Hexavalent hard chrome is under scrutiny because of environmental and worker safety concerns, which are real if hex chrome is improperly used.
Eric Svenson Sr.In reality though, chromic acid (hex chrome) is no more hazardous than many other industrial chemicals are.
Unfortunately, hex chrome was abused unknowingly — and sometimes inappropriately — by discharges into the waterways, atmosphere and terrain in the past. There could be several reasons for this:
- Ignorance of its negative effects.
- Lack of knowledge on better ways to manage hex chrome.
- Inability to adequately control these discharges.
Because of these abuses, the EPA and OSHA agencies enforced more restrictive limits on hex chrome. These regulations caused an unfortunate reduction by more than 50% of the hard chrome installations in the United States. The surviving operations adopted various compliance techniques including fume suppressants, improved ventilation scrubbers and waste treatment. Unfortunately, many operations are still struggling to meet the current standards and are concerned about what might happen if these regulations are further tightened.
Developments in Use of Safe, Sustainable Hex Chrome
Fortunately, a recent development in the use of safe and sustainable hex chrome has been demonstrated. This involves a 7-step method involving improvements to the hex chrome bath, using better agitation, ventilation and containment systems combined with zero-discharge recovery technology.
The result is zero air and zero water hex chrome discharge, with the workers testing over 50 times below the PEL for hex chrome and greatly reduced operational costs. This was achieved without the need for waste treatment equipment or fume suppressants.
But the attack on hex chrome continues by the regulators, partially because of unawareness on how it can be properly managed. They need to understand that a suitable, reliable, and economical hard chrome process is vital to the U.S. industry and defense.
Some have tried to replace hard chrome with other processes such as HVOF, electroless nickel, vacuum coatings (PVD), laser cladding and several alloy and composite electroplates. These may be suitable for specialized and limited applications, but none are as useful or economical for broad application across all industries like hard chrome, and none of these alternatives are a drop-in replacement for hex chrome. Hexavalent hard chrome has been widely used and accepted for over 100 years now.
If you need a coating to perform like hex-hard chrome then you should use hex-hard chrome. It's well developed, practical, user friendly, economical and now sustainable. In essence, it works – so what’s the point in replacing it with something that’s more expensive, more difficult to control and is not yet proven or accepted by industries?
Are we to assume that trivalent chrome is totally benign and non-hazardous? Actually, trivalent chrome has its own environmental and worker safety issues to contend with. The tri chrome process still uses chromium, and it’s still hazardous.
Tri-Chrome is Not New
The idea of using a Tri-Cr bath is nothing new, as investigations into trivalent hard chrome chemistry were attempted over 100 years ago; it didn’t work then and we question its application today. There’s a good reason the industry settled on using the hex chrome process; we just need to use it responsibly and sustainably which has been demonstrated.
Past attempts to electrodeposit trivalent chrome resulted in extremely thin deposits with the inability to produce thicknesses over 0.001 inch. These early attempts also experienced adhesion problems, ductility issues and troubles controlling the bath. Some required membranes that separated the anode and cathode areas and all of them use special anode materials. These processes operated within very narrow operational parameters adding their overall complexity and cost. Many of these issues still apply to the recently commercialized trivalent processes.
The Hex-Cr bath has been in general industrial use since around 1920. It’s an easy process to control, it’s relatively user friendly and its economical to apply. Its use has proven to be sustainable and its friendly to both environmental and worker safety concerns.
Our goal is doing an in-depth examination of the three trivalent chrome baths currently being promoted. None of these has yet seen an industrial application of significant quantity, although a few test sites are currently in operation.
Trion’s Safe Chrome may currently have several beta test sites. The only known Atotech Blue Chrome Beta test site is at Cardinal Plating in Aledo, Illinois.
The potential Tri-Cr baths currently or close to being available include:
- Trion Safe Chrome
- Atotech Blue Cr
- Faraday Tri-Cr (not yet commercialized)
More data is known about the Trion and Atotech Tri-Cr processes as the Faraday bath is still undergoing alpha research development.
The data shown for Hex-Cr uses the high-efficiency bath instead of the standard 100:1 bath because of its increased productivity, improved deposit properties and its decades long industrial acceptance.
Data on the various Tri-Cr processes will be presented when known. Much of the technical information on the various Tri-Cr process is unknown or is being withheld. Attempts to obtain this information from the Tri-Cr manufacturers have not been successful to date.
