Guest Editorial -For Plateworld.com                                                 

 Don Baudrand, Don Baudrand Consulting,   e-mail:donwb@tscnet.com

Jack Marty¹ doing research in the laboratories of Allied Research Products in the mid 60’s, studied the effect of the impurities on the deposit characteristics of the sulfamate nickel electroplating baths. His first paper was published in 1966 and another paper published with the work of Gian P. Lanza included^1,2 . These papers laid the groundwork for recognizing the profound influence of trace impurities. In this paper, we will include the work of Marti and Lanza (Figures 1-12) along with the original work of Vern Weinsenberger who researched the effect of magnesium and found that it had little effect on intrinsic stress, contrary to previous published information. An extensive reference section is included valuable to anyone researching the subject of sulfamate nickel plating-widely used but not widely understood process.

In the last fifty years, nickel sulfamate solutions have been used extensively in electroforming and in electroplating for engineering applications. The properties of the deposits or the operating factors that make those baths attractive in these fields are low residual stress, high deposition rates, and good ductility. In these respects the sulfamate baths are generally regarded as being superior to the conventional Watts bath. But, for the decorative deposits, both bright and dull, used for metal finishing, the Watts bath has continued to be favored.

The reasons for not using the sulfamate bath to a larger extent in metal finishing are not wholly clear. However the high cost of nickel sulfamate compared to nickel sulfate and nickel chloride is a factor. The enormous background of practical operating experience with the Watts bath, along with the multitudinous brighteners and other addition agents that have been developed, certainly favor the continued use of this solution. It would require a great deal of time and effort to develop a bath based on sulfamate to the degree of perfection regarding appearance and properties of the deposits now available from the Watts bath and the proprietary baths based on it. While Berglund³ prepared sulfamic acid over 120 years ago, Piontelli and Cambri⁴ first announced nickel electrodeposited from sulfamate baths in 1938. Commercial development of this process did not gain impetus until US patent for electrodeposition of nickel, copper and lead from the solution consisting of "metal cation and the anion of SO₃·NH₂ was issued to M. Cupery in 1943⁵. In 1953, Barret⁶ described the bath, and claimed the first commercial application in USA⁷. Mixed Watts/sulfamate bath⁸ is described in 1974. This bath can improve bath plating characteristics of bright Wattsbath while avoiding much of the expense of all-sulfamate solution. Having higher conductivity, the burning is reduced and throwing power increased.

Characteristics and properties of electroplated deposits from nickel solutions based on the sulfamate anion that are influenced by plating conditions and impurities are numerous. They include: ductility, hardness, tensile strength, intrinsic stress, deposit structure, porosity, smoothness (degree of roughness) density, (specific gravity), specific heat, coefficient of expansion, thermal conductivity, specific resistivity, modulus of elasticity in tension among others. High-speed nickel electroforming in general and sulfamate baths in particular has become popular as micro-electroforming techniques for manufacture of Microsystems technologies. Knowledge of the effects of impurities, and alloying metals such as cobalt, are important to the successful use of nickel sulfamate plating solutions.

Any description of sulfamate plating baths must inevitably start with an explanation of the distinctive properties of sulfamic acid and its salts.

Sulfamic (amidisulfonic) acid is a white crystalline inorganic solid, nonhygroscopic and nonvolatile. It may be conveniently handled and stored. In strength and chemical structure, it is very similar to sulfuric acid⁹ . The pH of 1% solution at 25^o C/77^o F is 1.18.

The substitution of an amino group (NH₂) for one of the hydroxyl (OH) groups of sulfuric acid undoubtedly gives to sulfamic acid many of its unique properties which are reflected in the quality of electrodeposits obtained from its metal salts in solution.

Sulfamic acid is monobasic and will react with metals, metal oxides or metal carbonates to produce the corresponding metal sulfamate by simple replacement of one hydrogen atom associated with the OH group.

Sulfamic acid is moderately soluble in water, yielding solutions that are highly acid and compare in pH range with those of the three common mineral acids, nitric, sulfuric and hydrochloric. The metal salts of sulfamic acid are extremely soluble and in many instances are the most soluble metal salts. Lead, ammonium sodium and magnesium are among the most soluble salts known and considerably more soluble than corresponding nitrates, sulfates, chlorides and acetates. Nickel sulfamate, Ni(NH₂SO₃)₂, is so soluble that it is not practical to crystallize from solution. Such high solubility make it possible to build the baths with exceptionally high metal content - a desirable condition for high current density operation. Only stannous and basic mercury sulfamates are insoluble⁹.

Sulfamic acid and its salts slowly hydrolyze in hot solutions (80^o C/176^o F) at low pH to form ammonium acid sulfate. As a result, the preparation of high purity nickel sulfamate requires attentively controlled reaction techniques and the presence of suitable inhibitors to prevent hydrolysis. Due to the extreme instability of sulfamate salts to heat, it is practically impossible to force crystallize or spray dry to produce a solid substance, which will not assay less than 98-99 % purity. Such a degree of purity is essential to operation of a sulfamate plating bath at full capabilities.

Acid sulfamate salt solutions can be buffered quite well with several of the weak acids such as boric, formic, acetic, citric, lactic and tartaric. For purposes of nickel plating bath formulations, it is preferred to use the usual concentration of boric acid as adequate buffer, in the operating pH range of 3.5-4.5.

Addition agents and impurities have a profound effect on deposit characteristics as is to be expected. These will be discussed in detail.

Changes in solution composition, operating temperature, current density (CD), solution agitation, can also alter the deposit characteristics.

Table 1 presents a typical solution composition and operating conditions and Table 2 mechanical properties of sulfamate nickel deposits.

