In this study, the specimens used were mild steel with an outer diameter of 16 mm, an inner diameter of 8.4 mm, and a thickness of 1.2 mm. The inner circle of the specimen serves to hang the specimen during testing corrosion. Before the immersion, the specimen was selected by inspection, then mechanically sanded with 400–1,200 grit sandpaper, then washed with acetone and toluene. Finally, the specimens were dried in an oven, and the initial weight was recorded using a precision balance with four decimal accuracies (Mettler Toledo, AB204). The composition of mild steel is shown in Table 1. After specified immersion time, the specimen was collected and rinsed using acetone and toluene, then dried in an oven. The morphology and composition of the corrosion on the specimen are observed using SEM-EDS. Then, the corrosion surface of the specimen will be rubbed using abrasive brushes to remove the corroded spot. Then, the specimen was washed using acetone and dried in an oven, and the final weight was recorded for corrosion rate measurement.

Besides mild steel, stainless steel A4 316 was chosen in this study for corrosion investigation, and these specimens were also purchased from Advance Bolts Supplier Sdn Bhd. Flat stainless steel with an outer diameter of 18 mm, an inner diameter of 8.5 mm, and a thickness of 1.2 mm was used. The specimens were treated the same as the mild steel. The inner circle of the specimen serves to hang the specimen during testing for corrosion. Before the immersion, the specimen was selected by inspection and washed with acetone and toluene. Finally, the specimens were dried in an oven, and the initial weight of the specimens was recorded. The composition of stainless steel is shown in Table 1.

A double jacketed reactor was used to produce esterified WC70CI30 [70 (v/v)% of waste cooking vegetable oil + 30 (v/v)% of C. inophyllum oil] oil. The FFA content of the WC70CI30 is 9.92 (w/w)%, which is highly undesirable because it will lead to low methyl ester yields due to saponification during the transesterification process. Therefore, in order to attain lower than 2% of FFA content, WC70CI30 was esterified using the following process parameters: 1) methanol/oil ratio: 70 (v/v)%, 2) H₂SO₄ concentration: 1.5 (v/v)%, 3) reaction temperature: 60°C, 4) reaction time: 2 h, and 5) stirring speed: 1,500 rpm. The FFA content of the esterified oil was found to be 1.50 (w/w)%, which is practically favorable in terms of the pretreatment process (Milano et al., 2018b).

Anton Paar Monowave 400 high-performance microwave reactor with Autosampler MAS24 was used for the microwave irradiation-assisted alkaline-catalyzed transesterification, where the esterified W70CI30 oil was converted into FAME. The optimized parameters for the transesterification process are 1) methanol/oil ratio: 59.60 (v/v)%, 2) KOH catalyst concentration: 0.774 (w/w)%, 3) stirring speed: 600 rpm, 4) reaction time 7.15 min, and 5) reaction temperature: 100°C (Milano et al., 2018a). The methyl ester yield produced by using the microwave reactor is 97.65 (w/w)%. The equipment used to measure the physicochemical properties of biodiesel is shown in Table 2. The properties of the biodiesel produced in this study are tabulated in Table 3, which all conforms to the specification stipulated in EN 14214 and ASTM D 6751 method.

The blending fuel B5 was prepared by blending 5 (v/v)% of W70CI30 biodiesel with 95 (v/v)% of diesel fuel. The other blending fuels were blended based on the ratios of 10, 15, 20, 25, and 30 (v/v)% of W70CI30 biodiesel with diesel fuel, named B10, B15, B20, B25, and B30 accordingly. Then, the physicochemical properties of the biodiesel-diesel fuel blends are measured, and the results are presented in Table 4.

