Two Case Histories: appeared almost perfectly benign at first sight. In the stunning visual setting of Hawaii a major professional football stadium seemed built to last forever. But subtle problems were plaguing the structure. Paint was peeling off the galvanized steel beams, creating an unsightly condition and giving evidence of underlying corrosion. MATCO was hired to determine what the root cause of the corrosion was and to prescribe ways of dealing with it.
Careful examination of metallurgical cross sections of small samples cored out of the steel members showed that there had been surface preparation problems right from the start. Prior to galvanizing the steel had been shot blasted to remove scale. The shot had deformed the surface steel and trapped small pockets of oxide under smeared out metal. After painting, the oxide continued to grow, aggravated by the relatively high sodium chloride loading in the winds off the nearby ocean. The galvanizing and the paint began to peel off. The solution: start over, completely remove the oxide, prime and paint the steel.
This is one of many examples of the effects of atmospheric pollutants on structural materials. Another major job involved investigating the cause of loss of lead from a lead-coated copper roof on a major state agency headquarters in the Mid-West. The broad expanse of roof was supposed to be a pleasing soft gray color. Instead it was striped and blotched in strangely irregular patterns. Here it turned out that the major problem was in the production of the roofing material: the lead had never properly coated the copper in the first place and atmospheric exposure simply opened up the original defects, dissolving the lead and sending it into a nearby receiving pond.The sediment in the pond was now heavily contaminated with lead.
An experimental regime was designed to test the solubility of the lead coating in water comparable to the local rain. An average composition was mimicked and water was flowed down over both top and bottom surfaces of fresh roofing panels. The lead concentration in the water was quite different in the top and bottom tanks, showing that the solubility of the lead differed depending on how the panels were installed.
MATCO scientists look not only at the effects of past exposures, but use a wide range of testing modes to predict the future durability of materials, including coatings, paints, specialty alloys and plastics.