It should be noted that variations in composition are used for special purposes. In concentrated sulfamate solutions, extremely high rates of deposition are possible with current densities up to 40 Adm^-2 (A/ft. sq.) without the necessity of violent agitation. For a given plating rate in this bath internal stress decreases rapidly with increase in temperature.

It is reported that when operating above 40 Adm^-2, 300 g/l baths can produce brittle deposits¹⁰. To avoid this, more concentrated bath (675 g/l) is employed, with temperature increased from 64 to 71^o C/147-160^o F and increased anode area to minimize anode sludge formation.

Kendrick11 disclosed the high-concentration, high-speed process, with 600 g/l of Ni-sulfamate, 40 g/l of H3BO3 and 10 g/l of NiCl2 as early as in 1964.

Nickel. Concentrations of 90 to 135 g/l of nickel metal are often used for high-speed plating, when coupled with very high solution agitation. Current densities of 400 to 4000 Aft^-2 (40-400 A/dm^-2) have been used where there is violent agitation. High metal content is also used to improve throwing power coupled with very low current density, (0.1-0.4 Adm^-2 /1-4 Aft^-2). High metal content does not appreciably change the characteristics or properties of the deposit. Low metal content coupled with moderate to high CD will cause deposition of basic nickel salts ("burning").

Boric acid. Its concentration should vary with operating temperature from 30 g/l (4 oz/gal) at ambient temperature to over 4.5 g/l (6 oz/gal) at 52^o C /125^o F. Low boric acid can cause "orange peel" type of pitting. High boric acid will tend to salt-out of solution at lower temperatures. Once crystallized (salted out) it is difficult to redissolve. 30 g/l boric acid will remain in solution at 20^o C/68^o F. The lowest practical operating temperature for current densities over 16 A/dm^-2 is 32^o C /90^o F.

High CD and/or low temperature could cause "burning". Lower nickel content (below 75 g/l) requires higher boric acid.

Chlorides/bromides. In small amounts they are useful to promote the best corrosion (dissolution) of the nickel anodes. Poor anode corrosion results in compressive stress, brightness, loss of ductility, increased hardness and increased porosity.

The addition of small amounts of chlorides to the plating solution slightly increases the tensile stress of the deposit, a trade-off that serves the purpose.

For most applications, the increased stress is too small to cause any problem such as distortion of an electroformed product. Others have found that for low or moderate CD's chloride is not necessary if sulfur depolarized anodes are use with sufficient anode area.

Traditionally, electroplaters and electroformers prefer faster deposition rates and higher cathode CD of at least 4 Adm^-2 /40 Aft^-2 . Since anode/cathode ratio is seldom bigger than 1, in order to plate fast, the chloride concentration is raised to 8-10 g/l to avoid the anode polarization, the forerunner of sulfamate decomposition. Now the stage is set to call for stress reducers to counter the influence of the higher chlorides. True?

b. INFLUENCE OF PROCESSING VARIABLES

pH. Acidity of the bath also has an effect on stress in the deposit (Figure 1). A solution with 76.5 g/l of nickel has the lowest stress at pH between 3.8 and 4.8. A solution with 107 g/l of nickel has the lowest stress at pH between 2.9 and 3.8. The stress increases rapidly at pH>5 due to the codeposition of basic nickel salts.

Hardness is affected by pH (Figure 2) on a similar way as other nickel plating

solutions. It is fairly constant until at critical pH (5.0) and raises sharply with a further increase of pH. A low pH, (< 3.5) reduces the cathode efficiency, slows plating rate slightly, but does not change the deposit.

For rack plating, the standard pH is 4.0. At this pH, iron contamination cannot remain in the solution. It precipitates as ferric hydroxide and is picked up in the filter. Deposit characteristics such as ductility remain good. At pH 5, hardness increases and ductility decreases due to the codeposition of small amounts of basic nickel salts.

For electrolytic low CD purification (dummy plating), a low pH (3 - 3.5) favors removal of metallic impurities.

Barrel plating must be done at a pH of 3.2 to 3.5 in order to prevent laminated deposits due to make and break of electrical connection as the barrel rotates. Low pH also results in faster accumulation of impurities.

TEMPERATURE. The influence of temperature on hardness is parabolic function. As temperature is increased or decreased from 39^o C/ 102^o F, the hardness of the deposit is increased, the increase being more rapid at lower than at higher temperatures^{13, 14}.

Hardness is not appreciably affected between 35-50^o C/ 95-122^o F.

Stresses has a pronounced minimum at 50^o C/122^o F with a value of about 1 Ton/in² tensile¹³, increasing uniformly above and bellow 50^o C/ 122^o F.

Mechanical properties such as tensile strength, elongation, hardness and stress for sulfamate and Watts baths are compared in Table 3 (Addendum).

CURRENT DENSITY. Within normal operating limits, CD and the temperature have little effect on the hardness and the structure of the deposits. On the other hand, CD has a varying effect on stress. Sulfamate bath may fluctuate from compressive to tensile, and the properties and characteristics of the deposit are also affected by current density¹⁵. Increase in CD causes a minor increase in tensile stress in chloride free baths.

Ductility deceases rapidly with increase in the temperature. In sulfate/chloride baths, as CD is increased, stress increases on an even rate (Figure 3).

As the concentration of Ni-sulfamate is raised from 300 g/l to 700 g/l, maximum plating rate without burning passes through maximum and deposit stress through minimum¹⁶.

Impurities in all plating solutions can cause changes in the deposit characteristics. Some of which can have a dramatic effect. The Ni-sulfamate formulations have found favor in electroforming where the ability to control internal stresses of the stresses of the deposit is particularly useful. Since stress is emphasized more in the use of sulfamate nickels, the effects of various impurities on this quality will be discussed next.

