The total amount of added metal can make up as much as 15% of the alloy. Alloys are categorized into series based on the alloying metals and are assigned a four-digit code (the first digit identifies a series). For example, the 1xxx series is comprised of almost pure aluminum, the 6xxx series is comprised of alloys containing silicon and magnesium and the 7xxx series is comprised of alloys containing primarily zinc.

Pure aluminum and the 3xxx, 5xxx and most of the 6xxx series alloys are corrosion resistant and can be used in the industry without surface treatment or painting. The higher-strength 6xxx alloys and all the 2xxx and 7xxx alloys typically do require coating.^{ref 2} Depending upon the alloy, and the ultimate use of that alloy, the types of surface preparation and paint utilized can vary somewhat. For example, the surface preparation and coating used for extruded windows, doors and other preformed shapes, which are commonly of the 6xxx series, are different than that used for the high-strength and corrosion-susceptible 7xxx series alloys widely used in the aircraft industry.

ALUMINUM OXIDE: A PROTECTIVE LAYER

The inert, tightly adherent aluminum oxide (Al203) layer that forms on both pure aluminum and most aluminum alloys provides excellent corrosion resistance in many environments. At ambient temperatures the thickness of the oxide layer ranges from about 2.5 to 5 nm (1 nanometer [nm] = 10 angstroms [A?] = 1 x10-9 meters [m]). At elevated temperatures the thickness is up to 10 nm. The resistance and stability of the oxide layer is a function of the environment, the alloy composition and the microstructure of the metal, which, in turn, is influenced by heat treatment. The protective aluminum oxide layer is stable over a pH range within approximately 4-9.

ANODIZING: INCREASING THE ALUMINUM OXIDE PROTECTIVE LAYER

The appearance and corrosion resistance of aluminum can be improved through an electrolytic process called anodizing. The anodizing process increases the thickness of the aluminum oxide layer. The resulting porous anodic film can be sealed and colored using organic and inorganic pigments in a variety of hues and shades. Anodizing entails immersing the aluminum item in an electrolyte bath (an electrolyte is a substance that when dissolved in water creates a solution that can conduct electricity) and passing a current through the electrolytic solution, so it must be done in a specialized facility.

ALUMINUM CORROSION

Corrosion occurs to aluminum in the same general fashion as corrosion occurs to most all other metals. The aluminum corrosion process and that of other metals in identical environments is essentially similar in mechanism but can vary significantly in the extent or degree of corrosion.

Seacoast environments are particularly corrosion aggressive for aluminum, as well as most other metals and materials. Wind-born salt spray can contaminate metal surfaces as far as one mile or more inland under storm conditions. The contaminating seawater contains dissolved ions (atoms or molecules with an electric charge due to the loss or gain of one or more electrons).

Cations have a positive charge, due to electron loss and anions have a negative charge due to electron gain. Because of their opposite electric charges, cations and anions attract each other and form ionic compounds, which dissolve in seawater. These ions increase the conductivity of water, accelerating the rate of corrosion. Furthermore, halide irons—notably chloride ions—are prevalent in seawater and severely aggravate metallic corrosion.

When alloyed aluminum is exposed to chlorides, and moisture, the aluminum oxide film can break down, initiating corrosion. The photographs in Figure 2 below depict aluminum pitting. The 6061 alloy was cut from a door in a South Florida marine exposure that was showing the surface effects of aluminum corrosion. The white material covering the alloy in the photo on the left was a light-colored paint, beneath which was white-colored aluminum corrosion products. The pitting was more clearly visible after the sample had been blast cleaned to remove the paint and corrosion products, as shown in the photos on the right.

From left: A door sample piece before blast-cleaning to remove paint and aluminum corrosion, the same sample after blast-cleaning that shows pitting, and another view of the sample after blast-cleaning.

FILIFORM CORROSION

Filiform corrosion cannot occur unless a metal is coated. It is a form of localized corrosion that occurs on the aluminum surface (and other metal substrates) beneath an organic coating. It takes the form of randomly distributed thread-like filaments, which, in time, grow and with aluminum alloys fill with the white-colored aluminum corrosion product. The corrosion product generated has a greater volume than the aluminum metal consumed and causes localized delamination of the paint at the head. Over time, filiform corrosion can result in severe aluminum pitting. Filiform corrosion occurs at temperatures between 68 degrees to 100 degrees Fahrenheit and 75-90% relative humidity. It results from an oxygen concentration cell with the propagating anodic head having the least oxygen and the aluminum areas behind the head, including the filiform tail with a higher oxygen level, becoming the cathode.^{ref 3}

As filiform corrosion occurs the thread diameters grow in size and often stress and break the coating, exposing the metal corrosion products. Aluminum corrosion products, principally as aluminum oxides hydroxides and chlorates, are white in color. Figure 3 below depicts this:

Typical filiform corrosion. The corrosion may have started at the crevice to the right or at a pinhole or discontinuity in the coating. Tendrils extend from the main area of corrosion which has been opened to show the white powdery aluminum corrosion product. (A fingernail at the lower left is for scale.)

