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Position Statement on Coatings for Aluminum Sculpture.

October, 2005

Prepared by Paul T. Amaral, President, Amaral Custom Fabrications, Inc.

In my seventeen years of association with art fabrication, I've become increasingly aware of concerns about paint failure and corrosion on aluminum sculpture, especially outdoor sculpture. In this statement, I'll attempt to convey a better understanding of the correct way to paint aluminum.

Nothing in this statement is either new, or unknown to those companies involved in coating aluminum everyday.  Companies such as Boeing Aircraft or Lockheed Martin would find this statement simple, straightforward and accurate. Unfortunately, in the world of large-scale, outdoor art, there exists a gap in knowledge evidenced by the poor condition of many outdoor aluminum sculptures.

Before beginning my explanation, I must advise the reader that beauty is ‘in the eye of the beholder’. I am often asked to prepare paint surfaces with marginal life span in order to accommodate the wishes of the artist. As fabricator for the work, it is incumbent upon me to fully advise the artist as to the risks involved. Having done that, it is the artist who must decide the course of action.

Also please note that I'll refrain from discussion of ‘natural’ or uncoated aluminum sculptures, as these tend to do very well as long as the artist is willing to accept the esthetics of the natural oxidation process. Polished aluminum also does quite well, if the sculpture is protected from excessive human and environmental contact.

ALUMINUM: commercial applications

Commercial applications for aluminum accelerated greatly in the late 1930's. Its light weight and ability to be strengthened by alloying with other metals made it particularly attractive to the airplane industry, which had been pushing the envelope of materials performance since the days of Orville and Wilbur. Not long after, aluminum was embraced by the marine industry as an alternative to steel or wood construction. It did not swell or rot like wood and is much lighter than steel. It was also thought to have superior corrosion resistance to steel.

GALVANIC CORROSION:

The Galvanic Chart lists some common metals according to their ability to resist corrosion. At one end of the chart are materials that rapidly corrode. At the opposite end, are materials that corrode very slowly, or not at all. Anode and Cathode are the terms given the extremes, with anode the more corrosion prone. ‘Noble’ and ‘less noble’ are self explanatory terms commonly encountered when comparing one metal to another. Aluminum resides at the less noble end of the scale. Metals like gold or platinum reside at the noble end.

Not long after the first aluminum hulls were launched, the phenomenon of ‘galvanic corrosion’ began to appear. For galvanic corrosion to occur, two dissimilar metals, one being more noble than the other, must be immersed in an electrolyte. In this case the electrolyte is simply seawater. On a typical ship, bronze, brass, aluminum, and steel are used extensively, and all reside at different positions on the Galvanic Chart. It is important to know that the further the span between two metals on the chart, the more aggressively corrosion will occur. When examining the chart it is noticed that bronze and aluminum are very far apart. Soon, marine builders began seeing significant problems with aluminum hulls disintegrating, particularly in areas around a bronze prop shaft or thru-hull fitting.

To solve the riddle of galvanic corrosion, three approaches are employed. First: is to use materials closer together on the chart; second: to isolate the metals in question; and third; place a less noble, sacrificial metal into the loop. With the advent of better ‘passive’ grades of stainless steels, the elimination of bronze or other red metals became possible. If, in the case of a large cast propeller where the elimination of bronze is not economically feasible, then isolation of mechanical contact between the two metals is regarded as an effective solution. Simply placing a non-conductive barrier between the two metals serves to break the galvanic circuit. A final method is to introduce an even less noble metal than aluminum, such as zinc. All galvanic flow will be directed to this sacrificial metal, saving the other metals from decomposition. The use of sacrificial anodes has long been a standard with the marine industry and is used in conjunction with isolation of metals, and employing metals closer together on the chart.

Along with substitution of more compatible metals, and the use of sacrificial anodes, a trend developed in the area of isolation materials. Early materials included thick leather, rubber, and asbestos gaskets. These were later replaced with modern plastics offering far better electrical insulation and resistance to water absorption. Eventually, it became apparent that even a high quality paint system could perform double duty as a cosmetic treatment and a galvanic isolator! It is this branch of coatings that are particularly interesting for their ability to adhere to aluminum, resist corrosion, and isolate galvanic activity under the most severe circumstances.

