 110 heating °C, showed experiment (Tair = 200 °C), at which the collected deposit had a temperature of 110 °C, showed ment (Tair = 200 °C), at which the collected deposit had a temperature of 110 °C, showed thatment the (T airproduct = 200 °C), consists at which of urea, the thiscollected is in agreementdeposit had with a temperature the colourless of 110 gel °C,state showed of the thatthat thethe productproduct consistsconsists ofof urea,urea, thisthis isis inin agreementagreement withwith thethe colourlesscolourless gelgel statestate ofof thethe samplethat the I-110 product (Table consists 3). At of higher urea, heatingthis is in temperature agreement with(Tair =the 250 colourless °C; Tsample gel = 181 state °C), of thethe sample I-110 (Table 3). At higher heating temperature (Tair = 250 °C; Tsample = 181 °C), the sample I-110 (Table 3). At higher heating temperature (Tair = 250 °C; Tsample = 181 °C), the collectedsample I-110 deposit (Table I-181 3). Athas higher slightly heating changed temperature its aggregate (Tair state= 250 to°C; solidified Tsample = 181gel °C),and theits colourcollected became deposit denser I-181 white has slightly (Table changed3), which its can aggregate be explained state byto solidifiedthe beginning gel and of the its colour became denser white (Table 3), which can be explained by the beginning of the ureacolour decomposition became denser process white (T (Tableurea dec. =3), 133 whic °Ch [22,26,27]) can be explained and formation by the ofbeginning biuret product of the urea decomposition process (Turea dec. = 133 °C [22,26,27]) and formation of biuret product urea decomposition process (Turea dec. = 133 °C [22,26,27]) and formation of biuret product (Tureabiuret decomposition form. = 150 °C [22,27]). process Sample (Turea dec. I-208 = 133 obtained °C [22,26,27]) at 208 °C and (T formationair = 300 °C) of as biuret a white product pow- (Tbiuret form. = 150 °C [22,27]). Sample I-208 obtained at 208 °C (Tair = 300 °C) as a white pow- (Tbiuret form. = 150 °C [22,27]). Sample I-208 obtained at 208 °C (Tair = 300 °C) as a white powder(Tbiuret (Table form. = 3) 150 consisted °C [22,27]). mostly Sample of biuret I-208 with obtained a small at amount208 °C (T ofair urea = 300 and °C) cyanuric as a white acid. pow- At der (Table 3) consisted mostly of biuret with a small amount of urea and cyanuric acid. At thisder (Tablestage, no3) consisted liquid urea mostly was ofpresent biuret in with the a sample small amount (Turea instable of urea = 210 and °C cyanuric [24]) and acid. biuret At this stage, no liquid urea was present in the sample (Turea instable = 210 °C [24]) and biuret this stage, no liquid urea was present in the sample (Turea instable = 210 °C [24]) and biuret reachedthis stage, its nofirst liquid decomposition urea was presentstep (Tbiuret in the1st dec. sample = 193 °C (T urea[22,24,27]), instable = 210 which °C [24])is explained and biuret by reached its first decomposition step (Tbiuret 1st dec. = 193 °C [22,24,27]), which is explained by reached its first decomposition step (Tbiuret 1st dec. = 193 °C [22,24,27]), which is explained by thereached appearance its first ofdecomposition cyanuric acid. step Sample (Tbiuret I-258, 1st dec. =heated 193 °C at [22,24,27]), Tair = 350 °C, which has isa milkyexplained colour by the appearance of cyanuric acid. Sample I-258, heated at Tair = 350 °C, has a milky colour the appearance of cyanuric acid. Sample I-258, heated at Tair = 350 °C, has a milky colour andthe appearancecrystallized of form cyanuric (Table acid. 3), whichSample ca I-258,n happen heated when at T airbiuret = 350 has °C, passedhas a milky the second colour and crystallized form (Table 3), which can happen when biuret has passed the second decompositionand crystallized step form (Tbiuret (Table 2nd dec 3), = 230which °C [24])can happenfrom solid when into biuretthe liquid has phase.passed The the IR second anal- decomposition step (Tbiuret 2nd dec = 230 °C [24]) from solid into the liquid phase. The IR anal- decomposition step (Tbiuret 2nd dec = 230 °C [24]) from solid into the liquid phase. The IR anal- Corros. Mater. Degrad. 