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Ask the Expert Question-and-Answer Archive
(Hard Chrome Plating)

by Larry Zitko, ChromeTech, Inc.
March, 2002

Single Catalyst Vs. Mixed Catalyst Baths

Q. What would be the speed of the chrome plating bath with a mixed-catalyst (fluoride) compared to a single catalyst bath. Also, what is your advice for converting conventional bath to mixed-catalyst (fluoride) bath.

A. I have designed and installed more than a dozen hard chrome plating lines for companies that built or repaired hydraulic cylinders. Even though the speed and deposit properties of the mixed-catalyst baths are superior to that of the single-catalyst bath, all of my installations used the single-catalyst baths. This statement is confusing, so I'll elaborate.

Here are the typical advantages and disadvantages of the two baths:

Single-catalyst bath

Advantages:

    • Low cathodic etch. Areas of the steel parts that are in contact with the chromic acid, but are not plated, are attacked by the acid at a very slow rate.
    • Long life expectancy with antimonial-lead anodes (6-7% antimony + 93-94% lead).
    • Reduced labor due to reduced masking requirements. Parts do not have to be protected with wax or stop-off lacquer to prevent fluid contact. The operator can prevent plating by shielding selective areas of the parts from communicating with the anodes.
    • Aluminum fixtures can be used. Aluminum has a good ampacity, and holds up pretty well in the bath,even when unprotected. 

Disadvantages:

    • Unfavorable microcrack structure: low population density of deep and wide cracks. This can lead to inferior corrosion resistance (see next), especially when the cracks extend through the entire deposit thickness to the base metal.
    • Inferior corrosion resistance when the plated steel part is placed into service in aggressive environments. This is not as much a disadvantage with hydraulic cylinders that retract frequently to recoat the cracks with oil. It would be a major disadvantage on shield support cylinders for longwall coal mining operations, for example, where the hydraulic rods are extended for months or years without retraction.
    • Slower rate of deposition. I have typically observed the following rates, using conforming anodes:

            - 25.4 u/hr (1.0 mil/hr) @ 31 amps/sq.dm. (2 amp/sq.in.);

            - 33.0 u/hr (1.3 mil/hr) @ 47 amps/sq.dm. (3 amp/sq.in.); 

            - 40.6 u/hr (1.6 mil/hr) @ 62 amps/sq.dm. (4 amp/sq.in.).

        These rates are usually slower when "stick" anodes are used, about

            - 19.8 u/hr (0.78 mil/hr) @ 31 amps/sq.dm. (2 amp/sq.in.) for example.

    • Lower cathodic efficiency. Cathode current efficiency might be only about 14% at a current density of 31 amps/sq.dm.This results in higher electrical consumption, slower rate of deposition and other consequences.
    • Reduced throwing power compared to some formulations containing fluoride.

Mixed-catalyst bath

Advantages:

    • Favorable microcrack structure: high population density of shallow and narrow cracks. This can promote  superior corrosion resistance (see next), and better lubricity.
    • Superior corrosion resistance when the plated steel part is placed into service in aggressive environments.
    • Faster rate of deposition. The rates, using conforming anodes, for formulations using fluoride might be:

            - 30.5 u/hr (1.2 mil/hr) @ 31 amps/sq.dm. (2 amp/sq.in.);

            - 50.8 u/hr (2.0 mil/hr) @ 47 amps/sq.dm. (3 amp/sq.in.); and 

            - 73.7 u/hr (2.0 mil/hr) @ 62 amps/sq.dm. (4 amp/sq.in.).

These rates would only be obtainable with newer, uncontaminated baths and optimized plating parameters and equipment.

    • Higher cathodic efficiency. Cathode current efficiency might increase to 21% at a current density of 31 amps/sq.dm. This results in reduced electrical consumption, faster rate of deposition and other beneficial consequences.
    • Improved throwing power. The baths are often able to plate into lower current density areas or regions of parts.

Disadvantages:

    • High cathodic etch. Areas of the steel parts that are in contact with the chromic acid, but are either not being plated or subjected to low current density, are attacked by the acid at a fast rate.
    • Shorter life expectancy and different alloy for anodes. Tin lead anodes are needed for fluoride baths (6-7% tin + 93-94% lead), and they do not hold up as well in the aggressive chemistry as the antimonial-lead anodes do in the sulfate bath.
    • Chemical attack of plating equipment: Titanium heaters and cooling coils cannot be used, more expensive Teflon units are used instead. Hood, scrubbers, tanks liners, etc. all have a reduced life expectancy.
    • Increased labor due to masking requirements. Parts have to be protected with wax or stop-off lacquer to prevent fluid contact. This can significantly add to labor and disposal costs, and is probably the single biggest factor that dissuades companies from using the bath for certain parts.
    • Aluminum fixtures cannot be used. Unprotected aluminum will dissolve quickly in the bath, forming complexes in the bath which tend to reduce the available fluoride catalyst. 

When you consider the labor and etch penalties associated with the mixed-catalyst bath, it may not be the best choice, even with the deposit characteristics and faster plating rate. If you decide to convert your bath, you may need to change heaters, cooling coils, tank liner and anodes. The health risks and dangers associated with adding fluoride from a liquid source (example: hydrofluosilicic acid) has prompted many companies to purchase more-expensive proprietary additives, which are often in solid form. You will also have to lower the sulfate concentration in your bath, since the mixed-catalyst baths often run only 1.2 g/L (0.16 oz/gal) sulfate.

Your initial questions asked how to improve the corrosion resistance of your deposits. For a given bath, even your single-catalyst bath, the following items should help:

    • Improve the preplate smoothness of the steel parts prior to plating. The preplate condition significantly affects the ultimate corrosion resistance of the plated part in service.
    • Keep the bath impurities as low as possible.
    • Keep the bath in chemical balance.
    • Plate-polish-plate. For parts requiring heavy buildups, try grinding or polishing between two chrome plating cycles. It's an extra step, but it can significantly increase the corrosion resistance of the part. I suspect one of the explanations for this is that the microcracks from the first layer are smeared by the polishing and then the cracks in the outer layer have a different layout or positioning. With a single layer, all cracks that penetrate through the entire coating will expose corrosion sites at their base. When you overlay two different crack patterns, only a relatively few intersections will form these corrosion sites.   

 



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