Rationale
The restrictions on using hex chrome caused several suppliers to develop a trivalent hard chrome process to replace the hexavalent chromic acid bath, some with U.S. government funding. This R&D work has been ongoing since around 1990. Various Tri-Cr baths are being promoted with claims of being a suitable replacement for the Hex-Cr process. But is that truly the case?
There’s more to it than simply considering the elimination of hex chrome. One also needs to consider the overall hazard and waste issues, the equipment requirements, the process durability, the chrome deposit differences, the investment requirements, and a comparison of the overall operating expenses.
Comparison
The objective is providing an honest in-depth side-by-side comparison of the two processes. You can then make the determination of which one is best for your operation. This study will look at comparing these differences in the processes using the following parameters:
- Deposit Properties
- Plating Bath Properties
- Substrate Activation
- Bath Chemistry
- Operating Conditions
- Chemical Supply
- Analytical Control
- Bath Impurities
- Process Durability
- Equipment Required
- Environmental & Waste Control
- Cost of Ownership (future work)
Deposit Properties
| Item | Hexavalent Chrome | Trivalent Chrome |
| Hardness | 70-72 RC (up to 1,245 DPH) | In general, Tri-Cr deposits are softer than Hex-Cr and this likely affects wearability. Most can be post-plate baked to increase the hardness, but this typically results in decreased wearability and a widening of the crack structure. Trion: 55 – 60 RC, can post-plate heat treated to 70 RC.; Atotech: 60-65 RC, estimated, can also be post-plate heat treated to around 70 RC. |
| Ductility | Excellent | Unknown |
| Adhesion | 130,000 psi (approx.) | Unknown, but generally believed to be considerably weaker than Hex-Cr. |
| Internal Stress (0.001”) | Compressive | Unknown |
| Fatigue Limit | 90,000 psi (approx.) | Unknown |
| Lubricity | Excellent with low friction | Unknown |
| Taber Wear Resistance | Excellent, only 1.5 grams lost on the abrasion test (see below). | Taber wear unknown. Most Tri-Cr vendors report wear using the Falex test which is not relative. (see note below). Generally, believed to be poor compared to Hex-Cr. |
| Corrosion Resistance | Excellent, 25-35 hour 5% salt spray test, 0.002”. | Poor when plated directly on steel, many applications require a nickel underlayer. |
| Crack Structure | Self-healing non-continuous micro-cracks, none extend down to the substrate, typically 800-1,200/lineal inch. | Typically wider and fewer macro-cracks with many extending down to the substrate. Macro cracking is common. |
| Maximum Thickness | Unlimited, over 0.100”/side is possible. | Unknown, but believed to be considerably thinner. |
| Deposit Color | Bright & shiny chrome. | Most are off-color, with duller or bluish hues. |
| Post-Plate Baking | None required for Cr adhesion. Sometimes used for hydrogen embrittlement relief if base metal is over 45 RC. | Same hydrogen embrittlement issues as Hex-Cr. Post-plate heat treating Tri-Cr deposits opens up their macro-crack structure degrading the corrosion resistance further. This may also result in reduced wearability. |
Notes
- The Taber test is an abrasive wear test that is used extensively for plated surfaces as it relates to actual wear conditions. The Falex test is generally used for fretting wear to test lubricants, its primary purpose is comparing the effect of various oils and greases.
- Some Tri-Cr deposits aren’t 100% chromium like the Hex-Cr deposit is. The Tri-Cr deposit might only be 88-98% chromium, with the balance being 2-12% Carbon. This may vary depending on the Tri-Cr bath being used.