2.1 METALLIC IMPURITIES.

Chromium. This is very harmful impurity either in cation (hexavalent) or anion (trivalent) form. As little as 3 ppm as hexavalent or 8 ppm as trivalent chromium can cause tensile (intrinsic) stress so high as to cause the cracking, dark deposits, peeling and low cathode current efficiency. Trivalent chromium can cause pitting (Figure 4). Chromium can be removed using a "high pH treatment", outlined in Table 6 in Addendum.

Copper. It has little effect on stress or ductility, but it can give unsatisfactory appearance. Also, adhesion can be affected with as little as 8 ppm copper when there is a delay in starting the current. Copper as high as 40 ppm has been tolerated when the parts are introduced in to the solution with the current on ("live entry") at plating voltages (Figure 5). Copper causes dark deposits in low CD areas. It is introduced from dissolution of buss bars, contacts and racking material or impure salts and anodes.

Copper is removed by low CD dummy plating at 0.5 Adm^-2 / 5 Aft^-2.

Iron. In concentrations of 5 -10 ppm reduces the ductility of the deposit. Limit is 0.3 g/liter. Iron usually comes from basis metal drops, improper rinsing or drag-in. Iron can cause roughness, pitting and dark deposits. It can be removed using a "high pH treatment" given in Table 6 (Addendum). Sulfamate nickel solutions, which operate at pH4, will not sustain iron in the solution. As iron is oxidized at the anodes, ferric hydroxide will precipitate and is removed by filtration. For that reason among others, continuous filtration is necessary (Figure 6).

Lead. Lead in the deposit increases tensile stress but not nearly as much as some other impurities. However, lead causes dark deposits in low CD areas, brittle and streaked deposits. Since lead deposits preferentially at 2-3 Aft^-2, lead is easily removed from the plating solution by electrolytic purification, also called dummy plating, and uses a low current density (0.2 Adm^-2 /2 Aft^-2) (Figure 7).

Magnesium. It may be introduced into a nickel sulfamate solution from nickel salts, anodes, water and other chemical additions. In some cases, it is added intentionally.

Recent studies¹⁷ have shown that up to about 2.5 g/l magnesium has very little effect on the internal stress or ductility of the nickel deposit. For the range studied an increase in the tensile stress of about 7 mPa or 1000 psi was observed. These results are not in agreement with previous reported results² that indicate a much greater influence on the internal stress of the nickel deposit. A previous study indicated stress caused by magnesium. We believe that impurities present in the magnesium salt used for the former tests actually caused the increase in tensile stress. Figure 8 shows the old and new results. Pure Mg salts are now available commercially. "These recent studies show that the influence of magnesium is in close agreement with the other alkaline earth’s and alkali metals. These ions have very little effect on the hardness of the nickel deposit. Sodium has the greatest effect on hardness with only a slight decrease"

Sodium. In relatively large amounts it will affect deposit. Limit is 30 g/l and when in excess can cause brittleness. Comes from water, impure anodes or plating salts or use of caustic soda to raise pH. The later should be avoided. There is no known removal method.

Tin. A stannous tin, (Sn⁺⁴), has little effect in low concentrations. Stannic tin, (Sn⁺²), can cause high tensile stress (Figure 7).

Zinc. When present in the deposit instigates tensile stress (less than lead) (Figure 5). Zinc in small amounts (50-150 ppm) is reported not harmfull¹⁸. In higher concentrations causes "burning", peeling and dark streaked deposits, which are brittle.

Zinc also lowers cathode efficiency. Zinc can be removed by electrolytic purification at 2-3 Aft^-2. Limit is 0.15 g/liter. Originates from impure plating materials and basis metal drops.

Impurities may preferentially deposit at different current densities. Consequently, the effect will be different at different CD's. Impurities such as lead that co-deposit at very low current densities may have only a small effect (or none) at high current densities and may have little effect in low current densities². Smith at al ¹⁹ verifies importance of the quality of sulfamate salts.

2.2 MANGANESE AS IMPURITY AND ALLOYING ELEMENT

When manganese is present in sulfamate solutions in small amounts it decreases stress to some small extent (Figure 6). It originates from impure salts or anodes or from residues remaining from potassium permanganate treatments. It is removed by dummy treatment. A difficulty of deposition of manganese alloys lies in the very negative (non-noble) potential of the metal in the aqueous solution (- 1.18 V), which is far removed from other depositable metals. The use of complexing agents is of limiting value for bringing codeposition of other metal, since the possible complexing agent capable of shifting the potential of less noble metal close enough would also likely complex Mn ions and prevent deposition of Mn altogether.

Despite of difficulties to deposit Mn in appreciable amounts, in small amounts, (less than 1%), it can be codeposited from simple acid baths, such as nickel sulfate or sulfamate plating solutions^20-23. The sulfur present in the organic stress reducing agents such as saccharin is incorporated in unalloyed Ni deposits and will cause embrittlement on exposure to temperatures higher than 200^o C/392^o F. Being able to preferentially combine with the sulfur during the heat treatment, it can prevent the diffusion of sulfur into grain-boundaries, followed by the formation of Ni-sulfide and improve high-temperature ductility of electrodeposited nickel. Atanassov and Schils^24-26 found the correlations between structural and mechanical properties of Ni-Mn deposits in the presence of sulfur containing additive: 2,4-dichloro-5-sulfamoyl benzoic acid and Malone²⁷ used pulse plating to obtain a Ni-Mn alloy from sulfamate solution and increased yield strength at the expense of ductility. Johnson at al ²⁸ suggested codeposition of manganese for overcoming intergranular cavity formations and fractures for thin (< 0.7 mm) nickel deposits exposed to elevated temperatures (400^o C).

