Electrodeposition of only molybdenum onto substrates is difficult, therefore molybdenum is typically deposited with iron-based alloys such as nickel. The deposition of such alloys is known as an induced codeposition mechanism. The electrodeposition of nickel-molybdenum alloys using alkaline plating solutions is covered in this chapter. The mechanism for deposition of nickel-molybdenum is reviewed, as well as the influence of the plating parameters on the coatings. Characterization of the coatings by scanning electron microscopy and x-ray diffraction is discussed and how deposition parameters affect morphology, composition, and crystallite size. Nickel-molybdenum alloys offer enhanced corrosion protection and mechanical properties as coatings onto various substrates. A survey of the resulting hardness and Young's modulus is presented for several research studies. Corrosion parameters for several studies are also compared and show the percentage of molybdenum in the coatings affects these values.
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Stainless steels are corrosion resistant in a wide range of environments due to formation of a passivating film on their surfaces. Despite passivity, extensive localized corrosion commonly occurs on stainless steels when exposed to harsh environments containing chloride ions, such as seawater. Chloride ions dislocate the passivating species at various sites within the passive film resulting in the initiation of pitting nuclei. Once the passivity has been destroyed, the reactions inside the growing pits propagate through an anodic process leading to a loss of strength in the overall structure (Caceres, 2009; Cvijović, 2006; Jung, 2012; Lu, 1993; Sedriks, 1996). Molybdenum, having been used as one of the fundamental metals incorporated in stainless steels, is of interest to alloy with nickel to enhance the repassivation behavior and deactivate pit growth in aggressive chloride media (Ahn, 1998; Bastidas, 2002; Ilevbare, 2001; Mishra, 2013; Tobler, 2006). Additionally, the presence of molybdenum improves the mechanical properties and temperature resistance of the alloys (Gomez, 2006). Molybdenum, in the same group as chromium and tungsten, has a melting point of 2620 oC. Molybdenum offers similar heat stability as tungsten and therefore is used to alloy with nickel leading to improved corrosion resistance, mechanical properties and heat stability. In addition, molybdenum is another alternative component to alloy with nickel to replace toxic components such as chromium which have been previously used for improved mechanical properties.
Due to the coating stability, nickel is commonly used to alloy with other metals to protect substrate materials under severe operating conditions such as corrosive environments, high temperatures and high stresses. Nickel can form intermetallic phases with some metals commonly used in nickel alloys leading to high strength for both low and high-temperature applications (Friend, 1980; Haubold, 1992). Nickel- molybdenum alloy coatings are of interest as a replacement for chromium due to environmental concerns (Brooman, 2000). Ni-Mo also exhibits high hardness and strength as well as good wear resistance, which makes it attractive as an anti-corrosion coating. Nickel-based alloys containing 9-16 weight percent molybdenum offer high corrosion resistance against chloride attack as well as protection from high temperatures (Chassaing, 1995; Friend, 1980; Sugimoto, 1977). Ni-Mo alloys have good resistance to corrosion in hydrochloric, phosphoric, and sulfuric acid solutions (Donten, 2005; Raj, 1988). Ni-Mo has been added to ferritic stainless steel to produce better interconnects in solid oxide fuel cells. Addition of 3 wt% Mo and 2 wt% Ni reduced the oxidation rate of interconnects (Safikhani, 2014). Thus Ni-Mo alloys are good choices for harsher environmental conditions such as high temperature, corrosive chemical attack, or high wear.