A corrosion test was conducted by static immersion tests on the specimens at 25°C–30°C (room temperature) for 7,200 h (300 days) and 14,400 h (600 days) for mild steel and stainless steel 316, respectively. Metal specimens (mild steel and stainless steel) were hanged using Teflon thread separately and placed in the fuel mixture of biodiesel-diesel fuel blends. The weight loss during corrosion tests was measured using an analytical balance with four decimal precisions by calculating the difference in weight before and after testing. Furthermore, observations and measurements were performed every 720 h (30 days) of immersion for mild steel and 1,440 h (60 days) of immersion for stainless steel 316 to study the effect of immersion time on the rate of corrosion. The rate of corrosion of metal specimens was investigated based on data measurement by using the equation below (Fazal et al., 2012; Fazal et al., 2013): C R = 8.76 × 10 4 × W D × A × T ( m m y e a r ) (3.12)

Here, CR represents the corrosion rate (mm/year) of the chosen materials, W is the weight loss (g), D is the density of the metal (g/cm³), A is the cross-sectional area of the specimen metal surface (cm²), and T is the time of immersion (h).

The partial observation of mild steel at 3,600 h shows that B5–B15 have a linear increment in corrosion rate. However, B20 (1.45 nm/year) has a lower corrosion rate than B15 (1.65 nm/year). A more stable corrosion rate is observed as the immersion period increases. At 7,200 h, which is full observation, the corrosion rate for the biodiesel-diesel blend of B5–B30 is 0.7270, 0.9694, 1.1632, 1.2117, 1.3086, and 1.5025 nm/year. The corrosion rate obtained in this study is much lower than that obtained by Jin et al. (2015), who performed mild steel observation in diesel and pure palm biodiesel (B100), whose maximum corrosion rate is 0.002 and 0.038 mm/year, respectively, for 120 days’ immersion time.

Moreover, corrosion of blended waste cooking oil with C. inophyllum oil is much lower than using rapeseed biodiesel-diesel blend (Hu et al., 2012). The corrosion rate of rapeseed biodiesel and diesel is 0.0182 mm/year and 0.0015 mm/year after being immersed for 60 days at 43°C. The biodiesel produced in this study has shown relatively low corrosivity compared with other researchers’ biofuel. The corrosion of mild steel can be reduced by pre-blending the biodiesel with diesel fuel to reduce the corrosivity of the mild steel. Storing biodiesel in its pure form will accelerate the corrosion.

Stainless steel 316 has shown its high compatibility with biodiesel-diesel fuel. B5–B20 have shown no mass loss when immersed up to 2,880 h with zero corrosion rate. B25 and B30 have shown the lowest corrosion rate of 0.0890 nm/year at the same immersion period. At the partial observation of stainless steel 316 at 7,200 h, the corrosion rate for B5–B10, B15–B20, and B25–B30 is 0.0354, 0.1062, and 0.1417 nm/year, respectively. Whereas for the full observation at 14,400 h, the corrosion rate for B5–B10, B15–B20, B25, and B30 was 0.1062, 0.1417, 0.1948, and 0.2125 nm/year, respectively. The stainless steel 316’s corrosion rate obtained after being immersed for 300 days is still much lower than that obtained by Hu et al. (2012), with a corrosion rate of 0.00087 mm/year after being immersed in rapeseed biodiesel for 60 days. Moreover, the corrosion rate of palm biodiesel (B100) immersed for 965 h is 0.000112 mm/year, showing a high corrosion rate compared to our study (Fasogbon and Olagoke, 2016). Stainless steel 316 has a low corrosion rate with various blended ratios of waste cooking oil with C. inophyllum oil biodiesel with diesel fuel. Exposure of stainless steel 316 with biodiesel has shown superior compatibility. Stainless steel 316 contains a high level of chromium that forms a very stable chromium oxide (Cr₂O₃) that shows a very low corrosion rate (Dalmau et al., 2018). The corrosion of stainless steel 316 is initiated with a higher concentration of biodiesel in the blending fuel and a longer immersion time.

To conclude, higher biodiesel content in the fuel blend will increase the corrosion rate of the metallic material. Similar trends were observed in other literature such as palm (Fazal et al., 2013), ghee butter (Ononiwu et al., 2015), Moringa oleifera Lam. (Fernandes et al., 2019), sunflower (Cursaru et al., 2014), Aegle marmelos Correa (Thangarasu et al., 2019), rapeseed (Hu et al., 2012), rice husk (Lu et al., 2008), Salvadora (Kaul et al., 2007), and poultry fat (Geller et al., 2008) biodiesel. The following section will discuss the surface morphology of the mild steel and stainless steel 316 coupon and the corrosion mechanism of metallic materials in the biodiesel.

