Corrosion of stainless steels in simulated diesel exhaust environment with urea

Abstract

Laboratory exposures have been performed simulating a selective catalytic reduction (SCR) system with urea injection for NOx reduction in diesel exhaust after‐treatment. The corrosion behaviour of three ferritic and one austenitic stainless steel was examined using X‐ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES). Continuous exposure to condensate did not cause any corrosion. Results show that cyclic interaction between high temperature and condensation aggravates the corrosion compared to isothermal exposure at 450 °C. All ferritic alloys exhibited more or less the same behaviour, while the austenitic steel performed better. In fact, the presence of urea decreased the corrosion compared to the environment without urea. The cyclic samples exposed with urea displayed iron sulphate on the surface. The sulphate appeared to decrease the oxide thickness. A sulphur enrichment in the form of sulphide also occurred in the inner chromium‐rich oxides of all cyclically exposed samples, both with and without urea. Thus, sulphidation is presumed to be involved in the corrosion process.

Corrosion of Stainless Steel by Urea at High Temperature

“…The corrosion of stainless steel by hot urea solution has not been largely studied; on the one hand, this may be due to the limited industrial demand, on the other hand, urea itself is considered as non-corrosive for stainless steel, while some of its decomposition products at high temperature may lead to corrosion [1][2][3].…”

“…Research work on corrosion involving urea mainly belongs to urea production, the fertilizer industry, and selective catalytic reduction (SCR) technology [2,4]. For example, during the manufacturing of urea under high pressure, the most critical intermediate 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 corrosion problems [2,6].…”

“…For example, during the manufacturing of urea under high pressure, the most critical intermediate 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 corrosion problems [2,6]. In SCR technology, urea is used as a source of ammonia to reduce the amount of NOx exhaust gases in automotive systems.…”

“…The corrosion mechanism in urea-related technology was explained in several ways; one such way was cyclic oxidation caused by thermal cycling [7], and another-external corrosion by road salts [8,9]. Additionally, 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-C 3 N 4 (all of them exist during urea decomposition process) reactions with metal oxides, which attract attention as nitrifying and carburization reagents [12][13][14].…”

“…Besides this, there are studies on melamine, ammonia, nitrogen and g-C 3 N 4 (all of them exist during urea decomposition process) reactions with metal oxides, which attract attention as nitrifying and carburization reagents [12][13][14]. However, more generally, it is concluded that the cracking and fracturing of metal is related to urea decomposition products and intergranular attack [2,4,[14][15][16][17][18]. By many scientists, an intergranular corrosion mechanism, sometimes attributed to chromium depletion, and nitride precipitation at grain boundaries, and sometimes without any indication of decrease or increase in the amount of chromium at grain boundaries, was described [4,19,20].…”