INTRODUCTION: THE CORROSION BATTLE
The battle of corrosion is a constant fight because of the curse of all coatings engineers – thermodynamics! The Second Law of Thermodynamics states, in simplified terms, that the naturally occurring state of matter is its lowest energy state, similar to humans on weekends. Metals ordinarily exist naturally as oxides (e.g. iron oxide, aluminum oxide, etc.) because oxides represent their lowest energy state. However, oxides are mined from the ground, and they are subjected to various unnatural acts – such as refining, casting, rolling, and forming into a variety of different shapes. These metals are now in an “activated” state, and do not want to stay there. They want to revert back to their naturally occurring state – oxides, or rust in the case of steel.
Coatings engineers fight the Battle of Corrosion by applying coatings, metal and/or paint coatings, to maintain metals in their processed state and prevent regression to their natural oxide (corroded) state.
FUNCTIONS OF COATINGS
A coating imparts two important functions to the underlying substrate:
- The aesthetic function gives the substrate a good appearance.
- The protective function protects the substrate from mechanical, chemical, and corrosion damage.
However, due to a myriad of reasons, coatings fail in service, causing the coating to lose its aesthetic and protective functions. Coating failure is a ubiquitous and on-going problem for the general public globally. It causes hundreds of millions of dollars of damage annually and compromises the safety, environmental, and appearance characteristics of the affected structures. When that occurs, I am often contacted by attorneys representing clients whose coatings have failed in service to act as expert witness and determine why a coating has failed.
CASE EXAMPLE ON WHICH I SERVED AS AN EXPERT WITNESS
Metal Coating. This case involved premature corrosion of copper roofing coated with a thin lead coating. Lead has been used in roofing and statues for centuries in Europe because it lasts hundreds of years in most environments. The lead is usually applied to copper sheet because the lead is not very strong as a stand-alone material, and the copper substrate gives the structural strength needed, particularly for roofing. Lead-coated copper (often referred to as LLC) is occasionally used on buildings in the United States because the lead gives a dull gray, antique look that is desired by some – and the lead should last decades. In this situation, LCC was used on a private residence because the owner wanted an antique look. Unfortunately, the lead corroded within two years and the roof was a visual nightmare – with red and white corrosion products all over the roof. So much lead corroded and washed off by rain that lead contaminated the lawn area of the residence. The property became a federal and state HAZMAT site and had to be monitored by federal and local environmental agencies. Needless to say, the owner was not happy and sued the contractor, architect, and LCC manufacturer. My role as an expert witness was to determine why the LLC failed so quickly. Using sophisticated analytical techniques, I determined the cause of the premature LLC corrosion was porosity and non-uniformity of the lead coating. The porosity exposed the copper substrate, creating an electrochemical corrosion cell which accelerated the corrosion of the lead. The corrosion of the lead resulted in unsightly red and white stains over a majority of the roof. This type of corrosion is often referred to as dissimilar metal corrosion, also known as galvanic corrosion. When dissimilar metal corrosion occurred in this case, the result was corrosion of the lead was accelerated while corrosion of the copper was reduced. A common example of dissimilar metal corrosion is when inexperienced plumbers connect copper pipe to a steel pipe or valve; in this situation, corrosion of the steel is accelerated relative to the copper.
The root cause of the premature failure of the LLC was is poor manufacturing by the producer of the LLC.
ABOUT THE AUTHOR
Robert A. Iezzi, Ph.D., is founder of RAI Technical Solutions®, Inc. (www.rai-technical-solutions.com), a technology company that provides expert witness and consulting services on metal coatings, paint coatings, corrosion, surface preparation/pretreatments for metals and plastics prior to painting, and plastics. He can be reached at riezzi@ rai-technical-solutions.com, or (610) 761-6721.