Figure 2A shows the mild steel specimen before immersion (0 day), Figures 2B–G show the immersion of mild steel in B5, B10, B15, B20, B25, and B30 at 3,600 h, and Figures 2H–M show the immersion of mild steel in B5, B10, B15, B20, B25, and B30 at 7,200 h. The oxidation process initiates the pitting formed on mild steel as the biodiesel contains acid compounds. This pitting is triggered by the metal ions released from the metallic surface that will later alter the biodiesel’s fuel properties and oxidation stability (Hu et al., 2012; Alves et al., 2019).

The corrosion on the mild steel is intense with a higher blending ratio (B30) compared to the lowest, as can be seen in Figures 2G and M for 3,600 h and 7,200 h, respectively, of the immersion period. The specimen immersed in B5 for 3,600 h (Figure 2B) shows a lower corroded spot, but it shows a relatively more corroded spot when the mild steel specimen is immersed for a more extended period (7,200 h), as shown in Figure 2H. There is pit formation (dark spot) in the immersed mild steel specimens with a blending ratio of B10–B30 for 3,600 h (Figures 2C–G). Therefore, as the immersion period increases, the corrosion is more intense, resulting in more pit formation and corrosion effect scatter around the mild steel specimens, as shown in Figures 2H–M. The weight percentage of the oxygen atom shown in Table 5 indicated the oxides formed on the specimen. Higher biodiesel concentration in the blending fuel showed more pits with various sizes (Figures 2A–G). After 7,200 h of immersion, more cavities and black spots were found on the mild steel’s surface, especially Figures 2L and M. B30 showed higher iron [77.6 (w/w)%], oxygen [14.36 (w/w)%], and carbon [8.04 (w/w)%] atom contents. EDS chemical characterization results indicate the presence of a metal oxide film on the specimen. The pit formation and black spots found on the coupons are due to oxidation. Biodiesel with highly unsaturated fatty acid content will cause higher reactivity with oxygen molecules. These molecules form active oxygen atoms that transform the metal oxides into other metal compounds by further oxidation. Metal oxides formed on the surfaces are commonly Fe(OH)₃, (FeO)₂CO₃, and Fe₂O₃, also reported by Fazal et al. (2017). The mechanism of corrosion in mild steel is described in Eq. (1)–Eq. (3).

Spontaneous oxidation of iron to iron (III) oxides: 4 F e ( s ) + 3 O 2 ( g ) → 2 F e 2 O 3 ( s ) (1)

The presence of oxygen molecules and water with iron (II) hydroxide will cause a substitution reaction to form iron (III) oxide-hydroxide. 4 F e ( O H ) 2 ( s ) + O 2 ( g ) +2 H 2 O ( l ) → 4 F e ( O H ) 3 ( s ) (2)

The presence of carbon dioxide and water molecules form H₂CO₃, then react with iron (III) hydroxide to form iron (II) oxide-carbonate 4 H 2 C O 3 ( l ) + F e O ( O H ) ( s ) → 4 ( F e O ) 2 C O 3 ( s ) (3)

Corrosion occurs more intensively in mild steel surrounded with a higher concentration of biodiesel blend. The chemical reaction produces oxygenated compounds due to active oxygen atoms, water, and other volatile compounds present in biodiesel blends that eventually corrode the mild steel. The small pits formation (micropits) will create a “protective layer” on the mild steel and decrease the corrosion rate. Biodiesel is more corrosive than diesel fuel (Ononiwu et al., 2015). High water content and higher acidity of biodiesel will increase the corrosive activity. Even high-quality biodiesel with prolonged exposure to mild steel will affect the metallic surface and piston parts (Cestari et al., 2021).

