Electroless Nickel Plating Formulation For Automotive And Mechanical Parts

Electro less plating processes are considered to be much convenient, cheaper and best for deposition on parts with lot of holes, cleavages, bents, curves, abnormal shapes, threading etc., effectively and economically.


D

Dr. KRISHNA RAM THOGULUVA SESHADRI

Electro less plating processes are considered to be much convenient, cheaper and best for deposition on parts with lot of holes, cleavages, bents, curves, abnormal shapes, threading etc., effectively and economically.

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Electro less plating uses a redox reaction to deposit metal on an object without the passage of an electric current. Redox reactions involve Reduction and Oxidation reactions. One metal is reduced from its salt and other one gets oxidized to its metallic salt. 

Electro less plating technique is being used for many decades. They involve reduction of a complexed metal using a mild reducing agent, typically formaldehyde.

For example, mirrors can be manufactured using this reaction:

R-CHO + 2 [Ag (NH3)2]OH 2 Ag(s) + RCOONH4 + H2O + 3 NH3   where R is an organic group or hydrogen. The reaction deposits a shiny coat of elemental silver on the walls of the container.

Electro less plating is an autocatalytic reaction. Autocatalytic reactions are chemical reactions, in which at least one of the reactant is also a product. And a chemical reaction in which a product (or a reaction intermediate) also functions as a catalyst. In such a reaction the observed rate of reaction is often found to increase with time from its initial value.

Being autocatalytic and electrochemical redox process, the formulation and control on the stability of bath, consistency of deposition and maintenance of bath become critical and sometimes difficult too. As probability of settling down of the bath leading to the deposition of metals-“Seeding”- on the walls, pipes and other parts of the plating vessel / tank is very high, resulting loss of metal, constituents of plating solution, forcing to clean the tank and pipes making loss in chemical cost and time. 

Of course, one need not require to use electrical energy and metallic anodes for electro- less plating of metal, as it is autocatalytic process with reduction of metal in presence of a catalyst of noble metal. But the formulation, maintenance of solution, plating tank design and arrangement of heaters, cooling coils, agitation system etc., become very critical, to obtain consistent plating quality, uniform rate of deposition of metal with constant productivity, as metal ion to be released and plated on the required part uniformly and the depleted metal to be added as per the depletion rate from the plating solution. Apart from these the surface preparation, catalytic preparation of surface receptive to get adherent electro less plating of metal etc., needs special attention.

Advantages with electro less plating include:

  1. No investment and maintenance cost on D.C. Power sources viz., rectifiers, dynamo etc., as this process does not use electric current.
  2. Even coating on parts surface may be achieved with uniform in thickness on uneven surfaces.
  3. No sophisticated jigs or racks are required.
  4. There is flexibility in plating volume and thickness.
  5. The process will plate recesses, bents, sharp edges, cleavages and blind holes with stable thickness.
  6. Chemical replenishment can be monitored automatically.
  7. Complex filtration method is not required
  8. Matte, Semi Bright or Bright finishes can be obtained.
  9. Rack and barrel plating is possible.
  10. No investment on metallic anodes as electrodes.

The following list of metals may be deposited by electro less plating technique on various substrates is as given below:

Metals deposited by electro less plating technique are: 1. Copper, 2. Nickel, viz., a. Nickel-Boron alloy b. Nickel-phosphorus alloy, c. Nickel-Teflon composite, Nickel-Sic,Nickel-Al2O3 etc., 3 Arsenic, 4.Cobalt, 5.Chromium, 6.Iron etc., on various substrates and are as depicted below:

1.Electro less copper is deposited on ferrous metals and non-ferrous metals,viz.,brass, bronze, apart from non-conductive substrates- ABS plastics and other plating grade plastics, Glass epoxy material of Printed Circuit Boards, Ceramics, SiO2 etc., and catalyst used is a solution containing Pdcl2 and Sncl2 with Hcl and reducing agent is formaldehyde-HCHO.

CuSO4 + 2HCHO + 4NaOH Cu + Na2SO4 + H2 + 2H2O + 2HCOONa

2a. Electro less Nickel is deposited on ferrous metals viz., iron, tool steel or mild steel with or without an undercoat of electro-less Copper. The catalyst used is a solution containing Pdcl2 and Sncl2 with Hcl. Reducing agents used are Sodium hypophosphite or Boron hydride.

