Controlling Chrome Bath Impurities

Hexavalent hard chromium plating baths are somewhat tolerant to many types of contamination.

Eric Svenson Sr.It is common for a bath to produce acceptable deposits with up to 5.0 grams per liter of metallic contaminants, typically iron and copper. Above this level the plating quality decreases and the operational cost increases.

To put this in perspective 5 g/l in a 1,000-gallon tank equates to almost 42 pounds of tramp metal. That’s a lot and shows just how forgiving the hexavalent chrome bath really is. However, that’s not to say it can or should be abused. Plating shops need to start thinking of their chrome baths as the lifeblood of their business, which it truly is.

Contaminated baths do not provide the degree of quality required in today’s business environment. The impurity levels in chrome baths should be closely monitored and controlled. The benefits of maintaining a bath with lower impurity levels s are higher production, lower electrical costs, less stripping and rework, and much lower operational costs. In short, it pays to monitor and control bath impurities.

Impurities have a negative effect on deposit quality and the objective should be eliminating or reducing their build-up as much as possible. A properly maintained chrome plating bath will have a life of over 20-30 years while an abused bath could last for only a few months.

Effect on Quality

It is very common for chrome shops to operate with high impurity levels. For unknown reasons, they ignore the negative effects impurities have. Some claim ignorance, others say that had always done it that way or they were too busy to manage the bath properly.

The degradation in plating is more pronounced in a standard 100:1 bath than it is in a Dura bath because of its greater impurity tolerance. The effects of high impurity levels include brittleness, peeling, burning, pitting, treeing, nodulization, roughness, poor deposit color, reduced coverage, skip plating, increased deposit stress, macro-cracking, soft deposits, slower deposition rates and much higher electrical costs because of the extra voltage required.

‘Active’ Chromic Acid

Bath impurities combine with chromic acid to form an ionized complex. The result is the "free" or available chromic acid is effectively lowered. This is one reason that impurities cause problems as the bath now operates with a much higher effective sulfate ratio than it should.

Trivalent, copper, and iron form a complex with the chromic acid by a factor of around 2.4 oz./gal. per unit of impurities present. This lowers the effective chromic acid level and upsets the sulfate ratio. This means a 30 oz/gal. 100:1 bath with 5% trivalent only has 18 oz/gal. of ‘effective’ chromic acid and an ‘effective ratio’ of 60:1. Nonetheless, you cannot simply lower the sulfate to ‘correct’ the ratio because this doesn’t work.

Impurity Sources

Impurity sources often include reverse etching in the plating bath, excessive ID plating, oils and machining particles left on the parts, chrome misting on the buss bars, fixture corrosion, drag-in from prior baths, debris-laden plant air, using impure chemicals and the plant water supply. Some of these involve issues with the equipment design while others are procedural problems.

Tolerable Levels

Any ion not intentionally included in the original bath formulation is considered an impurity. As impurity levels increase so does their negative effect. Some impurities, however, are more damaging than others.

Establishing an upper limit for a particular impurity is not realistic without considering the concentration of all other impurities. This is because combined impurities often have a greater negative effect on the bath than a higher level of any single impurity would. It is the accumulative concentration that is important unless any single impurity is already at its maximum level. Also, thicker deposits are more affected than thinner deposits are.

General limits are shown for the typical impurities. The chrome bath should be maintained to not exceed these levels.

See Controlling Impurity Levels below for details on the Total Contaminant Level (TCL). Other metallic impurities can include aluminum, nickel, tin, lead, and zinc. However, these are not as common as copper and iron and they don’t have as much of a negative effect on the bath. The same thing applies to carbonates, nitrates, silicates, and phosphates.

Production Costs

Contamination decreases the bath’s conductivity and increases its specific gravity (Baume). Lower conductivity means the voltage must be increased to maintain the same amperage. Contaminated baths require operation at much higher power levels. A relatively pure bath may normally operate at 6 volts DC while that same bath with high impurity levels may require 12 volts. If the DC current was 10,000 amps. that the 6-volt increase means that an additional 60 KW of electricity is being wasted, or about 120,000 KWH per shift year. Assuming a rectifier efficiency of 90% and an electrical rate of $0.15/kwh the additional electrical cost of operating this bath with high impurities amounts to $20,000 per shift year. This along with the extra rework, labor, and slower production means the additional annual operational cost could well exceed $100,000 per shift year for a small operation.

As an example, a recent study of a larger shop using a 2,000-gallon hard chrome bath and a 5,000 amp. rectifier operating with a single shift indicated that it saved over $442,000 a year by simply controlling its impurity levels. That savings came from the lower electrical costs, faster plating speeds (increased production), and a significant reduction in their stripping and re-plating costs.

Controlling Impurity Levels and TCL

Bath with low TCL with a better design.Bath impurities are a fact of life and they will never be zero, even for a freshly made solution. We can not stop them from entering the bath, but we can control their entry rate and then purify the bath when they reach an established upper limit.