Plating Bath Properties
| Item | Hexavalent Chrome | Trivalent Chrome |
| Throwing Power | Excellent compared to the standard 100:1 bath, but poor compared to Watts nickel. Conforming anodes typically used for low current density areas | Believed to be worse than hex-Cr. One user claimed it was much worse than hex-Cr; the only improvement was lowering the current density which then decreased the plating speed substantially. |
| Burning on Edges | Can be a problem, but less so with the high-efficiency bath and conforming anodes. | A much greater tendency for edge burning and nodulization. |
| Pitting Tendency | Not an issue with quality base metal and proper surface preparation. | Deposit pitting may be a problem with some Tri-Cr processes. |
| Current Interruption: | Not a problem for short periods with the high-efficiency bath. | Not a problem for short periods. |
| Able to plate Cr on Cr | Yes, with special procedure. | Unknown |
| Bath Make-Up Cost | $11.38/gallon (500 gal. bath, approx.) | Unknown, but expected to be considerably higher. |
Substrate Activation
| Item | Hexavalent Chrome | Trivalent Chrome |
| Process Cycle | Typically. DuraPrep scrub (or alkaline cleaner for high production), rinse and then reverse etch. | More extensive requiring an alkaline soak cleaner, an electrocleaner and an acid dip, with appropriate rinses for each, much like a Watts nickel process. Many more tanks are needed. |
| Primary Activation | Reverse etch in a chromic acid solution. Many operations do this in the plating bath, although not recommended. | Acidic activation with hydrochloric or sulfuric acid. Similar to Watts nickel plating. |
Bath Chemistry
Hexavalent Chrome
- Chromic acid 30 oz/gal.
- Sulfate 0.25 oz/gal.
- Secondary Catalyst A 2.5 oz/gal.
- Secondary Catalyst B 3 % volume
Overall, a simple straight-forward and easy to control bath chemistry.
Trivalent Chrome
Considerably more complex than a Hex-Cr bath requiring significantly more process control. Also, much less tolerant of bath impurities and process variances.
These baths generally consist of a chrome salt like chromium chloride, complexing agents, buffers and addition agents. This will vary from one Tri-Cr manufacturer to another.
- Trion: Uses an organic ionic liquid to complex part of the Cr ions. Chemicals like methylimidazolium chloride are typical. These are very sensitive to the water content in the bath.
- Atotech: Bath chemistry unknown.
Operating Conditions
| Item | Hexavalent Chrome | Trivalent Chrome |
| Bath Temperature | 140°F (120°F-150°F range) | Trion 122°F-150°F Atotech 113°F-140°F |
| Current Density | 2 ASI, typical (1-10 range); A broad current density range. | Generally, a much narrower current density range. Trion: 1.0 – 2.5 ASI; Atotech: 1.3 – 3.9 ASI |
| Deposition Rate (2 ASI) per hour per side |
0.00125” | Trion 0.0020” (est.) Atotech 0.0008” (est.) |
| Voltage | 6 VDC at 2 ASI, typical | Some Tri-Cr processes require 12-20 DC volts to achieve 2 ASI. More electrical energy, and cost, is needed compared to Hex-Cr. Trion: Unknown; Atotech: 12-16 V |
| Live DC Entry | Not required. | Typically required for most Tri-Cr baths. |
| Bath pH | Less than 1.0. Monitoring and control are not required. |
pH changes affect the throwing power, deposit color, plating speed and other properties. It continually increases during electrolysis and must be closely monitored and controlled. Trion: Less than 1.0; Atotech: 3.5 – 6.5 |
| Agitation | Air or eductor pumping recommended | Unknown, but believed to be similar. |
| Cr Deposit Limit | Deposits exceeding 0.100” are possible. | Trion claims up to a 0.008” deposit is possible. |
Chemical Supply
Hexavalent Chrome: Chromic acid and bath additives are a globalized market with many available vendors. Supply chain issues are not expected.
Trivalent Chrome: Uses specialty chemicals only available from a single manufacturer.