2b. Electro less Nickel on tool steel with or without an undercoat of copper, is to improve abrasion resistance and on brass, bronze or copper electrical parts and switches of auto parts, mobile phone components and other electrical and electronics devices, components etc., to improve conductivity with reduced contact resistance. Hence, the applications of electro less copper and electro less nickel are very wide.   

2c. Apart from these other specialty electro less plating is reported in the literature viz., Nickel with Teflon of 5, 7, 9 % , Nickel with SiC, Nickel with diamond etc. for special applications of   auto parts and other parts to improve their properties and applications-hardness, abrasion resistance, wear resistance etc.

2d. For aerospace applications, new developments are taking place on electro less plated composites.

 

History of Electro less Nickel plating:

The EN plating of metallic nickel from aqueous solution in the presence of hypophosphite was first noted as a chemical accident by Wurtz in 1844.

In 1911, Roux reported that metal was inevitably precipitated in the powder form; however these works were not in practical applications. In its early stage, progress in the field remained slow until World War II. In 1946, Brenner and Riddell developed a process for plating the inner walls of tubes with nickel-tungsten alloy, derived from the citrate based bath using an insoluble anode, which brought out the unusual reducing properties of hypophosphite.

The U.S. Patent Office says that the patent it issued in 1950 differed from the earlier patent in that Roux reaction was spontaneous and complete, while the Brenner and Riddell process was a controlled catalytic process so that deposition occurred only on catalytic surfaces immersed in the bath.

Brenner later wrote that his patent was an accidental discovery similar to the work of Wurtz and Roux, but said that he took out a patent to protect U.S. government rights. In fact, a declassified U.S. Army technical report written in 1963 goes on extensively about Wurtz and Roux work, and gives more of the discovery credit to them than to Brenner. This plating process was attributed to the action of chemical reduction of Ni ions.

During the 1954-59 period, Gutzeit at GATC (General American Transportation Corporation) worked on full scale development of electro less plating by chemical reduction alone, as an alternate process to conventional electroplating.

Initially, the co-deposition of particles was carried out for electrodepositing Ni-Cr by Odekerken, during the year of 1966.

In that study, in an intermediate layer, finely powdered particles like aluminum oxide, polyvinyl chloride (PVC) resin were distributed within a metallic matrix. A layer in the coating is composite but other parts of the coating are not.

The first commercial application of their work used the electro less Ni-SiC coatings on the Wankel internal combustion engine and another commercial composite incorporating poly tetra fluoro ethylene (Ni-P-PTFE) was co-deposited, during the year of 1981. However, the co-deposition of diamond and PTFE particles was more difficult, than the composites incorporating Al2O3 or SiC. The feasibility to incorporate the fine second phase particles, in submicron to nano size, within a metal/alloy matrix has initiated a new generation of composite coatings.

Probable Mechanism of eNi deposition:

As per Scholder and Heckel, the deposit consist of a mixture of pure metal and metallic phosphides. They suggest the following reaction:

NiO + H3PO2 Ni + H3PO3

3H3 PO2 → H3PO3 + 2P + 3H2O

2Ni + P → Ni2P6

The author did not connect the reduction of Ni with the evolution of H2.

Luke’s theory, gives a satisfactorily explanation of the simultaneous reduction of Nickel and hydrogen, as follows:

H2PO2-- + H2O HPO3 2-- + 2H+ + H-- (in acid solutions)

H2PO2-- + 2 OH-- HPO3 2-- + H2O + H-- (in alkaline solutions)

Ni2+ + 2 H-- Ni0 + 2H

H+ + M-- H2

Many theories are proposed for the mechanism of electroless deposition of Ni, as the deposition is complicated process.

Table.1: Types of Electro less Nickel Deposits with different Phosphorus Content

No

Type of Deposit

Phosphorus content in deposit %

Heat treatment temperature in ᵒC

Hardness in Rockwell C

Remarks

1

 

LPEN- Low Phosphorus e Ni

 

2 to 5

As plated

 

400 to 425ᵒC for 1 hour

Up to 60

 

Maximum Hardness

Uniform coating, most resistant to alkaline environments,best solder-ability

 

2

MPEN- Medium Phosphorus e Ni

 

6 to 9

 

375 to 400ᵒC for 1 hour

 

Maximum Hardness

Best appearance

 

3

HPEN- High Phosphorus e Ni

10 to 13

375 to 400ᵒC for 1 hour

> 65

Super corrosion protection, most resistant to acidic environments

  4

e Ni with PTFE

-----

As plated

----

Increased lubricity and low reflectivity

 

Electroless nickel can also be rendered non-magnetic, making it the optimal choice for electromagnetic shielding.

