Baume
The Baume scale is a pair of hydrometer scales (developed by Antoine Baume in 1768) to measure density of various liquids. One scale measures the density of liquids heavier than water and the other, liquids lighter than water. At 20C, the relationship between specific gravity (s.g.) and degrees Baume is: For liquids heavier than water: s.g. = 145 / (145 - degrees Baume).

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March 12, 2009

The process, which would soon be known as ‘electroless nickel’ was initially discovered by a French chemist named Adolphe Wurtz in 1845.  A. Wurtz’s first published paper dealt with the chemical hypophosphorous acid in 1841.  Further experimentation with Sodium Hypophosphite (NaPO2H2), which is the sodium salt of hypophosphorous acid, proved beneficial in the reduction of nickel metal ions.  Even though Wurtz discovered this process, he decided not to expand on the manufacturing capabilities of this find.  In 1916 a patent was filed by Roux, supporting the use of hypophosphite as a nickel coating method.  History at the beginning of the twentieth century is vague as to why this process was not further exploited.  As a result the use of hypophosphite again lay dormant until 1944.

In 1944 the team of Brenner and Riddell ‘rediscovered’ the process termed electroless nickel plating, and their investigations were published in 1946 and 1947.  Brenner and Riddell observed that when electrodepositing nickel from a bath containing sodium hypophosphite, the cathode current efficiency was much greater then 100%, and it was found that in addition to the nickel being electrodeposited, additional metal was being plated out by means of a chemical reduction reaction, which supplied the necessary electrons.

One of the more important features of the hypophosphite reduction of nickel ions is that

only certain metals provide the necessary catalytic effect to initiate deposition.  Further

research has found that all elements within Group VIII of the periodic Table, which

includes nickel, have this property.  Furthermore, once an initial coating layer of nickel

has been established, the electroless process is self-perpetuating, thus is better described

as auto-catalytic.  Layer after layer are formed spontaneously, and they then become the

catalyst for subsequent buildup.  The only way the process can stop, baring any

unforeseen mechanical problems, is when the ingredients in the solution deplete to where

the deposition slows down and eventually stops.  Therefore, the deposition reaction

from electroless nickel differentiates itself from the electrolytic deposition method, and is

usually specifically preferred because of the following 3 reasons:

·        Doesn’t require the application of external current.

·        Has no theoretical thickness limit.

·        The deposition rate is absolutely uniform across the initial coating layer, regardless of surface topography.

• Does not require rectifiers, electrical current, or anodes.
• Typical hardness is 55 – 70 Rc (equivalent to tempered steel).
• Superior abrasion/wear resistance, compared to chrome-plated steel that has the same surface finish.
• Low coefficient of friction.
• Can plate recesses and blind holes with stable thicknesses.
• Excellent corrosion protection.
• Non-magnetic properties, when phosphorous content is kept over 8%.  As the phosphorous level diminishes, the magnetic properties of the deposit strengthen.
• Excellent solder-ability properties.
• Can be plated directly to certain alloys of stainless steel without an electrolyte or electroless nickel layer. 

• Lifespan of chemicals is limited.
• Being auto-catalytic the internal effervescence of the solution never stops.
• Eventually the solution will plate-out to the liner of the tank, and the nickel will drop-out.
• Waste treatment costs are high.  The following methods are available for removing nickel from spent EN solutions:
• Reduction – Nickel ions (Ni+2) are reduced to form nickel metal (Ni0).  This is accomplished by electrolytically depositing nickel on special, high-surface-area cathodes, or by forming a fine precipitate of nickel.
• Precipitation – Nickel ions (Ni+2) are combined with sulfides, hydroxides and carbonates to form sludge’s.
• Separation – Metals can be separated from solutions by ion exchange, electrodialysis, Donnan dialysis and reverse osmosis.
• None of these above methods (all of which are being used somewhere in this industry of waste removal), can completely waste treat or remove nickel from every EN solution.  Controlled plate-out will lower the nickel concentration in most EN baths to 2-ppm.  However, current regulations demand a lower effluent discharge, in a majority of the manufacturing locations within the US.  Therefore, in order to meet the current regulatory demands, more than one method will be necessary to discharge a virtually zero-discharge nickel effluent.

• Electronics – The largest and most diverse market where EN is used.
• Aluminum memory disks that are found in many of the computer storage devices.
• Aluminum and Zinc connectors, serving to provide corrosion protection, wear resistance and is the easiest to plate on complex shapes.
• Components for semiconductor packaging, ceramic micro-devices, microwave devices, battery components and heat sinks.
• Nickel/gold applications on circuit boards.
• Automotive – This market is growing constantly, as consumers are demanding longer warranties on their vehicles.
• Brake pistons, wheel cylinders, caliper pins, steering column yokes, air bag components and transmission parts.
• Decorative applications, such as interior hardware and wheels, where overcoats of copper, electrolytic nickel and chrome are then applied.
• Oil, Gas and Chemical Processes – Corrosion and wear resistance, and chemical stability features are critical to this market.
• Housings, flanges, pipes, pumps, ball valve bodies, and connector sections are commonly plated with thicknesses of up to 100 microns.
• Low-phosphorous electroless nickel where exposure to strong alkalis, even at high temperatures and flow rates.
• General Industrial applications – Shafts for ink-jet printers, heddles for weaving machines, rolls for the printing of newsprint, are just a few examples.
• Food Processing applications – Evaporators in ice machines, bearings, gears, conveyors and chains, are some of the examples in the food industry.
• Aerospace – Compressor and stator components for jet engines.
• Airline – Drive trains and landing gear components.