Figure 3A shows the stainless steel 316 specimen before immersion (0 days), Figures 3B–G show the immersion of stainless steel 316 in B5, B10, B15, B20, B25, and B30 at 300 days (7,200 h), and Figures 3H–M show the immersion of stainless steel 316 in B5, B10, B15, B20, B25, and B30 at 600 days (14,400 h). Exposure of stainless steel coupons in B5 and B10 for 7,200 h shows less mass loss and a low corrosion rate. The SEM micrograph showed that slight corrosion spots were illustrated in B5 and B10. Little corrosion pits are seen in the SEM morphology for B15 and B20. More pits were found in B25 and B30 blending fuels. The corrosion rate for 7,200 h of all blending fuel is minimal. The SEM morphology has successfully captured pit formation in the coupons. The result has shown that higher biodiesel composition in blended fuel will stimulate corrosion on the stainless steel 316 coupon at the same immersion period.

The morphology obtained from SEM presented in Figures 3H–M shows that stainless steel 316 has higher corrosion resistance when the specimens are immersed in various biodiesel-diesel fuel ratios. Even though the immersion periods were extended beyond the immersion period of the mild steel specimens, the corrosion attack is still minor with little sediment formation. The EDS results indicate that the presence of carbon, oxygen, and chromium content for B20 at 600 h is 22.22 (w/w)%, 9.59 (w/w)%, and 8.14 (w/w)% (Table 5). Stainless steel 316 exposure to biodiesel-diesel blend causes titanium carbides (TiC) and chromium (III) oxide (Cr₂O₃) to form on the specimen. TiC particles formed a carbide stabilizer that reinforced the stainless steel 316 surfaces, while Cr₂O₃ forms a thin layer of oxides on the stainless steel surface, called the “passive layer.” Stainless steel exposed to oxygen molecules caused the formation of metal oxide films that prevented the metal’s oxidation and resulted in a lower corrosion rate, as also reported by Kugelmeier et al. (2021). Passive film is thin and not visible, and the metal remains lustrous and smooth with the film attached to it. The passive film will reform when the surface is scratched, and it prevents the corrosion from spreading into the metal’s internal structure. Stainless steel can prevent metallic contamination to the biodiesel-diesel blend due to the passive character of the film. The alloy can make storage tanks or tankers to transport biodiesel due to its corrosion resistance. It is also recommended to be used to make some common parts in the engine such as exhaust manifold, fuel pump, fuel filter, fuel lines, fuel tank, and even fastener, retainers, clips, screws, nuts, and bolts that can be used to secure the engine parts. The mentioned parts are consciously exposed to biodiesel-diesel, so it is vital to choose a suitable material to secure the engine compartment and routing lines and for detailing the engine.

Besides, biodiesel in this study was produced from non-edible oil (waste cooking oil blend with C. inophyllum oil) with a high unsaturated fatty acid content. In fact, high unsaturated fatty acid contents will cause high reactivity with an oxygen molecule. It will cause low oxidation stability of the biodiesel. According to ASTM D6751 and EN 14214 standards, the oxidation stability at 110°C should be at 3–6 h, respectively. The W70CI30 biodiesel has significantly higher oxidation stability (18.03 h). The superior oxidation stability has slowed down the corrosion rate of the metallic materials. The metallic materials were immersed in a surrounding with low active oxygen atoms that will create a less favorable environment for corrosion attack. The corrosion rate and morphology of mild steel and stainless steel have shown that the corrosion attack was milder than other studies (Jin et al., 2015; Cestari et al., 2021; Kugelmeier et al., 2021). The following section will discuss the behavior of the degraded biodiesel in terms of changes in fuel properties after each immersion period of the metal materials of various blending ratios of W70CI30 biodiesel-diesel.