A great importance of electroless Nickel plating is its ability to produce deposits with a very high degree of thickness uniformity. It is much beneficial when coating on complex parts with critical dimensions, such as ball valves or threaded components. This is because of the fact that no current is involved and the associated problems of current distribution do not arise.

Formulations on Electro less Nickel on copper:

Several formulations on EN plating are tried and in this technical paper, the most successful formulation on electro less Nickel is discussed in details after continuously using in large scale production of mechanical and automobile parts for more than one  year period.

The following Bath-I was selected from literature, as it was giving good results  for electro less Nickel plating on Copper and electro less copper coated steel parts and it is as given below: .

Bath-I :

1) Nickel Chloride Nicl2 .6H2O – 20 grams per liter

2) Sodium Hypophosphite, NaH2PO2.H2O– 15 grams per liter

3) Acetic Acid, CH3COOH – 20 ml per liter

4)  DL- Malic Acid, CH2 (COOH).CH (OH).COOH -20 grams per liter

5) Glycine NH2CH2COOH – 5 grams per liter

6) Boric Acid H3BO3              -- 2 grams per liter

pH – 6.5 to 6.7 (Adjusted with NaOH & Hcl) 

Temperature – 70 to 72 o C

Time to get 5 to 6 microns of eNickel on Copper – 25 to 30 minutes

Process Sequence:

The Process sequence followed for non-ferrous metal parts made of copper, brass and bronze, is as given:1) Degrease 2) Alkaline Cleaner 3) Swill 4) Acid dip 5) Swill thoroughly 6) Pre-Activator 7) Activator 8) Swill 9) Post-Activator 10) Swill thoroughly 10) Electro-less Nickel  Plating 11) Swill 12) Hot water dip 13) Dry.

The Process sequence followed for ferrous metal parts made of steel, tool steel etc., with electro less copper coating is as given: 1) Degrease 2) Alkaline clean 3) Swill 4) Acid dip 5) Swill thoroughly 6) Pre-activator 7) Activator 8) Swill thoroughly 9) Post activator 10) Swill 11) Electro-less copper plating 12) swill  13) Electro-less Nickel Plating 14) Swill 15) Hot water dip 16) Dry

The Process sequence followed for ABS plastics  with electro less copper coating is as given: 1) Chromic acid etching 2) Swill 3) Swill thoroughly 4) Pre-activator 5) Activator 6) Swill thoroughly 7) Post activator 8) Swill 9) Electro-less copper plating 10) swill  11) Electro-less Nickel Plating 12) Swill 13) Hot water dip 14) Dry. 

After electroless Nickel Plating further thicker deposit of bright nickel is deposited as follows:

1. Jig and load 2. Watts nickel Plating (5 to 10 minutes to get 4 to 5 microns) 3.Swill 4. Bright nickel plating for 25-30 mts to get 20 microns of Nickel 5.Swill 6.Bright  chromium plating for 10 mts. 7. Swill 8. Dry 9. Unload. 

This Bath.1- is further studied thoroughly by varying the various ingredient chemical compositions. Graphs are plotted to find out the optimum condition to get the best & fast results of deposition of electro less Ni deposit on copper.

Every time one chemical ingredient out of 6 chemicals are varied of concentration & studied at temperature of  70 o C, 75 o C and 80 o C .

Cleaned, pre-weighed 1”x1” sized copper specimens are used throughout the experiment & the deposit thickness of Nickel is obtained & calculated by weight gain method. This is cross-checked with X-ray fluorescence thickness tester & the results are within ± 5 %. difference. The graphs are plotted with the results of thickness obtained by weight-gain method, to find out the optimum condition of the chemical composition of the bath.

