Diffusion and Bonding Mechanism of Protective -Al 2 O 3 on FeCrAl Foil for Metallic Three-Way Catalytic Converter

. High pollutant level contributed by mobile sources/land transportation that become main problems for the human health. Improving exhaust emission system by improving catalytic converter properties is one of the most effective way to produce healthy air in our environment. It is conducted by two methods i.e. ultrasonic during electroplating (UBDEL) and electroplating process (EL) which are not fully investigated yet as catalytic converter coating process. UBDEL is conducted using sulphamate types electrolyte solution, Frequency of 35 kHz, current of 1.28A, Voltage of 12 V, and various time of 15, 30, 45, 60 and 75 minutes. Meanwhile El method is conducted using parameters of current of 1.28A, Voltage of 12 V, stirrer speed of 60 rpm and various time of 15, 30, 45, 60 and 75 minutes. Fully γ -Al 2 O 3 bonding to the FeCrAl substrate is shown by UBDEL 75 minutes samples proved by SEM images and Ra and Rq are 4.01 µm and 5.64 µm, respectively. Ni present on the FeCrAl substrate as other protective layer generated by Ni electroplating process that will improve thermal stability of FeCrAl at high temperature of 1000 0 C. From the results, can summarized that UBDEL technique is promoted as an effective catalytic converter coating technique.


Introduction
The first strict regulation to control automobile emissions was introduced through a US Clean Air Act, 1970, in the United States of America. In addition, Mobile sources contribute for about 44% of outdoor toxic emissions, approximately 50% of cancer risk and at around 74% of non-cancer risk health problems. The technology for removing pollutants in the exhaust system is catalytic converter that first developed in 1965 [1,2]. The location of the catalytic converter in the car is shown in Figure 1.  [3] Nowadays, the metallic catalytic converter of FeCrAl was interesting component to explore due to their advantages in the high-temperature corrosion resistance, including strong adherence of oxide film on the surface of substrate [4,5]. FeCrAl is consists of 5-8 wt% aluminium, 17-22 wt% chromium and reactive element to improve oxide adhesion and oxidation resistance [6]. Command washcoat material such as Al 2 O 3 , SiO 2 , TiO 2 , or SiO 2 -Al 2 O 3 [7]. However, γ-Al 2 O 3 washcoat material not fully explored even though it promising several advantages such as low price, high surface area, good chemical stability, high micro-hardness, and wear resistance at high-temperature [8]. Therefore, in this research the FeCrAl is coated by γ-Al 2 O 3 powder which can result higher surface area of about 100 m 2 /g.
There are several method for coating FeCrAl substrate such as dip coatings, which combined with some pretreatments, such as growing a number of textured alumina whiskers on the surface of the metal support and shortened the diffusion path before depositing the washcoat [9,10]. The other methods are co-precipitation, sol-gel and spray-pyrolysis methods [11], electrophoretic deposition [12], solution combustion synthesis (SCS) [13], aluminizing technique [14], and hydrothermal method [15,16]. That method presented good adherence to the substrate, decreasing surface roughness and closed the surface porosity of the substrate [17]. However, the existing methods still have some limitations, especially due to the rather complicated method to applying the catalyst which is in the form of powder.
Material coated by Al 2 O 3 promote high surface roughness which important to the filtration and catalyst application [17]. Roughness of the material indicates the surface topography which have several forms such as digs, pits, dust particle, scratches, machining marks on machine surface, polishing marks on optical surface, granularity, crystallite in film deposited in surface and electro-polishing [18]. According to Putrasari et al., (2010) [19] that surface roughness of FeCrAl coated by SiC and Al 2 O 3 is successfully developed using ultrasonic technique for 10 min in frequency of 35 kHz and it improve high thermal stability of FeCrAl substrate and homogeneity of Ni electroplating layer. However, it still need improvement and special treatments. In other hand, diffusion of aluminium alloy (FeCrAl) is very challenging to investigate since some phenomena occurred. Less protective γ-Al 2 O 3 powder is caused by slow diffusion of Al alloy to the metal oxide interface to compensate for the aluminum consumption. Oxidation behavior of the aluminium alloy is measure by diffusion coefficient [20]. The growth mechanism occurred by both cation and anion transport in the Al 2 O 3 scales on Fe-Cr-Al. The alumina scale grows exclusively by oxygen diffusion along grain boundaries. Diffusion results were also obtained in alumina developed on γ-NiAl by cationic and anionic self diffusion tests and also by an electrochemical method [21]. Therefore, the diffusion phenomena of the coated FeCrAl and its surface roughness treated by electroplating approach is very challenging to investigated. It believe that electroplating approach to promote fully embedded of γ-Al 2 O 3 powder to FeCrAl substrate that figured by SEM/EDS and surface roughness testing.

