One such case involved a large quantity of corrugated, painted steel panels destined for use as roofing and siding on a building that was being refurbished. The panels had been fabricated from prepainted, coil-coated, galvanized steel sheet, then shipped by truck to an industrial building site on the shore of one of the Great Lakes. All went well until part way through the construction, when serious problems were suddenly encountered with blistering of the paint and corrosion of the underlying metal.

The building owner was irate and blamed the installation contractor... which made the installation contractor irate... so he, in turn, blamed the panel supplier, which of course didn't make the panel supplier too happy. Now you might think this is where we came in. Not quite yet.

Following several preliminary, inconclusive technical evaluations (and the usual amount of back-and-forth finger pointing), the installation contractor resorted to that tried-and-true method of solving technical problems—he filed suit, in this case, against the panel supplier, claiming the panels were defective. That's where we came in. We were hired by the panel supplier to find out what was happening to his panels, which he indicated had been performing perfectly well in similar applications for years.

Right off the bat, we learned that the case was focused on a critical issue: Prior to installation, the panels had been stored in stacks, uncovered for up to seven weeks, during which time the temperatures had been quite high—often exceeding 90ºF. In addition, it had been rainy—to the tune of approximately 6 inches of rain. The primary issue was whether the corrosion was due to a coating deficiency or due to overly severe environmental exposure. Or put more simply, was the corrosion the result of bad material or good material, badly handled?

We started with a review of all the available information, including historical data, specifications, and results of a previous investigation conducted by another laboratory.

The panel material had been specified as 20-gauge corrugated steel, with a 0.76-mil thick zinc coating (hot-dip galvanized ASTM G 90), an epoxy primer, and a 1-mil thick silicone polyester topcoat.

We examined the surfaces of several panels that had been stored on site but not installed. The panels were severely corroded over most of their surfaces, although the areas next to edges exhibited less corrosion. From far away the coating simply looked blistered, but as we looked closer, we found that the damage looked like elongated little worms.

We selected the worst sample available for more detailed examination, both on the surface as well as in cross-sections.

Stripping the paint with methylene chloride revealed that beneath the elongated worm-like blisters were... No, not worms, but elongated deposits of white corrosion products on the zinc surfaces. Analysis in the scanning electron microscope via energy dispersive spectroscopy confirmed what our eyes already told us: the deposits were primarily zinc oxides, with only traces of iron, indicating the corrosion was primarily in the galvanized layer

So what was happening? The zinc coating on the steel had been attacked by filiform corrosion, a condition caused by wet storage conditions. Moisture causes the paint coating to soften, which then allows moisture and oxygen to permeate through the coating and locally attack the zinc, resulting in zinc oxide in the form of elongated filaments or threads (known as wet storage stain or "white rust"). These filaments grow due to a difference in oxygen concentration between the open tail and the closed-and-growing head.

 

Of Paint and Spaghetti What happens when paint cures? At its simplest, paint starts out as a liquid that consists of polymer molecules, along with pigment, all dispersed in a solvent. Polymer molecules are long chains, like very small spaghetti. Liquid paint is like spaghetti being cooked in a pot of water—the chains are free to move around. When the solvent evaporates and the paint cures (solidifies), the polymer molecules get all tangled together. (Sort of like unrinsed spaghetti left in a colander, or yesterday's leftover spaghetti in your refrigerator). After the paint cures, you have a solid, continuous, protective film, which can serve as a barrier between a substrate and the environment. Now, solid polymers behave two different ways. When they are cold, they are hard and brittle, like glass. When they are hot, they are soft and stretchy, like rubber. At an intermediate temperature, called the "glass transition temperature," there is a change between glassy behavior and rubbery behavior. In many paint systems this transition temperature continues to increases as the paint cures; thus, under controlled conditions, it can be used as a measure of the curing process. The problem in this instance is that the conditions within the stack of panels where by no means controlled; thus, the transition temperature was not a valid measure of the degree of cure. So, why not simply formulate the paint so it has a higher glass transition temperature, you ask? Because in this application, the paint is applied before the panel is corrugated. If the glass transition temperature is too high, then the paint behaves in a brittle manner at room temperature when the panel is being corrugated, and it cracks Why not use a different type of paint? Cost... this type of paint is cost effective... and it works just fine—that is, if it's stored and handled correctly!

Analysis of the paint primer and top coat layers as well as the underlying galvanizing layer and steel substrate showed that the steel, galvanizing, and coating system were all within specifications.

So why had the other side's expert claimed the coating was defective? Here's where things get a bit technical. The previous investigators had measured the glass transition temperature (see sidebar note right) of several samples and, based on these measurements, had concluded the paint had not been properly cured. However, a comprehensive review of the scientific literature revealed that even after they are fully cured, these particular types of coatings (silicone polyester) react with water, causing their properties to "revert" to what they were before the cure was complete. Thus, the conclusion that the coating hadn't been properly cured was unfounded.

Rather, the permeability of the coating was the key issue in the failure. Water and heat act together to increase permeability by a variety of mechanisms, such as plasticization and hydrolysis. The detailed mechanisms are not fully understood, but the overall coating degradation caused by heat and humidity is well documented.

At temperatures above the glass transition temperature (in this case, about 85ºF), a coating of this type has high permeability, which means that moisture can penetrate through it and attack the underlying metal.

Reviewing the weather data for the area, we found that the air temperature had exceeded 90ºF on many days, and with the panels in the sun, the panel temperatures were undoubtedly even higher.

