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

by Larry Zitko, ChromeTech, Inc.
January, 2002

Cracked Chromium Plating

Q. 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 would 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 (single-catalyst bath, fluoride bath, non-fluoride high-speed bath, catalyst ratio of your bath, etc.)

I will offer the following information, in hopes that it will help you.

Almost all electroplated hard chromium deposits are cracked. Cracking occurs during the plating cycle when internal stress exceeds the tensile strength of the chromium, which is hard and brittle. The width, depth and population density of these microcracks varies according to many variables, including:

• the type of plating chemistry used (single-catalyst, mixed catalyst, proprietary), 
• chromic acid concentration, 
• type and concentration of catalyst, 
• chromium-to-catalyst ratio, 
• plating current-density, 
• bath temperature,
• concentration of bath impurities (iron, copper, zinc, nickel, trivalent chromium, etc.)
• chromium deposit thickness
• surface condition of substrate

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 lubricity, good wearability and better corrosion resistance. 

If the conditions during plating cause the cracks 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 should be noted that macro-cracking can occur in chromium deposited over any type of substrate, not just those that are already stressed in tensile.

You inquired about shot peening. Certain substrates are stressed in tensile, even before the chromium plating cycle. Typically, these are high-strength or heavily alloyed materials. Stresses may also be induced into the substrate by machining operations or hardening techniques. These stresses can be reduced, prior to plating, by heat-treatment techniques or shot peening. It is important that the combined or compounded tensile stresses from the substrate, the electroplated chromium and post-plate hydrogen embrittlement relief (baking) do not reduce the fatigue strength of the plated part below its fatigue limit. Parts are typically designed with a healthy safety margin.

Here are some sources of information on peening:

• 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 of Metallic Articles Prior to Nickel, Autocatalytic Nickel, or Chromium Plating 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.
• AMS-2432.

To sum up, many factors can influence cracking in chromium, not just tensile stress in the preplate substrate. One of the first steps to solving problems which may be chemistry related is to send a sample of the bath(s) to a reputable laboratory that is experienced and knowledgeable in chromic acid electroplating solutions.

I hope this information helps.