Introduction

Stainless steels are widely used in various industrial applications, which include very aggressive environments. This has influenced the development and study of their properties, which strongly impact the service life of components and equipment [1]. In particular, the automotive industry uses a wide range of parts made from stainless steel. This is particularly the case for combustion systems, where mechanical and chemical stabilization properties play a crucial role in the nozzles’ performance and the low-pressure systems in contact with fuel.

Metals used for automobile production, such as copper, aluminum, and cast iron, are highly resistant to corrosion when immersed in diesel fuel. However, they suffer more accelerated corrosion processes when in contact with biodiesel and mixtures. Therefore, to extend the lifespan of these metals, the study of the corrosive nature of biodiesel is important [2]. This is particularly so given the advantages of biodiesel, which have led to a worldwide increase in its use. The main advantages of biodiesel are its low Sulphur content and the fact that it does not involve burning fossil fuels, given the current environmental crisis.

Biofuels extracted from vegetable sources have significantly different in particulate matter emissions and biodegradability compared to conventional biodiesel. Currently, the diesel fuel distributed and used in Colombia contains around 10% particulate matter [3].

Considering the above, this work focuses on studying the corrosion resistance of AISI 304 stainless steel, the type used in commercial nozzles, when exposed to pure biodiesel fuel and its mixture B50 (50% palm oil, 50% commercial diesel) under stationary immersion conditions. In order to separate the influence of the NOx species, potentiostatic polarization tests were done under concentrations of 0.1M, 1x10-3M, and 1x10-4M.

Under stationary immersion conditions, biodiesel suffered solidification, manifesting in the formation of wax, as reported in the literature [4]. This could restrict or reduce the use of mixtures with a high percentage of biofuels in locations with cold weather (<18°C). Furthermore, localized corrosion was observed on the nozzles, even though no degradation was observed under the electrochemical test, suggesting that the corrosion was caused by oxygen depletion.

Materials and methods

Preparation of the samples

Commercial nozzles were used for the immersion tests, along with AISI 304 rod, a common material used for nozzles. The nozzles were degreased in a basic solution and rinsed with distilled water. Later, they were subjected to ultrasonic cleaning in ethanol for 30 minutes. Once the nozzles were dry, they were subjected to the immersion test. Before performing the electrochemical tests, the same procedure was used to clean an AISI 304 disk cut from the rod.

Biodiesel Characterization

Commercial biodiesel samples were obtained and mixed with commercial diesel for the immersion test. However, pure biodiesel properties were analyzed according to THE tests described in Table 1.

Stainless steel performance under total immersion in biofuel

The stainless-steel nozzles and the AISI 304 disks were subjected to stationary immersion tests for 4 months. Solutions used in the tests were 100% biodiesel and a mixture of biofuel (50%) and commercial diesel (50%). This proportion was taken from a previous study, where the effect of different mixtures on the efficiency of engine combustion was analyzed [5]. In all cases, the samples were covered by biodiesel or the mixture. The environmental temperature was measured, obtaining a range between 14ºC and 20ºC.

Electrochemical description of the stainless steel in biofuel

NOx emission measurements were taken from the combustion of biodiesel in a test engine to measure the concentration of those species in the biofuel. The value recorded for parts per million was taken as the lower concentration in the electrolyte used to perform potentiostatic tests carried out in an Autolab 305 potentiostat. A standard 3 electrode cell was used, composed of an Ag/AgCl reference electrode, a platinum counter electrode, and a disk made of AISI 304 as a work electrode. HNO₃ solutions with concentrations of 0.1M, 1x10^-3M, and 1x10^-4M were used as electrolytes. The polarization curves were carried out between -1V and 1V.

Results

Biofuel Chemical characterization

In general, to determine the main properties of biofuels and their mixtures, the biodiesel volume fraction in the mixture is considered and reported [6]. Modern diesel engines can use pure biofuel (B100) or mixtures with diesel petrol [7]. In addition to being extracted from renewable sources, biodiesel offers several advantages: it is biodegradable, non-toxic, reduces toxic emissions, and is ecofriendly when mixed with diesel fuel [8]. Nevertheless, biodiesel has some negative aspects, such as oxidative instability, poor/low-temperature properties, and similar properties to solvents. It can also provide slightly low power and torque and, therefore, higher fuel consumption [9]. Given this, chemical characterization was performed to determine the stability and oxidative properties of the biofuel used.

