Improvement of the NiBrAl Casting Alloy Surface Properties by Electroless Ni-B Plating for Dynamic Marine Applications

The use of NiBrAl casting alloy for the construction of marine propellers is expanding due to the resistance to fatigue, corrosion and wear under the effect of particles suspended in fluid. In this study, the nanostructure of the Ni-B coating is created by the electroless plating method on the NiBrAl alloy surface. After plating, the specimens are heat-treated, and the temperature and optimum time of the heat treatment are found to be 410°C and 70 min. The microstructural study by scanning electron microscopy and X-ray diffraction shows that, by conducting the heat treatment after plating, the amorphous structure of the coating becomes crystalline. After heat treatment cauliflower colonies experience 17% growth due to their constituent crystallites. After plating and heat treatment, the hardness of the surface increased from 410 to 788 and 1365 Vickers, respectively. According to the pin-on-disk wear test results, the wear resistance improves due to plating and heat treatment. The microscopic study of the worn surfaces shows that the heat treatment turns the wear mechanism from adhesive to slightly abrasive. The corrosion resistance evaluation by the polarization test shows that heat treatment leads to the reduction of corrosion resistance. According to the results, the electroless Ni-B coatings can be considered as one of the best options to improve the properties and performance of NiBrAl alloys, under service conditions.


INTRODUCTION
Copper and aluminum compositions are known as aluminum bronze alloys and, together with other alloying additions produce a wide range of properties which are beneficial in a diverse range of industries. Nickel aluminum bronze alloys (NiBrAl) have been paid more attention so far. The use of NiBrAl alloys is expanding on account of many unique properties such as high wear and galling resistance, high strength, lighter density than steels, nonsparking, low magnetic permeability, excellent resistance to corrosion, stress corrosion, and cavitation, damping capacity twice that of steel, resistance to biofouling, self-repair due to an oxide surface film, and good cryogenic properties. The NiBrAl alloy is used in wide ranges from landing gear bushings and bearings for all commercial airplanes to seawater pumps and valves, also, propellers for naval and commercial shipping, nonsparking tools in the oil and gas industry and pleasing facades in architecture. NiBrAl alloys are important materials in which their microstructure and properties are controlled during manufacture by the addition of alloying elements and PHYSICAL MESOMECHANICS Vol. 23 No. 1 2020 subsequent processes like heat treatment. NiBrAl alloys are available in both cast and wrought products [1].
The main markets and targets of the manufacturers of NiBrAl parts are marine applications. Many dynamic pieces of the marine equipment are made of NiBrAl. In addition to the aforesaid, good antidamping properties, which are essential in submarines to suppress sound in silent operations, weldability, reasonable cost, and the ease of repair when damaged are the reasons of NiBrAl usage which lead to producing propellers and power shafts of military ships, commercial vessels, and cruise liners. Propellers are heavily damaged by corrosion, corrosion in seawater, erosion corrosion, wear, cavitation, nucleation, and growth of cavity and cracks due to the difference in air pressure caused by propeller rotation and fatigue [1,2].
The surface of NiBrAl alloys has a protective Al 2 O 3 -CuO 2 film, which due to high hardness and corrosion resistance, as compared to the substrate, increases the life of NiBrAl parts. Also, iron and nickel oxides are added to the protective film under prolonged exposure. In addition, the availability and mobility of copper ions at the surface provide excellent resistance to biofouling as they hinder the adherence, colonization, and growth of marine and microorganisms [1,3,4].
The propellers protective layer quickly gets damaged by seawater flows. This damage can be severe if there is a high degree of turbulence or the seawater contains many abrasive particles such as sand. If flow rates are mild and with enough oxygen present, in most cases, the protective film repairs itself. If there is a high or persistent turbulent flow, the protective film is unable to self-repair, and the local attack takes the form of erosion-corrosion phenomenon. Another action of seawater flows on the propeller is rapid changes in pressure. In so doing, small water bubbles are formed due to turbulence at the low-pressure point and tend to migrate to higher pressure areas. This migration causes damage to the surface at the bubble reference points. Stress that induced by the bubble migration leads to damage and finally removes the protective oxide film [49]. Thus, one of the results of this factor is fatigue that both can form microcracks and separate metal particles from the surface. The friction coefficient of the protective layer and substrate has increased the usage of NiBrAl at high rotation speeds situations. However, if rotation speed surpasses the optimum range the substrate will deform plastically due to the rise in temperature [10,11].
The usage of a coating harder than a protective layer can decrease the crack growth rate. This coating can protect the substrate and protective oxide film, which enhances the NiBrAl part life. In this study, the electroless Ni-B coating is used for this purpose. Wear and corrosion resistance of coated NiBrAl specimens are investigated to examine the coating efficiency. The application of NiBrAl alloys in marine propellers is considerable, and these parts require corrosion resistance and wear resistance when operating in seawater and exposed to abrasive particles suspended in water. This study aims to improve the corrosion and wear resistance properties to increase the efficiency of these alloys during service, and to provide a solution to deal with degradation mechanisms.

