The electrodeposition of chromium has
been used since the mid 1800s because of the
deposit’s unique combination of properties
that hasn't been matched by any other single
material. Commercially, chromium deposits
are typically divided into two groups, decorative
and hard (functional) chromium—based more on their applications than on
the chemistry of the processes.
Decorative Chromium Plating
Decorative chromium electrodeposits,
“chrome,” are almost exclusively used as
protective topcoats for decorative parts.
Many of the parts are fabricated from the
least expensive and easiest to form material
(substrate) that offers the required physical
properties. Typical chrome substrates are
nickel or copper plus nickel-plated plastic,
steel, zinc die-castings and aluminum.
Stainless steel is one of the few substrates
frequently plated directly with decorative
chromium as well as with nickel and then
chromium. Once plated, all of the parts
produced from the different substrates have
the same consistent, eye-appealing, metallic looking
decorative chrome appearance.
Decorative chromium is usually plated
in the thickness range of 0.2–0.5 μm. Even
though the deposits are very thin, they offer
the physical and chemical properties that are
required for most decorative parts. Thinner
deposits are very porous and offer minimal
physical properties. Thicker deposits tend
to be dull and form visible cracks. The
service life of chrome-plated parts can vary
from untouched ceiling light fixtures in
environmentally controlled rooms to truck
bumpers and wheels traveling thousands of
miles per year with minimal cleaning or other
care. The deposit’s appearance is retained
in service because of its hardness and its
excellent tarnish (surface corrosion), wear,
chemical and scratch resistance
However, all of these properties depend
on the chromium deposit interacting in
synergy with its underlying deposit, typically
nickel. Bright nickel deposits help give the
chromium deposit its bluish-white color and
the chromium deposits protect the nickel
from dulling due to surface oxidation.
While multiple or single layers of nickel
are primarily responsible for protecting the
substrate from corrosion, the chromium
deposit helps control the corrosion rate and
is responsible for maintaining the overall
reflective decorative appearance during
service. Nickel and especially copper plus
nickel also improve the appearance of the
part by leveling out the defects (scratches,
pores, etc.) in the substrate. This reduces
the need for expensive surface preparation
of the substrate.
Hexavalent Chromium
Hexavalent or trivalent chromium electrolyte
can be used to electroplate decorative
chromium. Hexavalent chromium use is
being increasingly regulated for environmental
and worker safety reasons. Hexavalent chromium ions are carcinogenic and
strong oxidizers while trivalent chromium
ions are neither. Also, hexavalent processes
are highly acidic while trivalent are mildly
acidic. Hexavalent chromium processes
produce the blue-white deposit that is considered
the standard for the appearance
of chrome.
Trivalent Chromium
Trivalent chromium processes can produce
deposits that vary from an almost
hexavalent chromium deposit appearance,
metallic white, to a pewter/stainless steel
appearing deposit that appears to have depth.
In almost all cases, the physical properties of
trivalent chromium deposits are equivalent to
those of micro-discontinuous (micro-pores or
micro-cracks invisible without magnification)
hexavalent chromium deposits.1 Decorative
trivalent chromium deposits are microporous
as plated while pre- or post-treatments
are used to produce micro-discontinuous
hexavalent chromium deposits.
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Compared to chromium deposits without
micro-discontinuities, micro-discontinuous
chromium deposits over electroplated nickel
significantly enhance the protection of the
part from substrate corrosion. Hexavalent
chromium deposits are not plated with sufficient micro-sized pores or cracks to offer
this protection. One method to increase the
micro-pore count is to lightly spray the dried
nickel plus chromium plated part with hard
particles such as aluminum oxide. This cracks
the brittle chromium deposit and produces
micro-porous chromium. A more frequently
used method is to deposit a customized nickel strike in between the bright nickel and the
chromium deposits. If inert particles are codeposited
within this nickel strike (particle
nickel deposit), the subsequent chromium
deposit will contain micro-pores where the
chromium does not plate over the exposed
particles. Another far less common approach
is to use a nickel strike that is designed to
micro-crack after being chromium plated.
This micro-cracks the chromium deposit.
An even less frequently used method is a
specially designed hexavalent chromium
process that forms micro-cracks during
plating. Control of this process is difficult.
Trivalent chromium deposits under 0.5
microns thick are micro-porous as plated.
Thicker deposits are micro-cracked.
