Influence of KCl and HCl on a Laser Clad FeCrAl Alloy: In-Situ SEM and Controlled Environment High Temperature Corrosion

Increasing pressure on the power industry to reduce carbon emissions has led to increased research into the use of biomass feedstocks. This work investigates the effects of HCl and KCl, key species influencing biomass boiler corrosion, on a laser clad coating of the FeCrAl alloy Kanthal APMT. In-Situ SEM exposure of the coating at 450 oC for 1 h was performed to investigate the initial effects of KCl on the corrosion process. The same coatings were exposed to 250 h exposures in both an air environment and a HCl rich environment. The influence of KCl was investigated in both. Evidence of a slow growing aluminium oxide was observed. It was found that HCl allowed chlorine based corrosion to occur suggesting it can interact from the gas phase. It was also observed that the presence of both HCl and KCl reduced the mass gain compared to KCl in an air environment.


Introduction
As global legislation pressures countries to transition to more sustainable energy systems, there has been an increase in the uptake of biomass combustion as an alternative to conventional fossil fuels [1]. Whilst using biomass in a conventional fossil fuel boiler is in many ways similar to conventional fossil fuel combustion, it still comes with specific challenges.
Biomass combustion can create a more aggressive combustion environment within a boiler, leading to increased material loss from boiler components and significant reductions in operational performance [2].
One of the techniques used to combat the aggressive high temperature corrosion environments within boilers is to use materials with higher corrosion resistance. Many of these materials are expensive, and as such it is not economically feasible to replace conventional materials in bulk. In these situations, a surface engineering solution or a corrosion resistant coating can be applied to the outside of a bulk material made from inexpensive, low performance alloy. One such technique for carrying out this coating process is laser cladding whereby the feedstock material is fed into a meltpool on the surface of the substrate generated by a laser. As the meltpool is moved across the surface a coating is built up.
Currently, materials used in heat exchanger components are mainly ferritic steels and the most commonly used coating materials for heat exchangers are corrosion resistant nickel based chromia forming alloys [3]. An alternative to these is high corrosion resistant iron-based alloys (FeCr, FeAl, FeCrAl, etc.) due to the lower cost and abundance of iron. The addition of small amounts of aluminium into the traditional FeCr alloys to form FeCrAl alloy has been shown to allow the formation of alumina at temperatures above 900 o C [4].
Other work has shown that a thin alumina layer will form at temperatures of 600 o C on a FeCrAl alloy, however this is most likely transient and so not stable and therefore non-protective [3]. The formation of stable alumina is beneficial as it forms dense, thermodynamically stable protective layer [5].
Salts play an important role in the corrosion performance of materials at high temperature. Corrosion performance can be influenced by these salts in both their solid and liquid phase. With typical meting temperatures in the rage of 500 o C to 1000 o C, these temperature ranges are the most studied. It is well established in the literature that chloride salts play a particularly important role in biomass induced fireside corrosion due to their high levels in many biomass feedstocks [6,7]. In contrast to coal combustion, where typical deposits contain zinc and lead chlorides, most biomass combustion deposits are dominated by potassium chloride [8]. Hydrogen chloride is the most common chlorine containing species in biomass flue gas [8]. It has been shown that combustion of feedstocks can produce HCl [9]; however, this concentration is proposed to be increased by additional production as a by-product in mechanisms such as the formation of alkali chromates in the presence of alkali salts and water vapour [10]. The protective oxides which form on the surface of metals are typically dense, and prevent further diffusion of oxygen to the scale metal interface. This pacifies the surface and prevents further oxidation. HCl is able to diffuse through this dense protective oxide towards the metal scale interface. Once at this interface, where the partial pressure of oxygen is low, the hydrogen and chloride dissociate as metal chlorides become more energetically favourable. Once formed, the metal chlorides can diffuse back through the protective scale towards the scale gas interface. Here in the higher partial pressures of oxygen, the metal chlorides are oxidised, releasing the chlorine to diffuse back through the protective scale in the same cycle as the hydrogen chloride.
FeCrAl alloys offer an alternative to conventionally used low alloy steels.
The addition of aluminium allows for the formation of alumina, as well as chromia scales. These scales are dense and thermodynamically stable. The current research into the effects of KCl and HCl on FeCrAl alloys is focused on two main areas: short scale, and long scale exposures. In short scale tests, it has been shown that pre-formed alumina scales, formed at 900 o C for 1 h, provide protection against chlorine attack. Flaws in this protective alumina layer provide pathways for chlorine ingress and the eventual failure of the coating [10]. Without pre-oxidation, at 600 o C, only an aluminium enrichment was detected at the scale coating interface, and potassium chloride was able to react with the FeCrAl alloy as well as O2 and H2O present in the corrosion environment to form potassium chromate [3,11]. The other area of research is at much longer time scales. Here the effects of chlorine are looked at in more complex environments and for much longer durations. Whilst playing an important protective role at short time scales, the concentration of chromium does not dictate the corrosion performance. However in longer scale tests, it is the concentration of chromium that will have the largest effect on corrosion performance, with aluminium concentration playing a less significant role [12].
At higher temperatures of 800 o C in these more complex chlorine riche environments the level of aluminium became important as alumina began to form providing protection and reducing metal loss [13].
For much of the previous work discussed above, In-Situ Environmental Scanning Electron Microscopy (In-Situ ESEM) was used. In-Situ ESEM offers opportunities to investigate the progression of corrosion as it occurs.
Environmental SEM chambers allow carful control of the gaseous environment surrounding a sample. This can be paired with real time imaging of the sample.
The technique becomes more powerful when a heated stage is added into the chamber, allowing for high temperature corrosion tests to be imaged in real time. When looking at iron at 500 o C and 700 o C, in-situ measurements allowed for the structure of the initial iron oxides that grow on the surface to be investigated, providing insight into the earliest mechanisms to act [14]. The initial heating conditions of a sample can play an important role in its oxidation mechanisms. The use of in-site ESEM means that these initial heating stages can be observed [15]. It was found that KCl crystals melted at temperatures below 400 o C, forming oxide shells in their presence. It was also found that a liquid film of these molten salts allowed the rapid transport of chlorine across the surface.  and a wire feed rate of 6.6 mm s -1 . Each successive pass of the laser deposited at a step distance of 2.618 mm producing a 40% overlap coating. These parameters were selected from previous work on optimising coatings for corrosion performance [16]. This section of coated substrate was cut into 10 mm Energy-Dispersive X-ray (EDX) spectroscopy was performed on the segregated areas to determine their composition. An 80 mm 2 spectrometer (Oxford Instruments, UK) was used to take these measurements and the analysis software INCA (Oxford Instruments, UK) to draw elemental information. This was again performed pre and post exposure.

