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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
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