MATERIALS AND METHODS
NiBrAl casting alloy was purchased and prepared in a disk form with a thickness of 5 mm and a diameter of 30 mm. The composition of the NiBrAl casting alloy is 0.02 wt % Sn, 0.04 wt % Si, 0.02 wt % Pb, 9.82 wt % Al, 1.2 wt % Mn, 4.26 wt % .e, 4.61 wt % Ni, 1.48 wt % Zn. Cut specimens are sanded with the SiC 1500 sandpaper. After sanding, the specimens are cleaned with acetone in the ultrasonic bath, and at the final stage, they were put into the sodium carbonate solution to remove oil from the surface.
In order to make the specimens active and sensitive, they were put into the solution containing 40 ml HCl, 10 g SnCl 2 , and 100 ml H 2 O for 1 min and then into the solution containing 30% HCl, 20% H 2 SO 4 , and 50% H 2 O [12].
After activation (etching), the specimens are immersed into the electroless solution consists of 20 g/L nickel chloride, 1 g/L sodium borohydride, 56 g/L ethylenediamine, 40 g/L sodium hydroxide, and 0.01 lead nitride for one hour which leads to precipitation with the rate of 15 µm per hour.
The electroless process was carried out at a constant temperature of 85°C. .or fixation of pH during the coating formation, diluted ammonia was added continuously to the bath. Therefore, pH stays at 13.5. After stabilizing the temperature and pH, magnetic stirring at 750 rpm was used during the electroless process in order to prevent particles from sedimentation in the bath. After plating, the specimens were annealed at the 410°C temperature under a commercial argon atmosphere (99.999% purity) for 70 min.
The morphology of the coated and annealed specimens was studied by Philips XL30 scanning electron microscope (SEM) operating with 25 kV. Phase analysis of the specimens was performed by Philips PW1730 X-ray diffractometer (XRD) before and after annealing. Microhardness tests were conducted on cross sections of the prepared specimens under a load of 50 g for 10 s.
To examine the wear resistance of the specimens, the pin-on-disk test was carried out on 52100 steel with the hardness of 75 Rockwell C. Also the wear test was carried out under the load of 5 N, constant speed 0.12 m/s and 1200-meter movement of the pin. The worn surfaces of the specimens were investigated by TESCAN Vega scanning electron microscope with the working voltage of 5 kV and the secondary electron method.
The corrosion test was performed in a solution of 3.5% NaCl at a scanning speed of 1 (mV s) 1 , which was done in the potential range of 250 to 250 mV with proportion to the open circuit potential at room temperature after three hours of immersion: The efficiency percentage was calculated according to Eq. (1). Here 0 corr E and corr E are the corrosion current density in the absence and presence of the coating which was obtained by the Tofel output selection method with NOVA 2.1 software.

RESULTS AND DISCUSSION
.igure 1 shows the SEM morphology of the coating after electroplating and a cross-sectional area of the coating. It was evident that the coating has a cauliflower structure with perfectly uniform clusters. This uniformity reflects the precise control of the bath and the optimal selection of process parameters. The cauliflower structure is a typical morphology of electroless coatings [13]. Self-lubricant is due to the porosity of cauliflower structures, which ultimately leads to a decrease in the friction coefficient [1016]. In .ig. 1, bright spots are clusters that consist of numerous nanosized crystallites, and dark points between clusters are porosities which presence is the nature of the structure of electroless coatings [14,16,17]. In past research, cracking and cavities in coatings are reported to be due to hydrogen interactions. The solution to this is to select the correct components of the bath or to carry out the hydrogen-removal heat treatment at temperatures below 200°C for 2 h [18]. .igure 1 exhibits an SEM image of a cross-sectional area of the coating. This monochromatic image exhibits excellent adhesion to the substrate with an average thickness of 4.5 µm.
Due to the absence of external flow, a perfectly uniform electroless coating was created on the substrate. This uniformity continues to be maintained even at sharp points and corners of the piece [19]. The results of the field emission scanning electron microscopy (.ESEM ) test show that the coating contains about 6.50% B and 93.50% Ni.
