From Chrome to Chrome-Free: Various Coating Processes Determine Optimum Use for Prepainted Aluminum Products
Published: Aluminum International Today May/June 2015
Prepainted aluminum parts are extensively used throughout many industries, from assembly, all the way to the marketplace and the after-market parts business. Industries which rely heavily on prepainted aluminum include, but are not limited to, automotive, aviation, electronics, marine, medical/dental, home furnishings, and commercial buildings.
Because of the extraordinary combination of properties, such as light weight, durability, and resistance to corrosion and the elements, the use of aluminum components in these various industries has continued to grow strongly. Prepainted aluminum can be used as a component in the manufacturing process, a part of a completed product, or a finished piece.
Due to its chemical properties, aluminum substrates, like many other metals, need an optimal protection to be safeguarded from environmental factors. For many decades, chrome-based coatings were the standard-bearer for aluminum pretreatment; however, chrome-based coating processes are no longer used in many markets because of regulations, changed market demands, and issues related to health, safety, and environmental protection.
For some years now, chrome-free technologies have successfully been introduced as a reliable alternative for aluminum coating. There is little difference between the chromium-containing and chromium-free technologies with regard to the process sequence, but it helps to know how the industry has changed over the decades with the switch to more chrome-free applications.
The beginning of chromate coatings
The use of chromate coatings began in the early part of the 20th century which roughly coincides with the use of aluminum for light-weight but strong parts for aircraft. To make the metal stronger, aluminum alloys were made (especially using copper); however, corrosion was a concern that was not address completely by normal coatings (paints).
Scientists and engineers discovered that a film of chromate could significantly reduce the corrosion of aluminum. Experiments also concluded that a film of chromate also greatly improved the adhesion and corrosion resistance of paint and other decorative coatings on aluminum.
As the rest of the 20th century unfolded, aluminum was found to be useful in many other applications, again because of the advantages of light weight and increased corrosion resistance when chromated. This was particularly true for the architectural industry and more especially the extrusion and coil industries. Decorative coatings (paints) applied over chromated aluminum had very good adhesion and corrosion resistance when compared to bare aluminum.
The chromating process in those times involved five to seven stages, consisting of clean, rinse, deoxidize, rinse, chromate, rinse, final rinse. The accepted cleaning step became an etching/alkaline cleaner step, requiring an acid deoxidizing (or desmutting step). The deoxidizing step was often used in the extrusion industry because the cleaning process needed a stronger solution and was measured in minutes rather than seconds; therefore, the extrusions had more opportunity to build a layer of oxides (smut).
The coil industry generally used a “final rinse,” another solution of chromate that was less aggressive than the chromating stage. This final rinse was added as insurance in case some areas of the coil strip were not adequately chromated because of the relatively shorter chromating time. Chrome-based coatings provided the substrate with excellent adhesion and corrosion resistance. These coatings were also very forgiving if the substrate was under or over cleaned.
Concerns about using chromate
Beginning in the 1950’s concern began to build over the toxicity of chromates. The EPA and OSHA began during the next two decades to introduce increasingly strict restrictive regulations over their exposure and release to the environment. Waste treatment systems were now required to remove chromium from plant discharges at much lower limits. The use of chromates therefore incurred a high cost penalty because the process generated a large volume of chromate wastewater.
Research began in earnest during this period to reduce chromium, at least on coil coating lines. The approach used was to apply a chromate treatment directly onto the metal surface, rather than spraying or immersing the aluminum into a reactive bath followed by rinsing. These new chromates were “pre-reacted” to resemble a chromate coating or pre-treatment reaction with the metal surface, so rinsing was no longer required.
As a result, the solution was “dried-in-place” onto the metal surface. Unfortunately, this approach was possible only for two dimensional applications, like sheets or coil coating. A 3D application used in the extrusion industry would have to use chromate solutions (chrome phosphate) for a while longer. The use of dried-in-place chromate solutions for coil-coated aluminum offered many immediate advantages:
- Greatly reduced amount of chrome-bearing wastewater. All of the chromium solution is at the chemcoater stage while the rinse and final rinse stages are eliminated.
- The chromium is carried out on the metal surface instead of continuously circulated as in the case of immersion or spray stage. The need for bath stabilization or continuous overflow is eliminated.
- Pretreatment coating weights can be controlled to a comparatively low range which results in more consistent quality.
- The only major source of chrome-bearing wastewater is from cleaning the chemcoater equipment (solution pan, applicator rolls, hoses, etc.), which is easily segregated into a reduced volume.
