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
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
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
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
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.
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
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
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.
In general, to determine the main properties of biofuels and their mixtures, the biodiesel volume fraction in the mixture is considered and reported
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
Likewise, the acid value measurements were done under the procedure described by ASTM D664
Finally, chemical stability was also determined using the iodine value, the maximum of which is around 120gI/100g
The chemical composition, described in
Chemical composition of the sample disks is shown in
Hardness values, shown in
Static immersion tests and mass loss control were carried out according to the procedure described in ASTM G31 – 72
As observed in
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
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
The metallographic characterization shows an austenite matrix in the equi–axial grains of austenite with the presence of carbon (black zone), as seen in
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
As shown in
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
In addition, complete polarization curves performed under concentrations of 0.1M and 1x10-3M HNO3 (
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.
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.
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.
Research paper (Artículo de investigación) - Article of scientific and tecnological investigation