Isothermal corrosion tests

Isothermal corrosion tests for carbon steel A516.Gr70, unexposed to the graphite layer (non-graphitized), with hydrated HitecXL.H 2 O salt produced a corrosion layer, which was observed by SEM cross-section analysis (Fig. 1a, b). XRD analysis of the samples surface showed that the layer is composed of iron oxides in the magnetite and haematite phases (Figure S3). On the other hand, the SEM analysis shows the formation of a protective layer on the surface of the carbon steel, which is an iron carbonate layer as discussed in our previous work.20 These layer is expected to be protective.38,39,40,41

Fig. 1 SEM images of cross-sections of carbon steel after 1500 h corrosion test with HitecXL·H 2 O. The cases of a separated and b attached protective FeCO 3 layer. Scale bars are 1 µm Full size image

When the carbonate layer separates from the carbon steel, the oxidation thickness is observed to be around 1.0 μm (Fig. 1a). However, when the carbonate layer remains attached, the oxidation thickness is around 0.6 μm (Fig. 1b). Hence the presence of a carbonate layer reduces the amount of oxidation by nearly twofold. Though, the case shown in Fig. 1b was rarely detected while statistically examining the carbon steel samples after the corrosion tests, and it was estimated that less than 1% of the surface was covered with FeCO 3 .

While being inspired by the protective properties of this carbonate layer, we decided to increase its formation during the corrosion test. In particular, the carbon concentration in the system was increased. To achieve this, graphite was added up to a total mass of 2 wt% into the salt prior to the corrosion tests (see section 'Salts preparation' for details). The isothermal corrosion tests revealed an unexpected and intriguing result. A dense firmly attached layer of calcium carbonate was formed on the surface of carbon steel (Fig. 2a, b). The formation of calcium carbonate was confirmed using XRD analysis with the spectra presented in Figure S3. The layer demonstrates the desired protective behaviour, by completely inhibiting the formation of the oxide layer (Fig. 2b). Only in a few rare zones, the corrosion layer was found on the steel surface. However, the thickness of the corrosion layer was reduced (Fig. 2a) when compared with the carbon steel without the graphite exposure (Fig. 1a). The same observations were seen on the surface of the carbon steel after the corrosion tests by SEM-EDX (Fig. 3). In this case, the surface of carbon steel is covered by the cubic crystals of calcium carbonate in contrast to carbon steel which was not exposed to graphite. There are some spots on the carbon steel surface, which present a much lower concentration of carbon and a higher concentration of oxygen, suggesting the presence of oxides. XRD analysis confirmed the presence of a mixture of CaCO 3 and iron oxide phases (Figure S3).

Fig. 2 SEM images of cross-sections of carbon steel after 1500 h corrosion test with HitecXL·H 2 O + 2 wt% Graphite. The cases of a presence and b absence of oxidation layer under CaCO 3 carbonate layer. Scale bars are 1 µm Full size image

Fig. 3 SEM images and EDX mapping of the surface of carbon steel after 1500 h corrosion test with HitecXL·H 2 O + 2 wt% graphite under different magnifications. Scale bar is a 25 µm, b 5 µm and c 2 µm Full size image

The obtained results illustrate the possibility of increasing the compatibility between carbon steel and molten HitecXL salt by raising the carbon concentration in the system. However, direct addition of graphite into the salt can cause a problem with the thermal decomposition of the unconsumed graphite at a higher temperature. The last point is demonstrated in Figure S5, where HitecXL.H 2 O salt with 2 wt% graphite shows a behaviour like pure HitecXL only until 450 °C. Nevertheless, the further increase of the temperature drives the decomposition process resulting with a pronounced mass loss at ~450 °C.

These experiments provide important information on the corrosion process of carbon steel in molten nitrate salts and build a strong basis for the improvement of corrosion resistance method. The accumulated knowledge was used to develop more application-suitable anticorrosion methods. In particular, a simple method of spraying the carbon steel surface with a very thin layer of graphite was tested (see section 'Carbon steel samples preparation' for more details on sample preparation). In this case, the mass of graphite in the system is very low—~5·10–3 g·cm–2 of graphite per area of carbon steel. This corresponds to about 0.08 wt% of graphite in the salt.

