Mining & Materials

Alternatives for significant uses of hexavalent chromium in Massachusetts

Introduction

In July 2005, the Commonwealth of Massachusetts requested that the Toxics Use Reduction Institute perform an alternatives assessment for five chemicals. For each chemical, the Institute was charged with identifying significant uses in manufacturing, consumer products, and other applications; reviewing health and environmental effects; and evaluating possible alternatives. The results of this study will serve as a guide for those seeking safer substitutes to the five chemicals discussed here.

Presented here is an executive summary of the findings of for high priority uses of hexavalent chromium in Massachusetts. The full report, the Five Chemicals Alternatives Assessment Study available from the link below, presents extensive factual information on each alternative.

Chromium is a metallic element found in nature in the form of chromite ore or the mineral crocoite. Chromium provides manufactured products with hardness, shininess, durability, color, corrosion resistance, heat resistance, and decay resistance. Important uses of chromium compounds include wood preservation, metal processing, leather tanning, and production of pigments. The major application of chromium is in the production of alloys, primarily stainless steel; historically, this has amounted to 50-60% of total chromium use.

There are several oxidation states of chromium, each with its own chemical characteristics. The most common forms are trivalent chromium and hexavalent chromium. Trivalent chromium compounds occur naturally, while the hexavalent compounds result primarily from industrial activity.

Hexavalent chromium poses far more health hazards than trivalent chromium. Short-term effects of hexavalent chromium exposure can include eye and respiratory irritation and sensitization. In large quantities, ingestion of hexavalent chromium compounds can result in acute gastroenteritis, vertigo, gastrointestinal hemorrhage, convulsions, ulcers, kidney damage or failure, and liver damage or failure. Acute skin exposure can cause burns, liver damage or failure, kidney damage or failure, and anemia. Effects of chronic skin exposure include dermatitis, hypersensitivity reactions, eczema, and kidney or liver damage. Hexavalent chromium is classified by the International Agency for Research on Cancer (IARC) as a known human carcinogen (Group 1).

Workers have the highest risk of adverse health effects from hexavalent chromium exposure. The industries with the greatest risk of occupational exposure are chrome electroplating, stainless steel welding, metal coating and painting, printing, textiles, leather tanning, wood preservation, and cement or masonry work.

The Institute assessed three general categories of use: decorative chrome electroplating; hard chrome electroplating; and chromate conversion coatings. The category of chromate conversion coatings was narrowed further to focus only on passivation of zinc and zinc alloy plated parts and zinc galvanized steel.

Decorative chromium electroplating of consumer and automotive products

Decorative chrome plating is used for consumer applications such as appliances, metal furniture, plumbing fixtures, knobs and hand tools, and for automotive trim. It creates an attractive blue-white finish and helps to reduce tarnishing.

The major advantage of decorative hexavalent chromium is its appearance, especially its blue-white color. It also presents some processing difficulties. These include poor throwing power (a measure of coverage in recessed areas of a part being plated), low resistance to burning during plating, difficulty in removing impurities from the plating bath, problems in rinsing the plating solution from the plated parts, and intolerance to interruptions or variations in the electrical current during plating.

Two alternatives for decorative chrome electroplating were assessed: trivalent chromium plating baths, and low temperature arc vapor deposition of trivalent chromium.

    1. Trivalent chromium plating baths use a very similar process to that used in hexavalent plating.
    2. Low Temperature Arc Vapor Deposition (LTAVD®) is a proprietary process in which parts to be coated are exposed to a vaporized metal that condenses on the parts, depositing a thin, solid film.

Results of the assessment:

  • Health. Both options are superior to hexavalent chromium plating from the perspective of carcinogenicity and occupational exposure standards. LTAVD® is superior from the perspective of skin irritation/sensitization, and trivalent chromium baths are either similar or superior to hexavalent chromium baths on this metric.
  • Environment. Both options are superior to hexavalent chromium plating from the perspective of waste generation. LTAVD® avoids the need for a lead anode; trivalent chromium baths may or may not use a lead anode.
  • Technical criteria. Criteria of interest include uniformity of coating, adhesion to substrate, hardness, color, and resistance to corrosion and wear.

