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Corrosion Basics: Testing Protective Coatings

Coatings tests may involve a wide variety of procedures, ranging from comparatively simple to highly sophisticated.

Coating operations of almost any size must include some type of meaningful test program. The program should evaluate materials, surface preparation, application, and inspection procedures. In the absence of extensive historical field performance data, such programs are the only objective basis on which to make decisions relating to future work. These may be done in-house or contracted to the many consulting/inspecting firms available to help those without the necessary personnel.

It is important that all parties having an interest in this work subscribe to the program to be instituted and agree on the basis by which decisions are to be made.

Whenever possible, standard test procedures and means of evaluation provided by NACE International, ASTM, SSPC, and others should be consulted and followed. Only in this manner can correlation with the results of other workers be established. The testing described below is typical for coatings that are subject to atmospheric exposure; however, the general approach for buried or submerged coatings is similar in most respects.

Laboratory Testing

The number and types of tests that can be conducted in the laboratory are almost limitless. Because field testing of coatings is so costly in time, labor, and facilities, it has been the persistent aim of the formulators, raw materials producers, and users of coatings to establish meaningful accelerated tests. These may involve a wide variety of procedures, ranging from comparatively simple to highly sophisticated.

The tests can define certain characteristics of a coating system. This discrimination usually is specific to the type of accelerated test and frequently does not indicate in advance how a given coating will perform in service. However, the tests are invaluable when developing a coating or appraising new concepts in coating application or use.

Field or Service Testing

Although the delays involved in field or service testing may seem interminably long in many cases (years may be required for completion of a full-scale field test), when coating expenditures run into the hundreds of thousands, and in some cases into millions of dollars, it pays to be sure about performance. Most field testing is done on site under the supervision of the customer’s testing staff. Ordinarily, coupons coated with the materials to be tested are exposed in multiples at several test sites in a plant as a first step.

After varying periods of exposure, depending on the design of the test program, coupons are removed from the exposure sites to laboratories where their residual properties are measured. At the end of such a program, it usually is possible to identify the best performance from among a group of coatings similarly exposed. This coating can be applied with some confidence that good service will be obtained.

As for any other tests, a field test panel and application procedure should be selected to represent common situations or other conditions of interest.

Once the panel testing is completed, the usual procedure is to apply the most promising two or three candidate materials on tanks or other large metal areas in the plant site, fully realizing that, when proceeding with this step, all surfaces in the field have far from the same exposure. The four quadrants of a tank are different in sunshine exposure, and the top and bottom halves of the tank are not the same because of the internal changes in liquid level.

In any large chemical or industrial plant, corrosivity at one plant location may be more or less severe by several orders of magnitude than the corrosivity at another location. Thus, exposure at a single location may not give results that can be anticipated with confidence everywhere in the plant. Furthermore, because some locations are less corrosive than others, they may not require systems as expensive as those used in the most aggressive locations.

This article is adapted by MP Technical Editor Norm Moriber from Corrosion Basics—An Introduction, Second Edition, Pierre R. Roberge, ed. (Houston, TX: NACE International, 2006), pp. 449-452.

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