Although the dried-in-place application of chromate pretreatment solved many issues, regulatory pressures continued to build rapidly where chromium had to be removed completely in the very near future. Additionally, dried-in-place chromium pretreatments still didn’t work for the extrusion industry.
Industry converts to chrome-free technology
Research work began in earnest on eliminating chromium entirely. By the year 2000, successful uses of chrome-free treatments were being demonstrated on aluminum extrusion lines. When it was demonstrated that a chrome-free process could meet the AAMA standards, more manufacturers and suppliers quickly adopted the chrome-free technology.
Some aluminum coil coating paint lines made the conversion to the non-chrome dry-in-place treatments with good results. These products were titanium based with additions of polymers. They were aggressive materials so the chemical coater had to change from mild steel to acid- resistant trays, pick-up rolls and other similar pieces of processing equipment.
The new chrome-free treatments typically replaced the chromate chemicals (which included phosphoric and hydrofluoric acid) with fluocomplexes of zirconium and titanium (which included specialty polymers). Surprisingly, typical pretreatment coating weights were an order of magnitude less than the chromate based versions. This industry typically ran the chrome phosphate coatings between 60 – 120 mg.ft2. The new chrome-free pretreatments were usually around 6 – 12 mg/ft2.
The chrome-free coatings were still usually applied as a recirculation spray, but the coating was nearly invisible and uniform, so the 3D nature of the processed parts were no longer a major constraint against its use.
Ironically, it was now the coil coaters who were slow to adopt this new technology for several reasons:
- Most coil coating lines are multi-metal toll coating lines that prepaint steel and zinc- coated steels as well as aluminum. Only the chrome dried-in-place coating could be run on all three major substrate types with proven results.
- Coil coaters were comfortable with the advantages of the chrome dried-in-place pretreatments and were not eager to be first in running chrome-free pretreatments.
Further research focused on extending the dried-in-place technology to the pretreating of all metals that were processed on coil coating lines without any reduction in quality. This work was mostly done during 2000 through to the present. This technology has been developed to the point that its acceptance is growing as a result of its proven success.
Various processes used for non-chrome pretreatments
Manufacturers and suppliers involved in the treatment of coil coated aluminum follow a variety of best practices to ensure the highest quality finished product, which can last for decades if the aluminum is cleaned, pre-treated and painted correctly.
Today, alkaline cleaners are the primary materials used in the United States for cleaning aluminum because alkaline is less expensive and can be used in mild steel equipment. On the contrary, acid cleaners with fluoride are more expensive to operate and must be used in an acid- resistant tank or spray chamber.
Alkaline cleaners can aggressively etch the aluminum surface, removing aluminum fines and oils from the metal’s surface. If operated at too high of a concentration, alkaline cleaners can over clean or over etch the surface, leaving smut. A high degree of smut (residual alloying elements and/or their oxides) can be detrimental to the adhesion and corrosion resistance of the final product.
Even though acid-based cleaners are more expensive they can be beneficial in the processing stage. Acid cleaners, typically sulfuric/hydrofluoric products, will dissolve aluminum fines from the surface of the metal while removing light oils. This provides an ideal surface for further applications.
The measurement of these pretreatments is generally by X-Ray Fluorescence (XRF); however, the extremely low levels of all these pretreatments make coating measurement a challenge. This is especially true since the elements being measured are generally alloying elements within the aluminum alloy itself. Careful blank measurements and subtraction of this blank is necessary.
What’s next in aluminum pretreatment? Advances in chrome-free development continue to this day. The previous use of chromium may have inhibited research since chromium enjoyed early success in aluminum pretreatment.
Since chrome-free research continues in many directions (zirconium, titanium, silanes, novel polymers, etc.), there are no set standards on their use. It may be another 10-15 years before chrome-free applications become standards in the industry.
Ed Musingo of Bulk Chemicals, Inc. serves as the NCCA Environmental, Health & Safety Committee Chair. Ed helped research and compile this article. The National Coil Coating Association (NCCA) is an established trade organization dedicated to the growth of coil coated products. Founded in 1962, the NCCA consists of more than 100 member companies that are at the forefront of the industry. Members include coil coating service providers as well as manufacturers and suppliers of metal, coatings, chemicals and equipment. Members of NCCA also process and warehouse the coil coated products prior to delivery to user industries. For more information, contact the NCCA at 216-241-7333 or email at ncca@coilcoating.org. Visit the NCCA website at www.coilcoating.org.