    Performance Factor

    Trivalent Chromium

    Hexavalent Chromium

    Throwing Power Good Poor
    Covering Power Good Poor
    Tolerance to Current Interruptions Tolerant Intolerant
    Tolerance to Rectifier Ripple Tolerant Intolerant
    Micro-discontinuous Structure Micro-porous or Micro-cracked Special Process Required
    Susceptibility to Burning Little Great
    Ease of Rinsing Easy Moderate
    Color Buffering Required Never Occasionally
    Removal of Impurities Easy Hard
    SO4,Cl,H3BO3 Contamination No Effect Very Detrimental
    • Decorative trivalent chromium plating has many processing advantages over hexavalent chromium plating. Examples of these advantages include superior throwing and covering power; tolerance of electrical current interruptions; low susceptibility to burning; and ease of rinsing and removing impurities. Trivalent chromium plating has a naturally micro-porous structure, which is advantageous for corrosion resistance. In the past, the color of trivalent chromium plating was a disadvantage, but recent developments now make it possible to produce a trivalent plate with an appearance equivalent to that produced using hexavalent chromium. See the following table:

  • The LTAVD® alternative operates at room temperature, making it possible to use it on a substrate with a low melting point, such as plastic. By using different combinations of gases and metals, a variety of coatings can be formed. Metals with dissimilar characteristics, such as titanium and aluminum, can be alloyed using this process, creating unique coating materials. Most of the technical assessments of LTAVD® have been conducted by the company that holds the patent rights. Findings of these assessments indicate that LTAVD® produces a very uniform coating with good adhesion to the substrate, corrosion resistance similar to or better than that of hexavalent chromium, color similar to that produced with hexavalent chromium, and hardness superior to that produced with hexavalent chromium.

 

  • Cost. Trivalent plating chemicals are more expensive than hexavalent plating chemicals, although economies of scale are likely to lead falling prices as trivalent systems increase in popularity. The cost of chemicals, however, is offset by the greater efficiency of the trivalent process and greatly reduced costs for exposure control and disposal. One study estimated that the volume of sludge generated by the hexavalent process is about 30 times that of the trivalent process. Another found that hexavalent treatment costs were nearly 10 times that of the trivalent process. While cost information for LTAVD® has not been published, the process is being used by several major manufacturers of consumer hardware, indicating that it is commercially viable. Since a wide variety of gases and metals are used, material costs also would vary accordingly. A major operating cost would be energy. Waste treatment costs are likely to be minimal.

 

Hard chromium electroplating of industrial components

Hard chrome plating, also known as functional or industrial chrome, typically is thicker than decorative chrome. It is used on industrial components that must perform under demanding conditions such as high temperatures, and repetitive grinding and impact forces (such as aircraft engines and landing gear, hydraulic cylinders, and drill bits). Unlike decorative chrome, appearance usually is not an important issue.

The two main reasons for using hard chrome are to provide wear and corrosion resistance, and to rebuild worn components to precise dimensions. It has a low coefficient of friction, is hard and heat-resistant, adheres well to substrates of various geometries, and provides corrosion resistance.

Hard chrome plating suffers from a number of technical limitations. The plating process involves numerous steps, which may need to be repeated in order to achieve an adequate coating. The coating can be brittle, leading to failure or reduced corrosion resistance. It can also be difficult to achieve even plating thickness.

The Institute assessed six processes that can serve as alternatives to hard chromium electroplating:

  • Thermal sprays include high velocity oxy-fuel (HVOF) and plasma sprays. Thermal spray is a coating process in which wire or metallic powder is melted by a high-temperature flame and sprayed as particles or droplets onto a substrate.
  • Weld facing is a dry method of joining a hard coating, edge, or point to a metal or alloy substrate to improve its resistance to abrasion, corrosion, heat or impact. It also is used to restore worn surfaces.
  • Heat treatments and plasma nitriding methods use heat to diffuse elements into the top surface of a substrate metal to form an alloy or layer with desired properties.
  • Nanocrystalline coatings use electrodeposition, vapor deposition, or spray conversion processing to deposit very small grains of crystalline alloys on a metal substrate.
  • Vapor deposition: In physical vapor deposition (PVD), parts to be coated are exposed to a vaporized metal that condenses on the parts, depositing a thin, solid film. Types of PVD processes include ion plating, vacuum evaporation, thermal evaporation, electron beam evaporation, and sputter deposition. Chemical vapor deposition (CVD) is similar to PVD, but uses gases that combine on a hot surface to form the hard coating.
  • Functional trivalent plating: The Faraday Technologies’ Faradaic™ process is similar to the wet hexavalent plating process, with the capability to plate a thick, functional chromium coating using a trivalent chromium plating bath. It is intended as a “drop-in” alternative to hexavalent baths.

Some of these categories include several related processes that differ in their functional details. In addition, the categories often overlap to a certain extent, so that a given process may be classified differently in different sources. Surface coatings of various materials, typically other metals, alloys, and metal carbides or nitrides, can be applied using these processes. Coatings that may be used to replace hard chrome include those based on titanium, tungsten, cobalt, aluminum and silicon.

