High temperature corrosion of low-alloyed and stainless steels: mechanistic study of chlorine-induced corrosion
Doktorsavhandling, 2019

The global demand on power generation is constantly increasing and so far, also its environmental impact. The environmental impact could primarily be directed to the power generation being based on fossil fuels, giving a net increase of CO2 to the atmosphere when combusted. The need of renewable fuels such as biomass and waste for power generation, leading to no net release of CO2, is therefore increasing. However, biomass and waste vary in composition and the considerably high amounts of alkali- and chlorine-containing species of these fuels result in a highly corrosive fireside environment for the metallic components of the boiler. Chlorine-induced corrosion is speculated to play an important role in the corrosion of these metallic components. However, the consequences of event leading to corrosion in the presence of chlorine is still not fully understood and the corrosion mechanism is under debate. Thus, this study aims at investigating mechanism of chlorine-induced corrosion. The study is divided into two parts; field exposures showing the extent and initiation of a chlorine-induced corrosion attack and laboratory exposures aiming at investigating the mechanism of the chlorine-induced corrosion attack.

The field exposures were focused on the startup sequence of probe exposures. The results showed that the initiation of breakaway corrosion is very rapid in this environment. Thus, the primary protection, i.e. the Cr-rich oxide scale on stainless steels, was immediately destroyed and the oxides and metal chlorides formed set the boundary conditions for further corrosion, i.e. secondary protection. The results showed that the different startup sequences had only a minor effect on the initial corrosion attack.

Based on the corrosion attack observed in the field-exposed samples, a set of laboratory exposures was designed. The objective was to investigate the mechanism behind chlorine diffusion through oxide scales at high temperatures. A series of pre-oxidations were performed in order to investigate the role of oxide composition, microstructure, and thickness on chlorine-induced corrosion.

The investigation showed that the presence of either KCl(s) or HCl(g) accelerates the corrosion rate of all the investigated materials. Both thickness and microstructure of the Fe-rich oxide, i.e. secondary protection, influences the incubation time to breakaway corrosion. In addition, cracking and spallation of the Fe-rich oxide, as well as the presence of metal chlorides at the oxide/metal interface below a crack-free scale, were observed. Thus, the corrosion attack may be driven by both for crack formation and chlorine diffusion through the oxide scale. Mechanisms for both the influence of crack formation on the corrosion attack and alternative diffusion paths for chloride is proposed. DFT calculations showed that the diffusion of chloride ions through the oxide scale is energetically favoured to occur via oxygen vacancies.

stainless steels

low-alloyed steels

pre-oxidation

High temperature corrosion

chlorine-induced corrosion

Seminar room 10:an, Chemistry Research Building 1, Kemigården 4
Opponent: Associate Prof.Dr.-Ing. Mathias Galetz, DECHEMA Forschungsinstitut, Germany.

Författare

Mercedes Andrea Olivas Ogaz

Chalmers, Kemi och kemiteknik, Energi och material, Oorganisk miljökemi

A. Persdotter, M. Sattari, E. Larsson, M. A. Olivas Ogaz, J. Liske, T. Jonsson Oxidation of Fe-2.25Cr-1Mo steel in presence of KCl(s) at 400 °C – crack formation and its influence on oxidation kinetics. Submitted to Corrosion Science

V. Cantatore, M. A. Olivas Ogaz, J. Liske, T. Jonsson, J.-E. Svensson, L-G. Johansson. I. Panas Permeation of iron oxide scales by chloride - new insights by atomistic modelling Manuscript

Förbränning av biomassa och avfall är av stor betydelse för att minska koldioxidutsläppen i Sverige. Förbränning av dessa typer av bränslen resulterar dock i en väldigt aggressiv miljö vilket leder till snabb nedbrytning av de metalliska komponenterna i pannan på grund av korrosion.

Vad tänker vi på när ni hör ordet korrosion? Förmodligen en rostig cykel, metalkedja eller förtöjningsring som lämnats ute i en fuktig miljö. Korrosion är en naturlig process där en metall eller legering omvandlas till en mer termodynamiskt stabil fas såsom en oxid (rost). Den här processen gör att vissa stål bryts ner med tiden. Men vad händer vid högre temperaturer? Även under dessa betingelser strävar metallen eller legeringar åt att uppnå den mest termodynamiskt stabila fasen (oxid) men de involverade mekanismerna skiljer sig mycket från de vid lägre temperaturer.

Vilken/vilka oxider som kan bildas och som är mest stabila beror på legeringens kemiska sammansättning och på omgivningen (temperatur och syrepartialtryck). Vissa av oxiderna anses vara skyddande och hindrar vissa korrosiva ämnen från att nå metallytan vilket bromsar korrosionsprocessen.

Det finns däremot vissa applikationer där material utsetts för väldigt aggressiva och komplexa miljöer. Det finns vissa ämnen som kan bryta ner även de allra mest skyddande oxiderna. Ett sådant exempel är klor vilket finns tillgängligt i stora mängder i biomassa- och avfallspannor. Jag har därför valt att fokusera min forskning på att öka förståelsen av korrosionsmekanismerna bakom den väldigt snabba nedbrytningen av metalliska material i närvaro av klor.

In Sweden, combustion of biomass and waste is of great importance contributing to the decrease of CO2 emissions. The combustion of these type of fuels results in an aggressive environment leading to rapid material degradation due to corrosion of metallic components in the boiler.

Corrosion? Every time we hear the word corrosion, what do we imagine? Probably a rusty bicycle, metal chain or mooring ring that was left exposed to humid conditions outdoors. Corrosion is a natural process where a metal or alloy is converted to a more thermodynamically stable form such as an oxide (rust). However this process will gradually degrade the steel. And what about corrosion at high temperatures? Also under those conditions the metal or alloy is striving to achieve a more thermodynamically stable form (oxide) but the mechanisms involved differ a lot from those at atmospheric conditions.

Which oxide(s) that is more stable and is able to form depends on the chemical composition of the alloy or metal as well as the environmental parameters (temperature and oxygen partial pressure). Some of these can be considered protective and inhibits further corrosion attack. However, in some applications, certain components are exposed to extremely aggressive and complex environments. There are some species that are able to break down even these very protective oxides. One such species is chlorine which is available in large amount in biomass and waste fired boilers. Thus, my research has been focused on improving the understanding of the corrosion mechanisms, regarding this very problematic species (Cl), responsible for the very rapid material degradation.

Ämneskategorier

Annan kemi

Annan materialteknik

Korrosionsteknik

ISBN

978-91-7905-104-4

Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 4571

Utgivare

Chalmers tekniska högskola

Seminar room 10:an, Chemistry Research Building 1, Kemigården 4

Opponent: Associate Prof.Dr.-Ing. Mathias Galetz, DECHEMA Forschungsinstitut, Germany.

Mer information

Senast uppdaterat

2019-04-09