CORROSION PREVENTION

There are a number of different ways to minimize or prevent aluminum (and other metal) corrosion, including equipment design, modification of the environment, selection of the proper metals and metal alloys, the use of corrosion inhibitors and cathodic protection.

While all of these are important, the painting and organic coating of aluminum (and other metals) exposed to an aggressive environment is by far the most common means of corrosion protection.

Coil Coating

Coil coating is a continuous, automated process for coating metal before fabrication into end products. Because of the high-speed throughput of a coil coating line, coil coating is perhaps the most widely utilized and most economical means of coating aluminum that is to be subsequently formed. A pretreatment, which is often a chromate or chrome phosphate conversion coat, is applied, and after drying/curing, a primer and one or more topcoats are applied. The metal substrate is delivered in coil form from the rolling mills. The coil is positioned at the beginning of the coating line, and in one continuous process, the coil is unwound, pre-cleaned, pre-treated, pre-primed and pre-painted before being recoiled on the other end and packaged for shipment. All of this happens at up to 700 feet per minute.

The conversion coatings most commonly utilized are chromates, chrome-phosphates, trivalent chromates or non-chromates. The prime coats and topcoats consist of pigmented organic resins of a number of commonly utilized paints including epoxies, polyurethanes, polyesters, alkyds, acrylics, polyvinylidene fluorides and many others.

Because of the high-speed throughput of a coil coating line, coil coating is perhaps the most widely utilized and most economical means of coating aluminum that is to be subsequently formed. Above shows a modern, high-volume coil coating line.

Cleaning and Coating of Fabricated Parts

Sharp edges from manufacturing or installation processes such as cutting, punching, drilling or forming must be removed prior to cleaning or painting. Otherwise, these areas will be susceptible sites for corrosion initiation due to insufficient coating coverage.

A fabricating shop process can be automated, and the bare prefabricated aluminum pieces are hung onto a conveyor belt and dipped into appropriate tanks for cleaning and coating as required. Similar to a coil-coating line, there are rinsing tanks and drying ovens in the automated line. The coatings applied are like those used in a coil-coating line and the application thicknesses are also approximately the same.

Light abrasive blast cleaning with a fine abrasive or mechanical abrasion to achieve a uniform and dense anchor profile of 1-3 mils on the substrate surface is sometimes recommended by paint manufacturers as an alternative to chemical pretreatment to increase adhesion of the coating to the substrate.

Alternatively, preformed metal is often spray coated, again in a production sequence, and commonly, combinations of dipping tanks and spray booths and drying ovens are utilized, depending upon the configuration of the part, the type and thickness of coating, and colors and finishes.

Liquid topcoating is customarily applied in-line by electrostatic spray (the item to be coated is usually grounded, and a positive electrical charge is applied to the paint spray droplets, attracting them to the item). The item is then baked, producing a finished extrusion (a complex pressed-die metal shape) ready for assembly into door and window casings and other formed assemblies. Subsequent coating adhesion and corrosion resistance is also excellent if the topcoat system is properly selected.

Powder coating is commonly done in fabricating or coating shops to architectural and pre-formed aluminum. The aluminum piece to be coated is grounded and a positive electrostatic charge is applied to the powder. Electrostatic spray guns and fluidized powder beds apply the powder, which is electrostatically attracted to the aluminum. The piece must then be heated to melt the powder. After the piece is cooled, often using air or water quenching, the melted powder gels and then hardens. The powder particulate consists of a number of generic resins, pigments and often curatives (as separate particles, or mixed within the powder) to enable curing upon heating. The most common powder is fusion-bonded epoxy, but many other powder coatings exist such as acrylics, polyesters, fluoropolymers and others.

In-Situ Field Cleaning, Coating Repair of Aluminum that has been Exposed in a Service Environment

After the aluminum has been exposed in a service environment and requires coating repair or replacement, cleaning and re-painting is often performed in-situ. Cleaning mechanisms utilized for painting aluminum (as well as other painted or unpainted substrates) installed around residential or commercial buildings are often significantly different than those utilized in fabricating shops and aluminum coating facilities.

