Aluminum Casting Nickel Plated

Nickel plating can be divided to pearl nickel plating, black nickel plating, mat nickel plating, bright nickel plating and brush nickel plating, is a very popular plating for aluminum and zinc die casting products.

Nickel is an expensive kind of metal, thus the thickness of nickel layer decides the cost of any kind of nickel plating. For common decoration purpose, a 7-8 micrometers nickel layer is enough, but for some die cast part subject to abrasion, the thickness may be increased to 17-20 micrometer. So when you ask for quotation for plating, the clarified coating thickness is the initial information we need.

Type of Electroplating in which chrome is deposited on the workpiece.

There are two types of Chrome plating depending upon the mode of application.

1) ¡°Hard Chrome Plating¡± (sometimes called ¡°Engineering Chrome Plating¡±) can be regarded as ¡°surface hardening¡±.

2) ¡°Decorative Chrome Plating¡± (sometimes called ¡®Nickel-Chrome Plating¡¯)

Hard Chrome Plating. Hard ChromePlating is an electrolytic process utilizing a chromic acid based electrolyte. The part is made the cathode and,

with the passage of a DC current via lead anodes, chromium metal builds on the component surface. Hard chrome plating has been applied as a fairly heavy coating (usually measured in thousandths of an inch) for :

¡ì - wear resistance

¡ì - lubricity

¡ì - oil retention

¡ì - high hardness (about 1000 HV)

¡ì - low coefficient of friction

Ultra Hard, 850-1000Hv (65-70Rc), harder than most industrial abrasives and steel counter faces. Unusually, it combines this hardness with a degree of toughness, so the deposit can stand up to high stress contact. It gives ultra high metal-to-metal sliding wear resistance. It produces very low friction.

Chemistry of Hard Chromium Electroplating. Hexavalent chromium (Cr6+) baths are used for hard chromium deposition. The main component of all hard chromium plating solutions is chromium trioxide (CrO3) referred also as chromic acid. The second component is a catalyst, which is either sulfate (SO42-) or fluoride.
By-product of the electroplating process in hexavalent chromium solutions is trivalent chromium (Cr3+). Ions of trivalent chromium continuously reoxidize to the hexavalent state at the anode. Normal level of the trivalent chromium is about 1-2% of the chromic acid concentration. Higher contents of trivalent chromium may cause reduction of throwing power and plating rate, pitting and treeing of the deposit. If the trivalent chromium is too high (more than 2%) reoxidation operation should be carried out at high anode area/cathode area ratio (30) at cathode current density 20 A/ft² (2 A/dm²).

Cathode current efficiency of hard chromium electroplating is low: about 10-20%. 80-90% of the electric current passing between the anode and the cathode is used for gaseous Hydrogen formation.

Conventional hard chromium plating process. In this process chromic acid is catalyzed by sulfate ions (SO42-). Chromic acid/sulfate ratio is one of the most important process parameters. It varies within the range 125 ¨C 200. Low ratio solutions are characterized by high plating rate but low throwing power. High chromic acid/sulfate ratio may cause gray or even ¡°burnt¡± deposition on high current density areas. Normally plating solutions with chromic acid/sulfate ratio 155 are used.

Bath formulation:

Chromic acid (CrO3): 20-35 oz/gal (150-263 g/l);
Sulfate (SO42-): 0.13-0.23 oz/gal (1-1.73 g/l). Source of sulfate ions is sulfuric acid.

Hard chromium plating in fluoride bath. In this process chromic acid is catalyzed by a mixture of sulfate and fluoride ions. Fluoride bath have higher than sulfate baths current efficiency. Additionally fluoride baths may operate at higher current density not causing burning and treeing. As a result plating rate in fluoride baths may be 50% higher than in conventional sulfate catalyzed baths. Fluoride ions are chemically active and may attack the unplated surfaces. In order to prevent etching of the areas, which are not to be plated, they should be masked.

Operating conditions of hard chromium electroplating process.

¡ì Temperature: 110-150¡ãF (43-66¡ãC)

High temperatures are used for deposition of crack-free chromium coating. Increase of the bath temperature causes reduction of the plating rate. Excessive bath temperature may result in formation of soft dull deposit.

