HISTORICAL BACKGROUND AND INTRODUCTION TO CHROME PLATING
The first electrolytes for industrial-level Chrome plating were introduced in the second half of the 19th century and improved in the early 20th century. The main component of the chromium plating bath is the Chromium trioxide CrO3 known as chromic acid when in solution. Chromium in this electrolyte is in hexavalent form and its salts are called Chromates.
Due to the high toxicity of Chromium in its hexavalent form (Cr VI) severe limitations are currently placed on its use. Metallic chromium, on the other hand, even when deposited from hexavalent chromium baths, has no limitations on its use since it is inert, and its dissolution in aggressive environments is also not problematic since it transforms from zerovalent chromium (Cr°) to nontoxic Cr trivalent (Cr III).
Due to the hazardous nature of hexavalent chromium, the REACh regulation restricted its use to companies that have obtained authorisation. For this reason, electrolytes using trivalent chromium, instead of hexavalent chromium, have been developed in recent years for decorative chromium plating. To date, there are no alternatives to hexavalent chromium for hard chrome plating.
The properties of electrolytically deposited metallic chromium are well known, and the use of chromium plating is still among the most popular methods of protecting metal objects from corrosion and wear.
Chrome plating is technically identified into two types of coatings used for different purposes:
- Hard chrome plating
- Decorative chrome plating
HARD CHROME PLATING
With the chromium plating process, a functional layer of Chrome is deposited directly on steel or other metal for the purpose of imparting high surface hardness and protecting the part from wear and corrosion.
Hard Chrome coating is very popular, especially on parts of simple geometry subjected to abrasive sliding wear such as hydraulic cylinders, rollers for paper mills, and many other applications where the hardness and sliding ability of Chrome are unbeatable.
Hard chrome plating is deposited in thicknesses ranging from a few tens to hundreds of microns depending on the severity of use.
In applications with high sliding loads or heavy wear, a layer with a high thickness is deposited and subsequent grinding is carried out to allow it to fit within the correct roughness and dimensions, smoothing out irregularities in the coating.
A fairly important limitation of the chrome plating bath is its poor penetration into areas of low current density. In other words, the current required for galvanic deposition tends to deposit the Chrome on the outer surfaces of the part to be coated. As a result, the Chromium layer results much thicker on the outer edges while it is low or absent on the inner parts of a complex mechanical part. For this reason, despite its exceptional characteristics, it is used mainly on cylindrical parts or those with simple shapes.
Hardness and wear resistance
Chrome hardness is high and ranges from 800 to 1000HV depending on the deposition mode.
Hard chrome plating is the main coating when very good sliding ability combined with maximum wear resistance is needed in very severe wear situations. The Chromium layer deposited at maximum hardness is micro-cracked with diffuse cracking and this characteristic, while reducing corrosion resistance, allows oily substances to settle in the cracks, operating a continuous light lubrication that is very advantageous in the case of sliding on seals.
Resistance to corrosion
Corrosion resistance is quite good although it is not excellent when not supported by an underlying coating, and this is due to micro-cracking of the layer that allows corrosion of the base material after a few hours of exposure to salt spray.
DECORATIVE CHROME PLATING
This is the classic polished chrome plating of faucets and parts used for decorative purposes, because of the shiny, attractive appearance that is achieved on the part after treatment.
It is also called Nickel-Chromium plating, as it is a double coating, consisting of a first layer of electrolytic nickel plating that gives smoothness and shine and a subsequent layer of chromium, which gives a consistent blue-white color over time and resistance to cleaning abrasion due to its hardness, thus allowing the shine of the part to be maintained over time.
The thickness of decorative chrome plating is usually about 10-15µm. Nickel plating has a thickness of about 10µm, and chrome plating is deposited at very low thicknesses, about 1-3 microns.
Resistance to corrosion
The micro-cracked chromium layer doesn’t have good corrosion resistance, so the electrolytic nickel layer comes to the rescue, which not only makes the part attractive due to its shining and leveling properties but also fulfills the role of protecting the base metal from corrosion.
HISTORICAL BACKGROUND AND INTRODUCTION TO ELECTROLESS NICKEL PLATING
In the second half of the 20th century were developed an industrial process for the deposition of Nickel-phosphorus alloy from solutions containing Sodium Hypophosphite and Nickel Sulfate or Chloride. The first industrially efficient process was patented in 1955 under the name Kanigen. The continuous modifications to the formulation of electroless nickel plating baths have made the process increasingly reliable, reaching to date the highest levels of quality with excellent surface characteristics.
