Ask the Expert Question-and-Answer Archive
(Hard Chrome Plating)
by Larry Zitko, ChromeTech, Inc.
Hard Chrome Plating Defects
Q. I am inquiring about troubleshooting for electroplating defects; in
particular adhesion, appearance, and corrosion. Do you know of any ways to
help pervent these three defects or of any good resources I could use to
further my research? I greatly appreciate your time and help.
A. The following comments assume that you are referring to hard (industrial or
functional) chromium electroplating on carbon steel substrates. This forum
does not address decorative chromium electroplating issues.
Adhesion of the chromium electrodeposits to the steel substrate is maximized
when two pretreatment conditions are met:
- The substrate is chemically clean, and
- The substrate is properly etched.
Cleaning of steel parts prior to plating may be accomplished using various
methods, including aqueous alkaline soak cleaning, aqueous alkaline
electrocleaning, ultrasonic cleaning in an alkaline medium, and mechanical
cleaning methods like pumice and abrasive pads. In order to test to see if a
part has been properly cleaned, the plating operator can perform the "water
break free" test. Water is sprayed onto the part, and the operator observes
whether or not the applied water film runs off the part in the desired
continuous sheeting action (clean), or is diverted in any areas around any
areas which may still be contaminated with dirt, oily films, etc. (not
Steel parts are easily electroetched in a chromic acid medium, which can
either be the plating tank itself, or a separate tank for this purpose.
Reverse polarity is applied to the part and anode for a brief period of
time, immediately prior to plating. During the "reverse etch" cycle, the
positive pole of the DC power supply is connected to the part, while the
negative pole is attached to the lead anode. Time can vary from a few
seconds to a couple of minutes or more, and current density can vary from
0.5 to 3 amperes per square inch, depending upon the part's alloy, heat
treatment and other factors. Parts that are either underetched or overetched
can suffer from reduced adhesion, incomplete coverage, roughness and other
defects. Experienced platers learn how to access the correct degree of
reverse etch for the particular parts, and use the successful formula for
subsequent identical or similar parts.
If a part is not properly cleaned, the reverse etch cycle may not adequately
remove the remaining foreign soils or films, and poor adhesion may result.
Direct-current (DC) power issues may also adversely affect adhesion. In
particular, power interruptions during the plating cycle may cause poor
adhesion and peeled chrome. The waveform from the power supply is also
important. The power supply should be heavily filtered to remove alternating
current (AC) ripple applied to the cell during the electroplating process.
Also, thyristor power supplies should not be "phased back" too far during
plating. That is to say, voltage and current levels during plating should
not be too small a percentage of the capacity of the power supply. A trained
troubleshooter can test the output with an oscilloscope to observe whether
or not the waveform is suitable for chrome plating.
There is no single appearance that is required or suitable for all parts
that are hard chrome plated. Part specifications may call out a required
surface profile or crack density, and the plating shop will have to provide
the preplate and/or postplate grinding or polishing, plating chemistry or
plating conditions that will provide the appropriate deposit. For example,
many parts are blasted with aluminum oxide or other media prior to plating,
and the as-plated part exiting the tank will not be as shiny as a part that
was belt-polished prior to plating. However, the blasting provides an
excellent surface condition for the chromium electroplating, and the part
will probably be postplate ground or polished anyway. Mirror-finish rolls
may be polished to a very smooth condition prior to plating.
Many factors influence the corrosion resistance of the chromium
electrodeposits, and a thorough discussion is beyond the scope of this
forum. Some of the influencing factors are:
- Bath chemistry (type or formulation)
- Catalyst ratios for the bath
- Bath purity
- Microcrack structure of the deposit
- Deposit thickness
- Number of chromium layers in the final coating
- Preplate "smoothness" of the part surface
- Plating conditions (current density, temperature, etc.)