Roger replies: Roger Lawson, product development manager at Schloetter Co Ltd, continues his popular question and answer column.
I have been given a contract to gold plate stainless steel components. I am getting real problems with adhesion of the gold. What should I do?
Achieving good adhesion on stainless steel is difficult due to the rapid formation of very stable oxide films. Literature sources give a wide variety of procedures, with nearly all utilising a woods nickel chloride strike for the initial electro-deposition. This traditional process is very acidic and consequently has a very low cathode efficiency, resulting in the formation of copious amounts of hydrogen on the work surface.
Nowadays, there are proprietary gold processes available which have been specifically developed for the direct plating of stainless steel. Such processes can be used purely as a strike, but are also capable of depositing the full required thickness.
Contact an appropriate supplier about such a process.
Can you please explain to me the differences between all the different names for nickel anodes. I don't understand how Durvanic is different from Depolarised and what do all those letters mean in front of the names for nickel pieces, e.g. 'S' Nickel.
The differences are as follows:
a) Electrolytic Nickel Anodes.
The purest (and usually least expensive) of the nickel anodes. It is available in strips or small pieces ('F'-flats, 'R'-rounds). However, as there are no 'depolarisers' added to aid solubility/corrosion, this type of material is the most critical with regard to anodic current density and electrolyte composition (pH and chloride).
b) Depolarised Nickel Anodes.
Made from high purity nickel, they additionally contain nickel oxide (<1%). This gives a myriad of fine crystals, separated at the grain boundaries by the nickel oxide. The addition of nickel oxide gives improved anode solubility and corrosion. The anode film is light brown. Usually supplied as rolled oval anodes, their use is generally restricted to dull nickel electrolytes, where the pH should be in the range of 4.0-5.8, with a minimum chloride concentration of 8.25g/l (25g/l NiCl26H2O).
c) Durvanic Nickel Anodes.
Made from high purity nickel with an addition of carbon (<1%) and available in rolled or cast formats. They form a tenacious, porous black film. The electrolyte must contain a minimum of 4.5g/l chloride (15g/l NiCl2.6H2O) and have a pH lower than 4.5. The anodic current density is also important and should be kept in the range of 1.5-3.5 A/dm2 (14-33 ASF).
d) 'S' Nickel.
Available as strip or small pieces (rounds, pellets etc.) it is very pure nickel incorporating 0.02% S (Sulphur) as a depolariser. This ensures a high degree of electrochemical activity, resulting in good solubility and uniform erosion. The high activity makes this type of material ideal for all electrolytes including those that are chloride-free and/or of a high pH. They work well over a wide anodic current density range. With good availability in small pieces, it is extremely suitable for use in titanium baskets.
Though there is no formation of a metallic sludge, nickel sulphide is formed and therefore as with all nickel anode material, bagging is essential, especially with air agitated solutions.
This is the most commonly used material today.
Can you please explain the differences between the different types of copper anodes. I am experiencing severe passivity in my decorative acid copper bath and suspect the anodes.
There are principally 2 grades of copper anode material:
This is pure copper and is suitable for use in cyanide copper solutions. It can also be used in additive-free acid copper processes.
b) Phosphorus Deoxidised Copper (PDO).
This material contains low traces of phosphorus, typically 0.05% P (0.04-0.08). It is a must for proprietary acid copper electrolytes formulated with organic additives. A satisfactorily working anode produces a dark grey, almost black film comprising Cuprous Oxide (CuO).
Anode passivity in an acid copper electrolyte can be caused by one of the following or combination thereof:
i) Over concentrated solution, limiting the solubility of copper. Confirm by solution analysis.
ii) Blocked anode bags, whereby the solution within the bag becomes super saturated.
iii) Excessive anode current density. Depending on degree of solution movement around the anode, do not exceed an anodic current density of 2.5 A/dm2. Check anode area and all electrical connections.
iv) High levels of chloride, usually denoted by a light grey to white anode film. Confirm by solution analysis.
v) Incorrect or poor quality anodes e.g. insufficient or no phosphorus, usually denoted by a reddish brown anode film.
vi) Low solution temperature.
In all cases of anode passivity, once the cause has been identified, the passive film must be removed. With bar/oval anodes this can be done by scrubbing. With anode baskets this is usually done by soaking in a mild copper etch such as sodium persulphate. The use of nitric acid is not recommended due to the risk of severe contamination of the copper electrolyte.
I have recently been drawn into the Lead-Free debate and have been asked to guarantee that my Pure Tin deposit is in fact totally free of lead. Is it wise for me to make such guarantees?
I would be very careful of making any guarantees as to the level of lead in your deposit. As a rule lead is present as a low level contaminant in both tin anodes and tin solution concentrates and has historically not been a problem.
With the latest pressure to be Lead-Free you will need to more vigilant over lead contamination in what you probably thought was a 'pure' tin bath.
It is not uncommon for tin anodes to contain 100 ppm of lead and for solution concentrates 50 ppm.
You should press your anode and concentrate supplier to provide a considerably higher purity grade of both.
It is difficult not to be commercial in this issue but my own employer can supply tin anodes with typically only 30ppm of lead (50ppm guaranteed max) and a maximum of 7ppm of lead in the solution concentrate.
If you require further details on these topics, please give me a call on 01386 552331 and I will be happy to discuss it with you and/or your customer.
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|Title Annotation:||Special Feature|
|Date:||Jul 1, 2003|
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