The question arises, “if a sculpture is installed far away from salt water, how is it that galvanic corrosion could possibly occur?” The answer can be found in atmospheric conditions in most urban environments. Acid rains, soot, and hydrocarbon, all combine to create a solution capable of acting as an electrolyte. Any crevice on the sculpture that might catch and retain water, becomes a prime target for galvanic corrosion to begin. The next question might be, “Isn’t a second metal required?” Answer: A second material is required. That second material could be the concrete base, the steel bolts fastening the sculpture to the ground, or a dissimilar metal employed as an armature within the sculpture. All that is needed, is the two be connected in a way to complete a circuit. It could be many feet away from the visual starting point of the corrosion.

ADHESION INHIBITORS #1: Oxides

In the absence of galvanic action, freshly exposed aluminum molecules will pull oxygen from the atmosphere to stabilize its outer electron sphere. Once a piece of raw aluminum is cut, the exposed surface immediately begins to oxidize much like steel. Unlike steel, once the newly cut surface has reached a saturation point, the process virtually stops. It is this coating of oxides that prevents further deterioration of the underlying aluminum. Anodized aluminum, is actually controlled oxidation of the aluminum surface under heavy electrical charge. The anodized coating can become quite thick, thereby imparting a high degree of corrosion resistance to the piece.

It would at first seem beneficial for the coatings applicator to leave the oxidized surface intact to preserve the aluminum’s natural corrosion resistance. In fact, the very opposite is correct. The natural coating of aluminum oxide, while protecting from further corrosion, inhibits any paint’s ability to chemically stick to the aluminum surface. Only a mechanical bond will result. Later in this statement, I’ll discuss the desired chemical adhesion properties of the initial primer.

ADHESION INHIBITORS #2: Alloying elements

In order to produce the multitude of aluminum alloys available to today’s end user, raw aluminum is alloyed with many different materials. Some of the elements used are titanium, manganese, nickel, chromium, zinc, molybdenum, and magnesium. Varying combinations of elements impart distinctive structural and workability characteristics to the aluminum. Some of these elements also increase aluminum’s resistance to adequate paint adhesion. Nickel, chromium, and molybdenum all possess a high degree of natural lubricity. The U.S. Army employs molybdenum additives in the crankcase of diesel equipment because of its ability to provide lubrication under extreme situations. It is easy to see why adhering paint to aluminum is sometimes tricky.

PREPARING: grinding and etching

Given the combination of aluminum’s natural oxide surface and the inclusion of alloying elements having high lubricity properties, preparing the surface of aluminum is the single most important step in adhering paint to it. Achieving a properly prepared surface is a two step process that must be completed in as short a period of time as possible.

The natural oxide surface must be removed in either of two ways. One way is to strip away the oxides with highly corrosive phosphoric acid. This method, while highly effective, is not an attractive one due to environmental and health considerations. Another way is to mechanically grind away the oxide layer with a wheel or sandblast the surface with glass bead. Both techniques offer very good results, but can leave behind small amounts of contaminants. A follow-up etching, with less aggressive acids finishes the process. The acid-etch also serves to strip away some of the alloying elements having high lubricity qualities.

CRITICAL STEP: ‘Fixing’

Now that the aluminum surface is clean and free of any contaminants, it is necessary to ‘fix’ the surface so no re-oxidation can take place prior to coating. To do this, a second acid is used to cross-link the open bonding sites on the newly cleaned aluminum. Chromic acid is employed to create an aluminum chromate layer on the surface. This acid is highly carcinogenic, and must be used with extreme caution. Recent developments in self-etching, chromium based primers have lessened the exposure to chromic acid fumes and are highly effective in achieving an excellent cross-link with the aluminum.

PRIMERS: Sealing and filling the surface.