Micro-discontinuities in the chromium
deposit spread the corrosion potential
over the chromium and underlying nickel
deposits, with the chromium the cathode and
the nickel exposed through the micro-discontinuous
sites, the anode. Because of the
very large number of nickel sites, the anode
current at any one location on the nickel is
greatly reduced, which retards the rate of
corrosion at any one site. North American
automotive decorative chromium specifications
require nickel plus micro-porous chromium
for all exterior applications because
of its increased corrosion resistance while
maintaining the after-corrosion appearance of the chromium deposit. These specifications
do not allow micro-cracked chromium
deposits but they are permissible in other
areas of the world. The importance of microdiscontinuous
chromium and the required
physical properties of the underlying nickel
deposits are reviewed in ASTM B 456, which
specifies a minimum of 30 micro-cracks per
mm or 10,000 micro-pores per sq cm.2
Decorative Chemistries
There are several different families of
decorative chromium processes, each offering
specific advantages. They are extremely
versatile, which blends the properties of
one process into the advantages of another
process. However, a plater may prefer one
process compared with another because it
offers advantages for his particular operation.
Decorative chromium processes are
most easily divided based upon their catalyst
system, as shown in Table I. Chromium
trioxide, CrO3 , commonly referred to as
chromic acid (the hydrated form of CrO3 )
is used to supply the hexavalent chromium
ions. The sulfate is commonly controlled byaddition of sulfuric acid (H2 SO4 ). Proprietary
additives are usually used to control
the fluoride catalyst. Proprietary trivalent
chromium processes are used in almost all
cases because of the complexity of the solution
formulation.
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The hexavalent chromium processes
listed in Table I can be further divided into
self-regulating processes and those regulated
by analysis. Self-regulating processes use
additives that dissolve at the level required to
maintain the proper catalyst concentration
for the process. Increased temperatures raise
catalyst levels resulting in improved plating
rates. The analytically controlled processes
use soluble additives that contain a specific
amount of catalyst. Catalyst adds based upon
analysis or amp-hours offer the advantage
of varying their concentration to alter the
bath performance
Some of the advantages and disadvantages
of hexavalent and trivalent chromium
processes are listed in Table II. Other than
the type and amount of catalysts used, most
hexavalent chromium processes have similar
operating conditions (Table III). This table
also contains some general operating conditions
for trivalent chromium processes.
Some of the more specific factors within
each process family also can be listed.
Single-Catalyst Hexavalent Processes. All
chromium processes use less than 100% of
their cathode efficiencies for chromium
deposition. Single-catalyst processes have
about 12% chromium plating efficiency. The
remaining current goes to the formation of
hydrogen and trivalent chromium, Cr3+ . The
chromium deposition effi ciency increases
proportionally with chromic acid concentration,
up to 250 g/l and decreases thereafter.
A CrO3 /SO4 ratio of approximately 100:1
is common. Chloride in any hexavalent
chromium process will over catalyze the
process and adversely affect the appearance
of the deposit.
Co-catalyzed Hexavalent Processes. The
inclusion of the second catalyst, fluoride,
can increase the average cathode efficiency
to about 22%. The efficiency increases with
increasing chromic acid concentration up to
300 g/liter. A CrO3 /SO4 ratio of about 190:1
is common. These processes are less sensitive
to current interruptions and can plate over
more passive substrates with reduced defects.
They also have an increased plating speed,
better coverage, wider bright range, and
more tolerance to impurities. However, they
will lose their fluoride catalyst if heated over
66°C. Also, the fluoride catalyst etches steel
substrates, resulting in iron contamination of
the plating solution. An increase in rectifier
voltage is required if metallic impurities (Ni,
Fe, Cu and Cr3 ) are permitted to build over
7.5 g/liter. Because of the second catalyst and the need for additional analysis, these
processes are more expensive and costly to
operate than single-catalyst processes.
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Tri-catalyzed Hexavalent Processes. These processes are similar to co-catalyzed
processes except for the addition of a proprietary
organic catalyst. This improves the
cathode deposition efficiency and chromium
coverage in the low-current-density areas.
This process is the most expensive decorative
hexavalent process to operate but the
improved low current density coverage justifies the cost in some operations.
Trivalent Chromium Processes. Platers
switch to these processes primarily
for increased productivity and improved
environmental/health/safety factors. The
improved coverage and throwing powers
(similar to nickel), faster plating speed,
the lack of burning and white wash, and a
complete tolerance to current interruption
all lead to reduced rejects and, in many
cases, more parts on the plating rack. These
factors along with a simplified method to
maintain the purity of the solution and to
waste treat it, if necessary, most times makes it
less expensive per part to plate even though
the operating solution is more expensive
to purchase. These processes are similar
to nickel processes in that they have to be
analyzed and maintained more frequently
than hexavalent chromium processes. The
slight difference in deposit appearance
and the need to passivate some thin nickel
(under seven microns) parts are cited by
some as reasons not to convert to trivalent
chromium processes.
