Protecting Protective Coatings

A wide variety of coating materials are used to protect everything from air conditioners to zoom lenses from corrosion, erosion, and sundry other hazards. Of course, in our diverse world, situations are often encountered where the coatings just don't perform as well as expected. In many cases, the cause is not apparent, and frequently, it's those cases that end up on our incoming project shelf.

Sometimes, the items themselves may be in perfectly good shape; they just look awful because the coating has deteriorated. Other times, the items may be in such bad shape because of corrosion that we are not even sure if there ever was a protective coating.

Reviewing our case histories, we found one of the more interesting themes is the ever-growing complexity of coatings. In the continuing quest for enhanced performance or loss mitigation, industries have come up with many new twists, not the least of which is a coating to protect the coating. Of course, this brings up the possibility of coatings to protect the coatings that protect the protective coatings, etc. Sounds crazy, but the following cases illustrate the trend toward both complexity and added layers.

One of the most common materials used to prevent corrosion is zinc. It's cheap, easy to apply, and fairly rugged. Thus, you frequently find zinc-coated, or galvanized, nails and other steel products for outdoor projects. Street sign posts, guard rails, and highway structures are just a few examples of items that are typically zinc coated.

The zinc coating you see on large outdoor structures has a gray matte appearance. It is formed by hot-dip galvanizing, a process in which the item is dipped in molten zinc, producing a thick, durable coating. If you want a shiny appearance, electroplated zinc can be used, but it is thinner and only suitable for lighter duty service, such as interior sheet metal construction, electronics, or interior nails and screws.

We have had several recent cases that involved galvanized nails rusting badly when used in outdoor wood construction, such as decks. To some of us old-timers this seems strange, since we have used galvanized nails with good success for years and years on fences, decks, porches, etc.

We found in each case the nails were electroplated, not hot-dip galvanized. Electroplating produces only a thin coating of zinc, which the acids in wood can consume fairly quickly. To enhance the corrosion resistance of the thin, electroplated zinc, the manufacturers sometime add a chromate coating after electroplating. Sometimes this still isn't enough, however, to prevent early rusting in our wet climate.

Of course, contractors like to use the chromated electroplate nails, since the old fashioned hot-dip nails are too irregular in shape to use in most nail guns.

The photo to the left shows one of several zinc-plated electronic instrument chassis that, after forming and plating, had been wrapped in a polyethylene bags and shipped across the ocean for assembly. Unfortunately, when they arrived at their destination, they were covered with a very unusual, wavy surface pattern, suggestive of corrosion.

To get to the root of the problem, the manufacturer sent us several examples for analysis. Somewhat surprisingly, a microscopic examination revealed the patterns were not corrosion-caused but instead were abrasion marks. With that information we then observed the marks clearly corresponded to the positions of folds in the polyethylene bags, where rubbing had occurred.

Further testing of the bags showed they were abrasive to the zinc. Sort of a "man-bites-dog" state of affairs, where a protective wrapping intended to preserve the zinc coating during shipment, instead ended up harming it.

Another zinc-plated chassis project is shown in the photographs at the top of the page. As is more commonly the case, the chassis in this project had actually corroded. Here, the coating had an iridescent blue and purple hue, indicating an additional chromate conversion coating had been added. Chromate conversion coatings vary in color from yellow, to blue, to purple, to olive, to black, depending on the composition and thickness. (Sometimes they're even colorless.)

In this case, the chassis had been shipped in cardboard boxes without polyethylene bag wrappers. It turned out high humidity was the cause of the corrosion. Cardboard can absorb large amounts of moisture, and then release it inside the boxes when the temperature rises, causing water to condense on articles inside the box.

Wrapping in polyethylene bags would have prevented this type of failure. Of course, we felt obliged to warn the client about the potential for abrasion damage with this fix!

Another coating problem we have encountered involves anodized aluminum. Anodizing is normally produced by rapid electrochemical oxidation of the aluminum substrate in a sulfuric acid bath. The anodized coating, however, is too porous to be used as-is, so platers have resorted to sealing the coating by various means. Most commonly, nickel acetate and hot water are added to form a layer of hydrated aluminum oxide to fill the pores.

The various colors of anodized aluminum are from dyes added to the bath before sealing. If the sealing process is incomplete, stains can develop, either as a result of external contamination or from bleaching and/or leaching of the dye itself. Most manufacturers who use anodized aluminum for their products are aware of this, and often, when they have a staining problem, they ask us to check the quality of the seal coat; however, we frequently find the blemishes or poor performance are due to problems with the base coat.

Wherever materials are put in extremely demanding environments, coatings on coatings are likely to be found. Most paint systems have multiple layers, usually a sealer or primer, then a base coat, and finally a top coat. These complex systems can have complex problems, which we will discuss in a later newsletter. Each case has a story to tell, which is usually only revealed through perseverance.

Getting back to zinc. Another way to protect a zinc coating is to paint it. Certain pitfalls must, however, be avoided. Chromate coatings on the zinc are incompatible with most paints and, if present, will cause paint to peel. (On the other hand, phosphate coatings are OK.)

The problem with painting zinc is the zinc itself always has a surface film of oxy-hydroxides and carbonates; thus, the paint needs to be compatible with that surface film. Oil-base coatings with drying oils, such as linseed oil, will react to form a zinc soap, causing delamination and peeling.