Heat treatment is usually being used to increase the hardness and wear resistance of electroless coatings. The heat treatment could be considered as the most crucial factor in the hardness of electroless coatings. Electroless nickel-boron coating, like any electroless coating, has an amorphous or semi-amorphous structure [15,20]. In .ig. 2, the SEM image shows the morphology of the specimen which annealed at the temperature of 410°C for 70 min.  As shown in .ig. 2, the cauliflower morphology of the specimens is retained after heat-treatment and clusters growth. The phenomenon of growth seems evident by a temperature rise. The high temperature is going to cause the growth of nanocrystallites, and since the clusters consist of numerous crystallites, then it is natural that the clusters grow after the temperature increase. Previous studies reported that increasing the temperature and time of heating leads to increasing crystallite growth rate and, consequently, cluster growth [21]. The researchers suggest that secondary phases in compositions are an excellent solution to prevent the growth of clusters [2224].
In .ig. 3, the XRD pattern of the specimens shows the heat-treated coating. The crystallographic structure of the electroless coating is semi-amorphous, i.e., a mixture of nanocrystalline nickel, amorphous nickel, and boron phases. As reported in the references, after electroless plating, nickel and boron are formed as amorphous or semi-amorphous phases on the substrate [15,20]. Since in the electroplated specimens broad peaks with low and high altitudes can be seen in the temperature range of 4347°C, this coating can be considered to be nanoscale semi-amorphous regarding the structural pattern, because fewer spike peaks along with amorphous structural patterns are visible. Previous studies state that the reason for the formation of the amorphous phase after electroplating is the difference in the crystalline lattice, the coordination number of nickel and boron, the chemical desire of these two elements to each other and the low temperature of the coating process [24,25]. In general, the properties of the electroless nickel coating depend on the amount of the alloying element to the extent that the crystallinity of these coatings is and also dependent on these elements. .or example, increasing the amount of boron in the coating increases the probability of crystallinity after the plating process [25]. In electroless coatings made of nickel-containing phosphorus or boron, the separation rate of these elements affects the crystallinity of the coating. Since the segregation of the boron element is negligible, after plating either these coatings, do not have a crystalline phase or their amount is low [26].
At the temperature of 410°C with the range of 46 47°C, nickel crystals were formed in the 〈111〉 direction. The peak of the annealed specimen at 410 and 510°C indicates that the coating is entirely crystalline due to high peaks. It is also possible to observe the peaks associated with the formation of nickel-boron bonds Ni 2 B and Ni 3 B. As the annealing temperature increases, depletion occurs, and the Ni 2 B phase converts to Ni 3 B. This was also reported in previous studies [2729]. During the heat treatment, boron is transferred from the center of the coating to its surface, which results in a high degree of crystallinity following the penetration [30]. As expected, the application of an electroless Ni-B coating increases the surface hardness of the substrate by 92%. After electroless Ni-B plating, the surface hardness of the substrate rises from 410 to 788 Vickers, which demonstrates that the heat treatment increases the coating hardness. The sufficient heating time and temperature lead to crystallization of the amorphous or semi-amorphous structure of the electroless coating.
Annealing at the combustion temperature creates intermetallic compounds, or in the best sense of the term nickel boride, which leads to the formation of the Ni 2 B and Ni 3 B phases. These sediments act as a hindrance to the dislocation motion and lead to hardening [2729]. The annealed specimen is entirely crystallized, and the formation of nickel boride in it leads to a dramatic increase in hardness; the average hardness of the annealed specimen was reported to be 1365 Vickers at 410°C. According to the results and comparison with the standard hardness data [1], the hardness of the specimens was increased. .igure 4a shows the weight loss chart of the specimens regarding the traveled distance. As demonstrated, the highest wear resistance rate for the annealed specimen is at 410°C. The crystallization and formation of the Ni 2 B, and Ni 3 B compounds can attribute to this issue [16,22,24,28,29]. It is clear that the application of electroless nickel-boron coatings in both conditions before and after heat treatment results in increased wear resistance. Resistance to wear in most cases is a function of the hardness of the material. It can be justified that an increase in the wear resistance after plating and heat treatment can lead to an increase in the hardness [21,29].
The SEM image in .ig. 4b shows the worn surface of the substrate. Deep scratches which were created on the surface indicate that severe wear occurred on the specimen and extensions around scratches indicate that adhesive wear has also occurred in the substrate. Due to a relatively low hardness of the substrate, in compared to the wear plate, at first the surface was expanded in the direction of force, and then after positional work and due to plastic deformation, the hardness of the surface has increased. Because of hardness and brittleness of the surface of the specimen, they scratched after passing some distance. These scratches usually reach the hardened depth.
.igure 4c illustrates the SEM image of the wear surface of the coated specimen where extensions are due to plastic deformation in the specimen. This elongation and transformation could be attributed to the dominant adhesive wear mechanism of the specimen. Limited scratches are observable in the specimen due .ig. 5. Polarization gradient, the data are obtained from the test on the substrate, coated specimen, and the heattreated specimen.