Specialty Processes. Dark gray to black
deposits are available from both hexavalent
and trivalent chromium processes. The
hexavalent processes are used for solar
collectors and both are used for decorative
applications. Other than the additives used
to darken the deposits, these processes operate
similarly to others in the same family of
processes. Hexavalent “cold chromium”
(16–21°C) processes are used where plating
efficiency is less important than coverage
in the very low-current-density areas. They
are more difficult to operate and use different sulfate (450–550:1 for CrO3 :SO4 )
and fluoride concentrations than typical
hexavalent chromium processes. Barrel
chromium processes are also available by
using special hexavalent formulations in
specially designed barrels.
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Hard Chromium Plating
Hard (functional) chromium coatings are
typically thicker than 1.2 μm and in some
applications can be more than 100 μm thick.
Chromium deposits impart functional qualities
of wear and corrosion resistance, lubricity
and release properties to a wide range of
substrates. Substrates that are chromium
plated include hardened and unhardened
steels, tool steels, stainless steels, cast iron
and aluminum. Chromium coatings allow
the use of lower-cost or lighter substrates,
also the unique engineering properties of
electrodeposited chromium that cannot be
attained by substrates. Chromium coatings
are very cost-effective compared to many
coating processes.
Chromium coatings resist
corrosion in various environments.
3 They are especially
resistant to corrosion in oxidizing
environments. Chromium
coatings are used in
the automotive, agricultural,
chemical, petroleum, and
manufacturing industries.
For more severe applications
duplex coatings are used.
First a layer of nickel or electroless
nickel is applied, then
chromium is plated. Some
duplex coatings consist of a
chromium layer that is plated
and ground and then another
chromium layer.
Hard Chrome
Chemistries
There are three predominant
hard chromium plating
processes: conventional, fluoride (mixedcatalyst),
and etch-free high-efficiency. All
of the baths contain chromium (VI) oxide
commonly referred to as chromic acid
(CrO3 ) and sulfate. The sulfate and fluoride
compounds act as catalysts. The etch-free
high-efficiency bath contains a non-halide
catalyst. Chromium cannot be electrodeposited
from an aqueous CrO3 solution unlessone or more catalysts are present. Depending
on which catalysts are present and the
plating parameters, between 10 and 30% of
the cathodic current will be used to reduce
hexavalent chromium (Cr6+ ) to chromium
metal. Their deposit properties and operating
conditions are summarized in Table IV.
The etch-free high-efficiency process will be
referred to as the etch-free process.
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| Figure 1. Photomicrographs of anodically etched chromium deposits from high-efficiency etch-free, fluoride, and conventional baths. The samples were plated under typical conditions for each bath. The cross sections were polished prior to etching. |
Corrosion resistance of a chromium
plated part is influenced by the substrate
quality, pretreatment, plating and posttreatment.
The optimum plating process
should produce the least amount of
nodules and the most microcracks
for the best corrosion resistance.
The microcracks of etched samples
are shown in Figure 1. These figures
show that as the microcrack density
increases the depth of the etched
cracks decreases. Corrosion resistance
of identical samples plated
under optimum conditions for
each of the three baths is shown in
Figure 2. The improved corrosion
resistance of the etch-free process is
due to a higher microcrack density
and less nodular deposit.4
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| Figure 2. The chromium deposit thicknesses were 25 µm (1 mil) and the corrosion testing was conducted according to ASTM B117. Samples were electroplated from etch-free, fluoride, and conventional baths, under optimum conditions for each chemistry. The samples were tested as plated without any post finishing. |
Wear resistance is related to the
deposits' hardness and toughness.
Abrasion and sliding wear resistance
data is shown in Figures 3
and 4. The etch-free process contains the
smallest grains or crystallites of the three
types of deposits and wears the least of the
three processes.5 Wear is also related to the
microcrack density and oxygen content of
the deposit.
The operating window is defined by the
ranges of temperature, current density, and
composition in which acceptable deposits are
plated. Conventional chemistry has the smallest
operating window, the fluoride process
has an intermediate operating window and
the etch-free process has the largest operating window. The fluoride process has limited practical temperature range due to the volatility
of the fluoride at higher temperatures.