Exposure of biodiesel-diesel blend with the metallic materials causes degradation of the fuel mixture. Viscosity represents the fluidity of the blended mixture; the viscosity changes will influence the combustion of the fuel mixture. High viscosity in the blended mixture will cause a burden to the fuel pump, which would result in more energy for the fuel pump to deliver the fuel to the injector. High-viscosity scenario will eventually reduce the engine’s power as it requires more input energy (Nautiyal et al., 2020). Besides, higher viscosity will cause larger droplets when fuel exits the injector. Large droplets will cause sudden temperature changes at the surface of the cylinder that induces incomplete combustion. The incomplete combustion will increase the chamber’s deposit and produce pungent gases forming aldehydes and acids (Malpass and Motheo, 2003).

The fuel kinematic viscosity should be within a range of 1.90–6 mm²/s and 3.5–5 mm²/s according to the ASTM D6751 and EN 14214 standards, respectively. The kinematic viscosity of WC70CI30 biodiesel-diesel blend mixture before and after immersion of mild steel and stainless steel 316 is shown in Figures 4A and B, respectively. The kinematic viscosity of the biodiesel-diesel blend increases as the concentration of the biodiesel increases in the blended mixture in all the cases. Besides, the kinematic viscosity increases with the time of immersion of mild steel and stainless steel 316 in the blended mixture. The kinematic viscosity for mild steel with an immersion period of 7,200 h for biodiesel-diesel blends of B5, B10, B15, B20, B25, and B30 is 3.2348, 3.3349, 3.4043, 3.4925, 3.5213, and 3.6267 mm²/s, respectively. The degradation of fuel in kinematic viscosity after being exposed for 7,200 h in various blending ratios is less than 9% compared with the fuels’ initial conditions. The kinematic viscosity for stainless steel with an immersion period of 14,400 h for biodiesel-diesel blends of B5, B10, B15, B20, B25, and B30 is 3.5123, 3.5032, 3.6959, 3.8023, 3.8121, and 3.9245 mm²/s, respectively. The degradation of fuel in kinematic viscosity after being exposed for 14,400 h in various blending ratios is less than 16% compared with the fuels’ initial conditions. The kinematic viscosity of all blended fuel fulfills the specification given by ASTM D6751, but B5–B20 did not meet the requirement by EN 14214 standards. B5–B20 do not degrade as fast as other blended fuels. The kinematic viscosity shows a similar trend for both mild steel and stainless steel material, with similar degradation percentages of the fuels based on time of immersion.

According to the ASTM D6751 and EN 14214 standards, the fuel density should be lower than 880 and between a range of 860–900 kg/m³, respectively. The density of W70CI30 is 861.8 kg/m³, which is well within the range specified in both standards. The density of W70CI30 is slightly higher than those for diesel (841.1 kg/m³), palm (858.9 kg/m³), and Moringa (859.6 kg/m³) biodiesel (Rashed et al., 2016). However, the density of W70CI30 is significantly lower than obtained by Dubey and Gupta (2017) (Jatropha biodiesel, 881 kg/m³) and Xue et al. (2016) (WCO biodiesel, 887.6 kg/m³). Variation of density is attributed to the difference of feedstock, quality of feedstocks, biodiesel production procedure, and the equipment used for biodiesel production (heating mantle, ultrasonic, supercritical method). The blending of biodiesel with diesel fuel will reduce the density of the blended fuel. Therefore, density is an equally important property, as it will also affect the efficiency of fuel atomization and combustion characteristics. The density will affect the fuel mass that reaches the combustion chamber, and then it will influence the air/fuel ratio and engine performance (Shekofteh et al., 2020).

The density for the mild steel and stainless steel 316 specimen immersed in the biodiesel-diesel fuel mixture is presented in Figure 5. The density of mild steel with an immersion period of 7,200 h of biodiesel-diesel blends of B5, B10, B15, B20, B25, and B30 is 845.9, 847.2, 847.9, 850.2, 851.5, and 853.9 kg/m³, respectively. The degradation in the density of all tested fuels at 7,200 h is less than 0.5% compared with the fuel’s initial density. The density of stainless steel with an immersion period of 14,400 h for biodiesel-diesel blends of B5, B10, B15, B20, B25, and B30 is 849.3, 850.3, 851.8, 854.5, 856.9, and 860.2 kg/m³, respectively, with degradation less than 1.3% compared with the fuel’s initial density. However, the density of B5–B30 (mild steel) after being immersed for 7,200 h is significantly lower than that stated in EN 14214. The density of B5–B25 (stainless steel 316) after being immersed for 14,400 h is also below the requirement stipulated in the EN 14214. However, density values of both metallic materials immersed in various biodiesel-diesel blends fulfill the ASTM D6751. The degradation of density is dependent on the FAME composition, molar mass, and water content. As observed, the changes in the density values are influenced by the metallic material and the time of immersion.