¡ì Cathode current density: 200-400 A/ft² (22-43 A/dm²)

Too low current density decreases economical effectiveness of the process. Too high current density may cause ¡°burning¡±. Optimal current density is determined by the bath temperature: higher temperatures require higher current densities.

¡ì Anodes.

Insoluble lead-tin alloy (93Pb-7Sn). Preferable ratio anode area/cathode area 1-2.

Properly operating anodes are coated with dark brown lead peroxide. If an anode has a lighter color (yellow-orange) it should be cleaned. Cleaning operation is immediately followed by immersion of the anode connected to the operating power supply into the bath. This operation results in formation of conductive lead peroxide coating on the anode surface.

¡ì Power supply: 9-12 V, ripple not more than 5%.

High ripple exceeding 5% and current interruptions may cause dull or even laminated deposit.

¡ì Agitation: either mechanical or by clean air.

Agitation helps to homogenize the bath temperature.

Examples.

¡ì - hydraulic cylinder rods

¡ì - rollers

¡ì - piston rings

¡ì - mold surfaces

¡ì - thread guides

¡ì - gun bores, etc.

Covers a wide range of engineering or high alloyed steels (including stainless steels and cast irons), lightweight aluminum alloys or titanium alloys, copper alloys, brasses and bronzes, as well as nickel-based alloys.

Hard chrome plating is almost always applied to items that are made of steel, usually hardened steel. It is metallic in appearance but is not particularly reflective or decorative.

It¡¯s suitable for most organic acids and gasses (not chlorine), excellent in hot oxidizing or reducing air, in commodities like beer, sugar, brine, coal gas, cyanides, fruit acids, molten glass, glue, milk, oils and fuels.

Decorative Chrome Plating.

Decorative chrome plating is sometimes called nickel-chrome plating because it always involves electroplating nickel onto the object before plating the chrome (it sometimes also involves electroplating copper onto the object before the nickel, too). The nickel plating provides the smoothness, much of the corrosion resistance, and most of the reflectivity. The chrome plating is exceptionally thin, measured in millionths of an inch rather than in thousandths.

When you look at a decorative chrome plated surface, such as a chrome plated wheel or truck bumper, most of what you are seeing is actually the effects of the nickel plating. The chrome adds a bluish cast (compared to the somewhat yellowish cast of nickel), protects the nickel against tarnish, minimizes scratching, and symbiotically contributes to corrosion resistance.

CHROMIUM COATINGS. (E¡ã = -0.74V)

As mentioned earlier, a thin overlay of chromium is applied on nickel coating to prevent the corrosion of nickel coatings. Chromium is generally applied in the form of electrodeposit. As chromium coating is resistant to corrosion and has a bright luster, it is used as a decorating coating. Chromium is more active than iron (E¡ã = ¡ª0.76 V) and has a tendency to become passive. The electrolyte for chromium plating has the following nominal composition:

¡ì - Chromic acid, 300 g/1

¡ì - Sulfuric acid, 2.6 g/1

¡ì - Sodium fluorides, 0.4 g/1

¡ì - Sodium fluosilicate, 0.2 g/1

¡ì - Temperature, 90¡ã C

¡ì - Current density, 30 A/dm2

The chromium deposits are quite porous, hence, corrodant can find easy entry. If the layer is more than 0.003 mm thick, microcracks appear in the layer. In modern practice, a microcracked layer (more than 0.003 mm) is laid over a microcrack free layer, which is laid on a nickel coating.

Corrosion Resistance. Microcracked chromium coatings are resistant to corrosion in the atmosphere. The coatings are not suitable for use in strongly acidic environments.The chromium coatings may not be impressive on their own merit but their contribution in increasing the life of nickel, copper and other coatings is significant.

Applications. Chromium coatings are mainly used in automotive applications. The other important application is in food industry. Tin coating of food cans have been replaced in several instances by chromium coatings. The steel is coated with chromium (0.008-0.01 xm) thin and an organic top coat is laid over the chromium coating. This practice increases the resistance of the outside of the containers to corrosion and increased adhesion to steel.

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Nickel plating is widely used for a corrosion- and wear-resistant finish. Typical applications, with a thin top coat of electrodeposited chromium, are decorative trim for automotive and consumer products and office furniture. Nickel deposits are also used for non-decorative purposes for improved wear resistance, for example, on pistons, cylinder walls, ball studs,and so forth.