[Technical] [Gumm Logo] [Article] --------------------------------------------------------------------------- Zinc-Nickel Alloy Plating [Zinc-Nickel Plated/Chromated Parts] by Nabil Zaki Posted 1/20/97 --------------------------------------------------------------------------- (Page 1) The automotive industry is leading has been in actual use in many the fight against corrosion as it applications world-wide for demands ever higher levels of several years. Precoated performance. Although high corrosion electrogalvanized zinc-nickel resistance materials are not new to sheet for instance, has offered the industry, they have been the automotive and computer certainly limited, and synonymous industries a superior with high price products. For prefabricated stock, resulting in example, the use of expensive much improved corrosion stainless steel and gold plated resistance. Zinc-nickel automobile components. An affordable electroplating of automotive high quality material will set the components such as fuel lines, competition apart in the near brake and transmission components future. is already a specification for some U.S. and European models, and Zinc coating of steel components has more extensively in Japan. been known for many years as a coating capable of providing high There are two basic types of standards for performance in zinc-nickel alloy processes corrosive atmospheres. The zinc, currently available. being sufficiently sacrificial to steel, corrodes preferentially to 1. The acid type. protect the functional steel 2. The alkaline type. component. Conversion coatings further protect the zinc layer by The acid type consists of a retarding its corrosion. Cadmium mixture of zinc and nickel salts, plating is also used for the same electrolytes, buffers, and reasons, as well as for its optional brightening agents. It exceptional lubricating operates at a pH range of 4-6 and characteristics, resistance to uses two rectifiers, one connected marine environments, and post heat to zinc and the other to nickel treatment corrosion protection. anodes as sources of metal in the bath. The proper amount of current Specifications written around these is supplied to each anode materials for salt spray testing independently and controlled in have ranged from 24 hours to white order to maintain the proper ratio corrosion, the point at which of zinc to nickel in the bath and chromates leave off and zinc or in the deposit. cadmium start to corrode, to 240 hours to red rust, where the steel The alkaline type is essentially substrate starts corroding. Although identical to alkaline zinc baths. there is still no consensus on what The electroIyte consists of sodium these numbers represent in actual hydroxide and a conventional zinc exposure to environment, one thing anode is used to supply the zinc is certain. The current level of to the bath by dissolution. Nickel corrosion protection afforded by the is supplied as a liquid additive, industry has to be improved. along with brightening agents. The nickel is plated out along with A host of alternatives have been zinc when current is applied. investigated with various degrees of Consequently, it must be added on success. Alloy plating offers one of an ampere-hour basis in the same the most practical and fashion brighteners are fed to technologically sound choices. plating solutions. The bath is Electrochemically, alloys have operated and maintained in the different corrosion potentials from same manner as alkaline baths, as their alloying elements. By shown in Tables I and II. selecting desired properties, alloys can be designed to produce these The immediate advantage of the properties. Alloys of zinc for alkaline bath is its inherent instance, if sufficiently high in noncorrosive nature to the ID of zinc content, can still maintain an tubular parts and recessed areas anodic potential to steel, yet that receive none, or very little remain less active than zinc alone. of the coating. Unlike acid type In a corrosive atmosphere, the zinc solutions, these baths do not alloy would still corrode promote or leave unplated areas in sacrificially to protect the steel, an active condition, prone to but at a much lower rate of oxidation and rusting after corrosion than zinc. plating. Many factors are involved in the Another advantage is the selection of a particular alloy for practicality and ease of a given application including bath application. The use of a single composition, ease of operation, rectifier and conventional zinc cost, and actual performance in anodes has been easily practiced corrosive atmospheres. Zinc alloys by alkaline zinc platers for many that have been considered feasible years. Insoluble steel anodes are alternatives are zinc-cobalt, also used as part of the anodic zinc-iron and zinc-nickel. The area in this bath to regulate the highest documented corrosion amount of zinc dissolved into the resistance was produced from tank. zinc-nickel. Basically, it is an alloy containing 90 to 95% zinc in the deposit and 5 to 10% nickel. The alloy (next column) --------------------------------------------------------------------------- (Page 2) DEPOSIT CHARACTERISTICS As the nickel is evenly Zinc-nickel is deposited at the rate distributed throughout the of 0.2 to 0.3 micron per minute at 2 coating, the chromating solution to 3 amp/dm2 from alkaline baths, reacts with and dissolves a small and at slightly higher rates from amount of zinc preferentially. acid baths. Ductility is similar to Although this is a similar that obtained from both baths mechanism to that of conventional respectively, as well as their zinc chromating, in zinc-nickel throwing