The Basics on Testing Protective Coatings

Without historical field performance data, coatings testing programs are the only objective way in which decisions relating to future work can be made. Whether done in-house or contracted out, any type of coating operations must include some type of meaningful test program. 

Protective coatings test programs should include the evaluation of:

• materials 

• surface preparation 

• application 

• inspection procedures 

Those interested in the operation’s coatings should subscribe to the program and agree on the basis by which decisions are to be made. Whenever possible, standard test procedures and means of evaluation as outlined in standards by NACE International, ASTM, SSPC, and others should be followed. Only in this way can correlation with the results of various workers be made/established.

Basically, there are two main types of tests for coatings subject to atmospheric exposure although, the general approach for buried or submerged coatings is similar.

Field or Service Testing 

While delays involved in field or service testing may seem interminably long -- with many cases requiring years for completion of a full-scale field test and hundreds of thousands, if not millions of dollars in coatings expenditures -- it pays to be sure about performance. Most field testing is done on site under the supervision of the customer’s testing staff and involves the following steps.

Ordinarily, coupons coated with the materials to be tested are exposed in multiples at several test sites in a plant.

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. Exposure at a single location may not give results that can be anticipated with confidence everywhere in the plant. Additionally, because some locations are less corrosive than others, they may not require systems as expensive as those used in the most aggressive locations.

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 laboratory 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. Nonetheless, the tests are invaluable when developing a coating or appraising new concepts in coating application or use.

 

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Source: MaterialsPerformance.com article adapted from Corrosion Basics -- An Introduction, Second Edition, Pierre R. Roberge.

 

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Published by Corrosion CONTROLLED, Corrosion Essentials, M&I September 7, 2021
Corrosion CONTROLLED, Corrosion Essentials, M&I