Nickel plating
Nickel plating is the collective term for various processes for producing a nickel coating on mostly metallic objects.
properties
Due to its special properties, nickel is particularly suitable as a coating metal for many applications. Nickel is resistant to air , water , diluted acids and most alkalis , but not to nitric acid , concentrated hydrochloric acid and ammonia .
Generation of the nickel layer
The nickel layer is deposited electrolytically from aqueous solutions containing nickel salts, so-called nickel electrolytes . Nickel electrolytes normally have nickel plates or nickel granules as an anode ; they are used to replenish metal ions .
Process efficiency
The processes for depositing nickel have been known for a long time and have now been greatly optimized. Most nickel electrolytes achieve an efficiency of> 98%, which means that over 98% of the electricity supplied is used for metal deposition. The remaining current is lost in undesired electrolytic processes, such as the evolution of hydrogen.
Appearance of the nickel layer
Nickel surfaces are not resistant to tarnishing; that is, dark discoloration may occur over time. Nickel has a light silver color, but differs from chrome surfaces by a characteristic, slightly yellowish hue. The appearance of the nickel layer is heavily dependent on adherence to the correct parameters during electroplating. The supply of gloss and additives, but also the metal ion content as well as the temperature and the pH value must be kept within tight tolerances. The respective parameters depend on the type of electrolyte.
Base body
Objects / base bodies / substrates / base materials that are nickel-plated often consist of / are steel , die-cast zinc , brass or aluminum , and more rarely of other metals, alloys or plastic .
Electroplated nickel plating
Nickel coatings are among the most important and most frequently used metal coatings produced by electroplating . With galvanic nickel plating (see DIN EN 12540, replaced by DIN EN ISO 1456, Dec. 2009), the objects to be nickel-plated are immersed in a nickel electrolyte after a suitable pretreatment and a nickel coating is deposited on the surface of the object by applying an electrical voltage from.
The nickel deposition on the cathode is simplified according to the following scheme:
In reality, however, the processes are more complicated, as the nickel ions first have to strip off their hydration shell before they are reduced to metallic nickel .
Electroplated nickel plating is available in numerous variants:
Bright nickel plating
Using special gloss additives in the electrolyte and setting certain process parameters, fine irregularities in the surface of the raw parts are leveled and a glossy surface is created, which is preferred for decorative applications. Bright nickel coatings are more brittle than matt nickel coatings.
Matt nickel plating
If you operate a bright nickel electrolyte without gloss additives, the nickel coatings will not be shiny, but matt. The matt appearance is not even, but rather uneven. In order to create an evenly matt surface, there are special matt nickel electrolytes that work with finely dispersed solutions or with suspended solids.
Semi-gloss nickel plating
Semi-gloss electrolytes are gloss electrolytes with only some of the gloss additives. These coatings are not used for decorative reasons, but to improve the corrosion protection in multi-layer systems.
Black nickel plating
Special additives lead to the deposition of an anthracite-colored to black coating.
Dispersion nickel plating
Small particles are added to the electrolyte, which are then deposited in the nickel coating. One of the oldest examples of these particles is diamond powder, the nickel coating then makes an excellent nail file. Other examples: corundum, quartz, titanium carbide ...
The nickel coatings acquire particularly interesting properties when particles made of Teflon or molybdenum disulfide are added: the surface then has a very effective and durable self-lubricating effect even without oil or other lubricants.
Multiple nickel plating
With double nickel plating and triple nickel plating, two or three different nickel coatings are combined, for example, semi-bright nickel and bright nickel. In the case of sandwich nickel, for example, a combination of nickel-chromium-nickel-chromium is used.
Double nickel is basically a base layer made of simple watt nickel and the top layer is a bright nickel (e.g. watt nickel with the addition of 6 g / l saccharine as a brightener and 5 mmol / l butenediol ). This results in high-gloss layers with good corrosion protection properties. Double nickel is an alternative to bright chrome-plated nickel; it is technically simpler, more environmentally friendly (no chromates ), but less corrosion-resistant. A typical application is simple household scissors: the shiny nickel has a very good appearance with moderate requirements for corrosion resistance.
Thick nickel plating
Older names for this process are hard or heavy nickel plating. Nickel coatings of approx. 200 to 3000 µm are deposited. With this process, for example, worn components can be made usable again. The thick nickel plating could replace the hard chrome plating in some areas.
Watts' nickel electrolyte
The first electrolytes for nickel deposition based on nickel sulfate have been described since 1840. However, these electrolytes were not suitable for surface finishing . In 1916 Oliver Patterson Watts described a mixture known today as Watts' nickel electrolyte or Watts electrolyte, which is based on most of today's nickel electrolytes. Watts electrolytes contain about 60 g · l −1 nickel.
