Results and Discussion

Hardness

Vickers’ hardness of the uncoated and coated manifolds is displayed in Figure 7.1. It clearly displays that the hardness of the coated manifolds was increased. The data points of the hardness were collected at various points of the manifolds. The experiments were conducted three times, and then, the average value was reported. The higher hardness was attained at 85 Ni-15 Cr compositions. The enhanced hardness was obtained because Ni-rich phase was coated over the manifolds. Hardness is directly proportional to the refinement of microstructure; it reflects the good bonding between the coating surfaces. It is apparent that the microhardness influences the higher dislocation density in the bare manifolds, because of the variation in their coefficient of thermal expansion.

Aging Behavior

Gray cast iron and coated material were placed in a furnace of 850°C for 5 min. It was suddenly cooled by air for another 5 min; thus, one thermal cycle was completed, and this process was again repeated for five thermal cycles. The microstructure of the

FIGURE 7.1 Vickers* hardness of uncoated and coated samples.

surface was investigated by SEM image. No cracks were identified on the surface of uncoated samples. Hence, it was decided to conduct another 15 thermal cycles for crack identification, but no cracks were found on its surface. Further, the thermal cycle was increased and the crack identification process was carried out for 20 thermal cycles, at which the cracks are clearly visible on the surface. Figure 7.2 displays the microstructure changes of various cycles for bare steel [11].

Figure 7.3 depicts the SEM micrographs of cast iron manifolds of different compositions before and after the thermal cycles. The microstructure of the surface was investigated using SEM. It was found that the cracks are obtained on its surface due to the brittle nature of the gray cast iron [12,13]. A similar test was carried for the coated samples under the same condition, and their microstructure was viewed, whereas no cracks were identified on the surface because nickel and chromium are non-brittle in nature. The presence of nickel and chromium in the coated samples of different compositions (i.e., Ni 75%-Cr 25%, Ni 80%-Cr 20%, and Ni 85%-Cr 15%) reveals the same results as no cracks were observed on the surface.

Corrosion Behavior

The corrosion behavior of the coated manifolds evaluated using the weight reduction method and Tafel exploration is discussed in this section.

Weight Loss Method

Table 7.1 shows the weight loss for uncoated manifolds and coated cast iron manifolds (85 Ni-15 Cr). The experiments were conducted for 15 days in an acidic medium (1 mole HC1 solution). It was noticed that the weight loss was reduced for coated manifolds. Ni and Cr act as a barrier for cast iron manifolds. But, bare cast iron

FIGURE 7.2 SEM images of bare steel, a) 5, (b) 15, and (c) 20 cycles.

reacts with HC1 strongly and leads to a reduction in weight. However, the weight loss was increased significantly, if the immersion time period of the samples increased. The ability of iron materials has led to excessive corrosion, and hence, it attacked the cast iron very violently.

Potential Dynamic Polarization

Figure 7.4 shows the potentiodynamic polarization (Tafel region) for coated and uncoated cast iron samples. From the graph, it is illustrated that for the coated sample of Ni 85%-Cr 15%, the curve shifts toward the positive side compared to the other samples. Using Tafel polarization, the corrosion rate was assessed for all the samples. OCP circuit reveals the thermodynamic parameter, which leads to the tendency of metallic materials to participate in the electrochemical reactions in a chloride medium.

The Tafel plot illustrates the logarithmic relation between current generated between electrochemical cells and electrode potential of a specific material. This plot was generated based on electrochemical reactions between the samples and medium at a controlled atmosphere. However, based on the dip value of the specimen, the plot was generated. More dip value creates less corrosion resistance and vice versa. The uncoated samples have more dip than the coated samples. Ni 85%-Cr 15% exhibits less dip value that leads to high corrosion resistance than the others [13]. As showrn in Figure 7.4, the corrosion potential of Ni 85%-Cr 15% coatings in the range of -0.5 to -0.63 V. Uncoated samples have -0.67 V, which is more negative than the coated samples. The maximum

FIGURE 7.3 SEM micrographs of cast iron manifolds of different compositions before and after the thermal cycles, (a) Ni 85%-Cr 15%; (b) Ni 80%-Cr 25%; and (c) Ni 75%-Cr 25%.

TABLE 7.1

Variation of Weight Loss for Bare Cast Iron and Coated Cast Iron Samples

FIGURE 7.4 Tafel plot for uncoated and coated samples.

corrosion potential was observed for the Ni 85%-Cr 15% sample. The corrosion potential is somewhat less for the uncoated sample, the value of which is mentioned before. The corrosion potential deviation was mainly depending on the chemical composition

FIGURE 7.5 SEM micrograph of corroded surface, (a) Uncoated cast iron, (b) 85 Ni-15 Cr, (c) 80 Ni-20 Cr. and (d) 75 Ni-25 Cr.

of Ni and Cr of the coating. During the experimentation, for the coated sample with Ni 85% and Cr 15%, polarization is shifting toward more positive potentials; thus, it becomes more noble when compared to the uncoated samples.

For superior investigation of this phenomenon, surface morphology (SEM) of coated and uncoated samples after polarization is essential and presented in Figure 7.5a-d. Uncoated cast iron sample is violently attacked on the acidic medium as shown in Figure 7.5a. More pitting holes are observed on the surface of the uncoated samples. Uniform corrosion damage was observed on the surfaces, usually called pitting corrosion [14]. Small cracks are observed and it is propagated along the surfaces and thus formed the continuous crack, which leads to stress cracking. Minor cracks are observed for the coated samples, which are evidenced in Figure 7.5b-d.

On the other hand, some of the dimples are also observed on the coated sample surfaces. Generally, dimples reduce the corrosion rate. From Figure 7.5d, it is perceived that the Ni 85%-Cr 15% samples have more dimples on the surfaces and exhibit good corrosion resistance than the other samples. From the SEM micrographs, it is clearly visualized that no significant attack is found on the coated surfaces and only minor corrosion is observed [15]. It can be concluded that the uniformity and homogeneity of the Ni 85%-Cr 15% composition are considered to be good candidates for coating on cast iron manifolds.

Conclusion

An intense glossy attractive Ni- and Cr-rich layer is formed on the exhaust manifold cast iron substrate. Vickers’ hardness was determined on the cross section of the uncoated and Ni- and Cr-electroplated cast iron samples which shows a significant increase in microhardness of 246 and 259 HV, respectively. Very closely packed Ni and Cr layer deposited on cast iron indicates the enhanced surface property of cast iron. It was also found that the crack was reduced in the Ni- and Cr-coated cast iron manifold when compared to the uncoated manifold. A significant weight reduction was observed on the coated samples compared to the uncoated cast iron manifold. Further, the corrosion resistance was considerably enhanced for the 85 Ni—15 Cr-coated samples. Thus, the nickel-chromium plating has been considered as an excellent corrosion resistance for mass production.

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