Nickel alloys electroplated for engineering applications include nickel-iron, nickel-cobalt, nickel-manganese and zinc- nickel. Iron is an inexpensive metal and solutions for plating nickel-iron alloys were developed mainly to reduce costs. However, they were also developed for special magnetic purposes. Cobalt and manganese are used to increase the hardness and strength of nickel plating. Additionally, nickel-manganese alloys have improved resistance to sulfur embrittlement when heated. Coatings of nickel-tungsten show high resistance to corrosion, but they are believed not to be true alloys and have not been used in practice.
Bright nickel-iron plating was strongly promoted as a substitute for bright nickel plating from 1970 to the early 1980s, when the relative price of nickel was high.
Advantages. The main advantage is the cost savings, because up to 35 pct of the nickel is replaced with iron. An additional advantage is that iron entering the plating solution through chemical dissolution of steel substrates, which is highly detrimental in straight nickel plating solutions, is readily dissolved and subsequently plated out. This feature is particularly relevant during plating onto tubular steel parts. Ductility is usually higher for the alloy coatings than for bright nickel, which may be advantageous if the plated parts are subject to deformation.
Disadvantages. The organic addition agents are more expensive than those for bright nickel, negating the savings on metal. The addition agent system is also more complex, so electrolyte control is more difficult. At equal thickness, nickel-iron plating is less resistant to corrosion than nickel. The higher the iron content, the lower its corrosion resistance.
Process Description. Preferred solutions for plating bright nickel-iron are slightly more dilute than nickel plating solutions in order to obtain a high iron alloy without using a high iron concentration in the solution. A typical solution is given in Table I. Addition agents include stabilizers for the ferrous iron, organic brighteners, leveling agents and wetting agents. Total iron includes ferrous and ferric ions. It is important to control the ratio, keeping ferric ions below 20 pct. Solution temperature is typically 130 to 140F. The pH must be kept at 2.8 to 3.6. It is better to use air agitation, since higher plating rates can be achieved than with cathode rod movement. A higher iron content can be altered by increasing or reducing the rate of air bubbling.
Properties of the deposit that are of interest include ductility, hardness, internal stress and magnetics.
Ductility depends on iron content, brightener concentration, solution temperature and pH.
Deposit hardness varies with iron content. When iron content increases from zero to about 10 pct, microhardness rises from 490 to 560 HK1. It falls to about 510 HK with 49 pct iron for coatings plated at standard conditions of 40 amps/sq ft, 140F and pH 3.5, with air agitation. Changes in solution pH and brightener concentration also influence deposit hardness, enabling values exceeding 700 HV2 to be achieved.
Internal stress is tensile, in contrast to that of most bright nickel deposits. It is influenced by iron content and more sharply by solution pH. Increasing iron content raises stress from 13,500 to 22,400 psi. Increasing pH from 2.8 to 4.5 raises stress from 2,500 to 33,600 psi.
Magnetic properties of nickel-iron are not important in the application of bright decorative coatings. However, similar alloys are deposited for magnetic applications from solutions not containing brighteners. The alloys with 18 to 25 pct iron are soft magnetic materials with low coercive force, low remanence and high maximum permeability. They can be used as coatings or as electroformed parts.
Some of the earliest solutions for bright-nickel plating contained cobalt, formate and formaldehyde. With the development of modern bright-nickel solutions based on organic addition agents only, the cobalt-containing solutions have fallen into disuse. Today, the cobalt additions are used when necessary to increase the hardness and strength of nickel plating, especially in electroforming applications.
Advantages. Compared with nickel itself, nickel-cobalt alloys are harder and stronger. In contrast to nickel hardened with conventional organic addition agents such as naphthalene 1-3-6 trisulfonic acid, nickel-cobalt alloys can be heated to high temperatures without embrittlement by sulfur.
Disadvantages. The need to maintain the cobalt ion level introduces an additional maintenance requirement. Also, internal stress is moved in the tensile direction, and there is a practical limit to the level of cobalt that can be used. Hence, the maximum hardness of about 400 HV is less than the 600 HV that can be attained using conventional organic additives.
