DC Power Supplies for Hard Chrome Plating

Hard chrome plating is an application of electrochemistry where electricity and chemicals work together to produce a chromium deposit.

Eric Svenson Sr.Electroplating requires a suitable direct current (DC) source because alternating current (AC) won’t work. We often focus on the chemical aspect of science while overlooking the electrical component. DC electricity is 50% of the process; it must be taken more seriously if the desired results are expected.

DC Power Evolution

The first electroplating used batteries as the power source. These were only used for thin deposits on small surfaces because the battery quickly became drained of its power. Commercial electroplating didn’t see a boost until 1872 when the first motor generator (MG set or dynamo) was developed. These MG sets provided continuous power but at a fixed voltage and current. They were bulky and required significant maintenance. While workable, they were inefficient because they needed rheostats to control the current, which wasted some of the power through resistance. MG sets were used continuously until rectifiers became readily available.

The first rectifiers, developed in the late 1920s, didn’t become popular in electroplating until after 1950. The early rectifiers used copper oxide, germanium, or selenium as semiconductors. The next development was the silicon diode rectifier (SCR-silicon-controlled rectifier), which became widely used around 1970. By the year 2000, virtually all rectifiers used in electroplating were SCRs.

Rectifier Requirements

The rectifier used for hard chrome must have sufficient amperage and voltage for the plated work and a suitable three-phase waveform free of voltage spikes. These rectifiers should have the following features:

Have a 3-phase AC input.
Use the full-wave design.
Provide less than a 5% ripple at any voltage (<1% is ideal).
Be free of voltage spikes (positive or negative).
Provide constant current or voltage.
Have user-friendly controls.
Have adequate cooling.
Provide automatic safety shutdowns.
Include fusing for the AC mains and all internal circuits.

These can be either air-cooled or water-cooled, with air-cooled being the most popular and safer, in the author's opinion. The safety shut-downs should include overtemperature, direct short, and electrical overload protection.

They should also include digital ampere and voltage meters, constant voltage and amperage controls, and a non-resettable ampere-hour meter. Other features that can be provided are automatic current density and polarity changes that eliminate the need for reversing switches or bussing adjustments.

The Newer Technology

The cutting-edge DC power development is the Switch Mode Rectifier, which became available to electroplaters in the late 1990s. These are highly developed today and provide the ultimate reliable DC power that’s infinitely controllable for specific applications.

Switch Mode Rectifier Advantages

Fail Safe: The switch mode rectifier runs even if a single module fails. This saves that batch and keeps production running.
Greater Uptime: These rectifiers offer the highest reliability for continual service, so downtime is minimized.
Maintenance: Your staff can easily and immediately replace individual modules whenever needed. While rare, this feature saves considerable time and cost. This alone makes these units extremely user-friendly.
Low Ripple: Guaranteed low ripple at any output provides better throwing power and higher-quality finishes.
Controls: Digital and programmable controls (on the unit or remote) provide pinpoint accuracy for the ultimate in-process control.
Upgradeable: Should your needs change, you don’t need to invest in a new rectifier. These rectifiers can easily be expanded for extra capacity by adding additional modules.
Floor Space: Typically has a 75% smaller footprint than SCR units.
Energy Savings: It operates at around 90% efficiency at any output, while many older SCR units operate below 20% efficiency. This can equate to significantly lower electrical bills.
Fast Service: If needed, 24/7 same-day shipment of stocked parts.
Cost: Lower cost than a comparable SCR unit.

What’s not to like? That’s a pretty impressive list of reasons for upgrading to a switch mode rectifier.

The newer switch mode rectifiers operate much more efficiently, converting AC into DC than the SCR units. The better models provide nearly a 90% conversion efficiency. This equates to lower monthly AC electrical costs, with some users reporting a 34% reduction in their electrical bills. They offer higher efficiency in a smaller size with less heat generation.

Switch mode rectifiers also have lower maintenance requirements, have longer uptimes, and require less floor space. They are built-in modules, with each providing 500-1,000 amperes. These rectifiers can be built up to 24,000 amperes at any practical voltage. If, for whatever reason, a module needs servicing, the rectifier continues to provide DC power but at slightly less amperage. This means the plating process will continue uninterrupted, saving the tank load and lessening rework. Your personnel can then replace that module, lessening the need for expensive service calls.

The Problem with Ripple

The chrome plating process doesn’t tolerate any form of current interruption. Ripple is the presence of AC that wasn’t converted to DC, essentially leaking past the rectification. The negative portion of the AC wave causes the current interruption (on a millisecond basis) that results in all sorts of problems. These defects include poor adhesion, dark deposits, macrocracking, dog boning, and reduced throwing power. The higher the ripple, the worse these problems become. The lack of throwing power is a major problem when plating parts with deep recesses. Throwing power is increased considerably when the ripple is around 1% or less.