2.3 COBALT AS IMPURITY AND ALOYING ELEMENT

Electrodeposited Ni-Co alloys are attractive due to the good ambient temperature tensile properties. Codeposition of these remarkable alloys has evolved from very hard, brittle deposits electroplated from Watts type Ni/Co sulfate baths to ductile deposits produced in Ni/Co sulfamate baths. The tensile properties are determined by Ni/Co composition, which in turn is controlled by plating variables²⁹.

Cobalt is added to control tensile strength and hardness. However, in significant quantities it will increase tensile stress at rapidly increasing linear rate². Trace amounts of cobalt have little effect. The limit is 0.1 g/l. Noticeable changes start to occur at about 1% of the nickel concentration, typically about 7 g/l. in the solution. Note that almost always an increase in stress (in either direction), results in an increase in hardness and a decrease in ductility (Figures 6 and 9).

Electrodeposition of Ni-Co alloys, whether from simple or complex salts, occurs in an anomalous type in which less noble metal is preferentially reduced³⁰. The Degree of anomalous behavior can be significantly changed by the influence of the operating variables, particularly CD. The inhibition of a more noble metal by less noble metal does not depend on anion composition of the bath. High-spin octahedral Co(II) complexes with three unpaired electrons are more labile than of Ni(II). Therefore they are expected to react rapidly resulting in preferential reduction of Co(II) as compared to Ni(II)³¹. Piontelli and Patuzzi ³² first described the sulfamate Ni-Co bath in 1942, while Tsu³³ published the data on NiCl₂-CoCl₂ bath without boric acid, producing the deposit with desirable magnetic properties. Malone³⁴ used a technique for controlling the Co content by using pulse rectifier and switching the current between separate banks of Ni and Co anodes. That was a simplification of the method used by Smith and Pawlak³⁵ where four rectifiers were used to drive Ni anodes, Co anodes, and inert anodes for stress reduction and purification cell.

There is potential merit of using cobalt as a hardening agent rather than conventional sulfur-bearing organic compounds. The Ni-Co alloys deposits, unlike nickel deposits containing incorporated sulfur, do not embritlle on heating. Heat treatment at 204^o C/400^o F increased the strength and ductility of electrodeposited Ni-Co alloy, believed to the reduction of internal stresses around grain baundaries³⁶.

The effects on temperature, pH, CD and boric acid on Ni-Co alloy were reported by Barrett³⁷. Of primary interest was the effect of CD in 2.7-8.1 Aft^-2 /25-75 Adm^-2 range on alloy composition and effect of Co concentration on tensile properties. Endicott and Knapp³⁸ performed first comprehensive study of the effect of plating variables and bath composition Ni-Co mechanical properties, microstructure and response to thermal treatments. They showed that alloy content is determined by relative concentration of Ni and Co and CD. Erickson³⁹ gave an account on the increase in hardness and tensile strength given by cobalt additions to a concentrated Ni-sulfamate solution. Belt at al⁴⁰ reported that by careful selection of and control of process variables, (CD and Co concentration), it is possible to produce the low stress Ni-Co deposit with a surface hardness of 400 HV 2.5.

McFairlen⁴¹ reported results from the first comprehensive study of mechanical properties of Ni-Co at elevated temperatures. Dini at al ⁴² determined that cobalt content and stress increases with solution agitation.

CD influences Ni/Co alloys with higher Co content and agitation, which in turn are influenced by concentration polarization of Co²⁺ ions^43-44. The alloy content is also influenced by relative concentrations of Ni and Co and agitation.

2.4 NONMETALLIC IMPURITIES

Ammonium. Ammonium ions can cause an increase in tensile stress and lower ductility of the deposit. Ammonia is present in even the best grades of sulfamic acid used to manufacture sulfamate nickel plating solutions. In addition, if the temperature is too high or the pH is too low during manufacture, more ammonia can be generated.

Ammonia is generated in a plating solution at high temperatures, (above 145^o C /145^o F) and at low pH, (below 2.2) by hydrolysis of sulfamic acid. Increased concentration of

ammonia increases the affected CD range and tendency for pitting⁴⁵. At concentrations below 500 ppm, ammonia seems to have little effect. At or above 500 ppm, a dark area is noted at 2 Adm^-2 /20 Aft^-2 in a Hull Cell plated panels. Ductility is mostly affected at the narrow band of CD around 20 Aft^-2 . However, at higher or lower current densities, the loss of ductility is very small if at all. At 2500 ppm and 3 Adm^-2 /30 Aft^-2 there is only a small loss of ductility.

At mixed current densities, as would be found in barrel plating, the effect is minimal.

At high CD, high ammonium can cause a more porous and brittle deposit as in wire plating (Figure 10).

Halogens.

All halogens will increase tensile stress. Bromides and chlorides have the least effect, fluorides next is iodide with a very large effect. (Figure 11). Precipitation with silver sulfate might be successful, but is expensive. Silver halogens can be reclaimed by regeneration to silver sulfate with sulfuric acid.

Nitrates. They cause the stress to be increased rapidly. Limit is 0.3 g/l. They should be avoided entirely, since there is no removal method known.

Sulfur. Presence of sulfur in the deposit causes compressive stress and increase in brightness. Sources of divalent sulfur (S²⁺) cause the most dramatic effects. High compressive stresses, brittle deposits, loss of ductility are all the result of even a small amount of sulfur (20 ppm) in the deposits (Figure 12).

Sulfur only in its higher oxidation state as sulfate (S⁶⁺) has no effect on stress.