A refractive index is a dimensionless number used to indicate light propagation through the medium. The refractive index measures how much the path of light is refracted when it passes through a material. The refractive index can precisely measure the degradability of the fuel instead. It is more accurate than observing the color changes on the biodiesel-diesel blends before and after the test. The mild steel and stainless steel 316 show no color changes after their full observation period, both exhibiting light yellow tones. However, no color changes reflect the excellent compatibility between the metallic materials and the biodiesel-diesel blended fuels. By referring to the changes in the refractive index shown in Figure 6, it can be observed that increasing the concentration of the biodiesel content in the blending ratio will reduce the refractive index of the medium. The refractive index of mild steel with an immersion period of 7,200 h of biodiesel-diesel blends of B5, B10, B15, B20, B25, and B30 is 1.4663, 1.4641, 1.4632, 1.4623, 1.4615, and 1.4607, respectively. For the stainless steel, the refractive index with an immersion period of 14,400 h of biodiesel-diesel blends of B5, B10, B15, B20, B25, and B30 is 1.4663, 1.4641, 1.4632, 1.4623, 1.4615, and 1.4607, respectively. The degradation based on the refractive index is less than 0.1% compared with the initial’s refractive index. Degradation has shown a steady increment in refractive index for all blended fuels. Hence, the refractive index can be considered another property to measure the degradation of the fuel. The degradation of the fuel is generally measured by referring to the changes in density, kinematic viscosity, and acid value.

Generally, the acid value determination is used to quantify acid content by adding a certain amount of potassium hydroxide to neutralize acidity in the oil sample. The limit of acid value in the biodiesel blend is 0.5 mg KOH/g for both ASTM D6751 and EN 14214 standards. The acid value of W70CI30 biodiesel is 0.46 mg KOH/g, which met the limit stipulated in both standards. The significantly high acid value of W70CI30 biodiesel is due to crude C. inophyllum oil, which consists of more FFA. The W70CI30 acid value is significantly higher than those for diesel (0.0017 mg KOH/g), C. inophyllum (0.41 mg KOH/g) (Ong et al., 2019), and waste cooking (0.37 mg KOH/g) (Xue et al., 2016) biodiesel. Blending with diesel has reduced the overall acid value of the blended fuels. The acid value for the biodiesel-diesel fuel mixture before and after immersion of both metal specimens is shown in Figure 7. The acid value of mild steel with an immersion period of 7,200 h for biodiesel-diesel blends of B5, B10, B15, B20, B25, and B30 is 0.2635, 0.2922, 0.3259, 0.3815, 0.4326, and 0.4835 mg KOH/g, respectively. The acid value of mild steel with an immersion period of 7,200 h is still within the stipulated limit set by the ASTM D6751 and EN 14214 standards. The acid value of stainless steel with an immersion period of 14,400 h for biodiesel-diesel blends of B5, B10, B15, B20, B25, and B30 is 0.3815, 0.4120, 0.4423, 0.4823, 0.5247, and 0.5867 mg KOH/g, respectively. However, the acid value for stainless steel 316 immersions of B30 is 0.5013 mg KOH/g at an immersion period of 8,640 h and is at the border of the limits. It can be observed that the acid value has increased when the time of immersion is extended beyond the limit. When the immersion time reaches 12,960 h, the acid value for both B25 and B30 is 0.5123 and 0.5548 mg KOH/g, respectively, exceeding the limits of 0.5 mg KOH/g. The acid value of the other blending ratios will take turns to exceed the limits of ASTM D6751 and EN14214 standards when the immersion time has exceeded 14,400 h. The acid value in this study is significantly lower than that obtained by Jin et al. (2015) [palm biodiesel, 2.2 mg KOH/g (120 days)]. Diesel fuel is more stable than palm biodiesel in terms of acid value. Hence, in this study, the blending of biodiesel with diesel fuels has reduced the drastic changes of acidity that will commonly happen to pure biodiesel. The changes in the acid value of the blending fuel represent the degradation trends of the metallic materials due to the oxidation of FAME. The increase in the content of FFAs will cause issues on the usability of the fuel in the engine system, especially at the engine fuel system, and it will cause fuel filter clogging (Monirul et al., 2015; Alves et al., 2019).

