Fireside corrosion on T24 steel pipes and HVOF NiCr coatings exposed to different salt mixtures

Abstract

Fireside corrosion of T24 steel and HVOF NiCr-coated counterparts is investigated. The samples were exposed to different deposits of NaCl, KCl, Na2SO4 and K2SO4 at 650 ºC for 360 h in dry air. The deposit with the highest chloride content (NaCl-KCl) was the most aggressive followed by the ones combining chlorides and sulfates. In contrast, the sulfate-based salts did not produce severe damage. The NiCr coating maintained the integrity of the substrate but the chlorides induced a great attack on the coating. The formation of K2CrO4 and the penetration of chlorides are identified as the main factors of this attack.

Corrosion behavior of T24, T92, VM12, and AISI 304 steels exposed to KCl–NaCl–K2SO4–Na2SO4 salt mixtures

“…In particular, in the presence of the S1 (NaCl–KCl), all the steels exhibited the highest mass gain (Figure 3b). This clearly confirms that the greatest content of chlorides in the S1 mixture significantly increased corrosion, as widely discussed in Reference [23]. The AISI 304 stainless steel showed the highest resistance, followed by the T24 and T92 steels but surprisingly, the VM12 steel behaved worse than the other two ferritic steels.…”

“…Figure 9 shows the cross‐sectional SEM micrographs of the tested steels exposed to NaCl–KCl salt mixture at 650°C for 360 h. The condition of T24 steel after the thermal test is shown in Figure 9a and was fully described in previous work. [ 23 ] The micrographs of T92 and VM12 steels with intermediate Cr content are presented in Figures 9b and 9c, respectively. Their oxide layer is less cracked than that of the T24 steel.…”

“…This is because sodium ferrite is accompanied by the formation of Cl 2(g) according to Equation (). [ 23 ] This Cl 2(g) diffuses through metal oxide layer and reacts with the Fe according to Equation () producing the formation of volatile FeCl 2(s) . [ 38 ] …”

“…[ 40,41 ] Therefore, the overall mechanisms proposed here are in line with previous works that extensively described these combined direct and active oxidation mechanisms. [ 23,42 ]…”

“…These steels were selected due to their different Cr content and maximum working temperature. The low‐Cr T24 steel was analyzed in previous work [ 23 ] and it was used as a reference for comparison purposes. In this way, the corrosion resistance of the intermediate‐Cr T92 and VM12 steels and the high‐Cr AISI 304 stainless steel were compared.…”