Tier Testing for Coating System Selection

Selecting a coating system that will perform well over time in a given environment is not always a clear cut decision. The type of substrate, service environment, durability, abrasive resistance, and long term appearance are all important factors that should be considered before a coating system is applied.

Matco Services' Tier Testing is comprised of four levels.  Each is designed to test a specific aspect of a coating system’s performance. Although coating systems must pass Tier 1 basic testing to proceed to Tier 2, and so on, all aspects of the Tier Testing can be performed at the same time if enough coated panels are supplied to Matco Services.   The following demonstrate Tier Testing objectives for many of our clients:

• Repeatable, standardized ASTM and NACE test methods that are selected based on the end user environment.   
• Independent and unbiased testing identifies the weaknesses and strengths of candidate coating systems.  Since testing is performed in Tiers, any coating system that doesn't pass the rigorous battery of testing is eliminated from any further testing - this equates to a large cost savings for our clients.  
• Distinguishes acceptable vs. unacceptable coating characteristics for above and below grade coating systems for electric utility transmission line structures.
• Flexibility – different coating types and service environments dictate changes in test protocols.
• Performance requirements are identified in terms of desired outcomes.
• Coating manufacturers are encouraged to formulate coating systems that achieve performance goals.
• Quantifiable performance criteria can be used to certify different coating systems.
• Redundant testing is eliminated if a coating system does not pass the Tier it is in.
• Tests can be performed simultaneously to meet client expectations.
• Weathering steel, aluminum, ductile iron and other metals are also candidates for Tier protocol.

In Tier 1, basic characteristics of a coating system are assessed including adhesion; cathodic delamination resistance; internal (defects, porosity, etc.) characteristics; substrate and coating thickness characteristics; and specification criteria.  Tier 2 provides a means to evaluate a coating system’s ability to resist corrosion and degradation over time when exposed to corrosive environments, while Tier 3 testing is used to evaluate a coating system’s ability to resist UV degradation, abrasion, and retain color and gloss.  Tier 4 testing ensures that coating system formulations don't change over time without the knowledge of the product purchaser. 

Tests that are included as part of the Tier Testing program are all based on ASTM or NACE standard practices.  As our client's needs govern the test selected for inclusion in the program to some extent, modifications to a protocol are often incorporated.  Tests commonly used include the following:

Our Tier Testing program utilizes the most sophisticated testing techniques available including electrochemical impedance spectroscopy (EIS), disbondment testing, cross section analysis, cyclic exposure, UV testing, gloss and color, gravelometery, and more. Tier Testing is usually performed in either three to four tiers, depending on a coating system’s requirements. Each Tier has specific criteria the coating system must pass in order to proceed to the next level. If a coating system doesn’t pass, it’s not only eliminated from further testing but further cost to the client as well. Clients pay only for the tests performed.

Each Tier is designed to test a specific aspect of a coating system’s performance. The first tier tests two of the most important coating characteristics: delamination resistance and coating uniformity, thickness, surface preparation and identification of internal defects. If a coating system doesn’t pass these two important performance parameters, we know the coating system will not provide adequate barrier protection and delamination resistance.

Tier Testing, Level 1: Cathodic Disbondment Testing. Cathodic delamination can be defined as “the destruction of adhesion between a coating and the coated surface caused by products of a corrosion reaction.” This basically means the coating delaminates or lifts and separates from the substrate even though it exhibits good adhesion prior to exposure service conditions .

Tier Testing, Level 1: Cross Section Analysis, above left. Internal defects, such as bubbles or pinholes in the coating, are identified in the first tier of testing. The image on the right shows a high resolution image of pinholes in a coating by SEM analysis for illustrative purposes.

If a coating system passes the first tier, it’s advanced to the second tier which involves cyclic exposure testing and electrochemical impedance spectroscopy (EIS), shown in Figures 1 and 2, to evaluate a coating’s performance and ionic barrier properties; adhesion testing prior to and after exposure testing and hot water temperature testing to determine how well the coating system performs when exposed to outdoor higher humidity temperatures in regards to corrosion resistance.

Figure 2: The image on the left shows the electrochemical behavior of a purely capacitive coating (Ideal), while the image on the right shows the electrochemical degradation (from Ideal) of an organic coating. A computerized electrochemical system and EIS software are utilized to collect data and help with analyses. This technique will clearly identify coatings that do not have good corrosion resistance in a relatively short period of time.