It consists of:
| Nickel sulfate | NiSO 4 · 6 H 2 O | 240-310 g / l |
| Nickel (II) chloride | NiCl 2 · 6H 2 O | 20-50 g / l |
| Boric acid | H 3 BO 3 | 20-40 g / l |
The nickel sulphate is the main metal supplier, nickel chloride improves the anode solubility due to the chloride content . The boric acid serves as a buffer substance , which means that it keeps the pH value within a certain range.
Nickel sulfamate
The further development of the Watts electrolytes are nickel sulfamate electrolytes, as they can contain up to 140 g · l −1 nickel. A current density of 10 mA cm −2 corresponds to a growth rate for nickel of 0.205 μm min −1
It consists of the manufacturer Enthone / Polyclad :
|
Nickel (nickel sulfamate concentrate with approx. 165 g l −1 nickel) |
Ni 2+ | 80 g l −1 |
| Additive EL (contains nickel (II) bromide ) |
NiBr 2 | 80 ml l −1 |
| Boric acid | H 3 BO 3 | 30 g / l |
| Additive K ( wetting agent ) |
5 ml l −1 | |
| Electrolyte temperature | 40 ° C | |
| Electrolyte pH | 3.2 | |
| Electric current density | 1-20 mA · cm −2 |
Chemical nickel plating
Electroless chemical nickel plating is one of the reduction processes. The objects to be nickel-plated are immersed in special electrolytes and a nickel coating is deposited on the surface of the objects without applying an electrical voltage. Chemical nickel plating is characterized by an even layer thickness, even on complex parts and on internal surfaces. In addition, the surfaces are characterized by very high hardness, good abrasion resistance and excellent corrosion protection. Chemical nickel coatings can be soldered and (although nickel is a ferromagnetic metal) not ferromagnetic .
The bright, shiny nickel coating is a nickel-phosphorus alloy that is applied pore-free. The layer properties of a chemical nickel coating depend on the purity of the base material, the pretreatment and the layer thickness. Mostly processes are used in which the phosphorus content is between 8.5 and 12%.
Corrosion protection
In addition to their decorative and technical function, nickel coatings also serve to protect against corrosion . In the case of a nickel-plated steel part, however, the mechanism of corrosion protection is different from that of a galvanized steel part: Due to its electrochemical properties, the nickel cannot function as a sacrificial anode compared to the steel . H. An effective protection against corrosion is only given with a tightly closed nickel coating. Otherwise pores and open areas will quickly become pitted . In order to produce a completely closed and pore-tight coating with a single nickel layer, the coating would have to have a considerable thickness of 25 µm or more, which is associated with high costs and technical disadvantages. For these reasons, a nickel coating is rarely used as a single coating, but very often as part of a multilayer system. Examples of nickel coatings in multi-layer systems:
- Intermediate layer copper, outer layer nickel
- Intermediate layer copper, top layer nickel, additional organic topcoat
- First intermediate layer copper, second intermediate layer nickel, top layer chrome
history
- In 1845 it was discovered that metallic nickel can be deposited by chemical reduction.
- OP Watts developed a powerful electrolyte in 1916.
- Up until about 1930, car parts were covered with copper and a top layer of nickel.
- Since nickel "tarnishes" under the environmental conditions, the multilayer system Cu-Ni-Cr gradually gained acceptance.
use
As a decorative corrosion protection layer, despite the risk of nickel allergies, nickel plating is still very important today, for example in the optical industry and for furniture fittings.
For parts with body contact (costume jewelry, zippers, buttons), nickel has largely been replaced by other metals or alloys.
See also
Individual evidence
- ↑ Thomas Fritz: Characterization of galvanically deposited nickel and nickel tungsten layers for microtechnical applications , dissertation 2002
- ^ Hans Gut: Process structures of chemical nickel plating , Springer-Verlag Berlin, Heidelberg 1993; doi : 10.1007 / 978-3-642-47870-3 Section 2.1.
literature
- Friedrich Hartmann: Tinning, galvanizing, nickel-plating, steeling, lead-coating and the coating of metals with other metals in general. An illustration of practical methods for the production of all metal coatings from tin, zinc, lead . (= Chemical-technical library ; vol. 76). Hartleben, Vienna 1881. Most recently in the 9th edition, revised and enlarged by Wolfgang Friedrich Hartmann: Hartleben, Vienna and Leipzig 1931