Now that the surface is clean and ‘fixed’ it is necessary to seal the surface from water migration to the aluminum. Water is known as the universal solvent, and can easily pass through the thin ‘fixed’ layer to begin its’ attack on the aluminum. Preventing this is imperative. Many types of primers have been developed for different paint systems. Some of these are lacquer, acrylic, polyester, enamel and epoxy. To achieve the very best waterproof membrane, it is advisable to use a primer having the best resistance to water migration. That material is epoxy, and to find the epoxy-based system having the best resistance of all, a marine epoxy is advisable.

Once the surface has been cleaned, fixed, and sealed, it is time to fill, fair and topcoat. There are many paint systems available which claim to have excellent results in adhering to aluminum. In my experience there are very few systems that are truly successful on a long-term basis. I find that a paint system using epoxy-based fillers and high-build primers are best in achieving long-term adhesion. Systems based on polyester chemistry should be avoided in the preparation of an aluminum sculpture. Polyesters (Bondo) have very good success in adhering to steel, but are limited in their ability to maintain adhesion to aluminum because of aluminum’s thermal expansion characteristics.

Aluminum tends to expand and contract at almost twice the rate of steel. In order to remain adhered to an aluminum substrate, the coating system must have elongation characteristics that are at least the same or greater than aluminum. The elongation properties of polyester-based compounds are not great enough. This is why so many aluminum sculptures lose the coating systems in a ‘sheet’ effect. 24-hour thermal cycles break down polyester’s mechanical bond to the aluminum substrate. Seasonal cycles, especially hot weather will cause polyester to continue to shrink over its lifetime. Aluminum and polyester simply do not perform well together. Elongation properties of epoxies, on the other hand, are upwards of 4 times greater than ester-based compounds.

WHY?: Economics

Unfortunately, polyester fillers have been used, and are still used, to a great extent on aluminum sculptures manufactured in the United States. The primary reason is their low cost. Preparation time is condensed, and that translates to quicker turn-around and fewer man-hours. On the other hand, epoxy can cost as much as $100 per gallon. Cure times are measured in days, sanding is tougher, and preparation time can take 3-4 times longer. This is a huge expense that cannot be seen and does not reveal its value to many artists and collectors…, until much later!

TOPCOATS: the artist’s decision

This is where I must step aside and let the artist determine the final finish. Again, I can only coach as to which is ‘best’ from a curator’s perspective. It is the artist who decides on levels of gloss, texture, color and everything unknown to me.

ALUMINUM SCULPTURE: = problems???

In my opinion, the conception of aluminum sculpture as ‘troublesome’ is as unfortunate as it is avoidable! Many builders underestimate the importance of preparing aluminum surfaces properly. The selection of a coating system designed to work with aluminum is even less well understood.

My earlier reference to the likes of Boeing Aircraft and Lockheed Martin is applicable in this discussion because they use aluminum every day building aircraft. And they have been doing so for decades. Indeed, it is not unusual to board a domestic jetliner still in everyday service that is over 20 years old. These craft are subjected to severe conditions that test coating systems on many levels, from thermal expansion to harsh chemical de-icers, hydraulic fluids and jet fuels. Consider the expense of painting a commercial jetliner, (a 737 costs approximately $300,000 to repaint, not including lost revenue from paying customers!) certainly the paint had better perform extremely well for many years. These companies approach aluminum coating systems from a very scientific and studied viewpoint and they have had great success.

CHOOSING THE RIGHT ALLOY: does it make a difference?

I believe it does. Marine Grade 5xxx series aluminum is designed to provide the best corrosion resistance of any commercially available aluminum. And, even though 5xxx series starts with lower engineering values than the more popular 6061 alloy, in its welded condition 5xxx series retains more of its strength than does 6061. In fact, 6061 loses 40% of it strength in the welded condition, and does not gain it back unless the structure is heat-treated in a giant oven. To date, I am unaware of ANY outdoor aluminum sculpture that has undergone heat-treating. 5xxx series aluminum is so superior in corrosion resistance and strength retention after welding that it is the only alloy series designation approved by Lloyds Registry of Shipping for use on aluminum vessels of any size.

If we are truly building sculpture for the ages, why should we choose anything less?



Paul T. Amaral
Amaral Custom Fabrications, Inc.