2021, 2 ysisdecomposition confirmed thisstep and (Tbiuret showed 2nd dec = the230 presence°C [24]) from of thermally solid into stable the liquid at this phase. temperature The IR471 anal- am- ysis confirmed this and showed the presence of thermally stable at this temperature am- melideysis confirmed (Tammelide this form. and= 250 showed °C [22,27], the presence Tammelide dec.of thermally= 360–410 stable°C [22,24,26,27]) at this temperature as the main ammelide (Tammelide form. = 250 °C [22,27], Tammelide dec. = 360–410 °C [22,24,26,27]) as the main melide (Tammelide form. = 250 °C [22,27], Tammelide dec. = 360–410 °C [22,24,26,27]) as the main productmelide (T withammelide a signform. =of 250 ammeline °C [22,27], (Tammeline Tammelide form. dec. = 250= 360–410 °C [22,27], °C [22,24,26,27]) Tammeline dec. = as360–430 the main °C product with a sign of ammeline (Tammeline form. = 250 °C [22,27], Tammeline dec. = 360–430 °C product with a sign of ammeline (Tammeline form. = 250 °C [22,27], Tammeline dec. = 360–430 °C [22,24,27]).product with Deposit a sign I-301, of ammeline collected (Tatammeline 301 °C form. (T air= 250= 375 °C °C), [22,27], was theTammeline first dec.sample = 360–430 of beige °C [22,24,27]). Deposit I-301, collected at 301 °C (Tair = 375 °C), was the first sample of beige intergranular[22,24,27]). corrosion.Deposit I-301, Although collected the at inspection 301 °C (T ofair grain= 375 boundaries°C), was the for first the sample presence of ofbeige colour[22,24,27]). (powder-crystalline) Deposit I-301, collected (Table 3), at which 301 °C is (T confirmedair = 375 °C), by thewas detection the first ofsample cyanuric of beige acid nitrides(itcolour has and (powder-crystalline)a characteristic carbides under beige high (Table colour) resolution 3), besides which did notis ammelide. confirmed reveal any The by precipitates, the last detection sample, this ofI-322, hypothesis cyanuric from acid the is not(it(it has fullyhas aa excluded characteristiccharacteristic yet. Therebeigebeige arecolour)colour) some besidesbesides studies, ammelide.ammelide. where g-C TheTheN ,lastlast NH sample,sample,,N and I-322,I-322, melamine fromfrom thethe isothermal(it has a characteristic set obtained beige at Tsample colour) = 322 besides °C (Tair =ammelide. 400 °C), has The3 a 4milkier last sample,3 colour2 I-322, than thefrom beige the isothermal set obtained at Tsample = 322 °C (Tair = 400 °C), has a milkier colour than the beige areisothermal used as nitrifying set obtained and carburization at Tsample = 322 reagents °C (Tair =to 400 convert °C), has metal a milkier oxides colour into nitridethan the and beige sampleisothermal I-301 set (Table obtained 3) because at Tsample the = 322cyanuric °C (T airacid = 400 was °C), decomposed has a milkier until colour gaseous than productsthe beige carbidesample nanoparticles I-301 (Table at 3) high because temperatures the cyanuric [12 ,acid13]. was On the decomposed other hand, until the gaseous regular attack,products (Tsamplecyanuric acidI-301 dec. (Table= 320–330 3) because °C [22,27]) the cyanuricand ammelide acid was could decomposed be directly until obtained gaseous from products biuret (Tcyanuric acid dec. = 320–330 °C [22,27]) and ammelide could be directly obtained from biuret which(Tcyanuric may acid be dec. associated = 320–330 with °C [22,27]) some cracking,and ammelide could could be the be result directly of cyclic obtained heating from and biuret or(T cyanuriccyanuric acid dec. acid. = 320–330 Thus, sample °C [22,27]) I-322 and consists ammelide of ammelide could be and directly contains obtained less cyanuric from biuret acid cooling.or cyanuric acid. Thus, sample I-322 consists of ammelide and contains less cyanuric acid inor comparisoncyanuric acid. to Thus,sample sample I-301. I-322 consists of ammelide and contains less cyanuric acid in comparison to sample I-301. Table 3. Characterisation of deposit samples from the isothermal experiment. Table 3. Characterisation of deposit samples from the isothermal experiment. Table 3. Characterisation of deposit samples from the isothermal experiment. Sample FTIR Results Sample T , ◦C T , ◦C FTIR Results Deposit Image Sample sampleTsample, °C Tairair, °C FTIR Results Deposit Image IndexSample Tsample, °C Tair, °C Main ProductFTIR Results Minor Product Deposit Image SampleIndex Tsample, °C Tair, °C Main ProductFTIR Results Minor Product Deposit Image Index Tsample, °C Tair, °C Main Product Minor Product Deposit Image Index Main Product Minor Product