Methodology
The samples were deposited with a saturated solution of KCl in water and industrial methylated spirit pipetted onto the surface. In-situ high temperature oxidation tests were carried out in an FEI Quanta 200 Field Emission Gun Environmental Scanning Electron Microscope (FEG ESEM) (Thermo Fisher Scientific, MA). The setup of the SEM is the same as that found in previous work [15]. The prepared samples were exposed in laboratory air at 1 Torr for 1 h. The After surface characterisation, cross-sections were made of the samples.
These cross-sections were cut, ground and polished using the same process as for the initial surface preparation, however non-aqueous lubricants were used to ensure chlorides were not removed. These sections were cold mounted to ensure the protection of any deposit formed and were again analysed with SEM and EDX using the same instrumentation to form a complete analysis of the samples.

3.1.
In-situ SEM exposure of APMT at 450 o C Beyond this region, across the boundary at B, there is a very rapid chromium depletion over a distance of approximately 6 nm. Here chromium concentration reaches 13.8 %. This is again matched by a change in iron as its concentration raises back to 74.5 % over the same region. As oxygen has been excluded from the EDX measurements, this transition region most probably represents the ironchromium spinel. Moving inwards from point B we see a small region in Figure 2 below the boundary at B, approximately 15 nm thick where chromium returns to its nominal bulk value of between 21.6 ± 0.1 %. Iron also returns to its bulk value over this region of 67.7 ± 0.1 %.

3.3.1.Without KCl deposit
After completing exposures in air for 1 h, long term exposures were carried out for 250 h. The top surface of the sample exposed without a KCl deposit can be seen in Figure 8. Figure 8 (A) shows a low magnification image of the top surface. Much like the surface of the sample exposed for 1 h, there are very few features that can be seen on the surface of the sample. The features that can be seen are shown in higher magnification in Figure 8 (B). Table 1 provides information on the composition of these phases. Whilst point 1 is iron and chromium deficient compared to the substrate measured at point 2, it is enriched in manganese suggesting a manganese oxide on the surface. This was not picked up in the XRD shown in Figure 6 however these oxides are very small.