The viscosity of biodiesel depends mainly on the biofuel chain composition and unsaturation, as well as on its temperature. An acceptable range for the kinematic viscosity can be between 1.9 and 6 mm²/s, according to ASTM D6751 [10]. The biodiesel kinematic viscosity was determined following the procedure described by ASTM D445 [11]. Values around 4.43 mm²/s were obtained. Although this value is within the acceptable range, it is localized around the mean. This can be explained by a higher number of double links [12].

Likewise, the acid value measurements were done under the procedure described by ASTM D664 [13], where the amount of potassium hydroxide reactive needed to neutralize 1 gram of methyl ester is determined[12]. According to ASTM D6751, the acid value could reach a maximum of 0.5 mg KOH/g of biodiesel [10]. The biodiesel reached a value of 0.23 mg KOH/g, meaning that it has low or no significant corrosion effects on the materials in contact with it. Thus, higher biofuel acid values lead to greater corrosive effects.

Finally, chemical stability was also determined using the iodine value, the maximum of which is around 120gI/100g [12]. The value obtained following the procedure explained in EN 14111 [14] was 59.3gI/100g. Furthermore, triglycerides were detected, implying a complete transformation of the starting composites. In turn, diglycerides and linked glycerin were identified as the main components in mass percentage (w/w).

Chemical composition of commercial nozzles

The chemical composition, described in Table 2, was obtained according to the procedure explained in ASTM A570 [15]. The atomic percentages identified correspond to UNS S3020 type 302 stainless steel.

Chemical composition of stainless-steel disks

Chemical composition of the sample disks is shown in Table 3. This corresponds to UNS S30400 type 304 stainless steel. The characterization test was done under the procedure described in ASTM A570 [15].

Hardness measurements of nozzles

Hardness values, shown in Table 4, were obtained according to ASTM E 18 – 03 [16]. As observed, the variability of those values is not significant. This may correspond to hardening due to the cold work done during the manufacturing process.

Static Immersion tests

Static immersion tests and mass loss control were carried out according to the procedure described in ASTM G31 – 72 [17]. The AISI 304 stainless steel disks and commercial nozzles were evaluated over 154 days (approximately 3696 hours) in B50 and commercial diesel under temperatures from 14ºC-18ºC, humidity (80% HR), and controlled static conditions.

As observed in Figures 1a and 1b, a solid wax or deposits were formed. As reported in previous studies [18], this wax is produced by separating biodiesel components below 13ºC. However, these waxes appear around 16ºC, which can obstruct the injector nozzles in cities where the environmental temperature can reach relatively low values, such as Bogotá.

Once the test was finished, rust and deposits were observed in the commercial diesel fuel and the B50. Moreover, localized corrosion was found inside the nozzle, as shown in Figure 2b. This degradation mechanism is attributed to oxygen depletion, which could occur because of a static solution’s decrease in oxygen evolution. The lack of oxygen affects the integrity of the stainless-steel passive layer. This phenomenon was found only under static immersion conditions.

Likewise, the AISI 304 stainless steel disks were subjected to immersion conditions. However, because of the ease of handling of the samples, metallographic analysis was performed before and after the test.

Three samples were prepared according to ASTM E3–01 [19]. In order to identify the microstructure, samples were chemically attacked for19 minutes with a solution composed of 5ml of HCl, 1 g of picric acid, and 100 ml of ethanol. The procedure was done following ASTM E407–07 [20]. The microstructure was confirmed by duplicated samples.

The metallographic characterization shows an austenite matrix in the equi–axial grains of austenite with the presence of carbon (black zone), as seen in Figures 3a and 3b. This microstructure was corroborated using the metallographic handbook of the ASM [21]. No evidence of changes in the microstructure was noticed, as observed in Figures 4a and 4b.