 

These optimum results are as depicted in Table.2

 

Table.2: Depiction of optimum composition of eNi Bath-II

No

Chemical constituents of Electro less  Nickel Bath

Grams/liter

or ml/liter

Optimum

Condition

Thickness in microns

Remarks

  1

NICKEL CHLORIDE

Nicl2 .6H2O

(in gram/liter)

15

70oC;

30 min

9.0

 

 

  2

SODIUM  HYPOPHOSPHITE

NaH2PO­2H2 O

(in gram/liter)

14

70oC;

30 min

7.19

 

 

 

  3

ACETIC  ACID   CH3COOH

(in ml/liter)

22

70oC;

30 min

7.0

80oC;30 min:

8 microns of Ni at 19 ml/l

  4

DL-MALLIC ACID

CH2(COOH).CH(OH)-COOH

(in gram/liter)

18

70oC;

30 min

9.0

80oC;30 min:

11 microns of

Ni at 15 gm/l

  5

GLYCINE   NH2CH2COOH

(in gram/liter)

7

70oC;

30 min

6.74

 

 

  6

BORIC ACID  3BO3

(in gram/liter)

2

 

------

 

-----

 

 

  7

Temperature

70 o C-72 o C

30 minutes

 

 

  8

pH

6.8 to 6.9

--------

 

 

  9

Time

30-40  minutes

 

 

 

 

Optimum Composition of eNi bath (Bath II):

Nickel Chloride, Nicl2 .6H2O – 15 gram per liter

Sodium Hypophosphite, NaH2PO­2H20– 14 grams per liter

Acetic Acid, CH3COOH – 22 ml per liter

DL-Malic Acid, CH2 (COOH). CH (OH). COOH-18 grams per liter

Glycine, NH2CH2COOH – 7 grams per liter

Boric Acid, H­3BO3 -2 grams per liter

pH – 6.8 to 6.9 (Adjusted with NaOH & Hcl) 

Temperature – 70 to 72 o C

Thickness: 6.74 to 9.0 microns of Ni in 30 minutes

Time- 30 to 35 minutes                                                

Final (optimum) Composition of eNi bath at 80 o C (after study on temparature:

Nickel Chloride, Nicl2 .6H2O – 15 gram per liter

Sodium Hypophosphite, NaH2PO­2H20– 14 grams per liter

Acetic Acid, CH3COOH – 22 ml per liter

DL-Malic Acid, CH2 (COOH). CH (OH). COOH-18 grams per liter

Glycine, NH2CH2COOH – 7 grams per liter

Boric Acid, H­3BO3 -2 grams per liter

pH – 6.8 to 6.9 (Adjusted with NaOH & Hcl) 

Temperature – 80 to 82 o C to get 9.44 microns of Nickel

Time- 25 to 30 minutes                                                                

This bath was prepared & used for 12 months to verify & establish the results and it was consistent in giving the thickness of 8-9 microns of Nickel at 80 o C in 25-30 minutes.

 

Composition of Nickel Deposit:

The deposited Nickel was subjected to EDAX analysis to know the composition. The results are as shown in Fig15 & 16. Inclusion of Phosphorus is clearly seen in EDAX plots. The deposit contains 12.23 to 13.74% of Phosphorus.

 

These optimum results are as depicted in Table.3

Table.3: Depiction of optimum composition of eNi Bath-III

 at optimum temperature of 80oC to 82oC

No

Chemical constituents of Electro less  Nickel Bath

Grams/liter

or ml/liter

Optimum

Condition

Thickness in microns

Remarks

  1

NICKEL CHLORIDE

Nicl2 .6H2O (in gram/liter)

15

70oC;

30 min

9.0

 

 Very Good

  2

SODIUM  HYPOPHOSPHITE

NaH2PO­2.H2 O(in gram/liter)

14

70oC;

30 min

7.19

     Good

 

  3

ACETIC  ACID   CH3COOH

(in ml/liter)

22

70oC;

30 min

7.0

80oC;30 min: 8 microns of Ni at 19 ml/l

  4

DL-MALLIC ACID

CH2(COOH).CH(OH)-COOH

(in gram/liter)

18

70oC;

30 min

9.0

80oC;30 min:

11microns of

Ni at 15 gm/l

  Very Good

  5

GLYCINE

  NH2CH2COOH (in gram/liter)

7

70oC;

30 min

6.74

Retards the speed

  6

BORIC ACID 

H­3BO3 (in gram/liter)

2

 

------

 

-----

 

 

  7

Temperature

80oC - 82oC

30 minutes

 

 

  8

pH

6.8 to 6.9

--------

 

 

  9

Time

30-40 minutes

 

 

 

 

This formulation is much useful for large scale production of Electro less Nickel   plating in rack and barrel plating of auto and industrial components.

