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Ask the Expert Question-and-Answer Archive
(Hard Chrome Plating)
by Larry Zitko, ChromeTech, Inc.
Cracked Chromium Plating
I am having problems of cracks in our chromium process. I think it is a
defect of the preparation before the chrome plating. Could you explain to me
the parameters to use for the shot peening process to reduce the steel
fatigue?. Thank you in advance.
A. Thank you for posting your question to
the "Ask The
Expert" feature at the National Metal Finishing Resource Center
website. Your email did not mention the following items, which
have helped me:
- How large the cracks were (are they
visible to the
naked eye, or do you need a microscope to see them),
- What type of base metal you are
plating on (carbon
steel, high strength steel, hardening techniques used, etc.),
- Plating bath chemistry
fluoride bath, non-fluoride high-speed bath, catalyst ratio of
I will offer the following information,
that it will help you.
Almost all electroplated hard
chromium deposits are cracked. Cracking occurs during the
cycle when internal stress exceeds the tensile strength of the
chromium, which is hard and brittle. The width, depth and population
these microcracks varies according to many variables, including:
- the type of plating chemistry used
(single-catalyst, mixed catalyst, proprietary),
- chromic acid
- type and concentration of
- plating current-density,
- bath temperature,
- concentration of bath
copper, zinc, nickel, trivalent chromium, etc.)
- chromium deposit thickness
- surface condition of
Generally speaking, a microcrack
structure which is
comprised of a high population density of narrow, shallow cracks is
desirable, because the deposit tends to have a lower stress, higher
good wearability and better corrosion
If the conditions during plating cause
to be coarse in nature, they are often referred to as macro-cracks,
which may be
visible to the naked eye. Usually, chromium with this type
of microstructure exhibits less desirable properties in service. It
be noted that macro-cracking can occur in chromium deposited over any
substrate, not just those that are already stressed in
You inquired about shot peening.
substrates are stressed in tensile, even before the chromium plating
Typically, these are high-strength or heavily alloyed materials.
also be induced into the substrate by machining operations
techniques. These stresses can be reduced, prior to plating, by
techniques or shot peening. It is important that the combined or
tensile stresses from the substrate, the electroplated chromium and
hydrogen embrittlement relief (baking) do not reduce the
fatigue strength of the plated part below its fatigue limit.
typically designed with a healthy safety margin.
Here are some sources of
- MIL-S-13165C-"SHOT PEENING OF METAL PARTS". This resource is available at www.nmfrc.com in facsimile form.
- ASTM B 851-94 "Specification for Automated Controlled Shot Peening
Metallic Articles Prior to Nickel, Autocatalytic Nickel, or Chromium
or as Final Finish"
- George Leghorn's "The Story of Shot Peening" is informative, and can be found at http://www.shotpeener.com/learning/1957006.pdf.
To sum up, many factors can influence
chromium, not just tensile stress in the preplate substrate.
One of the first steps to solving problems which
chemistry related is to send a sample of the bath(s) to a reputable
laboratory that is experienced and knowledgeable in chromic acid
I hope this information