Chromium Electrodeposits

Decorative chromium plating baths generally produce deposits that range from 0.13 to 1.25 pm (5 to 50 pin.) in thickness. These deposits generally reproduce the finish of the substrate, or, in a multilayer system, the undercoating that is applied prior to the chromium layer. Optimum luster of the final chromium deposit is obtained by plating the substrate coating to a uniformly bright condition. If the substrate is nonuniform, grainy, hazy, or dull, then it should be polished and buffed to a uniformly high luster before being plated with chromium. When a final chromium coating over a uniformly bright substrate is hazy in certain areas, these areas can be buffed on a wheel or the coating can be stripped and the substrate replated. Buffing of chromium is not allowed when corrosive service conditions will be encountered.

In addition to being lustrous, the final chromium deposit should cover all significant areas. When there is not adequate coverage, because of an improperly operated chromium bath, the chromium should be stripped, the substrate reactivated, if necessary, and the part replated.

Decorative chromium that has been applied over nickel, the typical undercoating, is readily stripped by immersion in a 1:1 solution of hydrochloric acid. An alternate method involves treating the part anodically in an alkaline cleaning solution. However, this method requires reactivation of the nickel surface prior to replating, which is typically accomplished by immersion in dilute sulfuric or hydrochloric acid. Cathodic, but never anodic, alkaline cleaning can also be used for activation.

Excessively high current densities, improper temperatures, and passivated substrates can produce hazy, nonuniform chromium deposits. Operating conditions for chromium plating should be in the specified ranges. Properly operated nickel baths and other similar precautions also are necessary to ensure uniformly lustrous chromium deposits.

The adhesion of chromium to an active or properly prepared substrate is usually not a problem. However, if chromium is plated on an undercoating that has been improperly applied and has questionable adhesion, then blistering or exfoliation can occur, either immediately after chromium plating or during storage or service. Organizations that generate standards, such as ASTM, can provide procedures for checking adhesion if a related method has not been specified in the purchase agreement for the part being plated.

Microporosity and Microcracking. The key to the corrosion durability offered by decorative chromium deposits lies in controlling the type, size, and distribution of microdiscontinuities that form in the deposit. These can occur as either pores or cracks. In an outdoor corrosive environment, as well as in accelerated corrosion tests, corrosion has been observed to proceed by galvanic cell action between the nickel and the chromium, with the nickel acting anodically. Microcracks or micropores in the chromium expose the underlying nickel through a uniform, diffuse network of discontinuities. Because the rate of corrosion penetration through the nickel layer is a function of the anodic current density of the corrosion cell, the reduction of current density that is obtained by the increase in exposed nickel area prolongs the time required to penetrate a given thickness of nickel. The advantage of such a system lies in its ability to provide long-term corrosion protection without developing easily visible fine surface pits in the nickel, which eventually become corrosion sites. The use of microdiscontinuous chromium makes the surface pits much smaller, which means that the substrate will be protected from corrosion for a longer time. However, after excessive corrosion, these fine pits will become visible as a haze on the corroded surface.

Chromium deposits, up to a thickness of 0.13 pm (5 pin.), that are obtained from hexavalent processes are somewhat porous. Because porosity decreases with increasing thickness, at approximately 0.5 pm (20 pin.), the deposits become nearly pore-free when plated (Fig. 1). However, because of the hard, brittle nature of the highly stressed chromium deposits, they quickly become cracked during storage or service. These cracks do not improve the corrosion resistance, as do deposits with intentionally developed micropores or microcracks.

Thickness of plate, pin.

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Fig. 1 Porosity in chromium plate as a function of plate thickness. Chromium deposited in low-temperature baths begins to crack at 75 pm (3000 pin.).

Except when special hexavalent chromium processes and conditions are used, hexavalent chromium deposits that are more than 0.5 pm (20 pin.) thick will have visible nondecorative microcracks. In contrast, chromium deposits from trivalent processes are microporous up to thicknesses of 0.5 to 0.6 pm (20 to 25 pin.), above which they become microcracked. Both features enhance the corrosion resistance of the part.

When hexavalent chromium is deposited from solutions operated below 50 °C (120 °F), the deposit will begin to craze when it exceeds 0.5 pm (20 pin.) in thickness, and a macrocrack pattern visible to the unaided eye will appear. This pattern generally has 5 to 10 cracks/cm (12.5 to 25 cracks/in.).

Either microcracked or microporous chromium deposits can be produced by altering the nature of the nickel undercoating. Microporous chromium can be obtained by plating over a thin layer of nickel deposited from a solution containing very fine, nonconductive particles. Chromium will not plate over these particles, which creates a microporous deposit with pore densities proportional to the amount of inert particles and additives in the nickel solution. An average pore density of

10,000 pores/cm2 (60,000 pores/in2) is the typical minimum specified for enhanced corrosion resistance. A disadvantage of this process is the addition of an extra nickel plating tank between the bright nickel and the chromium tanks. The pore count is also current-density dependent, and chromium deposits of 0.3 to 0.4 pm (12 to 16 pin.) in thickness have a tendency to bridge over the inert particles and reduce the pore count.

Trivalent chromium deposits provide pore counts of more than 16,000 pores/cm2 (100,000 pores/in2), without any special procedures. The pore count also is rather current-density independent.

Another very common method for obtaining microporous chromium deposits is to mildly blast the chromium deposit with an abrasive, such as sand or aluminum oxide. The brittle chromium fractures where it is hit by the particles, thus causing a microporous deposit to form. This method permits the pore count to be varied, based on the amount of particles used, and is independent of current density.

Microporous chromium is the most common microdiscontinuous chromium deposit used in North America to enhance the corrosion resistance of the decorative nickel-chromium type of electrodeposit. Microcracked chromium is somewhat more popular in the rest of the world.

Microcracking can be produced by using a thin layer, approximately 1.25 pm (50 pin.), of a highly stressed nickel deposit between the bright nickel and chromium deposits. Approximately 0.25 pm (10 pin.) of chromium is typically used with this procedure. A crack density of 275 to 790 cracks/cm (700 to 2000 cracks/in.) is typically produced. Thicker chromium deposits are required with other microcracking methods.

Microcracked chromium deposits can be obtained from systems using either a single or a dual specially formulated chromium solution. An example of the latter is duplex chromium. Although single-deposit systems are easier to operate, conditions that favor the formation of microcracks, such as high solution temperature, low chromic-acid concentration, and high fluoride content, usually have an adverse effect on the covering power of the chromium deposit. Duplex chromium systems have resolved this problem by using two successive chromium baths. The first obtains coverage and the second creates the microcrack pattern.

Satisfactory coatings are not too difficult to obtain on parts with relatively simple shapes, but complex parts can present a serious problem because it is difficult to obtain adequately thick chromium in areas of low current density. Auxiliary anodes can be used to increase the thickness in these areas.

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