Application of aluminum diffusion coatings to mitigate the KCl‐induced high‐temperature corrosion

Abstract

Pack cementation was used to produce Fe1−xAl and Fe2Al5 diffusion coatings on ferritic‐martensitic steel P91 and a Ni2Al3 diffusion coating on pure nickel. The performance of diffusion coatings against high‐temperature corrosion induced by potassium chloride (KCl) was evaluated by exposing the samples at 600 °C for 168 h in static lab air under KCl deposit. In addition, a salt‐free experiment was performed for comparison. Microstructure, chemical and phase composition of the samples were analyzed with scanning electron microscopy (SEM), energy dispersive X‐ray spectroscopy (EDS) and X‐ray diffractometry (XRD) before and after the exposures. It was found that all the diffusion coatings formed protective oxides under salt‐free exposure in air. Under the salt deposit, Fe1−xAl showed local failure while on large parts of the sample a protective layer had formed. Fe2Al5 was attacked over the entire surface and the dominant mode of attack was selective aluminum removal. Ni2Al3 showed excellent performance and no sign of attack was observed anywhere on the sample.

KCl‐induced high temperature corrosion of selected commercial alloys

“…The corrosion product on alloy HR160 is largely composed of potassium, chromium, silicon and oxygen (Figure ). This is consistent with previous observations that chromia and silica react with KCl in an oxygen containing environment . Limited thickness loss on this alloy (Figure ) indicates that the corrosion product retards the progress of corrosion.…”

“…In addition, the similar damage extent observed on both series 1 and 2 also discounts the presence of a thin oxide formed from annealing affecting the results. The lack of a significant protective effect from a preformed chromia and/or silica layer can be due to the reactivity of KCl with both silica and chromia .…”

Microstructural investigations of Ni and Ni2Al3 coatings exposed in biomass power plants

“…In biomass combustion, the corrosion mechanism due to the aggressive alkali chlorides has been extensively investigated [11][12][13][14][15]. The initiation of corrosion is based on breakdown of protective oxide due to reaction of KCl with the oxide [6], and the propagation of corrosion is due to reaction with Cl species [16,17]. However even in environments with high HCl content, increased corrosion is also observed [16,17] According to the element distribution in Figure 4, potassium was not present at the corrosion front.…”

“…They attributed the intergranular corrosion of the two coatings to chromium enrichment along the grain boundary. Kiamehr et al [6] investigated the performance of two pack aluminised iron-based coatings (Fe1-xAl and Fe2Al5) and one nickel based coating (Ni2Al3) at 600 ºC for 168 h in static lab air with a KCl deposit. It was reported that Ni2Al3 showed no sign of attack, however, Fe1-xAl showed local attack and Fe2Al5 suffered heavily from selective aluminium removal.…”

Danish Experiences in Biomass Corrosion and Recent Areas of Research

“…The reaction of KCl with different oxides was investigated and Al 2 O 3 was not observed to react with KCl 47 . Thus coatings basing their protective behavior on aluminium have been developed utilizing pack cementation to aluminize both steels and nickel based alloys 50 . After the promising results of Nialuminides obtained on a laboratory scale 51,52 , the demonstration of the performance of these coatings under real service conditions in a biomass fired plant was undertaken for two years.…”