Organic sulfur bearing compounds, where sulfur is lower than six-oxidation state, such as saccharine are added deliberately to change stress from tensile to compressive.

Compressively stressed deposit increases the fatigue life of the basis metal. Heating to 600^o F /316^o C reverts stress to tensile.

However, monitoring the sulfate in the plating solution gives a suggestion of the amount of hydrolysis, which may have taken place over time or due to low pH or high temperature. Monitoring sulfate is meaningful only when there is no drag-in of sulfate such as from a sulfuric acid or sulfate containing pre-plate solutions.

Organic addition agents. They can alter the deposit characteristics profoundly.

Examples of this type of organic agents are a long list of proprietary brighteners

designed to be used in bright nickel plating solutions⁴⁶. Primary brighteners can cause compressive stress, loss of ductility and loss of heat resistance. Primary brighteners such as saccharine are not removed by carbon treatment. Secondary brighteners cause tensile stress and loss of ductility. It is possible to balance the two types of brightener to achieve virtually zero stress. However, the result will be a more brittle deposit much like bright Watts nickel solutions. The advantages they bring are grain refinement and smoother deposits.

Saccharine and similar compounds act in a similar way to primary brighteners.

Brighteners add considerable hardness to the deposit. Proprietary products containing similar compounds also contain other organic compounds that impart heir hardness.

They are used in different amounts than when saccharine is used. The breakdown products are more easily handled and stabilized. Removal is accomplished by a series of carbon/hydrogen peroxide treatments, outlined in Table 7 (Addendum).

In Table 3 limiting concentrations of impurities are summarized.

Wetting agents. They are often used in sulfamate nickel solutions. Careful selection of the right type with very low level of impurities is essential for best results. Appropriate surfactants (a.k.a. anti-pit agents) have virtually no effect on the deposit properties, if used in the proper ranges.

For any soluble plating anode to perform adequately it should meet, among others, the following characteristics: a. It should dissolve (corrode) uniformly with high current efficiency. b. It should have low polarization. c. It should not introduce into solution any new compound that will have adverse effect on the mechanical or electrochemical effect on electrodeposits. The anodes used sulfamate baths typically consist of nickel, to which has been added low (e.g., 0.02 %) amounts of sulfur to promote dissolution. However, in the absence of chlorides, nickel anodes tend to form resistive passive films on their surfaces^50,51. Highly positive anodic potentials generally associated with high speed nickel electroplating tend to increase this passivity, resulting in the increase of applied bath voltage, power costs⁵², and nickel metal concentration of the bath. In addition unwanted changes in the pH can induce possible decomposition of sulfamate ion.

To minimize these problems, halide ions usually are added to the plating bath to promote the dissolution of the anode by destabilizing the passive film on its surface.

However, chloride or iodide ions tend to raise the tensile stress in the deposited nickel films¹; conversely, bromide ions at the same concentration tend somewhat to decrease it⁵³. Thus, any discussion of the effects of halide ion additions must take into account not only the reduction of anode passivity, but also the tensile stress in the electrodeposited films that are produced. Tsuru at al ⁵⁴ recently discussed effects of chlorides, bromides and iodides on stress.

For sulfamate nickel plating anodes should be sulfur depolarized type^46,55,56. Rolled depolarized anodes have been used, but being less active require much higher chloride content or bromide (that is occasionally used). Electrolytic anodes or "nickel 200" anodes are the least active and are not recommended. Use of these anodes will produce higher amounts of sulfur causing greater effects than rolled depolarized anodes.

Thus the oxidation potential at the improper anode would be higher and could lead to decomposition of the sulfamate resulting in the formation of a divalent sulfur products which will codeposit causing increase in brittleness. It could cause compressive stress in the deposit with the increase in hardness. High chloride (25 g/l) will minimize this effect.

Anodes should be contained in titanium baskets^57,58, which are double bagged using polypropylene bags in combination with polypropylene, felt bags. Bags should be removed and cleaned periodically to assure free solution flow through them. Particulate materials are retained in the bags and prevented from entering the plating solution where they would cause rough deposits. Symptoms of poor anode corrosion are increased brightness, loss of ductility, increased hardness, compressive stress, and increased porosity.

Titanium baskets are material of the choice, having numerous advantages:

a. High chemical resistance, made better when used as anodes.

b. Titanium forms an anodic film with "inert" properties (no gassing). The anodic film does not grow continuously but reaches a definite thickness for each voltage, unlike aluminum, which continues to build thickness.

c. Although the film is an electrical insulator, current will pass at a contact point, because nickel has a lower overpotential. Thus the nickel will become anodic. The degree of the contact with titanium is dependent upon the resistance offered by nickel™ anodic film at the point of contact.

d. Titanium is strong light in weight and tough. Good is seldom perfect, and with the good properties of titanium, they are also certain disadvantages:

a. The protective film breaks down on certain high voltages, the value of which varies with different plating solutions.

b. The protective film can be destroyed by arcing.

c. The electrical conductivity of metal is extremely poor. Compared with Cu, Al, and Ni the conductivity ratios (Ti=1) are 1:1.28, 1:1.18, and 1:1.5.

d. Titanium is relatively expensive metal and requires special welding techniques under inert atmosphere.

In some operations where insoluble anodes must be used, an anode phenomenon may occur whereby sulphur is produced and co-deposited on cathode, thereby increasing brightness and yielding harder more comprehensively stressed deposits. The deposit becomes brittle, and will deteriorate further on heating to 200^o C (394^o F).

Nickel basket anodes prepared from sintered sulfidized nickel powders are reported⁵⁹.