The results of the corrosion test for the graphite-treated (graphitized) carbon steel are summarized in Fig. 4. It was found that almost 99.9% of the surface is completely covered by the crystals of calcium carbonate. Only few spots exhibit lower concentration of crystals (Fig. 4b). The complete coverage of the carbon steel surface by the protective layer is mostly due to the initial homogenous distribution of graphite (Figure S11). The analysis of the cross-section revealed the absence of detectable corrosion layer and a very thin layer of CaCO 3 on the surface, which is confirmed by EDX (Fig. 4a) and XRD (Figure S3) analyses. This result confirms the protective activity of CaCO 3 . The thermogravimetric curve of the salt after the corrosion test with graphitized carbon steel showed a decomposition temperature like the pure HitecXL (Figure S5). XRD analysis of these samples could barely detect the residual amount of iron oxides (much lower when compared with 2 wt% graphite case) and showed a clear peak of CaCO 3 (Figure S3). The corrosion rates obtained for the experiments presented above are summarized in Table 1.

Fig. 4 SEM images and EDX mapping of a cross-section (scale bar is 1 µm) and b surface of graphitized carbon steel after 1500 h corrosion test with HitecXL·H 2 O (scale bar is 2 µm) Full size image

Table 1 Corrosion rates of carbon steel under different conditions Full size table

Other methods for creating the carbonate layer were tested in this work: (1) graphite doping of the salt to a concentration of 1 wt%; (2) ethanol-deposition of graphite onto the surface of carbon steel; (3) temperature pre-treatment under CO 2 atmosphere and (4) MgCO 3 doping of the salt. Details on the results of these experiments are presented in the Supplementary information (Figures S6–S10). For all the methods except for MgCO 3 doping, the formation of CaCO 3 protective layer was observed, however, complete coverage of the surface of carbon steel was not achieved.

Effect of oxygen and thermal cycling

Reactions which form the carbonate surface layer and the oxidation reactions on the carbon steel are competitive reactions.38,39,40,41 Thus additional corrosion test for the proposed method was performed under an air atmosphere to observe the formation of carbonate layer under more challenging conditions.

First, for demonstration purposes, half of the piece of carbon steel was sprayed with graphite, while the other half was not treated. After a 24 h corrosion test with HitecXL at 310 °C under an air atmosphere, crystals of CaCO 3 appeared and were observed under an optical microscope (Fig. 5). The line of separation between graphitized and non-graphitized surfaces is very clear (Fig. 5): while the graphitized part is well protected by CaCO 3 , the non-graphitized one exhibits clear signs of oxidation. XRD and SEM-EDX analyses further confirmed the formation of crystal CaCO 3 .

Fig. 5 Optical microscope images of the surface of partially graphitized carbon steel after 24 h corrosion test with HitecXL. Scale bar is 50 µm Full size image

Given the excellent results under an air atmosphere, we decided to try more challenging conditions by performing thermal cycling in the range of 300–500 °C with exposure to air, which is the typical condition for a CSP plant. The experiment consisted of 125 consecutive cycles over a 500 h period, the results of these tests are seen in Fig. 6. In the case of non-graphitized carbon steel, the corrosion layer thickness is high and layer detachment is observed (Fig. 6a). While the increased rate of oxidation can be attributed to a higher maximum temperature in the cycling test compared with isothermal at 310 °C, the layering of the material is probably due to thermal cycling and is caused by different dilatation of oxide layer and carbon steel. On the other hand, graphitized samples clearly demonstrate the formation of a layer of CaCO 3 crystal over the entire surface (Fig. 6b), with little evidence of the oxidation. XRD analysis reveals the presence of CaCO 3 phase and two iron oxides—magnetite and haematite (Figure S4). This suggests that some oxides form during the corrosion test, however, the corrosion rate decrease about three times compared with non-graphitized carbon steel (Table 1).

Fig. 6 SEM images of cross-sections of a carbon steel and b graphitized carbon steel after 125 heating-cooling cycles in the 300–500 °C temperature range. Scale bars in the main Figures are 5 µm. Scale bars in the insets are 1 µm Full size image

The results demonstrate the potential of a simple graphitization method to improve the corrosion resistance of carbon steel against molten nitrate salts. The final evaluation, however, requires further corrosion tests by using experimental conditions close to real conditions. In particular, it is necessary to verify the stability of the formed carbonate layer under flow conditions. It must be noted that the presence of graphite on the surface of carbon steel is also beneficial in terms of lowering the surface energy of the carbon steel, which is observed from the contact angle images of the molten salt (Figure S12). This property is beneficial for molten salt pumping under harsh conditions. It reduces the frictional dissipation associated with laminar flow near the walls while reducing the stress on the carbonate layer increasing its lifetime. More dynamic tests are required to further confirm this method.