An easy way to evaluate the impurity level in a chrome bath is by calculating the Total Contaminant Level, or TCL for short. This is simply the sum of the percent trivalent and the g/l of iron and copper combined. As an example, a bath with 3.5% trivalent, 5.5 g/l of iron, and 4 g/l of copper would have a TCL of 13 which is abnormally high and is well beyond the point of obtaining a quality deposit and keeping operational costs low.

For most operations, the TCL should not exceed 7.2 and there is no point in purifying the bath to below a 4.8 TCL amount, except for operations plating very critical parts. The TCL range of 4.8 - 7.2 therefore provides the best efficiency and deposit quality while also keeping the operational costs lower.

Decanting and remaking a portion of the bath has proven to be the least costly, fastest, and most efficient way to lower the TCL to 4.8 when it reaches the 7.2 level. In this case 1/3 of the bath would be removed and remade. The amount that needs decanting will be reduced if the trivalent is first dummied down to the 1% level using CR-3 Reducer and the proper technique.

Decanting for purification is ‘old-school’ technology, but there really is not a better method available for controlling impurities. Ion exchange is overly expensive and generates too much hazardous waste, electrodialysis is burdened with technical problems and porous pots don’t work in spite of what others say. This is a case where ‘Old-School’ technology still is the best and least costly approach.

Operating with high impurity levels is very expensive on many fronts. You should not look at impurity control and decanting as an expense but as an investment in plating quality and cost reduction. Plating Resources, Inc. can help by providing impurity analysis, supplying the CR-3 Reducer, and establishing a bath impurity control program for your company.

Trivalent Chrome

Trivalent chrome is the most common impurity found in hexavalent plating baths. It is usually measured as a percentage of the chromic acid concentration. A 1% trivalent chromium level is needed for optimum plating results, but trivalent acts as a contaminant at around the 2% level.

The conditions that generate excessive trivalent is a long list that is best left for another article. However, one of the most important issues in keeping low trivalent levels is maintaining the optimum anode:cathode ratio. Anything that adds metallic impurities like iron and copper also increases the trivalent concentration, so this acts like a double whammy. Each g/l of dissolved metallics also increases the trivalent concentration by about 1%.

The effect of excessive trivalent is 6 times that of iron contamination and it lowers the tolerance level of other bath impurities. Other trivalent issues include higher electrical costs, slow plating speeds, substandard deposits, and excessive chrome mist fuming.

Fortunately, trivalent is easily removed by dummying with the CR-3 Reducer. This should be done any time its concentration is 2% or higher. Baths with a 5-6% trivalent level are severely impaired.

Metal Ions

Iron and copper are the most common metallic impurities, but these can also include aluminum, nickel, tin, and zinc. Metallic impurities also increase the bath's resistance, increase the electrical costs, and contribute to poor deposit quality. The normal upper limit of total heavy metal impurities is 5.0 grams/liter, but this only applies if the trivalent level is lower than 2%.

Contrary to earlier beliefs, these metals do not plate out during electrolysis of the bath, so they never get lowered during use and they only continue to build up in the bath.

Copper

Copper is a very common impurity in chrome baths and it is quickly dissolved in the bath. Typical sources are copper immersion, reverse etching, rack tips, dropped copper parts, chrome misting on buss bars, etc. Dropped copper parts must be retrieved immediately. The negative effects of copper are similar to trivalent and iron, although copper has a much greater effect. The upper limit of copper is 5.0 grams per liter if no other impurities are present or around 2.0 g/l if combined with iron and trivalent.

Iron

Iron is also a very common impurity. Typical sources of iron include a gradual build-up from reverse etching, metallic chips or "fines" not cleaned from the parts, using steel anodes instead of a recommended lead alloy, dissolution of steel wire supports or robbers, and uncoated steel fixtures. Its upper limit is around 5.0 grams per liter if no other impurities are present or around 3.0 g/l if combined with other metals such as copper.

Chloride

Chloride is a very powerful impurity in the hexavalent chrome bath and its negative effect starts at very low concentrations. As little as 50 ppm acts as a strong impurity and it should always be maintained below 20 ppm. Chloride acts like a halogen catalyst that has the effect of interfering with the sulfate ratio.

Excess chloride causes dull, hazy, and gray deposits, roughness, burning, reduced covering power, white spotting, skip plating, and macro-cracking and it increases the trivalent level because it removes the peroxide coating on the anodes. It also attacks the lead alloy anodes causing them to deteriorate faster.

Typical sources of chloride include drag-in from a hydrochloric acid stripper or a prior nickel bath, the use of chlorinated solvents for precleaning, the plant water supply, or a breakdown of tank linings and other PVC items.

Fortunately, removing excess chloride is fairly easy. The recommended method is dummying the bath with a 30:1 anode ratio when used along with CR-3 Reducer. Chloride can also be removed by precipitation with silver oxide, but that approach is excessively expensive.

Other Ions

As mentioned, any ion not intentionally added to the bath can be considered an impurity. However, many of these do not have the negative effect that the primary impurities do, unless present in very large amounts. These ions include alkaline metals (barium, calcium, magnesium, sodium, potassium, and strontium), carbonates, hydroxides, nitrates, silicates, and phosphates. The hexavalent chrome bath is fairly tolerant to these and they do not normally cause problems.

