Ask the Expert Question-and-Answer Archive
(Hard Chrome Plating)
by Randy Taylor, Advanced Tooling Corporation.
Q. We are concerned about hydrogen embrittlement effects in 4130 & 4140 alloys.
I understand one method of alleviating this problem is to chrome plate the
steel. Does chrome plating completely alleviate this problem? The plated
tubing would be exposed to moist hydrogen sulfide solutions. Also we would
like to perform metallographic analysis upon the chrome coating. Can you
suggest a highly effective solution for etching chrome plated surfaces?
Thank you for your attention.
A. Hydrogen embrittlement - (definition) ... is a metallurgical phenomenon
that occurs in many different metals, however, high strength steel by far
seems to have the highest sensitivity to embrittlement. In high strength
alloys, the presence of hydrogen tends to block ductility.
(definition)...Ductility can be more commonly understood as the ability of
metals to deform under stress.
We know there is a predictable relationship between stress/load and
ductility/ elongation. Hydrogen is absorbed into the steel structure during
some electroplating processes, especially hexavalent chrome plating. (fact:
hydrogen gas is liberated at the cathode during electroplating)
Hydrogen inhibits the metals ability to deform, and the result is, the metal
can break or fracture at a much lower load or stress than anticipated. The
affects of hydrogen embrittlement on metals is a serious problem, especially
in aircraft & aerospace component manufacturing.
A proven method of removal is baking after plating. Thermal treatment of
metals for specified times and temperatures can drive the hydrogen out of
the metal and relieve stress. There are as many theories and methods as
there are alloys. Suffice to say that most high strength, high heat treated
(high tensile strength) materials are baked for as much 24 hours at 375
degrees F, within 2 hours after removal from the plating bath.
Your other experiments are ambitious. There are large amounts of
metallographic data available of chrome coating. (Atotech.com) Chrome etching
is best performed 1.) in the standard hexavalent chrome plating bath,(30-33
oz./gal. 100:1 ratio with sulfuric acid.) 2.) Sulfuric/ Hydrofluoric acid
etch solution, (H2So4, commercial grade Be' at 40-60% by volume of water,
1-2% Hydrofluoric acid)
Q. Any idea on the average size of the cracks typically present on hard chrome surfaces?
A. My answer is going to be "no" to your question. and here is my reason.
Most electrodeposited chrome plated structures have a significant population of micro cracks in the deposit. Depending on the chemistry and method of plating, cracks can be random or conform to a pattern and vary in depth and quantity. Regardless of pattern or structure, cracks in the plated layer are large enough to permit diffusion of hydrogen into and out of the base material. Hence, one of the beauties of "chrome" coating for use in aerospace, I.E., its ability to allow hydrogen to diffuse through the coating during a post plating bake.
Some hexavalent chrome plating baths are tailored to form a "crack free", thin dense chrome deposit, with high hardness and wear resistance. Despite the claim "crack free", some cracking, albeit far fewer cracks, still occurs.
"Avoiding introducing hydrogen into the article during plating?" This is essentially impossible in electrolytic immersion plating. Hydrogen can be absorbed through most masking.
Some non-electrolytic plating processes have minimum hydrogen generation, Example: 1.)Ti-Cadmium, non-cyanide cadmium, electroless nickel and vapor deposited metallic coatings to name a few.