Methodology
FeCrAl foils was treated using various novel Nickel electroplating technique i.e. electroplating ultrasonic bath during electroplating technique. It coated by γ-Al 2 O 3 powder with crystallite size of 121.68 µm which measured by X-Ray Diffraction (XRD) machine. In this research, electrolyte used in electroplating process is called by sulphamate types which consist of nickel (II) sulphamate 6-hydrate (NiSO 4 .6H 2 O), nickel (ii) chloride (NiC l2 ·6H 2 O), boric acid (H 3 BO 3 ), and sodium dodecyl sulfate (C 12 H 25 OSO 3 .Na) which believed that it can optimize the electroplating process.

Nickel (Ni) electroplating process (EL sample)
Nickel electroplating was carried out using some equipments and components such as DC power supply, anode (Ni plate), cathode (FeCrAl foil), electrolyte solution, magnetic stirrer and hot plate for adjust temperature and stir speed as well as electronic microbalance. Ni plate and FeCrAl foil is cropped at the fix size of 50 mm x 10 mm and 40 mm x 20 mm respectively. Preparation of electrolyte solution with the composition is shown in Table 1. Electrolyte solution is prepared with distilled water at constant temperature of 60 0 C, distilled water volume of 0.25 L, holding time approximately of 10 minutes and pH value is adjusted of 2.5-5. Electroplating process was conducted using various times of 15, 30, 45, 60 and 75 minutes, current of 1.28 A, voltage of 12 V, distance between anode and cathode of 25 mm, mass of γ-Al 2 O 3 powder of 5 g and total surface area of 1600 mm 2 in two sides. The schematic diagram of electroplating process is shown in Figure 2.   Figure 3. The specification of ultrasonic machine is listed in Table 2.     Figure  4 shows the uncoated sample with even surface that consist of C, O, Al, Cr and Fe. Figure 5 to Figure 9 show the samples that treated by ultrasonic during electroplating with the various times of 15, 30, 45, 60 and 75 minutes, respectively. There are many unrelated chemical is detected in those samples such as Sodium (Na) for 6-12 %mass and Sulfur (S) for 0-6%mass because the electrolyte used in this process is contain Na and S. Therefore, there is bonding activity between those chemical and coated FeCrAl. Nickel (Ni) present 11-32 % mass in the coated samples because Ni plate is performed during this process. From the UBDEL samples can be seen that the bonding activity for surface γ-Al 2 O 3 powder and electrolyte chemical is successfully achieved. Fully embedded of γ-Al 2 O 3 in line with the time increment. It approved by surface roughness analysis that the UBDEL 75 minute samples has highest surface roughness for 4.01 µm as compared to other UBDEL samples. The full bonding phenomena of UBDEL samples proof that the jet flush energy or high speed bubbles promote to the γ-Al 2 O 3 powder to bond on the FeCrAl substrate [19]. Fully bonding activity is related to the high diffusion of γ-Al 2 O 3 to the FeCrAl substrate since there is cation and anion transport which promote higher diffusion coefficient [20].     Figure 14 shows the EL samples with the various electroplating times of 15, 30, 45, 60 and 75 minutes, respectively. The influence of electrolyte to electroplating process is shown in EDS analysis that the Na (2-13 %mass) and S (0-4 %mass) chemical present on coated FeCrAl surface. It will play the important role when the coated material is performed in high temperature of 1000 0 C. The γ-Al 2 O 3 as main coated material is increased with time performed increased as well. It approved by surface roughness of 1.06 µm as the highest number as compared to other EL samples.

Influence of electroplating approach to the coating surface and composition
Besides that Ni has observed in the EDS analysis of coated FeCrAl for 6-25 %mass. It is promote the protection at high temperature [23] because appear intensive co-precipitated NiO-Al 2 O 3 system [24]. Interaction between γ-Al 2 O 3 and FeCrAl substrate is related to the diffusion activity where the alumina scale grows exclusively by oxygen diffusion along grain boundaries. Higher Al content and Oxygen content will promote higher diffusion phenomena of the samples [20].   The other parameter such as surface roughness mean squared (Rq) also presented by Figure 15 a to

Conclusion
Coating process of FeCrAl substrate by γ-Al 2 O 3 powder is successfully conducted using ultrasonic during electroplating and electroplating process. There are several phenomena tha has been observed such as: 1. Fully bonding of γ-Al 2 O 3 into FeCrAl is shown by UBDEL 75 minutes sample where increasing holding time will increase bonding activity of washcoat material to the substrate material.
2. Surface roughness of the samples is defined by how many particle of powder and chemical agent which ebbed into the substrate. surface roughness of the EL samples is lower than UBDEL samples where highest Ra and Rq are 4.01 μm and 5.64 μm.
3. There are several chemical observed by EDS analysis such as O, C, Ni and Na which promote developing NiO-γ-Al 2 O 3 , C-γ-Al 2 O 3 and NaO-γ-Al 2 O 3 form. That form believe that it can be increased thermal stability of developed substrate.