Properties of eNi deposits obtained:

  1. Plating thickness ranges from of 8.5 to 9 microns, obtained in 25 to 30 minutes.  Thickness was checked by weight gain method and verified by x-ray fluorescence thickness test equipment.

 

2. The phosphorus co-deposited is 12.23 to13.74 %.

Further the P content in the deposit may be reduced by way of change in sodium hypophosphite content in the solution.

 

3. Baking or heat treatment after plating has resulted in hardness as high as 70 Rc.

 

4. Salt Spray Corrosion Test (1000 hours for high phosphorous containing eNi deposits;

250 hours for mid-phosphorous containing eNi deposits )

5. Mechanical Wear:

The abrasion resistance of electro less nickel is very good in lubricated wear situations and the wear index numbers of different deposits are as in Table.4 obtained from the Taber Abrader/ Abraser.

 

Table.4: Comparison of wear index numbers from Taber Abrader:

 

No

Type of Deposit

Heat treatment Temperature C

Wear Index Number

Remarks

1

Electro less Nickel (as deposited)

-----

9.6

Good

2

Electro less Nickel

300

4.4

 

3

Electro less Nickel

500

2.7

 

4

Electro less Nickel

650

1.3

 

5

Electrolytic Nickel from Watts Bath (as deposited)

-----

14.7

 

6

Electrodeposited Hard Chromium (as deposited)

------

2.0

 

 

Advantages with this bath are:

1. Rack and Barrel plating are possible

2. Masking is possible for selective surface plating. Masking was performed by either screen printing with ink, peelable solder mask film or by photo-printing.

3. The deposited eNi contains 12.23-13.74 % P which is highly recommended for hardness requirement of the deposit after heat treatment.

This is achieved by heat treatment of the deposited eNi at C for 1.5 to 2 hrs.

4. Bath may easily be maintained and restarted after dummy plating at any time, after adding the chemicals after analysis and adjusting pH to working pH.

  

Maintenance of eNi Bath:

Analysis Methods

 

     The bath contents may easily be maintained by periodic analyses and addition of chemical ingredients of the bath. The critical constituents to be analysed are 1) Nickel or Nickel chloride and 2) Sodium hypophosphite, in this case.

    

1.Nickel Concentration:

 

Reagents Required:

a. Conc. Ammonia A.R.

b. Murexide indicator: Grind 0.2 gm of murexide A.R with 100 gms of Sodium chloride A.R in a mortar to a fine powder and store in a clean glass or polyethylene bottle; label properly with contents and date.

 c.0.05 M EDTA C10H14N2Na2O8 ·2H2O 

Disodium salt of Ethylene Diamine Tetra Acetic acid standard solution

 

Dissolve 10 gms of Sodium Hydroxide-NaOH A.R. in 200 ml of DI water placed in 1 litre volumetric flask. Weigh 18.6 gms of disodium ethylene diamine tetra acetic acid EDTA A.R. and record the exact weight. Transfer this to the flask and stir to dissolve fully. Dilute to the mark with DI water, mix well and transfer this to a clean polyethylene container, label it with molarity of EDTA with date.        

    M of EDTA = weight of EDTA / 372 

Procedure:

1. Pipette 2 ml of plating solution in a 250 ml capacity Erlenmeyer flask.

2. Add 100 ml of DI or distilled water.

3. Add 10 ml of conc. Ammonia.

4. Add 0.2 gram of Murexide indicator.

5. Titrate with 0.05 M EDTA standard solution with swirling the flask while adding the EDTA solution. Color change is from yellow to purple end point.

6. Note down the volume of EDTA consumed.

Calculation:

Amount of Nickel present = ml of EDTA solution X Molarity of EDTA X 29.35 = grams /liter --“A”

(Note: Atomic weight of Ni = 58.69 and Molecular weight of Nicl₂.6H₂O = 237.70 )

Amount of Nicl₂.6H₂O present: = “A” (237.70 / 58.69) = “A’ x 4.05 =   grams / liter

2. Nickel Chloride  Nicl.6HO

Reagents required:

a.20% Ammonium acetate-CHCOONH₄: Dissolve 200 gms of ammonium acetate CH₃COONH₄, in 1000 ml of DI water in a clean glass beaker, by stirring. Store it in a glass or polyethylene bottle; label it with contents and date.