Switch mode rectifiers provide improved throwing power and reduce the dog-boning effect, resulting in more uniform chrome deposits that require fewer after-plate finishing operations, primarily grinding. On average, the low current areas are reported to be 15% thicker, and the after-plate grinding is reduced by almost 13%. The lower the ripple, the better the chrome deposit will be.

One defect of SCR-type rectifiers is they produce extremely high ripple, sometimes around 50%, and require a choke (filter) to reduce the ripple to acceptable levels. However, the choke only lowers it to 5% at the rectifier’s full voltage output. For example, even though it's filtered, a 12-volt SCR might have a 25% ripple when operated at 7 volts. The ripple must be below 5% at the voltage being used.

The best switch mode rectifiers are guaranteed to have less than 2% ripple at any voltage output. Their ripple is always less than 1%, typically below 0.2%. This is a significant advantage that leads to improved chromium deposits.

The Control System

The older technologies used tap switches, variable powerstats and potentiometers to control the rectifier’s output. The newer switch mode rectifiers use digital controls that can be mounted on the unit or operated remotely. The rectifier can provide up to four (4) different programs, each with up to eight (8) steps (separate operations with time controls). These features are particularly beneficial for hard chrome operations and can include, among others.

Reversing.
Ramp up.
Step up.
Surging.
Pausing.
Pulsing.
Dual output option (for two separate operations).

These controls are simple to program and are user-friendly. Ideally, the controller should be installed close to the plating tank so that it can be easily operated. The control box is protected from moisture and chemical contact.

The operator can choose between the various programs depending on the type of work being plated. This is ideal for shops where the type of parts plated varies daily.

Rectifier Location

Some platers prefer locating the rectifier(s) as close to the tank as possible to save on bussing costs and voltage drop. This may be practical if it can be kept free of moisture and chemicals, including airborne mists from the plating tank. Most applications, however, prefer their rectifiers to be in a separate equipment room close to but behind the plating line where a cleaner and dryer environment is provided. Air-cooled units need an adequate supply of relatively cool and dry air for optimum operation. The equipment room is typically within 15-20 feet of the plating tank, so the voltage drop issue can be easily solved using adequate capacity bussing. This location also provides sufficient space to properly and safely service the rectifier(s) when needed.

Rectifier Evaluation

Several in-house tests can be done to evaluate the quality of DC energy produced by your rectifier. These involve the voltage drop, ripple, and DC waveform. These measurements should be taken at both the rectifier stub-outs and at the anode fixture in the tank while plating a typical load.

Voltage Drop: Use a digital multimeter and take DC voltage readings at the rectifier and the tank (preferably directly at the anode and the fixture). Subtract the DC volts at the tank from the DC volts at the rectifier to obtain the voltage drop. The voltage drop should be less than 0.5 volts if the bussing is sized properly and its joint connections are free of oxidation and corrosion.

Ripple: Use the digital multimeter again and take both a DC and an AC voltage reading at the rectifier and the tank (preferably at the anode and the fixture). Divide the ACV by the DCV and multiply by 100 to obtain the ripple percent. This should be less than 5% (ideally <1%) at the rectifier and the fixture. Ripple is the amount of AC that isn’t rectified, i.e., the AC that leaks into the bussing. SCR-type rectifiers need a choke built into the unit to limit this to below 5%. However, this depends on the voltage used as most chokes (filters) are designed for 5% maximum ripple but only at full voltage output. This means the ripple will be higher at lower voltages. There is no way around this unless a better choke is used.

DC Waveform: This test requires an oscilloscope. Your maintenance personnel may not have access to one or be familiar with its operation. In those cases, a local electronics engineer can be used. During plating, the oscilloscope checks the waveform at the rectifier and the anode fixture in the tank. This should indicate smooth three-phase DC sine waves without voltage spikes, either positive or negative. A positive spike at the wave's leading edge could indicate a failed capacitor in the rectifier. Unlike a normal sine wave, a tank-side wave that appears more square could indicate the rectifier is producing too much AC ripple. Any abnormalities found should be reviewed with the rectifier manufacturer for diagnosis.

Takeaways

Modern switch mode rectifier technology provides substantial advantages over the earlier SCR design. Anyone interested in upgrading their operation or replacing an older rectifier should take advantage of these many benefits. This has become incredibly important for obtaining high-quality hard chrome deposits.

DC electricity is 50% of the electroplating process, so it’s important. Switch mode units have become the go-to technology for improved operations in hard chrome applications.

Eric Svenson Sr. is CEO of Plating Resources and a Master CEF and IUSF member. Visit www.Plating.com. Please contact Vicky at Vicky@Plating.com with any questions or if you wish to explore this topic further.