A Microstructural and Kinetic Investigation of the KCl-Induced Corrosion of an FeCrAl Alloy at 600 A degrees C

The corrosion behaviour of a FeCrAl alloy was investigated at 600 A degrees C in O-2 + H2O with solid KCl applied. A kinetics and microstructural investigation showed that KCl accelerates corrosion and that potassium chromate formation depletes the protective scale in Cr, thus triggering the formation of a fast-growing iron-rich scale. Iron oxide was found to grow both inward and outward, on either side of the initial oxide. A chromia layer is formed with time underneath the iron oxide. It was found that although the alloy does not form a continuous pure alumina scale at the investigated temperature, aluminium is, however, always enriched at the oxide/alloy interface

High Temperature Corrosion of FeCrAl Alloys: The influence of water vapour and alkali salt

The corrosive environment in biomass- and waste-fired boilers has been known for several years. The traditionally used FeCr and FeNiCr steels show limited performance in the aggressive environment that contains more H2O and alkali salts than traditional fuel. A more sustainable material is needed to satisfy the constant strive for higher operating temperatures. FeCrAl alloys are well-known for their good oxidation resistance at elevated temperatures and might be a suitable candidate. In this thesis, the KCl induced corrosion of FeCrAl alloys is investigated at 600 \ub0C. The chosen temperature is higher than today’s operating temperatures in the superheater region (̴ 350-550 \ub0C) and was chosen to meet the increasing demand from industry. In the laboratory, the effect of adding 0.5-0.1 mg/cm2 KCl was investigated on polished samples in O2 or O2 + H2O for up to 168 hours at 600 \ub0C. Samples were exposed to the same environment in the absence of KCl as reference. Pre-oxidised samples were also exposed to O2 + H2O + KCl in order to investigate the interaction of alkali salts with an alumina scale. The samples were analysed using TGA, SEM/EDX, XRD, IC, AES, SIMS and BIB cross sections. The two investigated FeCrAl alloys form a protective oxide layer in O2 and O2 + H2O which is rich in alumina with minor amounts of chromium and iron. Water vapour accelerates the oxidation and a slightly thicker oxide scale is formed in O2 + H2O. No evidence of chromium evaporation was recorded, as can be seen, on stainless steels. KCl accelerates the corrosion and a rapidly growing iron-chromium rich oxide forms in humid and dry environments. Chromate formation and alloy chlorination are found to initiate the formation of the non-protective oxide scales. Pre-oxidation of the alloy inhibits the severe corrosion attack in O2 + H2O + KCl. The oxide from the pre-oxidation at 700 \ub0C is not completely inert to KCl but is nevertheless considered protective in the present environment. Keywords: FeCrAl, oxidation, water vapour, alumina, high temperature corrosion, KCl, pre-oxidation, kinetics

High Temperature Oxidation and Chlorination of FeCrAl alloys

AbstractThe constant strive for improved efficiency and lower-cost industrial processes often results in progressively higher temperatures and more aggressive environments. High- temperature corrosion is a well-known problem in biomass- and waste-fired boilers and is one of the obstacles to overcome in achieving a more sustainable society. Alkali- and chlorine- induced corrosion of chromia-forming alloys has been studied by several researchers. However, the literature is scarce on how alumina forming-alloys, e.g. FeCrAl alloys, perform in such an aggressive environment. In this thesis, the KCl-induced corrosion of FeCrAl alloys is studied through well-planned laboratory exposures, detailed analyses and corrosion tests in a full scale waste-fired power boiler. In the laboratory, the effect of adding small amounts of KCl was investigated on polished FeCrAl samples in O2 or O2 + H2O environments up to 168 hours at 600 \ub0C. Pre-formed alumina scales were prepared on the alloy substrate through pre-oxidation at various conditions. The pre-treated samples were subsequently exposed to O2 + H2O + KCl in order to investigate their corrosion resistance. The samples were analysed using TGA, SEM/EDX, XRD, IC, AES, SIMS, BIB cross sections and STEM/EDX. In order to evaluate the validity of the findings in the laboratory studies, corrosion tests were carried out in a full-scale combined heat and power (CHP) boiler.The study showed that KCl-induced corrosion caused a rapidly growing iron-chromium-rich oxide to form in O2 and O2 + H2O environments at 600 \ub0C. Chromate formation and alloy chlorination were found to initiate the formation of a non-protective oxide scale. Alloy chlorination was greater in O2 than in O2 + H2O resulting in a more porous scale with poor adhesion to the alloy substrate. Pre-treating the alloy prior to exposure to O2 + H2O + KCl mitigated the corrosion, since alumina, itself, is rather inert towards corrosion. However, the corrosion started locally, most likely at flaws/cracks in the alumina scale and then spread laterally until the entire surface suffered from breakaway corrosion. Similar results were obtained in the corrosion tests in the waste-fired boiler. The pre-formed alumina scale failed after only 24 hours in the boiler at both 600 and 700 \ub0C