The best way to tell if a paint is compatible with zinc? Well, short of doing your own research project, simply check with the paint manufacturer to see if the paint is recommended for galvanizing, and if so, whether any special surface preparation is needed.

Shipping and Storage Damage

In our last newsletter, we introduced our readers to the newest member of our staff, Dr. Richard I. Garber, who comes to us with over 20 years of metallurgical problem-solving experience. One of Richard's specialties has been assisting insurance adjustors and attorneys, where his metallurgical expertise and problem-solving abilities have proven to be invaluable. We thought our readers might enjoy hearing about some of the projects Richard has worked on in the past which relate to our lead story about protective coating problems.

The Rusty Cans

We'll begin our first tale with a question: How do you make a federal case out of rusty steel cans? Well, it's easy - just ship them across state lines by truck!

In this project, a can manufacturer shipped a rush load of 50,000 empty cans to a mothball manufacturer via an interstate trucking firm. Unfortunately, when the cans arrived at their destination and were being examined before filling, they were found to be rusted. As a result, a lawsuit was filed against the trucking company alleging they were responsible for the rusting.

The cans had tin-plated tops and bottoms and foil-coated paperboard sides. The can manufacturer retained a testing laboratory, which did a quick lab analysis and found chlorides on the surface. Based on this cursory analysis, the manufacturer alleged that the rusting was due to hydrochloric acid vapor exposure during shipment, and he sued the trucking company in federal court for the damages to his shipment.

The trucking company said that nothing else was even in the trailer with the empty cans, let alone hydrochloric acid, and furthermore they had previously only shipped breakfast cereal in that particular trailer. Richard's assignment was to sort all this out and determine why the cans had rusted.

The cans had been packed in four layers (with 56 cans in each layer) in paper bags. Unfortunately, when the cans were received and found to be rusted, they were unloaded and then reloaded into another trailer to be returned to the manufacturer; thus, by the time Richard became involved in the project, the overall damage pattern in the original trailer was lost. However, his examination of all the layers in a sample of four bags revealed a consistent pattern: The damage was always concentrated at the center of each package.

This was an important clue because it was the opposite of the pattern of outward-in damage that would be expected from external acid fumes; that is, fumes from outside would be expected to give a pattern with damage either concentrated at the outside, or, if the fumes had thoroughly permeated the packages, the damage would be randomly located. Instead, the damage pattern suggested the possibility of water vapor condensation from inside the bags.

In this case, the source for the water vapor was the glue used to laminate the paperboard. But where would the chlorides have come from? Well, after a bit of investigation, Richard learned the tops and bottoms of the cans had been applied by hand in a nonair-conditioned plant in the Midwest in August. And guess what the weather's like in the Midwest in August? That's right, hot and humid! Thus, the corrosive chlorides turned out to be simply from the workers' sweat as they packed and stacked the cans.

Glass Attack

As every homeowner knows, glass is unaffected by washing, rinsing, and drying. However, under some conditions, even a little bit of plain water will attack a stack of flat glass. How, you ask?

When glass is covered by condensation for an extended period of time, the film of water can leach sodium and potassium ions from the surface. If leaching continues long enough, and the surface isn't flushed with fresh water (as by a rain storm for example), the solution becomes sufficiently alkaline to slowly dissolve the glass.

The result is a permanent iridescent stain on the surface. Staining is more apparent on tinted glass than on clear glass. If the glass is rinsed to remove the alkaline solution, the corrosion will stop; thus, glass corrosion is not commonly observed either on washed glass or on glass exposed to rain. A typical example of glass corrosion is flat glass stored uncovered during building construction.

Corrosion Photo Gallery

This scanning electron micrograph shows the interior morphology of a corrosion tubercle formed in a galvanized steel, dry, fire-suppression pipe. The inadvertent presence of stagnant pools of water resulted in the loss of the zinc coating, followed by corrosion tubercle formation and leaks. The flocculent, voluminous, nodular products are primarily hydrated iron oxides resulting from dissolved oxygen-induced pitting corrosion.

MEI-People

Allen Sharp and his wife, Veleria, became the proud parents of a baby boy on 23 September 2001 at 7:49 am. Antoni Leander Sharp was about 14 weeks premature. He weighed in at 1 lb, 5 oz, was 12 1/2 inches long, and is doing fine. Congratulations, Allen and Veleria!

Dr. D.G. Chakrapani, President, MEI-Charlton, presented a paper entitled "Some Recent Case Histories of Corrosion Failures in Industrial and Commercial Systems" on 3 October 2001 at the NACE (National Association of Corrosion Engineers) Western Area Corrosion Conference, which was held at the Embassy Suites Hotel in Tigard, Oregon. The paper highlighted several interesting corrosion failures in a wide range of materials and applications. Dr. Chakrapani's presentation examined the underlying causes of the corrosion and explored appropriate corrective measures to avoid future failures.

Dr. Richard I. Garber, Senior Metallurgical Engineer, gave a presentation entitled "Were the Lights On? Lamp Analysis After Vehicle Accidents" at the October meeting of the Oregon Casualty Adjusters Association. The presentation highlighted the importance of vehicle lamp analysis in automobile accident reconstruction.