Figure 5 depicts the variation of efficiency
and microcrack density as a function of ratio
(sulfate concentration varied), compared
to values obtained at a 100:1 ratio, for conventional
and the etch-free processes. The
conventional process has higher variations
since it is a single-catalyst system.
Deposition rate is a function of cathodic
efficiency and the current density. Figure
6 shows the deposition rates for the three
processes. At the same current densities the
etch-free process is up to 60% faster than
conventional, and up to 20% faster than the
fluoride process. Due to catalyst stability, the
etch-free process can be operated at higher
current densities and temperatures than the
fluoride process.
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| Figure 3. Taber, dry abrasive wear with 1kg load on CS-10 wheels after 10,000 cycles. The results are the average of three tests. |
Energy Cost. At an electricity cost of $0.8/kWh the power cost is about $6/kg of CrO3
for an etch-free deposit. The power cost of
a conventional deposit would be $11/kg
and for the fl uoride process would be $8/kg. The etch-free process has lower power
costs due its higher effi ciency and lower
voltage (compared to the fluoride process).
The fluoride process will have a higher average
voltage due to the substrate etching.
The conventional and fluoride have 80%
and 30% higher power cost
respectively, compared to the
etch-free process.
Anode Cost. Anodes last
about three times longer in
a conventional or etch-free
process than anodes in the
fluoride bath. A large tank
using the fluoride process
can have an annual anode
cost of $60,000. The savings
with a conventional or
etch-free process would be
about $40,000/year. Additional
savings can occur by
switching from the more
expensive lead-tin alloy required in fluoride
processes to the less expensive lead-antimony
anodes.
Low-Current-Density (LCD) Etching. LCD
etching occurs at lower cathodic current
densities than where plating occurs. Etching
is minimal unless fluorides or chlorides are
present. These halides attack the oxide that
protects metals and allow the hydrogen to
etch the substrate. The LCD etch rates of iron
samples are shown in Figure 7 for the three
chemistries. The etch-free and conventional
chemistries etch 1,000 times less than the
fluoride process.
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| Figure 4. Lubricated sliding wear with 340 kg load for five hours. The results are the average of three chromium plated test pins against 1137 vees. |
Difficult to Plate Substrates. Nickel, high-nickel
alloys and tool steels can most easily
be plated in the fluoride process. Fluoride
provides very good activation for many substrates
with an immersion or anodic etch.
With the use of an activating solution, anodic
etch and or a hydrogen wash; nickel, high
nickel alloys, and tool steels can be plated
in conventional and etch-free processes with
the same adhesion strength as with a fluoride
process. When the fluoride process is used
the LCD areas must be masked to prevent
etching of the substrate.
Additional Processes
Higher-Speed Etch-Free Processes. The
etch-free catalyst is stable and non-volatile at higher temperatures, allows the use of
this type of chemistry at higher current
densities. Current densities of up to 200
A/dm2 can be used to obtain efficiencies
of 30% and deposition rates of 4.6 μ/min.
These deposition rates are about 10 times
faster than plating speeds obtained from
conventional chemistries. Higher plating
speeds allow smaller or fewer plating tanks
and allows inline plating with other production
processes.
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| Figure 5. Variation of efficiency and microcrack density as a function of ratio (sulfate) for conventional and the etch-free processes. |
Self-Regulated Processes. Numerous
self-regulated conventional and high-speed
fluoride processes are available. These
chemistries contain sparingly soluble compounds
to control the sulfate concentration
and or the fluoride concentrations.
The chemistries are partially or completely
self-regulated and in some cases platers
need only to maintain the chromic acid
concentration to maintain the plating
process.
Crack-Free Processes. Original deposits
from crack-free processes contain no microcracks.
These coatings have niche uses.
Time, mechanical, or thermal stresses can
cause these coatings to form macrocracks.
Crack-free deposits are softer than microcracked
deposits.
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| Figure 6. Deposition rate for etch-free, fluoride, and conventional baths. Fluoride data was from a customer sample plated in the laboratory. |
Equipment
Anodes. Insoluble anodes are used for
hexavalent chromium plating. Lead alloys
with 7% tin or 6% antimony or tin and
antimony are the most common anodes
used. Lead/tin alloy anodes are more
corrosion-resistant and last longer than
lead-antimony alloy anodes. Lead-tin alloy
anodes should be used in fluoride baths.
Lead-antimony alloy anodes are more rigid
than the lead-tin alloy and must be used
in deep tanks. Lead-tin-antimony alloys
give some of the advantages of both alloying
metals.