The source of the moisture was obvious. The panels had been stored outside in horizontal stacks, uncovered, exposed to the rain. Stacked one on top of the next, once water entered the crevice between sheets, it had nowhere to go, and the surfaces between the sheets remained wet, even after the sun came out and dried everything else off. In this condition, the sheets might as well have been immersed in water, a condition which this particular coating was not designed to withstand.

Under these conditions the coating chemically reacted with the water (hydrolysis), softening and becoming permeable, and the resulting transport of moisture through the coating and into the underlying material resulted in the observed corrosion.

The outcome? After both sets of experts had testified, the judge concluded the contractor had not demonstrated the material was defective, and he ruled in favor of our client.

Coatings on Furniture

_Speaking of industrial-strength finishes, we recently received some chairs from a furniture manufacturer who was receiving complaints from his customer about the perceived poor wear resistance of the chair varnish. It seems that the chairs were being used in a fast food restaurant, and the restaurant owners were complaining that the finish was starting to wear off after several years of use. Whoa!... we were thinking... the customer is concerned because the finish only lasts several years?… in a fast food restaurant? Since when is furniture varnish supposed to wear like porcelain, retain its gloss like a diamond, and magically sluff off encrusted salt, oils, and the footprints of countless sugar-charged kids?

Be that as it may, our task was to compare the different finishes they had been using for the last five years or so and determine which ones were best suited for fast food restaurants. The standard procedure in such restaurants is to clean everything daily, using a spray-type bactericidal cleaning and degreasing solution in conjunction with a damp cloth to remove the accumulation of catsup, grease, and other dried-on yuck.

So, how did we test the finishes? You guessed it, by applying various sticky and greasy things like aged vegetable oil, and stain-producing substances like mustard, then following up with various types of cleaning products. how did the various finishes fare? One chair was a standout winner. It resisted all our efforts, while the others all had problems with softening, staining, or loss of gloss. Some finishes didn't like ammonia-based cleaners, others didn't like alkalinity, others couldn't handle solvents, and some fell short on stains.

The formulation of the resistant coating was not known to our client (after all, the coating manufacturers need to have their own trade secrets). However, our FTIR analysis showed that the winner was a polyurethane.

In another project, cabinet finishes were developing a hazy, powdery appearance in service. We received an example on a cabinet door for examination. We rubbed off the powder and examined it by a variety of instrumental methods. To our surprise, the surface powder was almost entirely organic and was totally unlike the varnish topcoat material elsewhere on the door. Puzzled, we wondered, could the powdery residue be a food product? After all, the cabinet came from someone's kitchen, and the residue was only present on the exposed outer side of the door. We went back and examined the varnish surface at higher magnification, hoping to find some additional clues. At this point, we noticed the material was composed of extremely fine, colorless threads. Hmmmm... this was getting interesting. Obviously, we needed to take a look at higher magnification.

In the scanning electron microscope at several thousand times magnification, the mystery substance began to reveal its identity. An extensive mat of threads on the surface was apparent wherever there was haze, and in the really powdery area, little clusters of balls, obviously biological in nature, were visible. It was mold!

So, problem solved? No, not quite yet. To make a long story short, several conference calls and additional sleuthing revealed that what we saw on this particular sample represented an unusually advanced stage of deterioration. We ultimately determined the problem originated as a result of the varnish surface degrading in sunlight. Then, later, under some special conditions, it had become a viable substrate for the mold. The problem was solved by reformulating the varnish to make it more UV resistant.

And the mold? It had to find somewhere else to live.

Spring Cleaning

It's that time of year again… no, we're not talking about the birds and the bees and a young man's fancy... we're talking about our annual spring cleaning... well, except in our case, it's been more than just a few springs since we last did a thorough cleaning. Nevertheless, we've embarked on a major rehab of our laboratories, starting with the front office. So under the leadership and expert guidance of Holly Howard and Diane Clark, a group of 10 employee-volunteers spent a recent Saturday remodeling and painting the front office. And now, the clean-it-up bug is spreading throughout the rest of the laboratory.

We'll be 70 years old this summer (well, not us personally, but "us" the company), and we've managed to accumulate quite a few things over the years, many of which are no longer of any use to us. Some of these items went directly to the dump (er, landfill), others followed employees home, still others are finding new homes through the wonders of eBay. (Don't laugh—we got $25 for our old/going-on-ancient inductively coupled plasma analyzer... twenty-five whole dollars... plus, the buyer paid us another $100 to crate it up for him!)

All in all it's turning into quite a project, one that we're eager to show to our many friends and clients. So the next time any of you are in the neighborhood, stop by and see us.

MEI-C People

Durgam G. Chakrapani, President, presented a paper at the Tappi Digester Corrosion Information Meeting in Vancouver, British Columbia in February. Dr. Chakrapani's paper was titled "Corrosion Journal of a Stainless Steel Clad Kamyr Continuous Digester in Acid Sulfite Pulping Service" and discussed our observations during 17 years of semi-annual inspections of a Type 319L/904L clad digester that had been commissioned in 1984.

Richard Garber, Senior Metallurgical Engineer, received notification from the Oregon State Board of Examiners for Engineering and Land Surveying that his request for comity registration as a professional engineer has been approved. Dr. Garber is also registered in Ohio.

MEI-CHARLTON, INC. IS A CONSULTING ENGINEERING FIRM WHICH SPECIALIZES IN QUALITY ASSURANCE, FITNESS-FOR-PURPOSE EVALUATIONS, CORROSION, METALLURGY, WELDING, AND ENVIRONMENTAL AND ANALYTICAL CHEMISTRY
©1999 MEI-Charlton, Inc.