The Effects of KCl, K2SO4 and K2CO3 on the High Temperature Corrosion of a 304-Type Austenitic Stainless Steel

Abstract

The oxidation of 304-type (Fe18Cr10Ni) austenitic stainless steel was investigated at 500 and 600°C in 5% O 2 ? 40% H 2 O. Prior to exposure the samples were sprayed with KCl, K 2 CO 3 or K 2 SO 4 , the amount of salt corresponding to 1.35 lmol K ? /cm 2 . For reference, salt-free samples were exposed in 5% O 2 ? 40% H 2 O and in 5% O 2 (N 2 was used as carrier gas). The oxidized samples were analyzed with SEM/EDX, XRD, IC and FIB. KCl and K 2 CO 3 strongly accelerate the corrosion of 304L while K 2 SO 4 has little influence on the corrosion rate and on the morphology of the corroded surface. KCl and K 2 CO 3 react with the chromium-rich oxide on the sample surface, forming K 2 CrO 4 . The resulting chromium depletion of the protective oxide causes rapid oxidation and the formation of a thick duplex scale consisting of an outer hematite layer and a inner layer made up of FeCrNi spinel-type oxide. The differences in the corrosivity of the three salts are directly connected to their ability to form chromate on the surface and, hence, to the relative stability of the corresponding leaving groups (HCl, CO 2 and SO 3 ).

Microstructural Study of the Influence of KCl and HCl on Preformed Corrosion Product Layers on Stainless Steel

“…However, signs of steel grain boundary attack were seen in the presence of HCl(g). The growth rate of the corrosion product layers in the presence of either KCl(s) or HCl(g) after the initial pre-oxidation is not significantly higher compared to the expected growth rate in the absence of these chloride salts [15]. Hence, after the initial corrosion attack the presence of chlorine does not accelerate the attack, at least not during the first 24 h after applied to the corroded surface.…”

“…Due to the experimental setup used, where the gas flow has been set at 2.5 cm/s, it might not be possible to detect any KCl(s) on the surface after 24 h, i.e., the KCl(s) might be lost as KCl(g). In earlier work by Pettersson et al [15], the amount of KCl(s) remaining on 304L stainless steel sample after being sprayed ex situ with 0.10 mg/cm 2 KCl (corresponding to the same molar amount of potassium as for K 2 CO 3 ) was less than 1% after 24 h at 600°C. The same result was observed when 304L was pre-oxidized in the presence of KCl(s) and subsequently exposed in the presence of KCl(s); the amount of KCl remaining on the sample after the exposure was below detection [23].…”

“…K 2 CO 3 (s). The pre-oxidation parameters were selected based on earlier work [6,15,20] to achieve Fe-rich oxide. The mechanism for breakaway oxidation in the presence of K 2 CO 3 has previously been proposed to be caused by chromate formation on the sample surface [15].…”

“…The pre-oxidation parameters were selected based on earlier work [6,15,20] to achieve Fe-rich oxide. The mechanism for breakaway oxidation in the presence of K 2 CO 3 has previously been proposed to be caused by chromate formation on the sample surface [15]. The stability of the potassium chromate formed on corroding stainless steel samples has earlier been reported to be low, and after 168 h, it is seldom detected [6,21,22].…”

“…One of the corrosion mechanisms proposed for the onset of the corrosion attack is the ''chromate formation'' mechanism [3]. In this mechanism, the alkali chlorides, or alkali carbonate [15], react with the chromium in the protective oxide scale which leads to a Cr-depleted oxide and thereby breakaway corrosion. Other mechanism proposed for the accelerated corrosion attack in the presence of chlorine-rich environments is the ''active oxidation'' [5].…”