Process Description. Most of the data about nickel-cobalt plating were determined using a 600 g/liter nickel sulfamate solution. The initial charge of cobalt is added to the base solution as cobalt sulfamate. Replenishment during operation is usually made by metered additions of cobalt sulfamate. It can be achieved, however, by dividing the anodic current between a nickel anode and a cobalt anode, so that the percentage of total current passing to the cobalt is the same as the percentage of cobalt required in the alloy deposited at the cathode. Good control of cobalt content is necessary.
Processing variables and properties of the alloy plated from a 600 g/liter nickel sulfamate solution. With solution composition and pH standardized, the cobalt content and alloy properties depend on solution temperature and current density. The properties are modified by subsequent heat treatment and simultaneous use of sulfur-free organic addition agents.
Alloy hardness. The relationship between cobalt content and deposit hardness is shown in Figure 1. The brown curve shows microhardness versus cobalt content in the deposits formed at 50 amps/sq ft. The green curve shows microhardness versus cobalt content in the solution. A peak hardness of about 520 HV is attained with six g/liter cobalt in the solution. This provides an alloy containing about 34 pct cobalt. At peak hardness, internal tensile stress is too high for electroforming applications, although the alloy can be used as a coating on a solid substrate. For electroforming purposes, the limit of tolerable deposit stress is reached with alloys containing about 15 pct cobalt that have hardnesses around 350 to 400 HV.
Alloy hardness depends on both the cobalt content of the solution and the deposition current density. Figure 2 shows hardness plotted against current density for different concentrations of cobalt in a solution operated at 140F, pH 4. Deposit stress also depends on current density. Moving left to right along any of the curves, deposit stress changes from compressive through zero to tensile. The superimposed thin black line is a zero-stress contour linking the combination of cobalt ion concentration and current density for zero-stress alloys. The corresponding value of deposit hardness can be read from the graph for each combination. The overall relationship is that the higher the cobalt ion concentration, the lower the current density that can be used for zero-stress alloys, but the higher the alloy's hardness.
Effect of Heating. Heating at temperatures to 570F has little effect on the mechanical properties of the alloys. At higher temperatures deposit hardness falls, but nickel-cobalt alloys still retain greater hardness than that of nickel deposits similarly heat treated.
Tensile strength, 150,000 psi for a 15 pct cobalt alloy and 108,000 psi for a 10 pct cobalt alloy, falls progressively as heat treatment temperature is increased above 570F.
Deposit ductility increases on heating above 570F, from five pct elongation as-deposited to about 40 pct after heating at 1100F for both 10 and 15 pct cobalt alloys.
Properties of alloys from other nickel solutions. The preceding data apply to alloys from a 600 g/liter nickel-sulfamate solution. A nickel-cobalt alloy deposited from a 450 g/liter solution without chloride or organic additions, operated at 126F, pH 4 and cathodic current density 23 amps/sq ft has a hardness of 350 to 400 HV, ductility less than five pct and internal tensile stress of 7,000 to 9,000 psi.
Measurements of the effects of heat treatment at 400F on alloys containing 53 to 55 pct cobalt indicated a small increase in yield strength. Heat treatment at 500F or more reduces yield strength, as with alloys from the 600 g/liter nickel sulfamate solution. Prior heat treatment of the 53 to 55 pct cobalt alloys at 500F, 700F or 800F improves mechanical properties. Thus pretreatment for four hours at 800F, increases yield strength and elongation.
Alloys deposited from a sulfosalicylate-based solution exhibit at maximum hardness value above 700 HV.
Nickel alloys with 22 pct chromium can be prepared by co-depositing chromium carbide particles with nickel and heat treating for 24 hours at 1800F in hydrogen. Alloy hardness after heat treatment is 223 HV, compared with 55 HV for similarly treated plain nickel.
Alloys containing 19 pct cobalt or 20 pct iron in addition to chromium are produced by co-depositing chromium carbide from a nickel-cobalt or nickel-iron based solution and heat treating.
Heat treatment in hydrogen gives almost complete decarburization with the nickel-cobalt-chromium alloys. Hardness after heat treatment is 215 HV. Approximately 0.8 pct carbon remains in the nickel-iron-chromium alloy, which may account for its higher hardness of 332 HV.
For practical purposes, the environmental, health and safety considerations for these nickel-based alloys and their production are the same as those for nickel and nickel plating. PF Reprinted with permission from the ASM Handbook, Volume 5, Surface Engineering. To order a copy of the Handbook, or for more information, contact ASM at 219-blog comments powered by Disqus