Keeping Paint on Aluminum

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Early in my career I worked for a dealership that sold production boats, every one of which was equipped with painted aluminum arches. Invariably, the pigment would blister, in some cases before the vessels were offloaded from delivery trailers, and warranty claims were dutifully submitted, which the builder, to its credit, promptly paid. In an attempt to stem the greenbacks menstruum, the company earnestly focused on improving the training and pigment application processes. Yet the problem persisted.

A few years afterwards, I was managing a boatyard where painting aluminum spars was common. Leery of experiencing the same sort of failure, I began reviewing onetime spars and other painted aluminum hardware. I noticed that nearly all the failures occurred next to a hardware installation, a fastener, a spreader, a step, or some other fitting—an area where the paint coating had necessarily been upset or breached. I believed I was onto something.

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It'due south the rare marine industry professional who hasn't come across cruddy blistered paint on aluminum surfaces. Here, the screws damaged the paint when they were installed, assuasive water to drift nether the coating.

Poultice Corrosion

Amid other attributes, marine-grade aluminum alloys, those in the 5000 and 6000 series, are naturally corrosion resistant. When exposed to air, oxygenated seawater, or fresh h2o, aluminum quickly develops a tough, transparent oxide picture that is resistant to corrosion. In that location is, all the same, an aesthetic price for this durability. Left to the elements, aluminum develops a dull gray—and somewhen dusty, gritty—cease; for military, pilot, fishing, and commonsensical applications, that finish tin can be a affair of dazzler, as information technology lowers building and maintenance costs. Few recreational users have, however, learned to look beyond aluminum's blemished surface.

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The white powdery textile, aluminum oxide, forms when aluminum is robbed of oxygen in a wet environment.

Enter two-part paints. These high-performance paints enable boatbuilders and component manufacturers to go on to reap aluminum'due south rewards, including strength, ease of fabrication, and weight savings, as well as a resistance to corrosion for interior and engineering spaces, while enhancing the blend'due south visual appeal. Nevertheless, once painted, aluminum tin can no longer develop its tough, clear, corrosion-resistant oxide coating. Information technology doesn't matter, because when aluminum is non exposed to moisture, its corrosion clock stops, or at least slows down substantially.

The paint works well until it is breached, and water enters the rift. This breach could be acquired past a fastener, an antenna base, a navigation light, a hinge, a lockset, a nick, or a scratch—and it could exist microscopic. Initially the wound is self-healing: The oxide blanket forms, just instead of protecting the aluminum, it lifts the border of the paint and allows water to migrate further nether the blanket. At that point, the chemical equation changes, as moisture and aluminum, in the absence of air, are the ideal incubators for a phenomenon known equally poultice corrosion. When this happens, the aluminum pits and produces copious amounts of aluminum oxide pulverization or, when wet, aluminum hydroxide, which looks a chip like freezer-burned vanilla water ice foam. The formation of the oxide or hydroxide lifts the pigment, causing the familiar and unsightly blister, which somewhen falls off, exposing raw, pitted aluminum. The process doesn't cease there, every bit it begins anew at the interface between the pigment and exposed aluminum. While information technology progresses slowly, once it begins it's difficult if non impossible to arrest without removing the blistered pigment, cleaning, priming, and repainting the surface.

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It'southward a double whammy for corrosion: The elk hibernate spreader boot is a hygroscopic material, attracting wet and keeping the painted aluminum surface underneath moisture and oxygen depleted.

Corrosion is likewise a problem on sharp, or unradiused, edges. In these areas, paint applied to the abrupt edge tends to run off, making information technology too sparse to be protective. Those areas also tend to suffer from abrasion corruption.

Prevention

As a effect of my observations, both at the production boat dealership and in the boat yard, I concluded that the corrosion was the indirect event of breaches in the paint's otherwise contiguous coating, caused, in the vast bulk of cases, past hardware installations.

Others who have come to like conclusions accept tried to prevent corrosion by inserting plastic or another nonmetallic bushing or insulator between the hardware and the painted aluminum structure. While this can exist helpful, it's far from ideal: On heavily loaded structures such equally cleats and winches, it'due south often impractical to install these insulators; the loading will often cause them to split or crush. And, when water makes its way betwixt the insulator and the painted surface, as information technology always does, poultice corrosion will begin.

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The all-time solution is prevention; when installing hardware on painted aluminum surfaces, liberally bed with polyurethane or polysulfide sealant.

It'due south most impossible to forbid paint from being damaged or breached at hardware installations, ofttimes fifty-fifty with the addition of insulators. Therefore, instead of attempting to forbid the damage, an effort I believe is futile, assume it will occur and mitigate its effects. The solution is straightforward enough: protect the pigment "wound" with a resilient, pliable sealant to prevent water from inbound. Begin past dewaxing/degreasing both surfaces with a solvent such as mineral spirits or 3M General Purpose Agglutinative Remover, so liberally bed the hardware with a depression-adhesion polyurethane or polysulfide sealant. "Liberally" means the sealant should ooze out around the unabridged perimeter of the hardware base of operations, as harm to the painted surface will occur wherever hardware makes contact. The sealant prevents water intrusion, rendering the pigment wound inert. Fastener threads should be similarly treated. If insulators are used, the surface between the insulator and the paint should exist bedded.

Abrupt edges should be gently rounded before being painted. That lonely volition prevent the thin-paint phenomenon, coating failure, and subsequent corrosion.

All pigment applications benefit from proper surface preparation, priming, and best industry paint application practices, and aluminum is no exception. Using these, too as the liberal hardware bedding technique and quickly repairing damage, pigment applied over aluminum can last for a decade or more.

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Well-nigh the Author: For many years a full-service yard manager, Steve now works with gunkhole builders and owners and others in the manufacture as Steve D'Antonio Marine Consulting. He is the technical editor of Professional BoatBuilder, and is writing a book on marine systems, to be published past McGraw-Hill/International Marine.