3.3.2.With KCl deposit
When exposed for 250 h in the presence of KCl, the deposits which grow on the surface of the APMT, seen in Figure 9 are very different to those that are observed at shorter duration and without KCl. Figure 9 (A) shows a low magnification image of the surface. It can be seen that there is a continuous and varying covering over the entire surface of the coating. Higher magnification images show some of the complex phases present in higher magnification. Figure 9 (B) shows a thick, extruded tube like structure. These structures have a cracked layered surface and can be seen from Table 2 to be rich in chlorine and chromium. The point 1 is the underlying iron oxide layer, however it can be seen that this has been penetrated by chlorine in Table 2

3.4.1.Without KCl deposit
After investigating the behaviour of the coating for different exposure lengths in air, a synthetic flue gas rich in HCl was used for a 250 h test. This can be seen in Figure 10. At low magnification as seen in Figure 10 (A) a fairly regular deposit can be seen the entire surface, although it is not continuous. Figure 10 (B) shows a typical structure that makes up the majority of the deposits seen on the surface. This deposit is covered by point 1 in Table 3. It is very thin so shows mostly the underlying APMT coating however there is also chlorine present. Figure 10

3.4.2.With KCl deposit
The final sample analysed was in a combination of both HCl gas and a KCl deposit. It can be seen in Figure 11 (A) that there is a complete covering of the sample with deposit. Figure 11 (B) shows the two components that make this up. There is a thin, continuous oxide that covers the entire surface. This is very thin and as such, is difficult to detect with EDX. The other feature on the surface is the chlorine rich tubes similar to those seen in Figure 9 (B). Finally, the plume like structures originating from pores that were seen in Figure 10 (C) are longer and better defined under these conditions.

3.4.3.Mass change
Mass change was taken from the coated to the poste exposure samples.

Oxidation in air and HCl
In the TEM cross-sections of the APMT clad exposed for 1 h in air, in a region where the influence of KCl is not obvious, there were three major phases In the simplest environment, in air without KCl, the results observed were consistent with the short term exposures in that very little was observed on the surface within the limitations of SEM imaging. There is some evidence that manganese oxide formed on the surface. This is unexpected due to the minimal manganese in the coating (0.4 wt.%); however, this may be a region in which dilution of the substrate into the coating is high and as such the manganese concentration is higher in the coating than in the bulk.
Looking at the slightly more complex environment of HCl without KCl in long term exposures shows similar results after a 250 h exposure to that in air.
The mass change of these two sets is very similar, initially suggesting that the presence of HCl does not affect corrosion; however, like in an air environment, the observable reaction products are very small, and as such the similarities in mass change could be due to measurements of such small amounts of reaction product; despite this chlorine was detected on the surface of the sample under SEM analysis. This could be particularly important when looking at the plume like structures that have grown from pores in the sample (Figure 10 (C)). These features could represent the oxidation of metal chlorides that have formed at the lower oxygen partial pressures present inside a pore [18]. These plumes were extremely small but could signify the initiation of the reaction products found in exposures with KCl discussed later.

KCl deposit induced corrosion
Looking at the same TEM cross sections (Figure 4) of the APMT after 1 h exposure in-situ, but in close vicinity to a KCl crystal, the same three major components are spotted as in the regions far from the KCl crystal: an iron rich oxide, an iron-chromium mixed oxide and an aluminium enriched layer.
Additionally, a potassium chloride enrichment was observed at the original coating-environment interface. A chromium depletion zone was again observed within the coating after the innermost mixed oxide layer. When comparing the depth of this chromium depletion zone in the presence of KCl as opposed to areas far from the KCl crystals, it can be seen that both the depth of the depletion zone, and the overall thickness of the reaction products are increased in the presence of the KCl. This increased thickness is especially true of the iron oxide that has grown outermost. It is well established that the presence of chlorine can inhibit the growth of chromium oxides and as such could explain the increased thickness of the iron based outer oxide [3]. It can also be seen in With the presence of HCl instead of air, the most notable result is the reduced mass gain in the coating with KCl as can be seen in Figure 12. Whilst it can be seen that the mass gain is greater in both air and HCl when KCl is present, the mass gain in HCl with KCl is significantly lower than that observed in air with KCl. In an air environment, the only source of chlorine is from the dissociation of potassium and chlorine. The presence of HCl causes an increase in chlorine partial pressure which could slow the dissociation of potassium from chlorine and therefore cause the observed mass gain differences. It has been proposed that HCl can produce chlorine through the reaction shown in Equation 1 [19].