Electrochemical description of the stainless steel in the biofuel

As the identification of composites rich in N from biodiesel mixtures is challenging, some reference values of the emissions were considered to determine the range of NOx species that can be in contact with the biodiesel. Gas emissions measurements were done from the combustion by-products of an engine where a mixture of diesel and biodiesel was used. The efficiency of the engine and the combustion characteristics are reported elsewhere [22].

Figure 5a and 5b show the NOx content in the combustion gases for the different mixtures used. In all cases, such emissions are below 500 parts per million (ppm), corresponding to concentrations around 1x10^-3M and 1x10^-4M HNO₃. Polarization tests were done under those concentrations and 0.1 M HNO₃. The last of these was used to compare results from 1x10^-3M and 1x10^-4M HNO₃.

As shown in Figures 6a, 6b, and 6c, no significant variation in corrosion speed or in the passive behavior of the AISI 304 stainless steel was observed.

The highest corrosion speeds are associated with the material exposed to the 0.1M HNO₃ solution. This is observed in the corrosion parameters, as the corrosion current (icorr) and corrosion potential (Ecorr) are lower for concentrations under 1x10^-3M and 1x10^-4M, as shown in Table 5. This means that the corrosion potential occurs at higher potentials. Likewise, the corrosion current (icor) for 0.1M HNO₃ has two higher magnitudes than the other two concentrations. However, from the electrochemical test, it can be concluded that a greater proportion of the NOx species can lead to the potential corrosion displacement and drive the system to faster passivation of the stainless steel, as has been reported in some other research studies on 300 series stainless steel [23, 24].

In addition, complete polarization curves performed under concentrations of 0.1M and 1x10^-3M HNO₃ (Figure 7), show the Ecorr displacement mentioned above. Differences in the current could be related to ohmic fall, as the test was done without a supporting electrolyte to improve conductivity. The analyzed surface did not show localized corrosive phenomena in all cases because the material did not reach the breakdown potentials.

The passive layer peak formation was not observed under low concentrations (1x10^-3M HNO₃), as shown in the polarization curve performed under 0.1M HNO₃. This is attributed to the passive layer formation being reached under lower potentials, which means the passive layer is more stable under this concentration.

Conclusions

• Biodiesel extracted from plant oil shows stability and a low tendency to oxidation. The components of this biofuel are mainly diglycerides and linked glycerin.

• Under immersion, the commercial nozzles exhibited localized corrosion due to aeration of differential cells formed under static conditions.

• After three months of immersion, the AISI 304 stainless steel does not show significant metallographic changes. Differences in the attacked surface between the AISI 304 samples and the commercial nozzles can be explained by the difference in their chemical composition.

• Wax was formed around 16ºC. This can lead to obstruction of the nozzles when higher concentrations of biodiesel are used in the mixture fuel.

• According to corrosion tests, the NOx concentration of the biodiesel does not affect the passive layer or the self–protective performance of the steel.

• The equivalent concentrations of NOx species from the biodiesel show low corrosion potential and corrosion current, which means lower corrosion speeds. Furthermore, no degradation phenomena were identified under any of the concentrations of HNO₃ used.

Acknowledgment

The authors wish to thank the Department of Environmental Engineering of the Universidad Pedagógica y Tecnológica de Colombia and the Cooperativa Agrominera multiactiva de Paipa Ltda. (Cooagromin) collection center of Paipa-Boyacá, Colombia, for providing the materials and resources necessary for the proper development of this research. One of the authors (NRA) thanks these institutions for their invitation to contribute to this research.

References

Notes

* Research paper (Artículo de investigación) - Article of scientific and tecnological investigation

Author notes

^a Corresponding author. E-mail: david.blanco@escuelaing.edu.co

Additional information

How to cite this article: A. Bermudez-Castañeda, D. L. Blanco-Estupiñan, S. Márquez, “Performance of Nozzle Steels in Biofuel” Ing. Univ. vol. 26, 2022. https://doi.org/10.11144/javeriana.iued26.pnsb

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