The long-term corrosion behavior of FeCrAl(Si) alloys after breakaway oxidation at 600 \ub0C

The long-term corrosion behavior of three FeCrAl(Si) alloys has been investigated in two environments (K2CO3 and KCl+H2O) at 600 \ub0C. The FeCrAl alloy experienced breakaway oxidation in both environments but displayed a higher corrosion rate in KCl+H2O. The FeCrAlSi alloys retained the primary protection in the presence of K2CO3 but underwent breakaway oxidation in the presence of KCl+H2O. The FeCrAlSi alloys displayed considerably reduced corrosion rates, suggested to be an effect of the prevention of internal oxidation and the formation of non-continuous corundum-type oxide dispersed within the inward-growing scale

Beyond breakaway corrosion – Influence of chromium, nickel and aluminum on corrosion of iron-based alloys at 600 \ub0C

Breakaway corrosion remains a challenge for many high temperature applications. The oxide formed after breakaway is commonly considered non-protective. This study investigates the protective properties after breakaway on a wide set of (Fe,Cr,Al/Ni)-model alloys by thermogravimertric analysis, ion/electron microscopy and X-ray spectroscopy. The results show that the oxide scales formed after breakaway exhibit similar microstructural features on all FeCr(Ni/Al)-alloys, and that the growth rate is greatly influenced by alloy composition for some alloys while is has little influence on others. This observation may be of great help in the selection and development of materials for use in harshly corrosive environments

High temperature corrosion behavior of FeCrAlSi model alloys in the presence of water vapor and KCl at 600 \ub0C – The influence of Cr content

The corrosion behavior of Fe(5−20)Cr3Al2Si model alloys and the underlying mechanisms after breakaway oxidation was investigated at 600 \ub0C. Rapid breakaway oxidation was triggered for all alloys in the presence of KCl(s). Thermogravimetric analysis showed that alloys with a Cr content of ≥ 10 wt% transitioned into sub-parabolic oxidation kinetics at an early stage of the oxidation process after breakaway oxidation. Through advanced electron/ion microscopy, this behavior was attributed to high Cr-enrichment in the bottom part of the inward-growing scale, indicating the formation of a healing layer. Additionally, alloys with elevated Cr-content (≥15 wt%) were more prone to grain boundary attack

The influence of Si on the primary protection of lean FeCrAl model alloys in O2 and O2+H2O at 600 \ub0C: A microstructural investigation

The present study investigates the influence of Si on the high-temperature corrosion behavior of lean FeCrAl model alloys in O2 and O2+H2O at 600 \ub0C. The addition of Si prevented breakaway oxidation in O2+H2O which may be explained by the increased Al-content and reduced Cr-content in the oxide (O2) as well as the Si-enrichment in the outer part of the scale (O2+H2O). A proposed explanation for the impact of Si-addition on the Cr- and Al-content was given by thermodynamic calculations which showed that the presence of Si increases the activity of Al in the alloy while reducing the activity of Cr

Liquid phase migration in cemented carbides – Experiments and modelling

Liquid phase migration (LPM) and its potential impact on the phenomenon known as cobalt capping in cemented carbides have been investigated through experiments on fully densified, pre-sintered samples and simulations. A model for LPM was developed based on interface energies, grain size, and contiguity, and was implemented and integrated with Thermo-Calc software to make predictions about the final microstructure. The model\u27s predictions, when applied to cases with gradients in grain size and volume fraction of the binder phase, were found to somewhat match experimental observations. Additionally, it was obsereved that LPM can cause a phenomenon resembling cobalt capping due to gradients in carbon activity

The influence of oxygen partial pressure on the base oxide of chromia forming steels: The story prior to breakaway oxidation

The influence of pO2 on the base oxide formed on 304 L stainless steel has been investigated at 600 \ub0C. The alloy was exposed in 5%O2-95%N2 and 10%H2-20%H2O-Ar atmospheres and the initial stages of oxidation were analysed by TEM and EDX. In both environments, the thin scale consists of a Cr-rich oxide overlaid by a FeCrMn oxide. However, the subscale formed in H2-H2O is richer in Cr compared to the 5%O2-95%N2 case (∼90 cation% and ∼70 cation% respectively). The findings are in good agreement with thermodynamic calculations and can explain breakaway oxidation of marginal chromia forming steels in H2-H2O

High-Temperature corrosion of P91/T91, 304L, Sanicro 28 and Inconel 625 exposed at 600 \ub0C under continuous KCl deposition

This study investigates the corrosion attack after breakaway oxidation on four commercial alloys (T91/P91, 304L, Sanicro 28 and Inconel 625) in the presence of KCl(g)/KCl(s) at 600 \ub0C. The study suggests that an increase in corrosion resistant alloying elements (mainly nickel and chromium) results in the decrease of the thickness of the general oxide scale and not equally deep alloy grain boundary attack for the austenitic alloys. The corrosion attack in the presence of KCl is suggested to proceed by chromate formation and chlorine-induced acceleration of the diffusion of ions through the scale

Secondary corrosion protection of FeCr(Al) model alloys at 600 \ub0C – The influence of Cr and Al after breakaway corrosion

The influence of Cr and Al content on the oxidation behaviour of FeCr(Al) model alloys after breakaway oxidation at 600 \ub0C and the underlying mechanisms were investigated in detail with thermogravimetrical analysis (TGA), thermodynamic calculations and advanced electron microscopy. The results showed that a Cr-content of ≥18 wt% drastically reduced the growth rate of the Fe-rich oxide scale, formed after breakaway oxidation, for FeCrAl alloys but not for FeCr alloys. This was attributed to the ability of the Fe(18-25)CrAl alloys to prevent internal oxidation, which enables the formation of a healing layer

High temperature corrosion resistance of FeCr(Ni, Al) alloys as bulk/overlay weld coatings in the presence of KCl at 600 \ub0C

The long-term high temperature corrosion behavior of FeCr(Ni, Al) alloys, both in bulk form and as overlay welds, was investigated in a new experimental setup, in the presence of KCl at 600 \ub0C. All the alloys experienced breakaway oxidation, with the bulk materials exhibiting better corrosion resistance than the overlay weld coatings with similar compositions. All results indicate diffusion-controlled kinetics, and the analysis revealed an improved corrosion behavior connected to an increased Cr content in the inward-growing scale

High temperature corrosion memory in a waste fired boiler – Influence of sulfur

The selection of fuel for a Combined Heat and Power (CHP) plant can vary over time. By choosing less expensive fuels, operation costs are reduced, however, cheaper fuels generally increase corrosion maintenance costs. The corrosiveness of different fuels has been studied extensively while how the current corrosion attack is influenced by corrosion history, i.e. previous deposit build-up and oxide scale formation, is less studied. This phenomenon may be referred to as a “corrosion memory” effect (Paz et al., 2017). The present work investigates the influence of addition of sulfur to the fuel on the corrosion memory through air-cooled probes in the Waste-to Energy lines at M\ue5bjerg Energy Center (MEC) in Denmark. The results show a corrosion memory effect, i.e. as initially corrosive environment may increase the subsequent corrosion rate and vice versa