The purpose is the same, however: to remove oils, dirt and contamination that may interfere with paint adhesion and to slightly roughen the surface being painted, which will increase paint adhesion. Detergent cleaning is often done initially to remove contaminating dirt, debris and water-soluble salts and other materials deposited by marine and industrial environments, smog and atmospheric contamination. Detergent cleaning can be done by utilizing clean rags or paper towels or, on larger surfaces, mops, hoses or spray washers. If the surface is complex or has crevices, cleaning with a soft-bristle brush to work the detergent water into crevices to emulsify dirt and debris is advisable. After cleaning, it is important to remove detergent residues by washing with fresh water. Detergent residues will interfere with paint adhesion. In many cases, thorough detergent cleaning and water washing will sufficiently prepare a surface for painting.

Further cleaning and surface roughening, scraping, sanding, wire brushing, or utilizing hand or power tools, can be used to remove poorly adherent coatings and to roughen existing well-adherent coatings for repainting. To clean larger areas, it might be more efficient to utilize a power-wire brush, or to conduct blast cleaning, wet-abrasive blast cleaning or vacuum blast cleaning. Pressure water blast cleaning with and without an abrasive is also a common practice. The blast cleaning of small surfaces, such as aluminum door and window frames, can be done using a “pencil” blast cleaning unit, which uses a small-diameter orifice.

The properly prepared surface can be recoated with inhibitive primers and topcoats applied by brush roller or spray.

Aluminum Conversion Coatings

A conversion coating is a thin protective coating formed through a chemical reaction of the metal with the coating solution in which the native aluminum oxide is replaced with a mixed metal oxide, the composition of which is a function of the metal(s) in the conversion coating solution. On aluminum, conversion coatings prevent corrosion, but generally not as well as anodizing. However, conversion coatings are relatively inexpensive and can be easily applied. There are a number of aluminum conversion coatings presently available. Those utilizing hexavalent chromium and alternatives to hexavalent chromate conversion coatings are discussed briefly below.

Hexavalent chromate conversion coatings

Hexavalent chromium (Cr +6) is a carcinogen and, due to its toxicity and environmental impact, regulations in the United States and European Union restrict its use in coatings and other materials. However, to date, hexavalent chromium is still considered by many to be the most effective means of protection on aluminum and other metal surfaces. This is because hexavalent chromate conversion coatings can self-heal when scratched or damaged by mechanical action. The hexavalent ions migrate when wetted to the damaged areas where they are reduced to trivalent chromium (Cr +3) and absorbed onto the aluminum oxide surface, discouraging absorption of detrimental anions such as chlorides and other halides.

Chromium phosphate conversion coatings

These coatings are widely used in coating aluminum to provide good corrosion resistance and paint-adhesion properties. These conversion coatings are formed by contact with acidic solutions containing hexavalent chromium (-Cr0₃), phosphoric acid (H₃PO₄) and sodium fluoride (NaF) as primary ingredients in the conversion coating bath. The fluoride serves to dissolve the air-formed aluminum oxide and activate the surface and the hexavalent chromate is reduced to trivalent chromate in the presence of the phosphoric acid. Accordingly, then, there is little or no hexavalent chromium present in the applied conversion coating. However, these chromium phosphate conversion coatings do not possess the self-healing properties that the hexavalent conversion coatings possess and provide corrosion resistance by barrier protection only.

Alternatives to Hexavalent Chromate Conversion Coatings

To date, the most promising replacement for hexavalent chromium is trivalent chromium, which is nontoxic and is an essential human dietary element.^{ref 5}

A wide range of chromium-free chemistries have been investigated including those based on cerium, molybdates, zirconates, titanates, permanganates, vanadates, silanes and organics.^{ref 6} This research and testing is still ongoing; some of these technologies have advanced to commercial implementation.

SPECIFICATIONS FOR COATING ALUMINUM

While there are many private and industrial specifications and requirements used to coat aluminum, three of the major ones are:

1. The Department of Defense Detail Specification MIL-DTL-81706, “Chemical Conversion Materials for Coating Aluminum and Aluminum Alloys,” which defines two types of chemical conversion coatings:

• Type I: compositions containing hexavalent chromium
• Type II: compositions containing no hexavalent chromium

As of the 2017 update, the QPL for type II, class 1A (maximum corrosion resistance) had essentially four products, all based on trivalent chromate chemistries.^{ref 7}

2. Qualicoat, which is a quality label organization that maintains specifications and qualified product lists for liquid and powder coatings on architectural aluminum. Among their QPLs is a listing of alternative conversion coatings that currently includes approximately 90 Cr3+ and chromium- free chemistries.^{ref 8}

3. The American Architectural Manufacturers Association has prepared three organic coating specifications for use on aluminum extrusions and coil coating. The specifications are:

• AAMA 2603, “Voluntary Specification, Performance Requirements and Test Procedures for Pigmented Organic Coatings on Aluminum Extrusions and Panels;”
• AAMA2604, “Voluntary Specification, Performance Requirements and Test Procedures for High-Performance Organic Coatings on Aluminum Extrusions and Panels”; and
• AAMA 2605, “Voluntary Specification, Performance Requirements and Test Procedures for Superior-Performing Organic Coatings on Aluminum Extrusions and Panels.”