Analytical Control
| Item | Hexavalent Chrome | Trivalent Chrome |
| Chemical Variance | Fairly broad range assuming the sulfate ratio is acceptable. | Believed to require precise control of the various ingredient levels. |
| Primary Control Items | Chromic acid and sulfate. | All ingredients outlined under Bath Chemistry, and the pH. |
| Analysis Frequency | Once/week recommended with monthly tests on the impurities. However, some shops rarely have an analysis performed. | Requires continually monitoring certain control points like the pH and perhaps others. The bath ingredients and additives typically require daily analysis to stay within their desired concentration window. |
| Analysis Time: | Around 30 minutes, typical. | Unknown, could be extensive. |
Bath Impurities
| Item | Hexavalent Chrome | Trivalent Chrome |
| Impurity Tolerance | Excellent resistance to impurities. Baths with over 12 g/l of trivalent, 4 g/l of iron and copper and 50 ppm of chloride are still usable (but not recommended). | Impurity tolerance is quite low which can cause major process problems. Continuous ion exchange purification is typically recommended to keep the bath impurities in check. Apparently, Cu is a serious impurity and must be below 40 ppm. Trion claims ion exchange isn’t needed, but doesn’t explain how to deal with impurity build-up. |
| Effect of Impurities | Slower deposition rate and deposit issues if impurity levels are extreme. | Slower deposition rate and deposit issues like burning, reduced throwing power and skip plating. |
| Typical Impurities | Excess trivalent Cr, iron, copper and chloride. | Unwanted Hex-Cr build-up is a concern along with heavy metals like Fe, CU, NI, etc. |
| Source of Impurities | The water supply, chromic acid, fixtures and bussing. | The anodes generating Hex-Cr, the water supply, possibly the chemicals used themselves, the bussing and fixtures. The buss bars should be coated to reduce Cu contamination. |
Process Durability
| Item | Hexavalent Chrome | Trivalent Chrome |
| Successful Process Window (chemical control) (operating conditions) |
Broad range of chemical levels, temperature and current density ranges. Overall, an easy process to control successfully and repeatedly. | Very narrow range of bath chemical levels, impurities and operating conditions requiring much greater process control. |
| Typical Bath Life (bath stability) |
A very stable bath with an indefinite life that rarely requires dumping if properly maintained. | Considerably less stable likely requires dumping & remaking on a regular frequency, perhaps every 4-6 months. Trion Bath: dumping required every 3,028 ampere hours/gallon, estimated. Atotech: Bath dumping also required around 3,028 ampere hours/gallon, estimated. |
| Typical Rework Level | Rejects are typically less than 5% for most well-run operations that plate on reliable substrates. | Unknown, but expected to be considerably higher due to the complexity of the process and the greater difficulty of control. |
| Bath Additions | Easily made with just one primary ingredient. No need to remove a portion of the bath to make room for the chemical additions. | Many (or most) of the ingredients are liquids, so it may require dumping a portion of the bath in order to make room for the maintenance additions (a process known as bleed and feed). The bath portion removed requires waste treatment as a hazardous waste. This continual bath dilution may also be considered part of the process’s impurity control. |
| Water Evaporation and Solution Level |
Can vary widely without major problems. Evaporation is desirable for Zero Discharge. Water concentration isn’t an issue. | The Trion Tri-Cr bath is very sensitive to the water concentration and changes can greatly affect the deposit. Bath evaporation could upset this balance causing control issues. |
| Technology | Proven and accepted for over 100 years. | Unproven. |
| Zero-Discharge Capability |
Easy to adopt Zero-Discharge and the Sustainable Hex-Cr concept. | Zero-Discharge is virtually impossible and requires the operation of a waste treatment system. |
Equipment Required
| Item | Hexavalent Chrome | Trivalent Chrome |
| Number of Tanks | 2-3 depending if a separate reverse etch is used. | Could be up to 8 tanks, or more. This process is much like a Watts nickel line that requires a soak & electro cleaner, acid dip, and several rinse tanks. More tanks will be needed if a nickel underlayer is required to improve the corrosion resistance. |
| Tank Materials | Typically, Koroseal™ lined steel | Polypropylene or HDPE |
| Floor Space Required | Minimal with few tanks. | Much larger with the additional tanks required. |
| Rectifier Type | Any 3 phase such as Tap Switch, VAT, SCR or Switch Mode unit. The ripple should be 5% or less. | Trion & Atotech: Believed to require a high quality SCR or switch mode rectifier with very low ripple. Apparently, these processes are very sensitive to ripple variance. |
| Anode Materials | Lead alloy. Anode sludge build-up over the years may require disposal. | Typically, graphite or mixed-metal oxide (MMO) coated titanium. Anode sludge isn’t a reported problem; however, the expensive anode coatings require frequent replacement. |
| Fixture Materials | Aluminum or steel, typical. | Titanium clad copper is typically required for conductivity and resistance to corrosion causing bath contamination. |
| Bath Cooling | Typically, only needed if tanks use more than 3-5 amperes per gallon of bath. | Most Tri-Cr baths require continuous use of a chiller regardless of the bath volume or process efficiency. |
| Anode/Cathode Separation | Not required | Some Tri-Cr processes require separation of the anolyte and catholyte with a membrane. |
| Bath Filtration | Not required | Continuous filtration required, 1-5 µm |
| pH Control | Not required | Automatic control required. |
| Chemical Dosing | Not Required | Automated chemical additions are required. |
| Existing Hex-Cr Process Conversion |
N/A | Converting an existing Hex-Cr operation is not possible unless its expanded and completely rebuilt. In most cases an entirely new process line is required. |
Environmental and Waste Control
The difference between the Hex-Cr and Tri-Cr chemistries are important to understand if a waste treatment system is needed. Tri-chrome baths use complexed chemistry which tightly binds the Cr ion. These complexes precipitate the Cr at different pH levels while potentially leaving some Cr in solution. Adjusting the pH at both low and high levels doesn’t liberate (precipitate) all of the complexed chromium ions and some Cr remains dissolved in the waste. This makes tri-chrome waste treatment very difficult, possibly requiring some additional expensive equipment.