Equation 1
This is consistent with the chlorine detected in EDX on the sample exposed in HCl without a KCl deposit, suggesting that the HCl gas is providing some chlorine to the surface.

Generalised mechanism of corrosion of APMT
In the most simple environment of APMT clad in air, the formation of oxides can be explained by the following process [17]. Oxidation of aluminium to form Al2O3 happens very briefly until the oxygen partial pressure drops low enough that chromium and iron start to diffuse outwards. These form a Cr3O4 and Fe3O4 solid solution, which in time, form a (Fe,Cr)3O4 spinel in solid solution with the more abundant Fe3O4. This explains the multi layered oxides present and the formation of the spinel layer.
When HCl is added to the system, there is no reason to think that the above mentioned mechanism will not continue to take place. This is confirmed in the exposures seen where similar reaction products are formed. The formation of the plume like structures may be explained using the presence of HCl. It has been shown that Cl -, rather than HCl itself is the main aggressive species [20].
This can be formed through Equation 1. In low oxygen partial pressures, such as those found inside a pore, this newly formed chlorine ion can react with chromium or iron to form (Fe,Cr)Cl. These metal chlorides, as they diffuse towards the surface, are exposed to higher oxygen partial pressures where they undergo the following reaction according to Equation 2. The free chlorine ion is available to move back into the coating, allowing corrosion to continue as part of the chlorine cycle [21].

Equation 2
When KCl is added to the system again, it gets more complex. Looking at KCl in an air environment, these mechanisms likely change again, which accounts for the vast differences in the corrosion products formed with and In the most complex environment, a combination of all of these processes are in action. Initially, corrosion will follow the mechanism described above in Equation 3. The free chlorine ions originating from the KCl will play a role in the chlorine cycle, moving metal ions to the surface in the form of metal chlorides.
Under normal conditions in air, Equation 2, these metal chlorides would be converted to metallic oxides at the surface, releasing chlorine to continue the cycle. This reaction will occur at a slower rate in the presence of HCl, due to the higher concentration of chlorine present at the surface already, formed from Equation 1. The net result of this is that potassium chromate is still formed; however, chromium chloride is not converted to chromium oxide, causing chromium chloride deposits to form, and reducing the formation of chromium oxide. This reduction in the formation chromium oxide accounts in part for the reduced mass gain observed when compared to APMT exposed in an air and KCl environment alone. The reduction in formation of chromium oxide through this mechanism is paired with the production of chromium chlorides. These chlorides are volatile, and can be removed at the gas oxide interface. This in turn contributes further to the reduced mas gain. It must be noted that this reduction in mass gain does not correlate to a reduction in corrosion rate, as chromium is lost though the volatile species produced.
This proposed mechanism is consistent with the results observed. In all cross-sections, an outer iron rich oxide is seen on top of an inner mixed spinel.
The introduction of HCl shows the formation of a thicker oxide layer as well as evidence of potential potassium chromate forming from pores.

Conclusion
In this study, the corrosion mechanics of the iron-chromium-aluminium Kanthal APMT were investigated. The corrosion performance was tested in two environments. A short term corrosion test was carried out for 1 h at 450 o C in an air environment. These tests were carried out in an in-situ ESEM. Longer term exposures were carried out in tube furnace for 250 h in air. These tests were also repeated in a HCl rich environment. These were also carried out at 450 o C.
All tests were carried out with and without the presence of KCl. The resulting corrosion products were analysed and the following conclusions made:  Further evidence is found to support the role that chlorine plays in corrosion of APMT. Chlorine was found at the oxide coating interface after only 1 h when KCl was present.
 Aluminium is shown to migrate to the oxide coating interface suggesting that alumina may be slow growing in this region over longer time scales.
 Results obtained in-situ were able to be replicated in a furnace, observing similar levels of oxidation over the same time scales. This was replicated at longer time scales where oxide growth was greater but compositions were similar.
 In the presence of a hydrogen chloride environment the role of chlorine could still be seen suggesting that chlorine can interact with the surface from the gas phase as well as from the solid potassium chloride.
 The presence of hydrogen chloride provided protection to surfaces deposited with KCl. This is proposed to occur due to the breakdown of hydrogen chloride and oxygen into water and gaseous chlorine, slowing the dissociation of potassium and chlorine and hence limiting the rate of oxidation.