I-110I-110 110110 200 200 urea urea I-110 110 200 urea

urea, I-181 181 250 urea,urea, I-181I-181 181181 250 250 biureturea, I-181 181 250 biuretbiuret biuret

Urea, I-208 208 300 biuret Urea, I-208 208 300 biuret cyanuricUrea,Urea, acid I-208I-208 208208 300 300 biuret biuret cyanuric acid cyanuriccyanuric acid acid

urea, biuret, urea, biuret, I-258 258 350 ammelide cyanuricurea, biuret, acid, I-258I-258 258258 350 350 ammelide ammelide cyanuriccyanuric acid, acid, Corros.Corros. Mater.Mater. Degrad.Degrad. 20212021,, 22,, FORFOR PEERPEER REVIEWREVIEW ammeline 1111 ammelineammeline

cyanuriccyanuriccyanuric acid,acid, acid, melamine,melamine,melamine, I-301I-301I-301 301301301 375 375 ammelideammelideammelide ammelineammelineammeline

cyanuric acid, cyanuriccyanuric acid, acid, I-322I-322I-322 322322322 400 400 biuretbiuretbiuret ammelideammelideammelide

MetallographicMetallographic analysisanalysis ofof thethe stainlessstainless steelsteel samplesample corrodedcorroded duringduring 100h100h ofof cycliccyclic 5. Conclusionsheatingheating andand injectioninjection ofof aqueousaqueous ureaurea solutionsolution revealedrevealed aa mixedmixed typetype ofof corrosion.corrosion. Inter-Inter- granulargranularAfter 100 corrosioncorrosion h cyclic heating favouredfavoured and atat injection thethe crevicecrevice of aqueous areaarea couldcould urea notnot solution bebe confirmedconfirmed on the 1.4509duedue toto stainless thethe absenceabsence steel,ofof achromiumchromium uniform attackdepletiondepletion with oror some enrichmentenrichment intergranular andand anan morphologyyy nitridenitride oror carbidecarbide was obtained. precipitates.precipitates. The grainPartiallyPartially boundaryandand completelycompletely precipitates exposedexposed were grainsgrains either couldcould not present indicateindicate or aa extremelydissolvingdissolving small.charactercharacter FTIR ofof attackattack revealed typicaltypical the forfor cyanuricuniformuniform acid corrosion.corrosion. as one of ItIt the couldcould main bebe corrosive assumedassumed constituents. thatthat urea-relatedurea-related The temperatures productsproducts werewere with likelylikely maximum toto accumu-accumu- latelate atat thethe crevicecrevice supportedsupported byby itsits cavity,cavity, wherewhere severalseveral chemicalchemical reactionsreactions couldcould bebe re-re- sponsiblesponsible forfor thethe dissolutiondissolution ofof thethe oxideoxide lalayeryer andand basebase metal.metal. Thus,Thus, uniformuniform corrosioncorrosion underneathunderneath thethe depositsdeposits isis assumedassumed toto bebe thethe mamainin causecause ofof thethe attack.attack. Generally,Generally, toto obtainobtain uniformuniform corrosioncorrosion atat thesethese temperatures,temperatures, twotwo typestypes ofof conditionsconditions areare required,required, aa lowlow pHpH valuevalue andand thethe presencepresence ofof anan electrolyte.electrolyte. AfAfterter reviewingreviewing allall chemicalchemical transitionstransitions happen-happening during urea decomposition in the heating cycle, the liquid phase of biuret (Tbiuret 2nd dec ing during urea decomposition in the heating cycle, the liquid phase of biuret (Tbiuret 2nd dec == 230230 °C°C [24])[24]) andand constantconstant humidityhumidity werewere founfoundd toto playplay aa rolerole ofof thethe electrolyteelectrolyte necessarynecessary forfor uniformuniform corrosion.corrosion. TheThe regularregular injectioninjection ofof aqueousaqueous ureaurea solutionsolution andand releaserelease ofof waterwater moleculemolecule duringduring mostmost ofof thethe chemicalchemical transitionstransitions areare assumedassumed toto bebe thethe sourcesource ofof highhigh constantconstant humidityhumidity inin thethe exexperiment.periment. TheThe dissolveddissolved cyanuriccyanuric acidacid depositdeposit (pH(pH == 3.8)3.8) and/orand/or gaseousgaseous isocyanicisocyanic acidacid (pH(pH == 3.7)3.7) maymay continuouslycontinuously decreasedecrease thethe pHpH valuevalue inin thethe cavity.cavity. TheThe cyanuriccyanuric acidacid isis oneone ofof thethe mainmain productsproducts ofof thethe depositdeposit mixturemixture obtainedobtained afterafter 100100 hh cyclecycle experiment;experiment; thethe cyaniccyanic acidacid isis consumedconsumed andand releasedreleased duringduring oneone pathwaypathway ofof ureaurea thermalthermal decomposition.decomposition. BesidesBesides uniformuniform corrosion,corrosion, foundfound herehere onon aa lalargerge scale,scale, aa certaincertain preferentialpreferential attackattack alongalong graingrain boundariesboundaries waswas observed.observed. OnOn thethe oneone side,side, itit couldcould bebe aa noticenotice ofof notnot con-con- firmedfirmed intergranularintergranular corrosion.corrosion. AlthoughAlthough thethe ininspectionspection ofof graingrain boundariesboundaries forfor thethe pres-pres- enceence ofof nitridesnitrides andand carbidescarbides underunder highhigh resoresolutionlution diddid notnot revealreveal anyany precipitates,precipitates, thisthis hypothesis is not fully excluded yet. There are some studies, where g-C3N4, NH3, N2 and hypothesis is not fully excluded yet. There are some studies, where g-C3N4, NH3, N2 and melaminemelamine areare usedused asas nitrifyingnitrifying andand carburicarburizationzation reagentsreagents toto convertconvert metalmetal oxidesoxides intointo nitridenitride andand carbidecarbide nanoparticlesnanoparticles atat highhigh temptemperatureseratures [12,13].[12,13]. OnOn thethe otherother hand,hand, thethe reg-reg- ularular attack,attack, whichwhich maymay bebe associatedassociated withwith sosomeme cracking,cracking, couldcould bebe thethe resultresult ofof cycliccyclic heatingheating andand cooling.cooling.