Descriptive statistics describe the basic features of the data in this study and provide summaries about the data measured. The data in this study consist of a total of 120 cases collected from the corrosion behavior analysis. Table 6 described the descriptive statistics of this study for the mild steel and stainless steel 316. Correlation analysis was needed to verify and understand the relationship between corrosion rate and the investigated variables. The coefficient of correlation will measure the strength and direction of a linear relationship between two numerical variables. There are three types of correlation, which are neutral, negative, and positive correlation. The correlation values close to 0 indicate weak correlation (neutral correlation), whereas the negative values indicate an inverse relationship, and positive values indicate a direct relationship. The value of the variable must lie between -1 and 1; the minimum amount of 0.5 correlation is needed to judge the strong strength of the correlation variables with the corrosion rate. The Pearson correlation with two-tailed test significance was performed for the study of the variables in this part. The null and alternative hypotheses are shown below:

H₀: Variable is not significantly affected by corrosion rate.

H₁: Variable is significantly affected by corrosion rate.

By referring to Table 7, the Pearson correlation and covariance between corrosion rate and variables for both mild steel and stainless steel 316 are shown. In order to reject the null hypothesis in favor of the alternate hypothesis, the p-value of the model should be less than 0.05 or 0.01, which is the significance level chosen in this study. The Pearson correlation for mild steel is significant at 0.01 for the day of immersion, blending ratio, weight difference, outer diameter, and refractive index. While other variables that are significant at 0.05 are density at 40°C and 15°C and acid value. Since the p-value is less than 0.05, there is sufficient evidence to reject the null hypothesis in favor of the alternative hypothesis. Hence, the mentioned variables significantly affect the corrosion rate of the mild steel. For stainless steel 316, the variables that significantly affect the corrosion rate are the day of immersion, blending ratio, weight difference, outer diameter, inner diameter, dynamic viscosity, kinematic viscosity, density, acid value, and refractive index. The p-value of the aforementioned variables is less than 0.01, which gives great reason to reject the null hypothesis.

Regression analysis was conducted to analyze the potential predictors or variables used to estimate the mild and stainless steel corrosion rate. Table 8 shows the model summary performed to develop a suitable regression model for corrosion rate. The model was developed by adding one variable at each regression modeling. At the end of the modeling process, 12 predictor variables were added into the model, such as day of immersion, blending ratio, weight difference (mg), outer diameter, inner diameter, thickness, dynamic viscosity, kinematic viscosity, density 40°C, density 15°C, acid value, and refractive index. The later model shows improvement in R and R ² of 0.931 and 0.867, respectively. The later model has 86.7% of variability to predict the corrosion rate of mild steel. The adjusted R ² of 0.833 for the later model and the difference between R ² and adjusted R ² are very small (0.867–0.833 = 0.034 or 3.4%). The small difference of R ² and adjusted R ² model indicates that the model is derived from the population rather than a sample; it would show that the results for the corrosion rate will be 3.4% less variance. Besides, the Durbin–Watson statistic for the later model is 1.441, showing positive autocorrelation in the model formed. The regression model developed by stainless steel is based on 10 independent variables, such as day of immersion, blending ratio, weight difference (mg), outer diameter, inner diameter, thickness, dynamic viscosity, kinematic viscosity, density 40°C, and density 15°C with R ² of 0.95, which means that 95% of the variability can be explained by the model developed in this study.