In order to prevent high temperature corrosion, identifying the corrosion products is key. As with the analysis of aqueous corrosion products, a combination of methods such as SEM-EDS and XRD are required to identify the species that are present in scale formed at high temperatures. The major difference is that the scale is generally thicker and often multi-layered, which means that cross-sectional analysis is also necessary.
In this article, we'll examine corrosion products produced by high temperature exposures and explain how this information can be used in corrosion prevention.
Testing for Corrosion Products
Because high-temperature corrosion tends to produce scale that is multi-layered, line scans and spot maps may be used to illustrate the multi-layers but these techniques are qualitative. They rely on displaying total counts; density of the location can impact the results. Thus, it is also useful to supplement these analyses with quantitative or semi-quantitative data reductions.
Thermodynamic modeling for gaseous systems is becoming commonly used and is an important tool for the prediction of chemical reactions such as corrosion, oxidation, sulfidation and resulting corrosion products. Such calculations are more useful than an Ellingham diagram because multiple species can be included. The results can assist in identifying and confirming experimental results. Conditions can be easily changed as well, such as the inclusion of various gases with identification of the stable species.
For example, the temperature dependency of corrosion products or protective scale can be assessed for a specific set of conditions. On the other hand, it is also possible that by varying the concentrations of O2 and Cl2 for a given metal such as Fe, the conditions for various oxide and chloride species can be predicted for a given temperature. However, this modeling is based on equilibrium conditions and reaction kinetics can limit their utility. The advantage of such a tool is to compare the calculations to the observed corrosion products and verify the process conditions.
Many high temperature scales are layered and these can be noted by changing the corrodent concentration at a specific temperature because the concentration of the corrodent in the scale declines due to diffusion. High temperature oxidation of carbon or low alloy steel has a scale layer of FeO/Fe3O4/Fe2O3. Note that the oxidation state of Fe is the lowest next to the metallic phase, Fe(II) in FeO, and it increases towards the scale/environment interface, a mixture of Fe(II) and Fe(III) in Fe3O4, and Fe(III) in Fe2O3. The thermodynamic calculation will predict the outermost thermodynamically stable corrosion product in contact with equilibrium process stream.
The scale products noted below are typically what is observed but the exact scale layers are a function of the kinetics, temperature and species present. For that reason, the thermodynamic calculations for alloys are extremely useful to understand the composition of the scales. However, scales at temperature may transform to a different structure upon cooling to room temperature.
Scale Products with Low Alloy Steel
Below 400°C (752°F) the scale on Fe is magnetite (Fe3O4); while at 550°C (1022°F) in air a layered structure is found where Fe diffuses outward and O inward. Thus, the outer scale tends to be hematite (α-Fe2O3) and the inner scale magnetite. Under those same conditions for a 21/4% Cr 1% Mo steel, the outermost scale is hematite, while the inner scale is FeCr2O4 spinel, magnetite and hematite. Higher temperature oxidation of Fe results in a three-layered scale of hematite, magnetite and wustite (Fe1-xO). Increased Cr in the alloy can result in a mixed spinel (Fe,Cr)2O4. Composition of the scale varies with temperature and O2 partial pressure. Presence of H2O produces an inner (Fe,Cr)3O4, a middle scale of magnetite and an outer scale of hematite. An alloy requires a minimum of 14% Cr for a complete protective chromia (Cr2O3) layer that will prevent the outward diffusion of Fe and the inward diffusion of other species such as O. Thus, low alloy steel is limited to exposure temperatures of less than about 300°C (572°F).
The presence of SO2 can produce whisker growth and a slower growing magnetite layer. FeS will form as discrete grains in the inner magnetite layer while iron sulfate will form at the oxide surface depending on the partial pressure of SO2.
Scale Products with Austenitic Stainless Steel
The higher Cr content of the austenitic stainless steels provides sufficient oxidation protection to minimize scaling up to about 850°C (1562°F). Scales can consist of an inner scale of chromia, (CrxFe1-x)2O3 or Cr-rich (Cr, Fe, Mn)3O4 with an outer layer of hematite. Above 900°C (1652°F), the chromia-rich scales can react further with O2 to form CrO3, which is volatile. The presence of water vapor, if sufficient, reacts with the Cr in the oxide to likely form a volatile CrO2(OH)2, which results in an Fe-rich non-protective scale and the potential of breakaway oxidation.
The addition of HCl to O2 and H2O at 600°C (1112°F) produces a thicker and non-protective scale of (Fe,Cr)3O4, magnetite and hematite. Metal chloride particles may be embedded at the scale metal interface. Typical corrosion products from biomass or flue gas conditions containing varying amounts of O2, CO2, SO2 and HCl result in and inner layer of Ni3S2, a middle layer of spinel and hematite, and an outer SO4=, FexOy and metal chloride particles. The presence of H2O produces a thin (Fe, Cr, Ni)3O4 inner layer.
Scale products from sulfidation are a function of temperature and the partial pressure of the reducing S species. Under sufficient reducing conditions, the chromia protective layer can be sulfided to Cr2S3 or Cr5S6; however, at higher sulfur partial pressures a multi-layer sulfide scale will grow. The inner scale will be Cr-rich S scale with a middle scale of the spinel daubréelite (FeCr2S4) with variable Fe-Cr content and an outer pyrrhotite (Fe1-xS) scale. At higher temperatures and/or S partial pressure the outer scale can be (Fe,Ni)1-xS and pentlandite (Fe,Ni)9-xS8.
For the oxidation of FeCrAl alloys there is the initial formation of Cr2O3 and hematite and then the nucleation of corundum (α-Al2O3). The presence of water produces an outer layer structure of corundum with chromia rich particles between the layers. FeCrAl alloys possess improved high-temperature steam oxidation and are considered as accident-tolerant nuclear fuel cladding material.
Scale Products with High Temperature Nickel-Based Alloys
Ni alloys have a variety of different compositions and as such the corrosion scale can vary with the alloy. With higher Cr content, Ni-based alloys have higher oxidation resistance. In the early stages of oxidation, a continuous NiO layer forms, while Cr2O3 islands form in the grain boundaries. If Fe is present in the alloy, the layer can include NiFe2O4. As the outer NiO layer grows into the metal, it encounters islands of Cr2O3, which then form NiCr2O4 or (NiFe2-xCrx)O4 spinel islands. Since Ni-containing oxides are less protective than Cr2O3, the outer scale will be NiO with an inner spinel scale and a Cr2O3 layer. Depending on the Fe content of the alloy, hematite may also be observed in the outer scale. For alloys high in Al and at temperatures above about 1000°C (1832°F) an inner scale of corundum tends to develop, which when combined with the NiO and Cr2O3 may form the spinel.
Chlorination of Ni in either Cl2 or HCl produces a NiCl2 scale. Depending on the partial pressure of O2, NiO scale may also be present.
For a study with a gas of HCl, CO2, CO, H2 and H2S that is reducing and depending on the temperature, the corrosion products for Alloy HT were identified as FeCl2 (evaporates at higher temperatures), Cr2S3, Cr2O3 and NiS. Under these same conditions Alloy 600 had Cr2O3 and Cr2S3 as scale products.
With H2S/H2 above 645°C the (Fe,Cr,Ni)3S2 can form a liquid product. Combination with O2 with reducing SO2 may result in breakaway corrosion of Cr2O3 and Ni2S3. Such conditions can limit the applicability of these alloys.
High-temperature corrosion involves a number of key products. By identifying these, it is possible to prevent future corrosion.