In Situ ESEM Investigation of KCl-Induced Corrosion of a FeCrAl and a Model FeNiCrAl Alloy in Lab Air at 450 degrees C

The very early stages of KCl-induced corrosion of a FeCrAl and a FeNiCrAl alloy in an 0211420 environment were studied using in-situ Environmental Scanning Electron Microscopy (ESEM). The samples were KCl contaminated and then exposed to lab air at a total pressure of 4.8 Ton at 450 degrees C. After exposure, the samples were analyzed using SEM-EDX, FIB and STEM-EDX. For both alloys, fast oxide growth occurred at the KCl particles. Far away from the KCl particles, a thin base oxide formed with potassium chromate nodules on top. The base oxide was layered, consisting of iron-chromium oxide on top and alumina in the bottom part. The FeCrAl alloy suffered internal oxidation while the FeNiCrAl did not. Both alloys formed sub-scale chlorides. A mechanism is proposed for the reaction of KCl on the surface and for the formation of different surface features at the KCl particles

In Situ Investigation of the Initial Stages of KCl-Induced Corrosion of a Chromia-Forming Steel at 450 degrees C Using an Environmental Scanning Electron Microscope

In the present work, the initial stages of KCl-induced high-temperature corrosion of a chromia-forming alloy (42Fe25Ni22CrWCuNbN), UNS S31035, is investigated by means of in situ environmental scanning electron microscope (ESEM) exposures in an O-2/H2O gaseous environment up to 1 h at 450 degrees C. Prior to the exposures, KCl particles were sprayed on the samples. In order to perform reliable in situ ESEM exposures, temperature calibration of the ESEM hot stage was done. Additionally, the in situ results were validated by reference tube furnace exposures, and the feasibility of the ESEM technique for obtaining dynamic information regarding the chlorine-induced corrosion process of the steel is discussed. Relatively large oxide crusts, with sizes of several micrometers, formed in the vicinity of the KCl particles. In between the KCl particles, a thin double oxide scale (20 nm to 100 nm), consisting of a top layer rich in iron oxide and a bottom layer rich in chromia, developed all over the surface of the alloy. Metal chloride was present beneath the thin base oxide, at the alloy/oxide interface, which shows the corrosive nature of KCl toward UNS S31035 in the studied environment. However, no extensive corrosion occurred at these locations after the short exposure times used in this work (1 h)

Cyclic Corrosion and Chlorination of an FeCrAl Alloy in the Presence of KCl

The KCl-induced corrosion of the FeCrAl alloy Kanthal® APMT in an O<inf>2</inf> + N<inf>2</inf> + H<inf>2</inf>O environment was studied at 600 °C. The samples were pre-oxidized prior to exposure in order to investigate the protective nature of alumina scales in the present environment. The microstructure and composition of the corroded surface was investigated in detail. Corrosion started at flaws in the pre-formed α-alumina scales, i.e. α-alumina was protective in itself. Consequently, KCl-induced corrosion started locally and, subsequently, spread laterally. An electrochemical mechanism is proposed here by which a transition metal chloride forms in the alloy and K<inf>2</inf>CrO<inf>4</inf> forms at the scale/gas interface. Scale de-cohesion is attributed to the formation of a sub-scale transition metal chloride

KCl-Induced Corrosion of an FeCrAl Alloy at 600\ub0C in O2 + H2O Environment: The Effect of Pre-oxidation

The present study investigates the influence of H2O and KCl on the high temperature corrosion of an FeCrAl alloy at 600 A degrees C. Polished samples were exposed to O-2 or O-2 + H2O and to O-2 + H2O with KCl applied. The samples were investigated using SEM/EDX, XRD, IC, AES and SIMS. It was found that KCl accelerates corrosion and that a rapidly growing iron-rich oxide forms with time. Chromate formation is shown to initiate the formation of a non-protective oxide scale. Pre-oxidising the alloy before exposure in the presence of KCl had a strongly beneficial effect on the corrosion

The Oxide Scales Formed on a Dispersion-Strengthened Powder Metallurgical FeCrAl Alloy at 900 A degrees C in O-2 and in O-2 + H2O

Early oxide scale growth on an oxide dispersion strengthened rapidly solidified powder FeCrAl material, Kanthal(A (R)) APMT, was investigated at 900 A degrees C in an O-2 + N-2 and an O-2 + H2O + N-2 environment for up to 168 h. Gravimetry was used to follow oxide growth and the oxide scale was examined with XRD. Scale morphology was investigated in detail with SEM/EDX, TEM/EDX/CBED. The alloy rapidly formed a protective two-layered alpha-alumina scale containing oxide nodules. Between the top and bottom alumina layers there was a zone containing chromia-rich particles 5-20 nm in diameter, corresponding to the original sample surface. The alumina scale mainly grew inward after 1 h of oxidation. Alumina scale growth at 900 A degrees C was initially somewhat faster in an O-2 + H2O + N-2 environment than in an O-2 + N-2 environment