b. Sodium chromate-NaCrO indicator: Add 2.0 gms of Sodium chromate in 100 ml of DI water and dissolve it fully by stirring. Transfer it and store in a clean polyethylene bottle, label with contents and date.

c. 0.1 N Ag NO (silver nitrate) standard solutions: T ake 700 ml of distilled water in a 1 liter volumetric flask and add 5 ml of Conc. HNO₃. A.R. Add 17 gms of silver nitrate Ag NO₃, stir well to dissolve completely. Dilute to the mark with distilled water, after mixing well. Store in an amber colored glass bottle, label with concentration and date. This has to be standardized with 0.1 N KCl (Potassium chloride), as below:

1. Pipette out 25 ml of 0.1N KCl into an Erlenmeyer flask

2. Add 25 ml of DI water and 5 ml of 20% ammonium acetate

3. Add 1 ml of sodium chromate indicator.

4. Titrate with silver nitrate with swirling till the precipitated silver chromate turns pink.

Normality of Ag NO₃ = ml of KCl X Normality of KCl / ml of Ag NO₃

Procedure for Nicl.6HO:

1. Pipette 5 ml of plating solution in a 250 ml capacity Erlenmeyer flask.

2. Add 100 ml of De ionized or distilled water.

3. Add 10 ml of 20% ammonium acetate.

3. Add 2 ml of sodium chromate indicator solution.

4. Titrate with 0.1 N Ag NO₃ standard solution to a permanent faint pink end point, due to the formation of Silver chromate. Ag₂CrO₄.

5. Note down the volume of 0.1 N Ag NO₃ consumed.

Calculation:

 ml Ag NO₃  X Normality of Ag NO₃  X 23.75 = grams / litre of Nicl₂.6H₂O --“B”

Note: Molecular weight of Nicl₂.6H₂O = 237.70; Atomic weight of Ni = 58.69

2.Sodium Hypophosphite Concentration:

 

Reagents Required:

 

a) Concentrated hydrochloric acid, L.R. or A.R. Grade

 

b) 1% Starch indicator solution:

Make slurry of 3 gm of soluble starch in a small volume of cold water and add this to about 300 ml of boiling water and stir well to mix thoroughly. Boil for 3 to 5 minutes. Cool to room temperature and store it in a clean glass or polyethylene bottle with proper label with date. A pinch of mercuric oxide may be added to prevent mold growth.

c) 0.1 N Iodine (standard volumetric solution):

Dissolve 40 grams of KI (Potassium Iodide) in 30 ml of Distilled water in a 250 ml beaker. Weigh accurately 12.69 grams of Iodine crystals and dissolve in the above KI solution by stirring till it completely dissolves. Transfer this solution into a 1000ml capacity volumetric flask and make it up to the mark with distilled or DI water. Transfer this solution in a brown coloured bottle and keep in darkness inside the desk after labelling with the concentration of iodine solution with grams per litre and date.

 

Standardize this solution with 0.1 N As2 O3 solution as follows:

 

Standardization of I2  solution:

0.1N.As2 O3 solution:

Take 30 grams of Arsenic oxide in a weighing bottle. Heat this at 110oC for 2 hours. Cool this in a desiccator. Add 4 grams of KOH in 50 ml of DI water placed in a 250 ml beaker. Dissolve this by stirring. Weigh out 4.9 to 5.0 grams of As2 O3 and record the exact weight. Add and dissolve this in the above KOH solution, which is heated to luke warm temperature. When completely dissolved carefully add 50% H2SOsolution drop-wise, until the solution is neutral to phenolphthalein (dissolve 1 gram of phenolphthalein in 100 ml of isopropyl alcohol and store it in a dropper bottle) indicator and become colourless. Cool and transfer it to a 1000 ml capacity volumetric flask and make it up to the mark with distilled or DI water. Transfer and store this in a glass or polyethylene bottle with labelling indicating concentration, grams per litre and date. 

Procedure:

Pipet 25.0 ml of As2 O3 solution to 250 ml capacity Erlenmayer flask and add about 2 gms of sodium bicarbonate,50 ml of distilled or D.I water and 2 to 3 drops of 1% starch indicator. While swirling, titrate with iodine to first blue end point that remains at least for 30 to 60 seconds.