The functions of each of the known bath constituents are as follows:

1)      Nickel Sulfate is the source of nickel for plating.

2)      Sodium Glycollate is a buffering agent that prevents excessively rapid pH (acidity) changes while the bath is in use.

3)      Sodium Citrate is a complexing agent that prevents precipitation of basic salts and nickel phosphite that accumulate in the bath when the bath is used for prolonged periods.

4)      Sodium Hypophosphite is a reducing agent, providing electrons for discharge of nickel ions to form a metallic deposit.

Reagents required:

0.02M EDTA

Murexide indicator (mix 0.2gm of dry indicator powder with 100gm of Sodium Chloride).

Dilute 10 ml of plating solution into a 100 ml volumetric flask.

Pipette a 5 ml sample of the diluted solution into a 250 ml Erlenmeyer flask.

Add 50 ml of DI water.

Add 5 ml of Ammonium Hydroxide.

Add 0.5 gm of Murexide indicator.

Titrate with the 0.02M EDTA, until the color changes from a greenish-yellow to a bright magenta tint.

Calculation: Volume (ml) of 0.02M EDTA x 2.35 = g/l nickel

Reagents required:

0.1N Iodine standard solution

0.1N Sodium Thiosulphate standard solution

Starch indicator

Pipette a 2 ml sample of the electroless nickel plating solution into a 250 ml stoppered flask.

Add 5 ml of concentrated Hydrochloric acid.

Mix well (the solution must be acidic before the iodine solution is added).

Pipette 25 ml of 0.1N Iodine solution into the flask, stopper, mix well, and allow to stand for 30 minutes.

Titrate the excess Iodine with the 0.1N Sodium Thiosulphate solution, adding Starch as an indicator, near the end of the titration.

Calculation: (25 – titration value) x 2.65 = g/l Sodium Hypophosphite (NaH2PO2.H2O)

Reagents required:

0.1N Iodine standard solution

0.1N Sodium Thiosulphate standard solution

Acetic acid solution (10% v/v)

Starch indicator

Pipette a 2 ml sample of the electroless nickel plating solution into a 250 ml stoppered flask.

Add 20 ml of DI water.

Add 20 ml of Sodium bicarbonate solution.

Pipette 50 ml of 0.1N Iodine solution into the flask, replace the stopper, and allow the flask to stand for 20 minutes with occasional shaking.

Acidify the flask with 20 ml of 10% Acetic acid solution.

Titrate the excess Iodine with the 0.1N Sodium Thiosulphate solution, adding Starch as an indicator, near the end of the titration.

Calculation: (50 – titration value) x 5.4 = g/l Sodium Phosphite (Na2HPO3.5H2O)

• Temperature 85C – 90C
• pH 2.5 – 3.5
• Plating speed is dependant on the concentration of Phosphorous in the solution.
• Hardness of an electroless nickel deposit varies between 250 HV – 750 HV, and again depends on the percentage of phosphorous in the solution.
• Structure of an electroless nickel deposit:
• No stress of deposit is present, if % phosphorous is 1% - 15%, and hardness is 250 HV – 750 HV.
• Conductivity of an electroless nickel deposit is less then that of either a nickel or nickel-cobalt deposit.
• Thickness can range from 5 angstroms to 100 microns (or more).
• Thickness variation can range from +/-2 microns to +/-10 microns (depending on the substrate material).
• Plating time:  24 hours = 300 microns (depending on hardness requirements).

Step 1 – Cleaning process – Non-etching soak cleaner

Step 2 – Rinse

Step 3 – Activation process

Step 4 – Rinse

Step 5 – Desmut process – Nitric acid 50%

Step 6 – Rinse

Step 7 – Zincate – long immersion

Step 8 – Rinse

Step 9 – Strip in Nitric acid – 50%

Step 10 – Rinse

Step 11 – Zincate – short immersion

Step 12 – Rinse

Step 13 – Electroless nickel plate

Fabricating operations, such as lapping and machining, can cause defects in the base metal, before it enters the electroless nickel process line.  Inclusions in the substrate metal may cause the part to be hard to clean and not easily wetted (meaning the inability to hold water uniformly, resulting in water beading off the substrate).  This issue could make uniform coverage difficult, even when thick deposits are applied.  Furthermore pores in the substrate can entrap preplate chemicals, which then ‘bleed out’ during the plating cycle, causing inferior electroless nickel deposits at these sites, and can lead to a “poisoning” of the EN solution.

Bath life depends on more then just external contamination being carried in with the substrate; it also depends on how hard the solution is used.  In normal plating operations, there are two electroless nickel solution, which are used alternatively.  This method allows the one bath to stabilize, before it is used again.  Even with this ‘life-prolonging’ method, usage dictates a 30-day life.  However it is possible to achieve up to double this lifespan, by establishing clean-process-practices, and making sure air-borne contaminants are kept to a minimum.  Temperature control, proper filtration, daily bath checks and concentrated upkeep are essential to maintaining a ‘healthy’ solution.

References:

• Adolphe Wurtz, Wikipedia
• Brenner and Riddell – Nickel and Chrome plating, Dennis and Such, 1972
• Enthone – Cookson Electronics, Electroless Nickel Plating summary
• PF Online: Electroless Nickel – Deposit Properties, Specifications and Applications
• David J. Kunces, “How To Treat Spent Electroless Nickel Baths”, Products Finishing magazine, August, 1990.