Hexavalent Cr on the other hand precipitates at a single distinct pH level, making waste treatment (rarely needed) more effective and less costly. The reality of Hex-Cr though is it’s an easy and inexpensive process to zero-discharge thereby avoiding the need for waste treatment.
This begs the question: Is tri-chrome really safe for the environment and the workers?
An examination of the SDS forms indicates that many of the restrictions that apply to Hex-Cr also apply to Tri-Cr. Another consideration is the larger number of hazardous chemicals used in the Tri-Cr process thereby making environmental and safety administration more involved and costly.
| Primary Pollutant(s) | Chromic acid (Hex-Cr) | Alkaline cleaners, Acid activators, Trivalent Cr (chrome chloride), Chrome complexing agents, Buffers. |
| Waste Disposal | Not needed with Zero Discharge. | Required for the plating bath rinses, bath bleeds and dumps and the pre-plate chemicals used. These all must be waste treated. This could pose costly problems for Cr compliance due to the complexing agent(s) used. |
| Waste Generation | Minimal | Considerable, especially considering the bath dumps involved. |
| Zero Discharge | Easy and inexpensive. | Can’t be used for the Tri-Cr bath. Expensive waste treatment is required. |
| Stack Test Required | Yes | Unknown |
Is Hex-Cr Really ‘Dirty’?
It is if handled and used inappropriately, but so are many other industrial chemicals. The secret to using Hex-Cr responsively is adopting the 7-Step process for Sustainable Hard Chrome Plating. This involves implementing several relatively inexpensive equipment and process modifications.
The result of using Hex-Cr responsively is true Zero Discharge with no liquid waste, no Cr air emissions and a cleaner-safer work environment. These features also lower the operational costs because all chemicals are recycled for reuse. Waste treatment isn’t needed as there’s no sewer connections and no need for a fume suppressant.
Newer Developments
Faraday Technology, Inc. of Clayton, OH is developing another trivalent hard chrome process based on chromium sulfate chemistry that incorporates pulse plating to achieve their desired deposit. Apparently, this process is still in the research stage and not yet ready for commercialization. If interested, Plating Resources, Inc. has a paper available that covers what’s currently known about this process.
Cost of Ownership
Future comparison work will involve the cost of ownership difference between the Hex and Tri hard chrome processes. This will involve the capitalization requirement (CAPEX) to install a new plating system, and the operational costs (OPEX) to operate it over a typical one-year period.
Apparently, both the capitalization requirement and the operational costing for a Tri-Cr operation are considerably higher than for a Hex-Cr operation. The difference in magnitude between these two processes will be the objective of future work.
The following hard chrome application will be used for this comparison. This is a hypothetical yet ‘typical’ hard chrome plating line for either a job shop or an OEM operation. Different size operations can extrapolate the cost difference between these two processes based upon the amount of mil. square inches plated per year. A mil. sq. inch is a surface 1” x 1” x 0.001” thick.
- Parts: Hydraulic Rods
- Base Metal: Steel, various alloys and hardness
- Size: 2” diameter x 72” long (452.39 sq. in each)
- Cr Deposit: 0.002”/side
- Production: 40 rods per 8 hr. day, 250 days/year
- Annual Production: 10,000 rods/year
- Annual Mil Sq. Inch Plated Area: 9,047,808
Eric Svenson Sr. is a Master CEF / IUSF and President of Plating Resources in Cocoa, Florida. Visit www.Plating.com