Different types of insoluble anodes
are used for trivalent chromium plating
depending upon the formulation. One
process uses special graphite anodes that
will last “indefi nitely” if not mechanically
damaged. Another process uses titanium
anodes coated with metal oxide that needs
to be recoated every several years. Both
anodes eliminate the creation of hazardous
lead salts in the plating solution.
Rectifiers. Direct-current rectifiers
should typically have a voltage range of
between 9 and 15 V and sufficient amperage
for the tanks. In hexavalent chromium
plating it is critical that ripple (alternating
current), over the full range of amperage
that will be used, is less than 5%. High
ripple can cause soft and dull chromium
and poor adhesion.
Trivalent chromium processes do not
require low-ripple rectifiers. Nickel plating
rectifiers with a voltage range of 6–12
V can be used. Fifteen-volt rectifiers are
sometimes used to optimize productivity
by increasing rack density and amperage
without burning.
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| Figure 7. LCD (2 A/dm2) etch rates of iron for etch-free, fluoride, and conventional baths. |
Tanks and Auxiliary Equipment. Hexavalent
chromium tank linings may be a
flexible polyvinyl chloride material of an
approved type, either sheet or sprayed.
Lead-lined tanks are not recommended
for fluoride or etch-free baths. Bricks
are often used to line tanks. These must be compatible with the solution. Glass-containing
bricks should not be used with
fluoride solutions.
Metallic temperature controlling coils
or heat exchangers must be electrically
isolated from the steam and water lines.
Titanium cannot be used with fluoride
baths, while tantalum or niobium can be
used. Kynar or PTFE coils can be used in
all baths. Pumps, pipes, and ventilation
equipment should be made from the
above materials or from other chromic
acid or fluoride/chromic acid resistant
materials.
Trivalent chromium processes can use
plastic, PVC, ABS, polyethylene or rubber
tank liners. Copper anode rails should be
nickel-chromium plated or plastic coated
to avoid possible copper contamination
of the plating solution. Under some plating
conditions, heating and even cooling
might be necessary. Titanium coils with
thermostat-controlled water can be used.
All equipment used to store or transfer the
solution must be plastic lined to avoided
metallic contamination. Ventilation is frequently
not used due to a lack of misting
but plastic air ducts with airfl ow similar to
what is employed on nickel electroplating
tanks may be used.
Purification. Some trivalent chromium
processes use ion exchange resins to
remove nickel, copper, iron, and zinc contamination
from the plating solution. The
operating solution may be passed directly
through the resin even while plating. The
regeneration solutions for the resin goes
directly to a normal nickel and chromium
waste treatment system along with the rinse
water containing trivalent chromium solution.
Plastic lined fi lter pumps containing
carbon can be used to remove organic
contamination if necessary. Continuous
solution filtration is not required. Since
contaminants can be easily removed, atmospheric
evaporators are frequently used to
evaporate water from the plating solution
so that dragged out plating solution can
be returned to the plating tank.
Filtration is used in plating solutions that
are used for plating chromium on optically
flat rolls. Filtration is occasionally used
in other applications. The proper filters
must be used and must be precleaned
and replaced on a regular schedule. If
they break down they can cause pitting in
the chromium.
Decorative hexavalent chromium solutions
are very seldom filtered because
particulate matter in the solution typically
does not cause a plating problem. PFD
References:
- “A Comparison of the Corrosion Characteristics
of Trivalent and Hexavalent
Chromium Electrodeposits,” D. L. Snyder,
Plating and Surface Finishing, June 1979.
“ Fifteen Years of Outdoor Corrosion of
Trivalent and Hexavalent Chromium Electrodeposits”,
D. L. Snyder, Metal Finishing,
1993, p 113-118.
- ASTM B 456, “Standard Specification
for Electrodeposited Coating of Copper
Plus Nickel Plus Chromium and Nickel Plus Chromium,” ASTM International, 100 Barr
Harbor Drive, West Conshohocken, PA.
- ASM Metals Handbook 9th Ed. Vol. 13 Corrosion,
1987, p. 871-5 A.R. Jones (revised
in 10th Ed. 2005).
- “Functional Chromium Deposits with
Superior Wear and Corrosion Properties," K.R. Newby, Interfinish 92 Surface
Finishing Congress Sao Paulo, Brazil, 1992,
p 1089-1103.
- “Hard Chromium: Microcrack Formation
and Sliding Wear,” A.R. Jones, Trans.
Inst. Metal Finish., 70(1), 8 (1992).
