# Electroless nickel

Electroless nickel is a chemical coating . It is usually deposited on metallic materials as protection against wear or corrosion . This creates chemical nickel layers .

## Difference to electroplated nickel

The difference to galvanic nickel is, among other things, that no external electrical current , for example from a rectifier, is used for the deposition, but the electrons necessary for the deposition (reduction) of the nickel ions are generated in the bath itself by means of a chemical oxidation reaction. In this way, with chemical nickel plating, coatings that are true to the contours are obtained, the dimensions of which can range from 8 µm to 80 µm with a tolerance of ± 2 µm to ± 3 µm. However, from 50 µm onwards, stresses in the layer must be expected.

## Non-conductive body

Due to the electroless deposition, it is possible to also use electrically non-conductive bodies, e.g. B. from plastics such as polyamide to coat. On leichtesten can be ABS ( acrylonitrile butadiene styrene ) good adhesion coat: the ABS After etching with a chromic acid - pickling , the nickel is deposited by a seeding with a noble metal ( palladium ) in the fine holes on, the chromic acid by dissolving of butadiene forms. The coating interlocks in the plastic. Germination is necessary in the case of a non-conductive base body, as the electrolyte begins to deposit (almost) only on bare metal surfaces. Otherwise the electrolyte would self-decompose.

## Layer properties

It is in this coating is a nickel - phosphorous - alloy which is mainly used in functional areas. The layer properties can be controlled via the phosphorus deposited in the layer. A distinction is made between a high (10–14%), medium (9–12%) and low (3–7%) phosphorus content.

The corrosion protection of the layer is mainly based on a high phosphorus content and the deposition of a pore-free layer, which is also always dependent on the base material and its processing. For example: polishing , grinding , turning , milling . The pre-processing of the material in turn influences the adhesive strength of the coating. The layer thicknesses are generally at least 30–50 µm, depending on the base material and its processing.

The deposition hardness increases with decreasing phosphorus content and can be increased to values ​​of 800 to 1100 HV 0.1 by heat treatment of the layer at a maximum of 400 ° C and a holding time of one hour  . The layer thicknesses are between 10 µm and 50 µm depending on the application.

The adhesive strength of the layer essentially depends on the base material and the pretreatment of the material. The adhesive strength can also be improved by heat treatment, for this purpose lower temperatures are used with somewhat longer holding times.

The appearance of the layer depends on the pre-processing of the base material on which the layer is deposited: blasted surfaces remain matt, polished surfaces remain shiny. Unlike galvanic coatings, the appearance of the layer cannot be optically adjusted using additives in the electrolyte (e.g. brighteners). In addition: In this regard, possibilities have been developed to achieve specific layer properties with selected components, e.g. B. optics and grain boundary density can be influenced.

Due to the high cost of this coating, layers higher than 50 µm are seldom deposited. The deposition of 10 µm chemical nickel takes about 1 hour.

## Chemical reaction

As a reducing agent for the deposition of nickel-phosphorus layers, hypophosphite (H 2 PO 2 - ) is usually used. As nickel electrolyte as nickel sulfate can be used.

First the RedOx reaction for the deposition of nickel is shown:

${\ displaystyle reduction: \ mathrm {Ni ^ {2 +} \ + \ 2 \ e ^ {-} \ longrightarrow \ Ni}}$
${\ displaystyle Oxidation: \ mathrm {[H_ {2} PO_ {2}] ^ {-} + H_ {2} O \ longrightarrow \ [H_ {2} PO_ {3}] ^ {-} + 2H ^ {+ } + 2e ^ {-}}}$
${\ displaystyle total: \ mathrm {[H_ {2} PO_ {2}] ^ {-} + H_ {2} O + Ni ^ {2 +} \ longrightarrow \ Ni + [H_ {2} PO_ {3}] ^ {-} + 2H ^ {+}}}$

The reaction equations show why the pH value has an influence on the reaction rate. The more acidic the electrolyte, the slower the separation takes place. At the same time, however, one must not get into the basic range, since poorly soluble nickel hydroxide would precipitate there.

In addition to the reactions shown, there is also a disproportionation of the hypophosphite, which is the reason for the high phosphorus content in the layer.

${\ displaystyle \ mathrm {3 [H_ {2} PO_ {2}] ^ {-} \ longrightarrow \ 2P + [H_ {2} PO_ {3}] ^ {-} + 4H ^ {+}}}$

In contrast to the deposition of nickel, a more acidic electrolyte leads to a faster phosphorus deposition and thus to an increased phosphorus content. This is one reason why the phosphorus content in a layer can vary by a few µm if the pH value is not monitored and adjusted throughout the deposition.

## Systems for surface coating

In order to reliably prevent the system parts such as tanks, pumps, etc. from being coated, these system parts are often made of stainless steel and artificially passivated and subjected to a low positive voltage (bath protection) so that any deposited nickel is immediately released again. A passivated stainless steel cathode is usually used as the counter electrode, on which nickel is then deposited as in a purely galvanic process. In the event of an incorrect approach, foreign matter being introduced or the destruction of the passivation layer of the system, for example due to mechanical damage, the electrolyte can nevertheless spontaneously decompose with heavy metal deposition.

The systems and baths in which "chemical nickel" is used for coating are more complex than with galvanic processes. Since the nickel ions are contained in the bath, the electrolyte "bleeds" out. The stabilizers, the temperature and the pH value in the bath must be kept constant within certain tolerances; this is done using replenishing pumps and regular checks of the values. After about 1–2 weeks the nickel in the bath is used up and a new bath has to be prepared. The bath age is expressed in so-called MTOs ( M etall T urn O ver). The costly bath management and the deposition speed lead to much higher costs compared to galvanic layers.