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

by Larry Zitko, ChromeTech, Inc.
April, 2003

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:

Adhesion of the chromium electrodeposits to the steel substrate is maximized when two pretreatment conditions are met:

1. The substrate is chemically clean, and
2. 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 clean).

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.

APPEARANCE:

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.

CORROSION

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.)