It should be noted that there is a distinct hierarchy in these three specifications, because AAMA 2603 has lesser performance requirements than AAMA 2604 (high-performance) and AAMA 2605 (superior performing). None of the AAMA specifications designate a specific generic type of coating. However, to meet the specification requirements, notably, for exterior gloss, color retention and weathering, many coating manufacturers use a resin containing a minimum of 50% PVDF for AAMA 2604 and 70% PVDF for AAMA 2605. Manufacturers complying with the AAMA 2603 requirements often use less expensive acrylic and polyester-resin-based coatings.

CONCLUSION

Aluminum, like steel and most other metals, has a number of different alloys, some of which are highly corrosion resistant and others that are more susceptible to corrosion in specific environments. Extreme care must be taken to ensure that the proper conversion pretreatment and coating system is applied to an aluminum alloy used in a specific environment for its intended purpose.

THE AUTHOR

Tator is the Chairman of the Board of KTA-Tator Inc., the son of KTA’s founder and an industry expert with over 45 years of experience in the field of protective coatings. He is a registered Professional Engineer (California, Florida, Michigan, Pennsylvania), is accredited as a Corrosion Specialist by NACE, is a NACE Certified Coatings Inspector Level 3 (Peer Review) and is an SSPC Certified Protective Coatings Specialist. Tator serves as a senior consultant and expert witness on many projects and contributes articles to various technical publications. He was selected by the JPCL in 2009 as one of 25 Top Leaders and Thinkers in the Coatings & Linings Industry for the past 25 years. He holds a B.S. in Chemical Engineering from Lafayette College and an MBA from Columbia University.

REFERENCES

1. “Aluminum Alloys 101,” The Aluminum Association, www.aluminum.org/resources/industry- standards/aluminum-alloys-101,accessed Oct 23, 2017
2. Talate Lecture 2101.01 Understanding Aluminum as a Material; Sigurd Støren, The Norwegian Institute of Technology, Trondheim and by Skanaluminium, Oslo
3. ASM Handbook 2A Aluminum and Aluminum Alloys; Painting and Organic Coating of Aluminum, Kenneth B. Tator, P.E.; Robert Leggat PhD; and Cheryl Roberts, KTA-Tator, Inc.; ASM International, Materials Park, Ohio, 2018
4. National Coil Coaters Association, Cleveland Ohio https://www.coilcoating.org/how-to-paint- metal-coils
5. “Assessment of Chemical Conversion Coatings for the Protection of Aluminum Alloys—A Comparison of Alodine 1200 with Chromium-Free Conversion Coatings,” A.M. Pereira, G. Pimenta, and B.D. Dunn, STM--276, European Space Agency, Feb 2008.
6. M.W. Kendig and R.G. Buchheit, Corrosion Inhibition of Aluminum and Aluminum Alloys by Soluble Chromates, Chromate Coatings, and Chromate-Free Coatings, Corrosion, Vol 59 (No. 5), 2003 p 379–400; and A.E. Hughes, Conversion Coatings, Encyclopedia of Interfacial Chemistry: Surface Science and Electrochemistry, Elsevier, 2017, p 1)
7. “Chemical Conversion Materials for Coating Aluminum and Aluminum Alloys,” MIL-DTL- 81706 (Rev B), Oct 2004.
8. Specifications for a Quality Label for Liquid and Powder Organic Coatings on Aluminum for Architectural Applications, 15th ed., QUALICOAT, Zurich, Switzerland, Sept 1, 2017

GENERAL REFERENCES

9. Aluminum and the Sea; Henning Johansen; ALCAN Marine; http://www.ansatt.hig.no/henningj/materialteknologi/lvd-aluminium%20and%20the%20sea.htm
10. “The Case of Corrosion in Paradise; K.B. Tator,” Presented to NACE Eastern Regional Conference (Panama City Beach, FL), 2013
11. E. Musingo, From Chrome to Chrome-Free: Various Coating Processes Determine Optimum Use for Prepainted Aluminum Products, Alum. Int. Today, May/June 2015
12. NAVAIR Discovering Alternatives to Hexavalent Chromium and Cadmium, Currents, Fall 2014, p 58–62
13. J. Qi et al., Chromate Formed in a Trivalent Conversion Coating on Aluminum, Electrochem. Soc., Vol 164 (No. 7), 2017, p C442–C229
14. Inhibition of Aluminum and Aluminum Alloys by Soluble Chromates, Chromate Coatings, and Chromate-Free Coatings, Corrosion, Vol 59 (No. 5), 2003 p 379–400