The influence of the presence of the sulfur on the kinetic of nickel dissolution is interpreted in terms of development on anode surface of Ni₃S₂ from nickel is extracted to leave a sulfide of lower content; the later sulfide then react with underlying elemental sulfide to reform Ni₃S₂.

Subramarian at al ⁶⁰ used nodules and residues of nickel as a secondary source of anode materials. The necessary sulphur to convert to sulfur activated anode.

Anodic decomposition products of sulfamate ions have been known to exert significant effects on the quality of electroformed nickel deposits in nickel electroforming baths.

Compared with Watts type nickel plating solutions, the anodic behavior of sulfamate based solutions is more complex. Klingenmaier⁶¹ reported that an oxidation product, which decreased tensile stress and increases the sulfur content of the deposit, is formed during the anodic polarization. Marti² noticed a similar phenomenon in a nickel sulfamate bath with passive or insoluble anodes and suspected that it was due to the effect of sulfite, which was produced during the anodic decomposition. Greene⁶² detected the products of anodic reactions as azodisulfonate (SO₃N = NSO₃⁼), which affects a deposit in a similar manner to that other sulfur containing addition agents e.g. decreasing the stress and increasing the sulfur content. Effect of sulfur is also influenced by usual variables, pH, current density, and agitation. This explanation has been well accepted^46,48.

Kendrick¹¹ and Kendrick and Watson⁶³ claimed that the decomposition reaction was different at a passivated nickel anode from that at the platinum electrode. Although the tensile stress of the deposit was decreased in both cases, the sulfur content of the deposit was increased only with the platinum anode.

Li, Zeng, and Zhang⁶⁴ detected hydrazine-disulfonate and dithionate in the anodic decomposition products using differential pulse polarography and infrared spectroscopy.

They also found there was another unknown stable species. The species was found to be produced at both the nickel and platinum anodes. In this research, they have investigated the anodic process of sulfamate at a platinum electrode and report results on the separation and identification of the anodic decomposition product. In the later work, Zhang and Park⁶⁵ , separated decomposition products via paper chromatography. From cyclic voltammetry and mass spectrography data they identified the unknown substance as diimide S-sulfonate (H₂N=N-S-SO₃).

Table 4 shows the effect of hydrolysis of the sulfamate ions. The products of hydrolysis are ammonium bisulfate The rate of hydrolysis is a function of concentration, temperature and pH. The rate is of first order kinetic, and as such is not time constant⁶⁷.

Hydrolysis takes place very slowly in the plating solution when operated normally⁶⁹.

However, local overheating, and localized very high anode current density, with pH drop can set the stage for runaway hydrolysis.

Stress in electrodeposits is subject that has plagued the plating industry for over century and a half. Residual stress is a term used to describe the inherent force present in an electrodeposited layer of metal that is free of external force or temperature gradient. Platers are usually concern with tensile stress, as the compressive stress is more often than not beneficial except under some extremely critical conditions.

If a highly stressed deposit is plated on a rigid object and it is unable to relieve stress by distorting the basis metal, poor adhesion, blisters may result or cracks may form in the deposit.

The Ni-sulfamate solution has found favor in electroforming where the ability to control internal stresses of the deposit is particularly useful. Two other electroforming processes, iron and cooper, can have limitations that which nickel electroform often overcome.

Internal stress-reducing reagents that contain sulfur atoms may be divided into two groups, according to their chemical behavior in the nickel plating solutions. First, there are compounds capable of providing traces of sulfur in electrodeposited nickel without causing detectable accumulation of the corresponding decomposition products.

Examples are 1,3,6-napthalenetrisulfonic acid (NTSA) and para-toluene sulfonamide (TSA). Sulfur deposition by NTSA and TSA generates, respectively, naphthalene and toluene as byproducts of the cathodic reaction⁷⁰.

An important representative of the second group of stress reducing, sulfur-containing compounds is saccharin (benzoic acid sulfimide). The electrolytic cathode reaction has been recognized as having several alternate paths. Each of these reaction paths results in detectable levels of organic compounds delivered to the plating solution. According to a comprehensive study carried out⁷¹, saccharin is to be considered one of the most effective and universal stress-reducing agents. Not only is it capable of influencing internal stress at levels much lower than NTSA and TSA, but it exceeds these compounds in versatility as well. At higher current densities, in certain bath formulations (e.g., Ni-Co), saccharin is the only known internal stress-reducing agent available. High-Performance Liquid Chromatography (HPLC) advantageously carries out simultaneous monitoring of saccharin and its decomposition products.

One of the early widely publicized baths, the Diggin bath⁷² uses sodium naphthalene trisulphonate as a stress eliminator.

At the low (0.8 - 1.6 Adm^-2 / 8 - 15 Aft^-2) electroforming CD's needed for nearer adequate throwing powers, zero tensile or even slightly compressive stresses can sometimes be obtained from nickel sulfamate baths without organic stress reducers. But this is only in the absence or near absence of chlorides and such baths will have far from optimum plating speed and will not adequately dissolve the anodes.

The accepted method of stress reduction in nickel deposits is the addition of certain sulfur-containing organic compounds to the plating solution. Stress reducers act in proportion to quantity; a small addition will decrease stress, the 'correct' amount reduces stress to zero, while an 'excess' induces compressive stress in the deposit.

Typical stress reducers, given in the order of their effectiveness, are: saccharin, TSA, benzene-disulfonate, and NTSA. Unlike properly functioning wetting agents, which are subject to drag-out losses only, stress reducers enter the deposit, possibly after partial decomposition and recombination. Brighteners and levelers enter the deposit in a similar manner. Most stress reducers are minor or secondary brighteners and refine the grain of the nickel. Saccharin also has the advantage of not only hardening but also of improving the ductility of the deposit under most conditions.

