PROPERTIES OF ITS COATING

COMPOSITION

- Nickel 90 – 92%
- Phosphorus 8 – 10%
- Carbon 0.04%
- Oxygen 0.0023%
- Nitrogen 0.0047%
- Hydrogen 0.0016%

The Electroless Nickel plating process has been improved significantly since its conception and can now provide high performance coatings that meet the needs of each workpiece.

There are two main types of coating that vary in terms of their phosphorus content:

1) So-called Classical Nickel, which contains 6 – 8% phosphorus and presents average to good corrosion resistance with high wear and abrasion resistance.

2) Technical Nickel, which contains 9 – 13% phosphorus. These two types are the most commonly used due to their high corrosion, wear and abrasion resistance. Both the density and magnetic susceptibility are reduced. These specifications can be improved by applying post-treatments.

PHYSICAL PROPERTIES

Property Value
Melting point 890 ºC  (pure Ni = 1,455 ºC)
Specific weight 7.92 g/cm3 (pure Ni = 8.9 g/cm3)
Electrical resistivity 60 micro-ohms/cm2/cm at 15 ºC
Thermal conductivity Weakly ferromagnetic (4%), coatings with a phosphorus content of more than 8% are non-magnetic but become magnetic after heating to 200 ºC for 6 hours.
Elastic modulus 2.00 - 0.10 x 104 kg/mm2
Ductility The maximum elongation of a coating containing 7 - 10% phosphorus with a thickness of 25 microns is 2.2% without cracking, although this can reach 6% after heat treatment.

HARDNESS

The hardness of nickel upon removal from the bath is 480 – 550 Vickers = 49 Rockwell C = 470 Brinell HB. This hardness can be increased to 900 - 1100 Vickers = 67 - 69 Rockwell C for coatings containing more than 10% phosphorus by heating to 290 ºC for 10 hours.

A maximum temperature of 200 ºC is recommended for nickel-plated workpieces that work under movement, friction, etc.

This temperature is 300 – 350 ºC for static workpieces.

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The hardness of Ni-P in its deposition state is relatively high compared with normal construction materials.

In any case, this hardness can be increased with heat treatment.

The hardness can only be measured using micro-hardness meters with loads of no more than 100 g.

BINDING

Electroless nickel coatings bind perfectly to substrates (base material) provided the surfaces have been correctly prepared beforehand.

Heat treatment often improves the binding. The temperature and duration of the treatment depend on the desired results.

The following table provides values for these parameters for guidance purposes.

Desired effect Temp (ºC) Duration (Horas)
Improved binding to light alloys 150 1
Improved binding to alloyed steels 200 1
Dehydrogenated 200 4
Maximum hardness 290 10
Improved binding to titanium alloys 400 1
Maximum wear and corrosion resistance 650 2

Annealing above 300 ºC must be performed under vacuum.

Minor stains that may appear on workpieces after heat treatment CANNOT BE CONSIDERED A REASON FOR REJECTION.

This resistance reaches a maximum after heating to 650 ºC, although this only corresponds to a hardness of 600 Vickers.

POROSITY

Due to its amorphous structure, an Electroless Nickel coating is practically pore-free, meaning that a layer of only 10 microns on a perfectly polished steel test piece is sufficient to ensure no porosity. The relationship between the absence of pores and the layer thickness depends mainly on the type of bath used.

ABRASION RESISTANCE

The high hardness of the coating generally results in good wear resistance, especially if the workpiece is lubricated and the surface temperature of the contact zones is no more than 200 ºC.

Its behaviour with regard to wear is intrinsically linked to the phosphorus content of the deposited alloy.

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Coating type Taber index
Freshly deposited electroless nickel 9,6
Electroless nickel treated at 200 ºC 8,7
Electroless nickel treated at 400 ºC 3,2
Electrolytically chrome plated 2,0
Electroless nickel treated at 600 ºC 1,3

Wear resistance is determined using a TABER-ABRASER apparatus (CS 10 abrasion cylinder, load 9.8 N, 1000 revolutions).

The specific wear must be no greater than 30 mg/1000 revolutions.

Necessary layer thickness, according to the required wear resistance:

- Mild wear resistance > 10 microns
- Moderate wear resistance > 25 microns
- High wear resistance > 50 microns

FRICTION COEFFICIENT

In contrast to nickel, which has a poor coefficient of friction, nickel-phosphorus alloys have interesting lubrication properties.

The following table provides friction coefficients for electrolessly plated layers in contact with some of the materials used (nickel, steel and chromium).

Materials in contact Dry friction Lubricated friction 
Ni-phosphorus/Ni Wear 0,26
Ni-phosphorus/ Ni-phosphorus 0,45 0,25
Ni-phosphorus/Chromium 0,43 0,3
Ni-phosphorus/Steel 0,38 0,21
Ni-phosphorus/Cast Iron 0,16 0,08
Nickel/Nickel Wear Wear
Nickel/Chromium Wear 0,2
Nickel/Steel Wear 0,2
Steel/Steel Wear 0,2
Chromium/Chromium 0,43 0,2
Chromium/Steel 0,21 0,13

Friction between two stainless steel parts often leads to seizure, although this can be prevented by applying an Electroless Nickel coating to one of the parts.

Parts of prolonged service should be lubricated.

WELDABILITY

Nickel plated workpieces are easily welded with silver or lead/tin alloys. A 5 – 10 micron thick layer is sufficient for welding aluminium or stainless steels to each other.

Arc or torch welding is not recommended once the workpieces have been nickel plated as the phosphorus in the coating diffuses into the weld, weakening it. In this case welding should be performed prior to nickel plating.

LAYER UNIFORMITY AND HOMOGENEITY

Provided the plating solution can circulate freely and reach the entire surface of the workpiece, layer deposition will be perfectly uniform irrespective of its shape. In the most unfavourable cases, variations in thickness of ±10% may occur. However, in the majority of cases, if the workpiece is rectified, further rectification after nickel plating will not be necessary.

These properties should be taken into account for those workpieces which cannot be rectified due to their complexity.

CORROSION RESISTANCE/LAYER THICKNESS

Numerous tests have shown the superiority of Electroless Nickel plating with respect to traditional electrolytic plating in terms of both atmospheric and chemical corrosion. Indeed, in many cases, this behaviour is superior to that of laminated nickel as it can be considered to be more "noble" than pure nickel.

Similarly, as porosity is practically non-existent, it provides efficient protection to the underlying metal, thereby explaining the results obtained.

Depending on the conditions to which the coatings will be exposed, and the surface configuration of the workpieces, four layer thicknesses for the following four types of

corrosion resistance can be recommended:

- I Very low corrosion resistance > 2u
- II Low corrosion resistance > 10u
- III Moderate corrosion resistance > 25u
- IV High corrosion resistance >50u

(DIN 50 966/RAL660)

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Depending on the degree of corrosion resistance required, the Ni-phosphorus layers deposited autocatalytically should have the following values:

Degree of resistance DIN 50021 DIN 50018
(Esp.) S.S ESS 0.2 S 2,0 S
I 5u 12h - - -
II 10u 192h 96h 1R -
III 25u 480h 240h - 2R
IV 50u 960h 480h - 4R

As can be seen from the preceding tables, a higher phosphorus concentration in the alloy leads to greater corrosion resistance, although heat treatment of the Electroless Nickel plating reduces these values markedly.

Generally speaking, it can be stated that Electroless Nickel exhibits excellent resistance to bases and good resistance to non-oxidising neutral, organic and inorganic compounds, as well as against the majority of dilute organic and inorganic acids - as long as the pH is greater than 5.

 

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