The Watts Solution and Deposit Properties

The nickel plating solution described by Watts in 1916 was a major milestone in the development of nickel plating technology. The solution eventually replaced all others in use up to that time. It remains the basis of most decorative nickel plating processes, and it is used for engineering applications and for electroforming. It is operated at elevated temperatures and is capable of being used with high current densities.

The composition of the modern Watts bath is included in Table 2. The constituents of the Watts bath have several functions.

• Nickel sulfate is available in commercially pure forms, is relatively inexpensive, and is the major source of the nickel ions in solution. A high nickel sulfate concentration is used when high current densities are required.

• Nickel chloride serves primarily to improve anode corrosion, but it also increases conductivity and uniformity of coating thickness distribution. Excessive amounts of chloride increase the corrosivity of the solution and the internal stress of the deposits. (Internal stress refers to forces created within the deposit as a result of the electrocrystallization process and/or the codeposition of impurities such as hydrogen, sulfur, and other elements. Internal stress is either tensile [contractile] or compressive [expansive] and may cause plating problems if excessively high.)

• Boric acid is used in nickel plating solutions for buffering purposes; its concentration may affect the appearance of the deposits. The deposit may first become frosty in high current density areas at 30 g/L (4 oz/gal) of boric acid, and then as the boric acid concentration approaches 15 to 23 g/L (2 to 3 oz/gal), the deposit may be burnt and cracked. No effect on appearance is observed at high boric acid concentrations up to saturation (45 g/L, or 6 oz/gal).

• Wetting agents or surfactants, formulated specifically for nickel plating solutions, are almost always added to control pitting. Their function is to lower the surface tension of the plating solution so that air and hydrogen bubbles do not cling to the parts being plated. Table 2 Nickel electroplating solutions

Electrolyte composition, (a) g/L

Watts nickel

Nickel sulfamate

Typical semibright bath(b)

Nickel sulfate, NiSO46H2O

225 to 400

300

Nickel sulfamate, Ni (SO3NH2)2

300 to 450

Nickel chloride, NiCl26H2O

30 to 60

0 to 30

35

Boric acid, H3BO3

30 to 45

30 to 45

45

Operating conditions

Temperature, °C

44 to 66

32 to 60

54

Agitation

Air or mechanical

Air or mechanical

Air or mechanical

Cathode current density, A/dm2

3 to 11

0.5 to 30

3 to 10

Anodes

Nickel

Nickel

Nickel

pH

2 to 4.5

3.5 to 5.0

3.5 to 4.5

Mechanical properties(c)

Tensile strength, MPa

345 to 485

415 to 610

Elongation, %

10 to 30

5 to 30

8 to 20

Vickers hardness, 100 gram load

130 to 200

170 to 230

300 to 400

Internal stress, MPa

125 to 210 (tensile)

0 to 55 (tensile)

35 to 200 (tensile)

(a) Antipitting agents formulated for nickel plating are often added to control pitting.

(b) Organic additives available from plating supply houses are required for semibright nickel plating.

(c) Typical properties of bright nickel deposits are as follows: elongation, 2 to 5%; Vickers hardness, 100 gram load, 600 to 800; internal stress,

12 to 25 MPa (compressive).

Good-quality nickel deposits can be produced within the ranges of solution pH, temperature, and current density given in

Table 2. Although the maximum current density given in the table is 11 A/dm increased solution agitation and flow rates.

higher rates of plating are possible with

The physical and mechanical properties of nickel deposited from Watts solutions are affected by the operating conditions and chloride content of the solution as shown in Fig. 1, 2, 3, and 4. Figures 1, 2, and 3 show how pH, current density, and temperature affect properties such as internal stress, hardness, percent elongation, and tensile strength. Figure 4 shows how the chloride content affects those properties; the maximum ductility and softest deposits are produced when 25% of the nickel in solution is present as nickel chloride. Reference 2 is a comprehensive source of mechanical property data for electrodeposited nickel, nickel alloys, and nickel composite coatings.

Stress Elongation, lb/in * x 103 MN/in2 % reo

Hardness, Tensite strength HV MN/m2 lb/in.2 x Itf3

100-

__J___

—*

ELONG

UlTION

-

1 f

l/j

1

»

HARP

1ESS

>

iT

1 ■

TENS

LE STBEft

, „ SIRE GTH

SSjy

Fig. 1 Variation in internal stress, tensile strength, ductility, and hardness with pH. Watts bath operated at 54 °C and 5 A/dm2. Internal stress is tensile (indicated by a positive number). Source: Ref 1

STRESS Ib/ln'riO' MN/m5

CUHRENT DENSITY, A/ft'

50 75 100

CUHRENT DENSITY, A/ft'

50 75 100

STI

ESS

HARDNESS^^

200 150

CUHRENT DENSITY, A/dm?

CUHRENT DENSITY, A/dm?

350 300

200 150

Fig. 2 Variation in internal stress and hardness with current density. Watts bath operated at 54 °C and pH 3.0. Internal stress is tensile (indicated by a positive number). Source: Ref 1

ELONGATION Vt

HARDNESS HV

E

.ON GAT

ON

IRDNE!

s/

TENS

IT

INGTR

TOO 120 140 160 °F TEMPERATURE

TOO 120 140 160 °F TEMPERATURE

TENSILE STHENGTH MN/m'

to/in'xIO1 llOi

100 90

60 50

Fig. 3 Variation in elongation, tensile strength, and hardness with temperature. Watts bath operated at 54 °C and 5 A/dm2. Source: Ref 1

Fig. 4 Variation in internal stress, elongation, tensile strength, and hardness with chloride content in deposits from Watts solutions operated at 55 °C, pH 3.0, and 5 A/dm2. Internal stress is tensile (indicated by a positive number). Source: Ref 1

The nickel plating processes used for decorative, engineering, and electroforming purposes are discussed in the following sections.

References cited in this section

2. W.H. Safranek, The Properties of Electrodeposited Alloys--A Handbook, 2nd ed., American Electroplaters and Surface Finishers Society 1986

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