Molten salt and carbonate layer chemistry analysis

In this section, we present how the proposed method of graphitization affects the molten salt and its interaction with the CaCO 3 protective layer. Especially, the solubility of CaCO 3 in the molten nitrate salt is a concern. In general, it is known that carbonates are soluble in molten salts. To explore this phenomenon, which directly affects the stability of this protective layer for long-term use, several tests were performed.

Firstly, the thermophysical properties (heat capacity, enthalpy of melting and melting temperature) of the salts, both before and after corrosion tests with carbon steel and graphitized carbon steel, were compared and found to be similar within the precision of equipment (Figure S13). These observations suggest that there is no significant change in the composition of the salt.

Additionally, FTIR spectra of the HitecXL salts after the corrosion tests with carbon steel and graphitized carbon steel are shown in Figure S14. It was observed that there are no changes related to the main nitrate peaks, however, the presence of traces of CaCO 3 is evident in the HitecXL salt after the test with graphitized carbon steel. This observation is in favour of the limited dissolution of CaCO 3 into the molten salt during the corrosion test. It should be also noted that no detectable traces of iron were present in the salts after the corrosion tests.

Finally, to further explore the stability of CaCO 3 in the molten HitecXL salt under the same conditions of the corrosion test, a controlled solubility experiment was carried out. Pellets of pure CaCO 3 were immersed into the molten HitecXL salt and kept at 310 °C for 1 month. A small concentration of CaCO 3 was used (0.05 wt%). Next, the mixture was quenched by immersing in liquid nitrogen. In this way, the crystals of CaCO 3 are easily distinguished from glassy HitecXL by SEM. The CaCO 3 crystals remained in the crystalline form is the indication of their limited solubility in HitecXL. This approach was used by Su et al. to study the solubility in molten salts.42 The observation of the stability of CaCO 3 micropellets in a quenched salt, (Figure S15a and S15b) as well as CaCO 3 crystals (Figure S15c) are considered as an evidence of its insolubility (or very limited solubility) in the molten HitecXL salt under the test conditions. Figures S15a and S15b show the cross-section of the CaCO 3 pellet in the quenched HitecXL. It is clearly observed that the pellet retains its sharp edges and does not show signs of dissolution. Separated crystals of CaCO 3 have been also found in the salt and support the limited solubility in molten HitecXL salt (Figure S15c).

We consider the above-presented observations (Figures S13–S15) combined with the well-formed CaCO 3 layer on the surface of carbon steel (Figs. 2–6) as a good sign of the stability of CaCO 3 protective layer in the molten nitrate salts. However, longer tests are required for further confirmation of the proposed method. It must be noted that limited solubility of CaCO 3 can be eliminated by including minor additions of CaCO 3 up to the saturation concentration i.e. Le Chatelier principle.

In this work, the mechanism of corrosion for carbon steel A516.Gr70 in molten HitecXL nitrate salt was investigated and reported. It was shown that upon direct contact of carbon steel and molten nitrate salts, two competitive reactions take place, namely oxidation (corrosion) and carbonization (protection). This finding has allowed the proposal of a simple method to improve the resistance of carbon steel to corrosion by spray graphitization. The proposed method improves the corrosion resistance of the carbon steel to molten nitrate salt under both air and inert atmospheres, in presence of humidity and under the stress of thermal cycling up to 500 °C. In particular, corrosion was not detected for graphitized carbon steel after 1500 h isothermal immersion tests at 310 °C under Ar in the presence of humidity, while non-graphitized carbon steel demonstrated the corrosion rate of 8.8 ± 0.6 µm/year under similar conditions. After the cycling immersion tests in the 300–500 °C temperature range under air atmosphere, the corrosion rate of graphitized carbon steel was found to be almost three times lower than non-graphitized one (11.4 ± 1.2 against 31.5 ± 1.6 µm/year). Further corrosion tests under dynamic (flow) conditions are required to confirm the proposed method for practical use or to set points for its improvement. Particularly the complete coverage of the protected surface is a factor to be considered to avoid localized corrosion.