# A Comprehensive Review of the Application Characteristics of Biodiesel Blends in Diesel Engines
## 7 Compatibility
### Corrosion  
Fuel is stored, transported and injected through a fuel system containing a fuel tank, fuel pumps, fuel filters, fuel injectors and pipelines. Most components made of iron (cast iron), steel and non-ferrous metals (copper, alloy, etc. ), and sealing elements made of elastic materials (rubber, polymer, etc.) are generally in direct contact with fuel. Therefore, the corrosion characteristics of the fuel will directly affect the service life of engine components. Generally, biodiesel is more corrosive than fossil diesel under similar conditions. The composition of biodiesel, moisture content, conductivity, temperature and contact time may all be factors that affect corrosion [148]. [PERSON] et al. tested the corrosion of copper (Cu), brass (BS), aluminum (Al) and cast iron (CI) in diesel and palm biodiesel [149]. The results showed that the corrosion order for these metals is copper, brass, aluminum, cast iron. The TAN (Total Acid Number) value of biodiesel in the corrosion process exceeded the ASTM D6751 standard. The corrosion of various metals can be seen in Figure 20. [PERSON] et al. tested the corrosion rate of jatropha biodiesel on copper, aluminum and stainless steel [150]. They found that the corrosion rate of copper was the fastest, and the acid content, density, and water content of biodiesel increased. Another study [151] also reached similar conclusions with metals, especially copper. The corrosion situation of metal in biodiesel is related to the atmosphere and fuel composition. Taking copper as an example, the corrosion process may follow the following mechanisms [149; 152]:  
\[2\text{Cu}+\text{O}_{2}+2\text{H}_{2}\text{O}\to 2\text{Cu}(\text{OH})_{2} \quad 2\text{Cu}+1/2\text{O}2\to\text{Cu}_{2}\text{O} \tag{9}\]  
\[\text{Cu}_{2}\text{O}+2\text{CO}_{2}+1/2\text{O}_{2}\to 2\text{Cu}\text{CO}_{3} \quad\text{CuO}+\text{CO}_{2}\to 2\text{Cu}\text{CO}_{3} \tag{10}\]  
\[\text{Cu}^{2+}+2\text{RCO}\bullet\text{Cu}\text{CO}_{3}+\text{R-R}+\text{CO} \tag{11}\]  
Figure 19: The changes recorded on the oxidation stability of the nine biodiesel samples [142]. To alleviate the corrosion of biodiesel, more and more attention has been paid to anti-corrosion additives. [PERSON] et al. found that tert-butylamine (TBA) and butylhydroxyanisole (BHA) additives can greatly delay metal degradation under the same conditions [152]. As shown in the Figure 21, copper corrosion rate is significantly reduced. In addition, other additives such as tert-butylamine (TBA), propyl gallate (PG), pyrogallol (PY), and butylated hydroxytoluene (BHT) effectively improve metal degradation [153]. Some scholars have also explored the compatibility of biodiesel with elastomers. [PERSON] et al. used the Hansen solubility parameter method to evaluate the compatibility between main degradation products of biodiesel (methyl hydroperoxide and acetaldehyde) and five common elastomers containing acrylonitrile butadiene rubber, fluorocarbon, neoprene, ethylene propene diene monomer and silicone rubber [154]. The partial degradation of biodiesel will increase the volume expansion of the elastomers and affect the compatibility of the two. [PERSON] et al. pointed that the degradation of biodiesel also deteriorated the mechanical properties of the elastomers, whose mass has also increased accordingly [155]. Other studies reached similar conclusions, but these are not absolute results. Figure 21: AFM (Atomic Force Microscopy) images of copper surface after exposure to biodiesel and biodiesel with tert-butylamine (TBA) additives [152]. Figure 20: SEM (Scanning Electron Microscopy) micrographs of metal surfaces before and after exposure to diesel and biodiesel at room temperature for 2880 h. (**a-l**) in this figure indicate the tested metals: (**a,e,i**) Cu, (**b,f,j**) BS, (**c,g,k**) Al and (**d,h,l**) Cl [149]. As [PERSON] et al. pointed out, most existing test methods (immersion, LPR and rotating cage etc.)