5.5. ConclusionsConclusions AfterAfter 100100 hh cycliccyclic heatingheating andand injectioninjection ofof aqueousaqueous ureaurea solutionsolution onon thethe 1.45091.4509 stain-stain- lessless steel,steel, aa uniformuniform attackattack withwith somesome intergranularintergranular morphologymorphology waswas obtained.obtained. TheThe graingrain boundaryboundary precipitatesprecipitates werewere eithereither notnot presenpresentt oror extremelyextremely small.small. FTIRFTIR revealedrevealed thethe cy-cy- anuricanuric acidacid asas oneone ofof thethe mainmain corrosivecorrosive constituents.constituents. TheThe temperaturestemperatures withwith maximummaximum damagedamage werewere found,found, wherewhere aa liquidliquid phasephase ofof biuretbiuret diddid present,present, whichwhich werewere aroundaround 193193 °C°C (first(first decompositiondecomposition step)step) andand 230230 °C°C (second(second decompositiondecomposition step).step).

AuthorAuthor Contributions:Contributions: Conceptualization,Conceptualization, G.M.,G.M., A.G.;A.G.; methodology,methodology, G.M.,G.M., A.G.,A.G., F.K.,F.K., H.W.,H.W., S.H.,S.H., B.S.B.S. andand S.B.;S.B.; validation,validation, G.M.,G.M., A.G.,A.G., F.K.,F.K., H.W.,H.W., S.S.H.H. andand S.B.;S.B.; investigation,investigation, A.G.,A.G., F.K.F.K. andand S.H.;S.H.; datadata curation,curation, G.M.,G.M., A.G.,A.G., F.K.,F.K., H.W.,H.W., S.H.S.H. andand S.B.;S.B.; writing—originalwriting—original draftdraft preparation,preparation, A.G.,A.G.,

Corros. Mater. Degrad. 2021, 2 472

damage were found, where a liquid phase of biuret did present, which were around 193 ◦C (ﬁrst decomposition step) and 230 ◦C (second decomposition step).

Author Contributions: Conceptualization, G.M., A.G.; methodology, G.M., A.G., F.K., H.W., S.H., B.S. and S.B.; validation, G.M., A.G., F.K., H.W., S.H. and S.B.; investigation, A.G., F.K. and S.H.; data curation, G.M., A.G., F.K., H.W., S.H. and S.B.; writing—original draft preparation, A.G., S.H.; writing—review and editing, G.M., H.W. and S.B.; visualization, G.M. and H.W. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: The data that support the findings of this study are available from the corresponding author upon reasonable request. Conflicts of Interest: The authors declare no conflict of interest.

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