power. Stress in the alloys a nickel-rich superficial deposit is low for thicknesses up to layer results as part of the 12 microns, then increases with chromate film. Consequently, a thickness; however, most different, highly resistant applications are in the range of 5 coating is obtained, increasing to 8 microns, which is ample for the overall corrosion resistance 5 superior performance. to 10 times (See Fig. 3). CORROSION RESISTANCE Another aspect of this unique Effect of Nickel Content: chromate film is its unusual The corrosion resistance of iridescent appearance. Compared to zinc-nickel alloys is derived from conventional iridescent yellow their anodic potential in relation zinc chromates, zinc alloy to steel. The nickel content in the chromates have a richer deposit controls this potential. As purple-green iridescence (See nickel content increases, the Figs. 4 and 5). corrosion potential increases to a maximum then shifts to the cathodic Various chromates can be obtained, side in relation to steel. If steel such as clear, yellow, bronze and becomes anodic to a nickel-rich black. Table III compares the alloy coating, its corrosion would corrosion resistance of these accelerate at pore sites. It has different types of chromates. been found that maximum corrosion Specifications aimed at maximum resistance is obtained in the 8 to corrosion protection are usually 12% nickel content range (See Fig. based on bronze or iridescent 1). At that level, zinc-nickel has a chromates. corrosion potential of -- 1.0 V versus SCE compared to -- 1.1 V for RESISTANCE AFTER zinc and -- 0.6 V for steel. HEAT TREATMENT Another unique feature of Effect of Current Density: zinc-nickel chromates is their The current density affects the ability to withstand exposure to nickel content in zinc-nickel high temperature and still plating baths depending on the type maintain a high corrosion of bath used. As can be seen in Fig. resistance value, even after 2 acid type baths produce high dehydration of the chromate film. nickel content deposits at low It should also be noted that current densities before stabilizing adhesion of the zinc-nickel at the higher range. Alkaline baths coating after heat treatment at maintain a fairly even level from 300°C for two hours remains low to high current densities, excellent, whereas conventional resulting in better corrosion zincs may show blisters or loss of resistance in recessed and low adhesion at those temperatures. current density areas. Table IV compares results of salt spray and cyclic accelerated Effect of Post Treatment: corrosion testing for conventional Chromate conversion coating of zinc and zinc-nickel before and zinc-nickel alloys is carried out in after heat treatment. a conventional post treatment dip similar to regular zinc plating. The coating obtained, however, assumes different and interesting characteristics that add to the uniqueness of the zinc-nickel alloy process. (next column) --------------------------------------------------------------------------- (Page 3) RESISTANCE AFTER Waste treatment of zinc-nickel MECHANICAL DEFORMATION plating effluents is possible in Extensive corrosion resistance existing treatment plants with testing of components after 8 mm minimum modifications, if any. In Erichsen Test extrusion showed that addition to neutralization, the chromated zinc-nickel coatings amount of nickel in the bath (0.9 maintained a high level of to 1.1 g/L) is readily reducible resistance to both white and red to acceptable effluent limits by corrosion. This is of significant typical treatment techniques. importance, particularly in plating automotive components such as fuel CONCLUSION and exhaust lines that require Zinc-nickel alloy plating offers bending or crimping after plating, the industry a substantial while maintaining the deposit's improvement in corrosion integrity. resistance over existing technology based on zinc and PROCESS CONTROL cadmium plating. Solution control consists of analysis of electrolytes, zinc and In addition to solving serious nickel using traditional analytical ecological problems with cadmium, methods. A typical control method is it is readily adaptable to summarized in Table V for an existing equipment and processes. alkaline zinc-nickel bath. Deposit Alkaline zinc-nickel baths, a thickness is checked by coulometric recent development in alloy or XRF after a calibration factor is plating, add easier operating established, using cross section conditions and controls, as well measurement as a reference. as improved corrosion resistance. The technology, already in use in Alloy composition of the deposit is Europe and Japan, is rapidly easily established by dissolving a making inroads in the US and small plated sample in an acid Canada. medium such as hydrochloric or nitric and measuring the metals by Biography atomic absorption spectroscopy or Nabil Zaki is Vice President, similar methods to establish their Technology at the Frederick Gumm ratio. Chemical Company in Kearny, NJ. He has an MS degree in chemical ECOLOGICAL ADVANTAGES engineering from New Jersey A decisive advantage to zinc-nickel Institute of Technology and has alloy plating is in its position as worked in the metal finishing a replacement for cadmium plating in industry since 1966 in plant most applications, other than those supervision, product development requiring exceptional lubricating and technical service. He is a characteristics. Since cadmium has member of ASTM B08 committee, become seriously restricted or AESF, SME, and ASM. banned, zinc-nickel has offered the industry an ideal alternative. 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