Nickel Iron

Bright nickel-iron plating was strongly promoted as a substitute for bright nickel plating in the period from 1970 to the early 1980s when the relative price of nickel was high (Ref 3, 4, 5, 6).

Advantages. The main advantage of the alloy is the significant saving in the cost of metal, because up to 35% of the nickel is replaced by 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 needed for bright nickel, substantially negating the saving on metal. The addition agent system is also more complex so that electrolyte control is more difficult. At equal thickness, nickel-iron plating is less resistant to corrosion than nickel, and the higher the iron content, the lower its resistance. The corrosion product is rust-colored, and there is no international standard for the alloy coatings.

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 1 (Ref 4). Addition agents include stabilizers for the ferrous iron, organic brighteners, leveling agents, and wetting agents. Total iron includes ferrous and ferric ions, and it is important to control the ratio, with ferric usually below 20%. Solution temperature is typically 54 to 60 °C (130 to 140 °F), and solution pH must be kept low at 2.8 to 3.6. The solution is preferably used with air agitation rather than cathode-rod movement, because higher plating rates can be used, a higher iron content can be obtained in the deposits, and iron content can be altered at will by increasing or reducing the rate of air bubbling.

Table 1 Typical nickel-iron solution composition

Constituent

Amount, g/L (oz/gal)

Ni+2

56 (7.46)

Iron (total)

4 (0.53)

NiSO46H2O

150 (20.00)

NiCl26H2O

90 (12.00)

FeSO47H2O

20 (2.67)

H3BO3

45 (6.00)

Stabilizer(a)

15 (2.00)

(a) Concentration will vary between 10-25 g/L (1.3-3.3 oz/gal), depending on the type of stabilizer used.

Properties of the alloy deposits that are of interest include ductility, deposit hardness, internal stress, and magnetic properties.

Ductility depends on iron content, brightener concentration, solution temperature, and pH.

Deposit hardness varies with iron content. With iron content increasing from zero to about 10%, microhardness rises from 490 to 560 HK, then falls to around 510 HK with 49% Fe for coatings plated at standard conditions of 4 A/dm2 (40 A/ft2), 60 °C (140 °F), pH 3.5, and air agitation. Changes in solution pH and brightener concentration also influence deposit hardness, enabling values exceeding 700 HV 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 from 10.8 to 27.6% raises stress from 93 to 154 MN/m2 (13,500 to 22,400 psi). Increasing pH from 2.8 to 4.5 raises stress from 17.5 to 230 MN/m2 (2500 to 33,600 psi) (Ref 4).

Magnetic properties of nickel-iron are not important in the application of bright decorative coatings. Similar alloys are, however, deposited for magnetic applications from solutions not containing brightening additions (Ref 7). The alloys with 18 to 25% Fe are soft magnetic materials with low coercive force, low remanence, and high maximum permeabilities (Ref 8). They can be used as coatings or as electroformed parts (i.e., freestanding electrodeposited shapes detached from the substrate after being deposited).

0 0

Post a comment