N = ml of As2 O3 X N of As2 O3 / ml of I2.

 

d) 0.1 N Na2S2O3.5 H2O- Sodium thiosulphate  solution:

Dissolve 24.8 grams of Na2S2O3.5 H2O in 700 ml of previously boiled and cooled distilled or DI water in 1000 ml volumetric flask. Stir, dissolve and dilute it to make up to the mark. Mix well and transfer it to a glass or polyethylene bottle. Label it with normality and gm/L of Na2S2O3.5 H2O with date and keep.

 

Standardize 0.1 N Na2S2O3 with 0.1N KIO3 solution as follows:

 

Preparation of 0.1 N KIOsolution:

 

Pipet 25.0 ml of KIO3   Potassium Iodate standard solution in a 250 ml Erlenmeyer flask.  Add 30 ml of distilled or DI water and 1 gm of KI-Potassium iodide and 10 ml of 25% H2SO4   solution. Titrate with swirling with the above O.1 N Sodium thiosulphate  solution to a pale yellow colour. Add 2 ml of starch reagent and continue titration until blue colour disappears.

Calculation: N of Na2S2O3= ml of KIOx N of KIO/  ml of Na2S2O3

Preparation of Standard  KIO3 solution:

Dry about 30 gms of KIO3 AR in a weighing bottle at 180oC for 2 hours and cool it in a desiccator. Dissolve 1 gm of NaOH and 10 gm of KI in 200ml of distilled water in a 500 ml beaker. Weigh out 3.5 gms of KIO3 and add it to the beaker and stir to dissolve. Transfer this solution to a 1000ml volumetric flask and make it up to the mark. Store this in a glass bottle and label it with N. of KIO3 and gms / L of KIO3.

N of KIO3 = weight of KIO3 / 35.67

 

Procedure for analysis of Sodium Hypophosphite :

 

1. Pipette 5.0 mls of bath (previously cooled) into an iodine flask.

2. Add 50mls concentrated hydrochloric acid.

3. Pipette 50.0mls 0.1 N iodine into flask. Stopper flask and shake.

4. Leave in a dark cupboard for 30 minutes.

5. Titrate with 0.1 N sodium thiosulphate to a pale straw colour.

6. Add a few drops of freshly prepared 1% starch indicator solution and continue titration   

    to a clear end point.

7. Record ml of thiosulphate.

 

Calculation:

g/L sodium hypophosphite = {(ml of I2 X N of I2) - (ml of thio x N of thio)} x 10.6

 

Abbreviations used:

DI = De-ionized water; A.R. = Analytical Reagent; wt = weight; gm(s) = gram (s); ml = milliliter; N = Normality; M = Molarity; gpl or gms/L = grams per liter; mpl or ml/L = milliliter/Liter.

     Maintenance of working solution:

 

     Once analysed the lesser amount may be compensated by adding the solution of Nickel chloride and Sodium hypophosphite.

 

     Stock solution A:

     Dissolve the following chemicals in 10 litres of DI water and store in 2 X 5 litres capacity PVC carboys with lid in a cool place with label as Solution “A” with date:

1. Nickel chloride- 150 gms.

     2. DL- Malic Acid- 180 gms

     3. Boric Acid       -   20 gms

 

     Stock solution B: Dissolve the following chemicals in 10 litres of DI water and store in 2 X 5 litres capacity PVC carboys with lid in a cool place with label as Solution “B” with date:

    

     1. Sodium hypophosphite – 140 gms

     2. Acetic Acid                    – 220 ml

     3. Glycine                          --  70 gms 

    

     The stock solutions “A” and “B” can be kept separately for 3 to 6 months if properly stored with lid.

    

     Solution “A” and “B” to be added as per analysis report, after removing the load. Adjust the solution to the required pH.

    

     Not advisable to add chemicals when the load or job is in the tank, which may lead to rough deposits of eNi and or spontaneous precipitation of Nickel as sponge, at high temperature of working bath.

Precautions to be taken before shutting down and start up of the working bath:

1. Before shutting down, cool the solution below the working temperature, say 50o to 55oC and add 100 ml of Glycine solution (1gm per liter of Glycine stock solution) to the working electro less Ni bath to avoid the possible auto-deposition of Ni on the walls of the process vessel, pipes, cooling coils etc. With this precaution, the bath can easily be maintained for more than 100 days.