.ig. 4. Weight loss chart of the specimens in terms of the distance traveled during the wear test on the disk (a), SEM images of the wear surface of the substrate (b), the coated specimen (c), and the heat-treated specimen (d). to limited abrasive wear. In .ig. 4d, the SEM image shows the surface of the specimen treated at 410°C. This specimen has no signs of elongation and plastic deformation, which is due to the resistance to the mechanical work applied during the wear test. This strength and resistance are due to the crystallization and formation of nickel boride, which hinder the dislocation motion, thus ultimately increasing the wear resistance.
Some scratches are caused by adhesive wear, and the actual surface of the specimen has bumps and ditches. The scratches were created because of the detachment of the surface during the wear process. Also, higher pressure consequently creates cold work and brittleness in these spots. Continuous detachment causes these scratches [14,21,22,28,29,31].
.igure 5 shows the electrochemical polarization gradient of the tested specimens. .rom these gradients, potential corrosion values corr , -corrosion density It is visible that different specimens have various flow densities, corrosion potential, and cathodic and anodic kinetics reactions. In general, the plating process reduces the corrosion rate of the substrate from 1.53 to 0.267 µA/cm 2 (.ig. 5 and the table). In fact, anodic reactions are delayed in the plated surface, and the alloy has more protection against corrosion phenomena.
According to the obtained data, the coated specimen has a lower corrosion rate and, as a result, higher corrosion resistance is found in the heat-treated specimen and substrate. .igure 5 and table state that the heat treatment increases the corrosion rate of the coated specimens from 0.267 to 1.47 µA/cm 2 and as a result reduces the corrosion resistance. By comparing the results of the XRD analysis of the coated specimens and the heat-treated specimens, it can be concluded that intermediate nickel boride (Ni 2 B and Ni 3 B) phases are formed due to the crystallization phenomenon. The determination of the necessary conditions for the in-termetallic compound transformation and crystallization of the coating is the reason for lower corrosion resistance of the heat-treated specimen [2729]. Nevertheless, intermetallic compounds act as microcathodic places, and the substrate acts as microanodic areas. As precipitates increase, more microcathodic places are formed. As a result, more corrosion reactions occur in these areas and lead to lower corrosion resistance [3033]. The values obtained from calculating the anodic and cathodic Tofel gradient also show that the specimen with lower corrosion rate and higher corrosion resistance has lower anodic and cathodic Tofel gradient. Therefore, the lowest Tofel gradient values are for the coated specimen, and the highest Tofel gradient is for the uncoated specimen.
According to the data obtained from .ig. 5, table and the corr E amount, it can be concluded that the Ni-B electroless coating reduces the corrosion rate and increases the corrosion resistance of the substrate. In addition, although annealing of the Ni-B coating increases the hardness and wear resistance of the specimen, it reduces the corrosion resistance by raising the corrosion rate. Therefore, the best specimen is that with the electroless Ni-B coating without heat treatment.
Therefore, the electroless Ni-B coating can increase the service life and improve the properties of wear and corrosion resistance of marine propellers.

CONCLUSION
In the present study, we can make the following conclusions by analyzing the data.
The electroless Ni-B coating is successfully applied to NiBrAl alloys. The coated and heat-treated specimen has the cauliflower morphology. After heat treatment, the cauliflower cluster structure grows in the orientation of crystallite growth.
After electroless plating, the coating structure is semi-amorphous. Annealing at 410°C leads to coating crystallization and the formation of nickel borides Ni 3 B and Ni 2 B. Application of the electroless Ni-B coating leads to a 2-fold increase in the surface hardness of the substrate. With the electroless Ni-B coating, the surface hardness of the substrate increases from 410 to 788 Vickers. Due to annealing, the semi-amorphous structure of the coating transforms into a crystalline structure, and the intermediate nickel phases Ni 2 B and Ni 3 B form, which increases the hardness of the electroless coating 577 Vickers. By coating and heat treatment, the wear resistance increases. Evidence shows that the substrate wear is due to a mixture of adhesive and abrasive mechanism, which turns to the adhesive mechanism due to plating. With increasing hardness, this mechanism moves toward the abrasive one. As a result of electroless Ni-B coating, the corrosion rate of the substrate is reduced by half and the corrosion resistance increases.
Heat treatment increases the hardness and wear resistance while reduces the corrosion resistance of the coating and increases the corrosion rate. As a result, the corrosion resistance of the substrate is lower than that of coated and coated plus heat-treated specimens. The results of the polarization test show that electroless Ni-B plating increases the corrosion resistance of the substrate, and the heat treatment after plating reduces the corrosion resistance of the substrate. Based on the results, it can be claimed that the application of electroless Ni-B coatings can have a positive effect on the performance of marine propellers regarding corrosion and wear resistance, and therefore it is a good candidate for NiBrAl marine propellers.