Stress tends to go from tensile to compressive as the temperature goes up, with approximately zero stress at 54^o C/130^o F. The tensile stress reaches minimum between pH 3.5 and 4.5. The pH should never be allowed to go below 3.0, and the sulfamate nickel solution is best operated at pH = 4.

Clear understandings of the stress profile of the bath are needed together with operable process window.

Too-high temperature can cause sulfamate decomposition with consequent increase in the stress. A temperature of 71^o C/160^o F must not be exceeded. If immersion heaters are used, circulation of the solution must be sufficient to prevent overheating of the solution in the vicinity of the heaters. The best temperature is 54-60^o C /130-140^o F.

From operational point, number of the ways to control the stress is as wide-ranging and assorted as the number of variables than affect it. Amid foremost modus operandi are: stress reducing agents, average current density adjustment during electrodeposition, temperature adjustments, agitation rates, etc. With this approach it is vital to keep all processing variables, except the controlled one, constant. Control variable are changed only in deliberate response to observed stress changes of the system. From steady, habitual stress readings, the adjustments of control variable are made to maintain stress levels in desired range.

A methodical approach⁷³, coupled with unyielding process control and good housekeeping makes stress control entirely possible⁷⁴ and from tip to toe less stressful.

Steady stress measurements will also help detect increased level of contaminants in the bath or other process deviations that usually result in abrupt unexpected stress changes.

6.1. STRESS MEASUREMENT

Since stress is emphasized more in the use of sulfamate nickels, compared to customary sulfate-based decorative nickel plating, stress measurement methods will be briefly discussed. Stress measurements can be done in several ways. Recently a test was described based on silicone wafers substrates.⁷⁵ Two basic types of instruments for measuring internal stress of plated deposits emerged as most useful or at least most widely used: The Spiral Contractometer method, and the Rigid Strip method.^76,77 They are compared in Table 5.

When performing an internal stress test, it important to establish the solution parameters and the test procedure because any deviations will effect the resultant internal stress to the deposit. The solution concentration pH and temperature can all affect the internal stress. Active anodes, sulfur depolarized, must be used to avoid an "anode phenomena" which may effect the internal stress while performing the test.

This is simplest, yet sensitive enough method for most industrial applications.

Intricacy allied with accurately measuring the radius of curvature of a single strip may have contributed to the fact that this simple, sensitive and clear-cut method had been nearly forsaken in favor of the spiral contractometar that had become the approved instrument for stress testing of electroplating baths.

Improved popular variety^78,79 uses disposable two legged brass strips who’s opposite sides are plated. The resulting leg deflection (curvature) caused by deposit stress can be measured on a simple scale and from the simple formula residual stress is calculated. Since tests can be performed directly in the plating tank, it should not alter established electrochemical and hydrodynamic patterns in the tank.

The Spiral Contractometer stress measurement is suggested for a reliable method to determine intrinsic stress in sulfamate nickel deposits.

When conducting stress measurement with a Spiral Contractometer, small changes in stress can be detected. These small changes can be observed because the movement of the helix is magnified through gears. These small changes are especially important when the plating solution is operated at or near zero internal stress.

The test procedure must also be established. Such items as helix calibration, helix preparation, cleaning cycle, plating CD and deposit thickness must all be specified to minimize any effect on the stress readings. Good scientific practice must be followed to provide consistent and reproducible results.

This method is based on plating the outside of a stainless steel helix, which has been masked on the inside. A pre-masked helix may be used.

The free end of the helix is attached to an indicating needle through gears that

magnify the movement of the helix. The other end is fixed to the instrument. As the helix is plated, the stress causes the helix to wind or unwind depending on the type of stress (tensile-wind, or compressive, unwind). From the amount of deflection of the needle when plated to a specific thickness and at a specific CD the stress free from external forces. The procedure for using the spiral contractometar is outlined in Table 8 in the Addendum.

Factors affecting accuracy are: The solution must be adjusted to standard operation concentration each time the test is performed, i.e., the solution composition must be the same each time. The pH, time, temperature, and current density must be the same, and according to ASTM specifications in order to achieve results comparable to other characteristics and on adhesion, it is best to select proprietary products specifically designed for use in sulfamate nickel plating solutions.

A refinement of the instrument is proposed⁸². Greater sensitivity was achieved by using jeweled bearings and optical read-out in order to measure films plated from Ni-sulfamate solutions, as thin as 40 Å.

Periodic plating with unipolar pulses (PC) or with alternate cathodic and anodic pulses (PCR) offers new dimensions in the electrodeposition field. Limiting current densities can be greatly increased, resulting in higher practical current densities. Anode passivation are reduced or eliminated, and mechanical and chemical properties of alloys are changed, usually for the better⁸³.

The theory behind the influence of PCR variables is much more complex compared with DC. However, in the last decade or so, new contributions are coming from different researchers around the world. This will help to further popularize this extremely promising technology.

Better corrosion resistance⁸⁴, finer grain size and increased hardness are obtained in PC plated sulfamate solutions using rectangular ramp-down wave form^85,86. PC improved throwing power and made it possible to increase or decrease stress values⁸⁷.

Kleinekathofer and Raub88 discussed a mechanism of Ni deposition from Ni- sulfamate baths in connection with current efficiency.

For PCR sulfamate nickel deposits, under specific PCR pulses⁸⁹, deposits become bright and smooth, even with small amounts of organic brighteners, due to the change of surface morphology. PRC can dramatically improve the metal distribution in low current density areas⁹⁰, in addition to improving crystallographic and magnetic proprerties⁹¹, stress⁹², and yield strenght⁸⁹.