2. Reduce the pH below the working pH before shutting down the bath.

3. Always check the pH by electrometric method by using a suitable pH meter.

4. When the solution is not in use, it has to be covered with a suitable lid and do not allow dust particles falling in the bath.

5. No job or dummy should be left unattended in the bath, while shutting down.

6. For better results and stable life of bath, the solution may be cooled and transferred to a clean storage tank with mild air agitation, to store when the bath is idle.  

7. No traces of activator should enter the bath.

8. Before start of the bath, analyze the Ni content & sodium hypophosphite content and pH. As per analytical report, add required Nickel chloride solution, Sodium hypophosphite solution, adjust pH with 10% NaOH or 20% Hcl solution and heat the solution to working temperature.

9. Do not add maintenance chemical solution when the job is inside the solution. Remove the job and add necessary chemical solutions with stirring to maintain the bath.

10. Keep the hanging hooks and jigs clean; strip them and clean before loading the job.

11. Do not carry drag out solution along with the job to the working solution. This is to maintain pH of the working solution.

12. Check the pH of solution for every batch of plating and if required adjust it with NaOH   or hydrochloric acid solution.  While adding 10% NaOH solution, stir well and do not allow the formation of nickel precipitate of Ni(OH)2;this may dissolve by continuous stirring with agitation.

13. Start plating on the dummy for 20-25 minutes and then the regular job may be loaded for eNi. plating.

14. Mild air-agitation with rod movement are recommended.

Conclusion:

The addition of chemicals and life of bath wholly depend on the volume of production taken out and the careful maintenance of the bath after chemical analyses with at most care of freedom from dust and dirt falling in the bath leading to unwanted “seeding” phenomenon.

When the bath is not in use, reduce the working pH, and keep it covered with lid.

This formulation is much useful for large scale production of eNi plating of automobile, electrical, electronic and mechanical parts and components.

Tiny components- washers, screws etc., may easily be eNi plated by barrel plating and bigger components-bumpers, mechanical parts of machnes by rack plating.

About Author:

Dr. Krishna Ram Thoguluva Seshadri is an expert with 4 decades of experience in Electroplating, Metal Finishing, Anodizing, Surface finishing, Electro less Copper, Electro less Nickel Plating, ENIG for PCBs, Immersion Tin, Immersion Silver etc. He is presently engaged as specialist consultant in PCB, Plating and Surface Finishing in India and South Africa.

Earlier he worked as Deputy General Manager in ECIL- Electronics Corporation of India Limited- A Govt. of India undertaking-under Department of Atomic energy for 33 years and as Chief Executive (Techno-Commercial) in Meena Circuits Pvt. Ltd., Vadodara, India for 3.5 years; now stays in Johannesburg, South Africa as a senior Plating and PCB consultant. He is Vice President of Impex Innovations, India and South Africa, a Green House of export/ import business of excellence. Contact: drtskram@yahoo.com

References:

1. Arthur K. Graham, ed., Electroplating Engineering Handbook, second edition, 1962.

2. Kenneth E. Langford, Analysis of Electroplating and Related Solutions, 4th edition, Teddington, Draper, 1971.

3. Jothi Sudagar, Jianshe Lian, Wei Sha, “Electroless nickel, alloy, composite and nano coatings - A critical review” Journal of Alloys and Compounds, Volume 571, (2013),183-204.  http://dx.doi.org/10.1016/j.jallcom.2013.03.107

4. A. Brenner, G.E. Riddell, Proc. Am. Electropl. Soc. 33 (1946) 16–19

5. G.G. Gawrilov, Chemical (Electro less) Nickel Plating, Portcullis Press, Red hill, England, 1979

6. G.O. Mallory, J.B. Hajdu, Electro less Plating: Fundamentals and Applications, William Andrew, 1990

7. http://corrosion-doctors.org/MetalCoatings/Electroless.html

8. ASTM B733 - 04(2009) Standard Specification for Autocatalytic (Electro less) Nickel Phosphorus Coatings on Metal

9. Electro less Plating by Glenn O Mallory and Juan B Hajdu, ISBN 10: 0815512175;

10. Electro less Nickel Plating by Wolfgang Riedel.  ISBN-10: 0904477126;

        ISBN-13: 978-0904477122

11. Chemical (electro less) - Nickel Plating, Georgi G Gavrilov, 1979

 

 

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