For each of these alternatives, the Institute assessed human health, environmental, technical, and cost criteria.

  • Health. All the alternatives are superior to hexavalent chromium from the perspective of carcinogenicity. However, there are health hazards associated with the alternatives as well. For example, thermal sprays may contain cobalt powder, which is classified as possibly carcinogenic to humans. This is an improvement over hexavalent chromium, which is classified as a known human carcinogen.
  • Environment. All the alternatives are superior to hexavalent chromium from the perspective of waste generation.
  • Technical criteria. All of the alternatives have the potential to offer equivalent or better performance compared to hard chrome plating, although several have some limitations in their application. However, given the range of alternative processes and coating materials, there is likely to be at least one alternative that can meet the technical requirements of every hard chrome plating application.
  • Cost. Many of the alternatives require a significant capital investment. On the other hand, the manufacturers of these systems claim that operating costs are significantly reduced. In some cases, new equipment may pay for itself within a few years through reduced operating costs.

Passivation of zinc plated parts and zinc galvanized steel

Passivation is a surface treatment that provides resistance to corrosion. The protection is afforded by a film or thin coating that interacts with the underlying metal. Hexavalent chromium is a standard passivating chemical for zinc and zinc-alloy plated parts, and zinc galvanized steel.

In passivation with hexavalent chromium, zinc plated parts are dipped into an acidic solution containing a mix of chemicals. The solution reacts with the plating to form a film of zinc chromate and other chromate compounds in both the trivalent and hexavalent state. This is referred to as a “conversion coating” because the hexavalent chromium solution converts the surface to zinc chromate. The hexavalent chromium reacts with the metal, forming an inert trivalent chromium layer with “releasable” hexavalent chromium ions that inhibit corrosion. The residual hexavalent chromium in the film will repassivate any areas on the surface that become compromised due to chemical or mechanical damage to the area. This property is referred to as “self-healing.”

The Institute selected three alternatives for study: molybdates, trivalent chromium compounds, and mineral tie-coat.

  • Molybdate-based coatings inhibit corrosion by forming a protective oxide layer on metal.
  • Trivalent chromium passivates exist in several types. They vary in appearance, performance characteristics, thickness of the coating, and other characteristics.
  • The mineral tie-coat process is a patented method of applying a thin mineral film on the surface of metal parts. It involves cleaning and conditioning the surface to be plated, immersing it in a sodium silicate solution, and then electrodepositing a mineral coating. The reaction between the coating and the metal surface forms a new protective surface.

For each alternative, the Institute assessed health, environmental, technical, and cost criteria.

  • Health. All the alternatives offer significant improvements over hexavalent chromium from the perspective of carcinogenicity and occupational exposure. Chemicals used in the trivalent chromium passivation process may pose skin irritation/sensitization hazards similar to those used in the hexavalent chromium process.
  • Environment. All of the alternatives offer significant improvements in terms of their environmental impact, although chemicals used in some molybdate formulations are toxic to aquatic life.
  • Technical criteria. Performance criteria of interest for passivation of zinc include corrosion resistance, heat resistance, torque/tension performance, and appearance.
    • Several technical evaluations have concluded that molybdates do protect against corrosion, but do not perform as well as hexavalent chromium passivation on this metric. Trivalent chromium may be inferior, equal, or superior to hexavalent chromium on this metric, depending on the thickness of the coating, the plating method, the additives, and whether a topcoat was used. According to the manufacturer, mineral tie-coat has superior corrosion resistance when used with a topcoat.
    • Trivalent chromium compounds do not have the “self-healing” properties of hexavalent chromium, and require a sealer/topcoat in order to offer the same level of corrosion resistance. The manufacturer of the mineral tie-coat process claims that it is equal to or better than hexavalent chromium in corrosion resistance (with topcoat), heat resistance, and torque/tension performance.
    • Trivalent chromium coatings differ in appearance from hexavalent chromium films. For most applications, color is a matter of user preference rather than of performance. In cases where a specific color is required, topcoats or sealers can be used to achieve the desired effect.
    • The molybdates offer better heat resistance than hexavalent chromium.
  • Cost. Little cost information is available for these alternatives. One analysis indicated that a molybdate-based process would be similar to a hexavalent chromium process in terms of labor and capital, more expensive for chemicals and energy, and less expensive for waste processing.

Additional Information

 

 

Further reading

 

 

Glossary

Citation

Institute, T., Harriman, E., & Ellenbecker, M. (2013). Alternatives for significant uses of hexavalent chromium in Massachusetts. Retrieved from http://www.eoearth.org/view/article/51cbece77896bb431f68e5f4