PROPERTIES OF ELECTROLESS NICKEL PLATING
Electroless nickel plating is a process that deposits a coating of nickel-phosphorus alloy, without the use of electricity. It differs from electrolytic processes because of the absence of an external energy source, allowing all surfaces and complex geometry to be uniformly coated. The main surface characteristics of electroless nickel plating are: uniform thickness, corrosion resistance, high hardness and wear resistance.
Electroless Nickel can directly be deposited onto all metal alloys commonly used (Steel, Stainless Steel, Aluminum, Copper, Brass), except Zinc alloys such as Zamak which must necessarily be copper plated before nickel plating.
Uniformity of thickness
The electroless Nickel bath begins to deposit metal the moment the workpiece to be coated is immersed in the nickel plating solution, by a chemical reaction between the hypophosphite anion and the Nickel cation. Its deposition occurs regularly on all surfaces of the immersed workpiece, with a constant deposition rate. The tolerances of the thickness are within ±10% of the required nominal thickness, with a minimum of ±2µm. Assuming a thickness of 20 µm, the thickness variation between one point and another of the parts will be ±2 µm on all surfaces reached by the nickel plating solution.
Inside blind holes, the coating can be poor or absent due to lack of liquid replacement, especially when they are small and deep.
Uniformity of thickness is a unique feature among various coatings and allows to maintain tight tolerances without subsequent surface rework.
Resistance to corrosion
Electroless nickel plating has the enormous advantage of uniformly protecting all the surfaces of the pieces. The degree of protection given by the coating is slightly different among the various types of electroless nickel and is however superior to electrolytic nickel plating and chromium plating for the same thickness. The degree of protection depends greatly on the metal alloy, machining and roughness.
Taking aluminum as an example, resistance to corrosion will depend greatly on the alloy used, the method of production, and the surface machining. Parts machined from solid will certainly have better corrosion resistance than die-cast parts with rough surfaces. Cast iron will resist less than steel because of its porosity.
The thickness of the electroless nickel plating is usually between 5µm and 50µm.
Hardness and wear resistance
Ni-P alloys, depending on the type of electroless Nickel deposited, have hardnesses ranging from about 500 HV to 700 HV with excellent wear resistance proportional to hardness. They can be further hardened by heat treatment at temperatures above 250°C, up to 400°C, which changes the structure of the Ni-P alloy metal coating, creating crystalline aggregates of Ni3P (Nickel Phosphide), which increase the hardness of the layer to over 1000 HV, also greatly increasing wear resistance.
The hardness similar to chrome plating, together with the uniformity of thickness, makes electroless nickel preferred in many applications, as it avoids subsequent grinding and related costs.
For wear resistance needs, the most suitable electroless nickel is NIPLATE 600®, which has a hardness of about 700 HV and can be hardened to 1000-1050 HV. For many applications the hardness of about 700HV meets wear resistance requirements and avoids reaching high temperatures that for some materials, such as aluminum alloy 7000, can be deleterious.
There is also an electroless nickel plating treatment NIPLATE 600® SiC with embedded Silicon Carbide particles, which achieves hardnesses of 1100 HV, with superior wear resistance even to hard Chrome.
- High hardness, varying between 800 and 1000HV depending on the deposition process.
- High deposition thicknesses over 100 µm for heavy abrasive wear applications.
- Higher wear resistance than the hardened electroless Nickel (although lower than that of the co-deposit of Nickel + Silicon Carbide NIPLATE®600 SiC)
- Cheap on big parts.
- Poor penetration of the deposit into holes or complex geometry with the need to use special anodes to overcome this limitation.
- Need grinding rework due to layer unevenness on high thicknesses.
- Fair corrosion resistance, not excellent.
- Limitations on the use of hexavalent chromium in industrial processes
DECORATIVE CHROME PLATING
- Cheap treatment.
- Bright and shiny appearance for decorative uses.
- Thicknesses poorly controllable and limited to a few microns.
- Not suitable for use in mechanical fields.
- Poor corrosion resistance for complex-shaped parts.
ELECTROLESS NICKEL PLATING
- Uniformity of thickness with calibrated thicknesses and tight tolerances.
- Excellent resistance to corrosion.
- High hardness and wear resistance.
- Cost of treatment non-competitive for decorative purposes.
- Difficulty in depositing thicknesses over 100 µm.
- Need heat treatment to achieve maximum hardness.