INTRODUCTION: THE CORROSION BATTLE
The battle of corrosion is a constant fight because of the curse of all paint coating technologists – 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.
Paint coating technologists fight the Battle of Corrosion by applying paint coatings to maintain metals in their processed state and prevent regression to their natural oxide (corroded) state.
FUNCTIONS OF PAINT COATINGS
A paint 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, paint coatings fail in service, causing the paint coating to lose its aesthetic and protective functions. Paint 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 paint coating has failed in service to act as expert witness and determine why the paint coating failed.
CASE EXAMPLES ON WHICH I SERVED AS AN EXPERT WITNESS
Case Example 1. This case involved corrosion of painted aluminum extrusion door and window frames. Dozens of residential building in coastal areas showed excessive and premature paint blistering and corrosion, and home owners filed suit. My role as an expert witness was to determine why they failed so quickly – usually within a few years. Using sophisticated analytical techniques, I determined the root cause of the premature paint failure was that the paint primer and topcoat were lifting off the pretreatment layer due to poor manufacturing practices by the company who pretreated and painted the extrusions. The failure was exacerbated by un-coated extrusion edges, and use in severe marine environments.
Case Example 2. This case involved premature paint blistering and delamination of paint coatings applied to exterior wood window and door frames. The root cause of this paint failure was poor formulation. The paint manufacturer substituted a lower cost resin for one that they had been using for years. The lower cost resin had much poorer properties that the original one, counter to the claims of the resin supplier.
Case Example 3. This case involved the use of an antimicrobial paint coating on galvanized steel for use in hospitals. The paint delaminated from the galvanized surface prematurely, causing the galvanized to be replace shortly after installation. The root cause for the premature paint delamination failure was inadequate cleaning of the galvanized surface prior to applying the paint.
TYPICAL REASONS FOR PAINT FAILURE
There are a huge number of reasons why paint coatings fail in service. To illustrate, The U.S. National Institute of Standards and Technology (NIST) Report “Methodologies for Predicting the Service Lives of Coating Systems” describes what they call “Fault Trees” to identify the many factors that can contribute to coating failure, and the interactions of these factors. The report explains “The service life of all commercial products [coatings – Iezzi clarification] is affected by many variables, including weathering variables and variables associated with the materials, manufacturing, and design of the product….Failure of a coating system (see Figure 3) can normally be attributed to a number of root faults which are associated with the (1) coating application technique, (2) design of the structure to be coated, (3) manufacturing and processing of the coating system components, (4) properties of the materials comprising the coating system, and (5) exposure environment.”
All of the possible reasons for paint coating failure cannot possibly be discussed in this article. However, I will discuss a few of the primary reasons in the following, in approximate order of importance.
Poor Formulation and Choice of Raw Materials
There are typically several raw materials that make up a paint formulation. These include major components such as the binder (i.e., the main resin like acrylic, urethane, polyester, etc.), pigments, and solvents. Minor, though still critical, components include additives like rheology modifiers, surfactants, defoamers, coalescents, drying agents, ultraviolet light absorbers, corrosion inhibitors, and biocides. All of these component determine the properties of the paint coating. They must be chosen carefully, both in quality and proportion of each in the formulation. Poor choices almost always result in poor paint properties. To emphasize, the book Paint Formulation – Principles and Practice explains “The absolute amounts and relative proportions of the various ingredients play a major role in determining the performance of the resultant coating formulations although, obviously, the characteristics of the selected components must also be suitable for the coating in its service environment. In fact, the incorrect choice of only one component can reduce a paint’s performance even if the other additions and general formulation requirements are otherwise satisfactory. This is a frequent cause of premature paint failure in practice.”
Inadequate Surface Preparation
Inadequate cleaning and chemical pretreatment steps prior to painting are one of the primary reasons for paint failure in service. The cleaning and chemical pretreatment steps are critical to obtaining a paint coating that will perform its intended functions for the long-term. If not done properly, cleaning and pretreatment steps can leave residual soils on the substrate surface, can leave hard water salts from rinse or process water, and/or chemical contaminants from unreacted pretreatment chemicals that are not adequately removed by rinsing. These soils and chemical impurities can reduce paint adhesion, and can attract moisture permeating through the paint that can lead to blistering and other defects in the paint.
Improper Curing of the Paint Coating
Incomplete curing/baking the paint can result in residual solvents or water remaining in the paint that can cause poor adhesion, blistering, too soft a coating, and other defects. Over-baking the paint can affect the appearance of the paint, and the paint will likely be very brittle. Paint brittleness is a particular issue if the metal is to be formed after painting. Curing the coating too fast or too slowly can result in blisters or other defects in the paint coating.
Poor Design/Misuse of Product
Poor design or misuse of the product can lead to premature paint failure. Examples of poor design include non-coated edges of painted metals used severe service environments, tight forming bends that crack the paint, small crevices that are prone to retention of moisture and other corrodents which accelerate corrosion, and connection to unpainted dissimilar metals. Examples of misuse include placing the product in a severe environment for which the paint formulation is not suitable, such as near or in salt water, and prolonged exposure to extreme heat, cold, humidity, chemicals, pollution, or intense ultraviolet light.
Defects in a Metal Substrate
Defects on a metal surface such as grinding or polishing marks, mill roll marks, nonmetallic inclusions, oxides, grain boundaries, nicks, and scratches are high energy sites which can accelerate corrosion under the paint coating.
Many factors can and do contribute to why paint coatings fail in service.
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 paint coatings, metal coatings, corrosion, surface preparation/pretreatments for metals and plastics prior to painting, and plastics. He can be reached at firstname.lastname@example.org, or (610) 761-6721.
 Martin, J.W. et. al., “Methodologies for Predicting the Service Lives of Coating Systems”, published by the Federated Societies for Coatings Technology, June 1996, p. 8-9.
 Boxall, J., & J.A. Von Fraunhofer, Paint Formulation – Principles and Practice, Industrial Press, Inc., 1981, p. 35.