Corrosion at the urea injection in SCR‐system during component test

Abstract

The corrosion behaviour of the austenitic stainless steels 304L and 904L was investigated after component testing in diesel exhaust urea environment. Cross‐sectional analyses and depth profiling of corrosion products were performed with Auger electron spectroscopy (AES) and X‐ray photoelectron spectroscopy (XPS). The samples of both materials displayed high amounts of silicon oxide on their surfaces, though this did not appear to have an active role in the corrosion. The higher alloyed 904L exhibited only small signs of corrosion and the inner oxide below the silicon oxide was chromium rich. Substantial nitrogen uptake was detected, presumably due to the presence of NH3, cyanates and similar species. The lower alloyed 304L showed more severe corrosion. The oxide on the 304L samples mainly consisted of iron oxide apart from the silicon oxide present. The nitrogen uptake was less pronounced and possibly connected to presence of nitride precipitates. The most pronounced corrosion was detected at sites where deposits frequently form and decompose. Those deposits are a probable nitrogen source and may also form compounds enabling the breakdown of the oxide.

Role of Nitrogen Uptake During the Oxidation of 304L and 904L Austenitic Stainless Steels

“…For 304L-L bcc formed during oxidation. The narrow peaks at 550 and 600°C could be consistent with a phase transformation S  bcc + fcc + CrN (1) This confirms that the S phase is less stable than austenite since, in comparison, the minor amount of bcc in as received 304L did not increase during oxidation. Several factors could contribute to the broad peak for bcc at 450°C.…”

“…The first nitriding condition, H, resulted in N concentrations almost twice as high as those on corroded components in 904L [1]. Moreover, for 904L a multitude of cracks formed both through the layer thickness and in parallel with the surface.…”

“…This study was initiated to examine the role of the substantial uptake of N in connection with corrosion of the alloys at the urea injection in diesel exhaust systems [1]. In the application the corrosion of 304L was severe and that of 904L limited, but noticeable.…”

“…In this method, an aqueous solution of urea is often used as ammonia source to react with NO x , forming N 2 and H 2 O. After component testing substantial N uptake has been found in exhaust systems at the urea injection where the corrosion was severe on 304L and limited on 904L [1]. The effect of the N uptake on the oxidation of stainless steel can thus be of great importance in an application running at varying temperatures up to 600°C where N might stay in solution or form other phases.…”

Corrosion of Stainless Steel by Urea at High Temperature

“…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].…”