New developments in rectifiers technology and the use of primary SCR's are making the new generation of PCR rectifiers more affordable and easily interfaced with the computers. This, in turn, can make dialing appropriate parameters quite simple. There is no doubt that the plating with periodically reversed current has a bright future.

Control and maintenance has been reduced to a minimum in the sulfamate nickel plating bath. Because of the simplicity of the bath composition, in which the sole nickel salt comprises more than 90 % of the dissolved solids, routine analysis need be nor more complicated than the taking of a hydrometer reading. The accompanying chart (Figure 13- Addendum) indicates the relationship between specific gravity and degrees Baumé at 21 C^o /70^o F and nickel metal concentration at specified boric acid content.

Boric acid control is not critical and it is sufficient to analyze for this constituent only at monthly intervals, using standard analytical methods for the determination of boric acid^93,94. The pH should be checked periodically, preferably daily. In normal operation, pH tends to rise slowly with use and may be adjusted quickly with small additions of sulfamic acid. Methods are published for determinations of ammonia and TSA⁹⁵. Weber⁹⁶, Maner at al⁹⁷, and Vaaler at al⁹⁸ discussed automation in solution chemistry control. Application of HPLC as suitable methods for monitoring organic additives and their decomposition or reaction products in Ni- sulfamate solutions have been developed⁶⁵.

Since ductility is sensitive to metallic impurities and to the variations of internal stresses, monitoring will help to maintain best performances of the bath and plated deposit. The tests can be as simple as vise test, or more precise micrometer with ball method⁹⁹, or displacement transducer and microprocessor control¹⁰⁰.

Parameter design experimentation was used by Bellows at al¹⁰¹ and Tang at al¹⁰² in order to optimize Ni-sulfamate process.

The sulfamate nickel plating bath has a relatively low sensitivity to contaminations. However, in many instances they can tolerate higher amounts of metallic and organic impurities than other conventional nickel plating baths. Good engineering practice is to provide for solution circulation with continuous electrolytic purification at low current density either in compartmented tanks or separate cells, in lieu of which to "dummy" periodically during shut-down time. In applications involving extra heavy deposits, continuous filtration is advised. Activated carbon should not be used, as it will remove the organic wetting agent or stress reducer(s). Severe cases of organic contaminations can be removed by the usual activated carbon treatment along with subsequent replenishment of the addition agents removed. In more severe cases of organic contaminations, permanganate treatment is required.

Sulfamate nickel plating solutions can be used with any equipment normally used with high chloride Watts' solutions, providing however that the lead be excluded from contact with the solution. Lead sulfamate is very soluble, and lead must not, therefore, be used for heating coils and thermostat control bulbs.

Although analytical methods can be used for the additives, the use of the time-honored Hull Cell¹⁰³ provides a quick, practical method of control of chemistry for sulfamate nickel baths. (The wetting agent concentration can be determined with a stalagmometer).

Hull Cell tests are typically operated according to the following conditions:

Time - 20 minutes

Panel - Polished brass

Current - 1 Amp

Temp. - 100^o F (38^o C)

Agitation - None.

A common practice is to determine any required additions with Hull Cell tests and add about half this amount at one time to the plating tank. These conditions can be varied, if desired, to simulate tank conditions more closely. Hull Cell solution should not be used for several consecutive tests. It is recommended that the effects of variations in the basic chemistry, additive concentrations, and any possible contaminants be studied in the Hull Cell to aid in interpretation of results that may occur later in production solutions. With experience, Hull Cell testing will prove to be a rapid method of control of additives and useful in troubleshooting. The 267 ml Hull Cell is the most common size because of the ease of calculating additions. If the plated Hull Cell panel shows dark low CD areas, impurities such as lead, (very low CD), copper (about 5 ASF), zinc (2-5 ASF), and certain organic contaminants. Other organic contaminants may be seen in middle and high CD areas on the panel. Carbon treating the solution is suggested followed by another Hull cell test to verify organic contamination. Chromium contamination sometimes causes a dark low CD area and can cause blisters on a properly cleaned panel.

Sulfamate nickel fills the requirements for engineering applications for the reasons given above, namely low residual deposit stress, high deposition rates and good ductility. Parameters that can change the deposit characteristics from sulfamate nickel solutions, are numerous (Table 9-Addendum). It is important to start with the purest possible sulfamate nickel plating solution. Colloidal matter and other insolubles should not be present. It is important to keep impurities out of the solution by careful attention to keeping the pre-rinse water clean and free from contaminants. Characteristics can be altered deliberately to achieve desired results by changing operating conditions or additives designed to produce specific changes. For example, cobalt, manganese or certain organic compounds to harden or strengthen the nickel deposit.

Sulfamate nickel requires different nickel anodes than the Watts nickel bath, unless chloride is included in the solution make-up. The use of chlorides increases the tensile stress in the deposit, however. A depolarized anode works well in the sulfamate solution, while an ordinary nickel anode will become passivated and cause a decrease in pH of the solution. Ultimately, use of such anodes will destroy the sulfamate radical in a solution. Sulfur-depolarized anodes give excellent corrosion.

The use of the stress measuring instruments in real time is needed to prevent problems before they take place.

Sulfamate nickel is widely used but not widely understood process.

102. P. Tang, H. Dylmer and P. Moller, "Nickel Coatings and Electroforming Using Pulse Reversal Plating", Proceed. Amer. Electroplaters’ Soc., 82, 529 (1995).

Dr-Ing N.V. Mandich, CEF, AESF Fellow, is founder, president and research director of HBM Electrochemical & Engineering Co., 2800 Bernice Road, Lansing, IL 60438.