Oils and Organics

Bath contamination from oils and organics is fairly common. Generally, the chrome bath is somewhat tolerant to these items but organics are quickly oxidized by hot chromic acid which increases the trivalent level. Typical sources include oils on parts, agitation using unfiltered plant air, unsuitable stop-offs, non-permanent fume suppressants, paint chips, breakdown of tank linings, or any other organic material that comes in contact with the chrome bath.

Also, certain saturated aliphatic hydrocarbons are not easily oxidized and can form scum on the bath surface and tank walls. These should be removed by skimming so they do not coat the parts on entry and cause skip plating.

Fume Suppressants

Most fume suppressants are organic in nature and some can break down forming trivalent chromium. This is particularly a concern for the non-permanent type of suppressants.

Particulates

Foreign particles are solids that get introduced into the bath; they do not dissolve. Their introduction into the solution typically comes from uncleaned parts, airborne debris from machining or polishing operations, and sludge from the anodes.

Airborne debris can come from nearby blasting, grinding, honing, or polishing operations. Ideally, the plating shop would be separated from any polishing or machining areas. Besides causing plating issues, they also tend to plug the mesh pads and HEPA filters used in modern ventilation systems. Foreign particles are a common source of roughness in the deposit.

The use of Dura-Prep scrubbing prior to chromium plating effectively removes these particles and helps to prevent deposit roughness. Although not a very common practice, bath filtration is a good remedy for removing particulate matter from chrome baths. Particulates are broken down into two categories, magnetic and non-magnetic.

• Magnetic Particles: Magnetic particles typically come from improperly cleaned parts. These get dislodged in the bath are attracted to the cathode and get plated over causing roughness and nodules. A common source is the inside of hollow cylinders that get plated on the OD and are not adequately cleaned in the ID. The following grinding or polishing operation breaks off these nodules thereby causing a pit. A magnet run through the bath using agitation will show the presence of these particles.
• Non-Magnetic Particles: Non-magnetic particles can come from the plant atmosphere, abrasive debris, and anode sludge. Any of these can be a source of roughness. Anode sludge, however, is not usually a problem as this has a much higher specific gravity than the bath so it often settles harmlessly to the bottom of the tank.

Barium sulfate sludge present in the bath from sulfate treatments with barium carbonate can also cause roughness. These treatments are, therefore, best performed in a separate tank.

Water Supply

The plant water supply is another common source of bath impurities. Chlorides and sulfates are typically found in city tap water and these add to the impurities problem. Chlorides tend to poison the bath and sulfates continually increase in concentration thereby upsetting the ratio whenever city water is added. Therefore, a city water purification system is highly recommended which typically consists of both RO and DI purifiers.

Contaminant Chelation

Dura-76 is an additive developed to chelate the typical impurities of iron, copper, and trivalent thereby helping to improve the bath’s performance. It can be used in all hexavalent chrome bath types except for those with a high fluoride level. Chelation ties up these impurities and helps the bath perform better. These ions do not precipitate but instead become less active in the cathode reaction.

Dura-76 is especially useful in new baths to chelate impurities before they become a problem and it buffers the bath to improve the throwing power. Dura-76 is not a ‘cure-all’ for high impurities, they still need to be controlled and reduced.

Controlling Impurities

This should be approached as an ongoing quality and cost reduction issue and not taken for granted or ignored. Bath contamination is very expensive in terms of poor deposit quality, increased stripping and rework rates, and high electrical costs. Many shops struggle with these issues without gain when they are eventually forced to dump and remake the bath, all the while suffering from high costs and poor quality.

These frustrations and high costs could have been avoided by simply controlling their impurity levels. Some of the direct benefits include:

• Improved deposit quality.
• Reduced rejects and rework.
• Increased production and income.
• Significant reductions in overhead costs.
• Better customer satisfaction and loyalty.

The efforts to prevent and control impurities should not be viewed as an expense, but rather as a cost-savings program and an investment in your company’s ongoing success.

Dealing with High Impurities

Many hard chrome platers are dealing with high impurity levels and this is hindering their business. Plating Resources, Inc. can help forward-looking companies deal with this problem.

We start with a complete bath analysis to identify the impurity levels and then provide a plan to reduce the impurities that are causing problems. A monitoring program can be established to determine the build-up rate and a routine purification program to keep the impurities within the recommended limits.

The goal is to address these issues before they become a major production problem. Understand, however, that some baths are so contaminated that purification can’t be fiscally justified. Continuing production in this condition is pointless and only leads to further problems and ongoing higher costs. The least expensive approach is making up a fresh solution. The bath will eventually be dumped anyway so it’s better to deal with it now before it causes even greater difficulties.

The bottom line is the time, effort, and minimal cost associated with maintaining bath impurities within their recommended range is easily justified.

Eric Svenson Sr. is CEO of Plating Resources, Inc. Please contact the author at PlatingResources@yahoo.com  with any comments or questions on this information, Visit http://www.plating.com