Published on 04/02/2021 by Aldo Bordiga
Galvani invented the battery in 1791, which later enabled the development of electroplating: the deposition of metal on conductive surfaces. The first industrial chrome-plating electrolytes were introduced in the second half of the 19th century and refined in the early 20th century. Apart from minor variations in electrolyte formulations, the principal component of chromium-plating baths remains chromium trioxide, CrO3, known as chromic acid in solution. Chromium is present in this electrolyte in its hexavalent form, and its salts are known as 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 non-toxic trivalent Cr (Cr III).
Due to the hazardous nature of hexavalent chromium, the REACh regulation restricted its use to companies that have obtained authorization. For this reason, electrolytes using trivalent chromium, instead of hexavalent chromium, have been developed in recent years for decorative chrome plating. To date, there are no alternatives to hexavalent chromium for thick hard chrome plating.
The properties of electrolytically deposited metallic chromium are well known, and the use of chrome plating is still among the most popular methods of protecting metal objects from corrosion and wear.
Chrome plating is technically divided into two types of coatings used for different purposes:
With the chrome plating process, a functional layer of chromium is deposited directly on steel or other base metal for the purpose of imparting high surface hardness and protecting the part from wear and corrosion.
Hard chrome is widely used, especially on simple-shaped parts subject to abrasive sliding wear, such as hydraulic-cylinder piston rods and calender rollers for paper mills or printing applications, as well as many other applications where chromium’s hardness and sliding performance are unmatched.
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 significant limitation of the chrome-plating bath is its poor throwing power in low-current-density areas. The current required for electrodeposition concentrates chromium on the outer surfaces of the part. As a result, the chromium layer is much thicker at external edges but thin or even absent in internal areas of complex parts. Despite its exceptional properties, hard chrome is therefore used mainly on cylindrical or simple-shaped parts.
Chromium hardness is high, ranging from 800 to 1000 HV depending on the deposition conditions.
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, providing continuous light lubrication that is very advantageous in the case of sliding on seals.
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.
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-chrome plating, as it is a double coating, consisting of a first layer of electrolytic nickel plating that gives smoothness and brightness 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.
Electrolytic nickel-chrome has a reduced cost and a bright decorative look, unmatched aesthetically by other galvanic treatments. It may suffer from less than excellent adhesion to the base metal and therefore, also due to its non-uniform thickness, typical of all galvanic treatments, it is almost never used for functional purposes on precision mechanical parts.
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, on average about 1 micron.
The micro-cracked chromium layer does not provide good corrosion resistance for the base material, so the electrolytic nickel layer provides support; in addition to making the part attractive thanks to the coating’s brightening and leveling properties, it also protects the base metal from corrosion.
The formation of metallic nickel from a solution containing nickel salts and hypophosphite was described as early as around 1844. However, an industrial process for depositing nickel-phosphorus alloy from solutions containing sodium hypophosphite and nickel sulfate or chloride was not developed until the post-war period. The first industrially efficient process was patented in 1955 under the name Kanigen. Since then, although the basic composition of electroless nickel solutions has remained essentially unchanged, continuous refinements to bath formulations have made the process increasingly reliable and capable of delivering very high quality and consistent surface properties.
Electroless nickel plating deposits a nickel-phosphorus alloy coating on the part without using external electric current for deposition. This distinguishes it from electrolytic processes and enables uniform coating of all surfaces, including those of parts with complex geometries. Electroless nickel plating provides uniform thickness, corrosion resistance, hardness and wear resistance.
Electroless nickel can directly be deposited onto all metal alloys commonly used in mechanics (Steel, Stainless Steel, Aluminum, Copper, Brass), except Zinc alloys such as Zamak which must necessarily be copper plated before nickel plating.
The electroless nickel bath begins to deposit metal the moment the workpiece to be coated is immersed in the nickel plating solution, triggering 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, making it possible to obtain a uniform nickel-phosphorus alloy deposit whose thickness varies from one point to another of the part within ±10% of the required nominal thickness. Assuming a thickness of 20 µm, the thickness variation between one point and another of the parts placed in the electroless nickel plating bath will be ±2 µm on all surfaces reached by the nickel plating solution and where it can circulate. Only inside blind holes can the coating be poor or absent because these holes are affected by the lack of nickel plating liquid exchange, especially when they are small and deep.
Uniformity of thickness is a unique feature among various metal coatings and makes it possible to establish in advance a thickness suitable for the type of use of the mechanical part, calculating, during the part design phase, the allowance to be taken into account in order to remain within the tolerances of the final dimensions with the coated part.
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 base metal alloy and the surface finish.
Taking aluminum as an example, resistance to corrosion will depend greatly on the alloy used, the method of production, and the surface finish. 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, and machined surfaces will resist better than rough as-drawn surfaces.
More or less high coating thicknesses can be selected to withstand more or less aggressive environments. The thickness is usually between 5µm and 50µm.
The NIPLATE® 500 is the most suitable for protecting parts made of Iron alloys and Copper alloys, while for Aluminum the most suitable is the NIPLATE® eXtreme .
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.
NIPLATE 600® SiC is also available: this electroless nickel coating incorporates silicon carbide particles, reaches a hardness of 1150 HV and provides wear resistance that exceeds even that of hard chrome.
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