Coatings

Organic
1 a (2):  of, relating to, or derived from living organism
1 b (2):  of, relating to, or containing carbon compounds
Inorganic
1 a (1): being or composed of matter other than plant or animal: Mineral
1 b: of, relating to, or dealt with by a branch of chemistry concerned with substances not usually classed as organic
Merriam-Webster https://www.merriam-webster.com/dictionary

Basic Coating Components
Binders
In order to perform in a practical way in any given environment, a coating, after its application, must con­vert to a dense, solid, adherent membrane with all or most of the properties that have been previously dis­cussed. The binder is the material which makes this possi­ble. It provides uniformity and coherence to any coating system.
Pigments
While binders are responsible for many of a coating’s primary properties, pigments also contribute several properties important to their effective use. In fact, proper or improper pigmentation can either make or break a coating in terms of corrosion resistance. Several different pigments may be used within the same coating, all of which contribute to the coating’s general characteristics and perform several functions.
Solvents
Most painters do not realize the importance of solvents, or thinners, in the formation of the coating and in the development of the most effective coating film. Although solvents do not remain in the coating, they can affect the coating in many different ways, i.e., by creating porosity, discoloration, poor gloss, floating of pigment, fish-eyeing, poor coating strength, and lack of adhesion. All of these things can happen if the proper solvent or solvent combination is not used in a protective coating. The proper use of solvents will create a smooth, clear resin film with a good gloss, and the coating film will have the in­herent strength and other properties of the basic resin.
Other Coating Terms
Comparisons of various coatings are often made according to composition. The composition of a coating is often expressed by dividing the total weight between the pigment and the vehicle as percentages. In this case, the pigment includes both the hiding and the reinforcing or extender pigments, and also any material used to regulate the gloss of the coating. The vehicle is the complete liquid portion of the coating. Normally, it consists of both nonvolatile matter and volatile materials.
Thermoplastic Coatings
Thermoplastic coatings, or lacquers, dry solely by the evaporation of the solvent (the resin is already in its final form), and there is no chemical or physical change in the nonvolatile portion of the coating that forms the film. In this case, the film-forming process is merely the evaporation of the solvents from the liquid leaving the thermoplastic resins on the surface as a continuous film.
Conversion Coatings
Conversion coatings, on the other hand, dry or react in a whole series of steps. All such coatings undergo a chemical and physical change in the process of film formation.
Epoxy
A much more important conversion reaction, from a corrosion standpoint, is catalyst conversion or cross-linking at ambient temperatures. The epoxy coating forms by this process in which the epoxy resin is mixed with an amine just prior to application.
Moisture
A third familiar process of film conversion takes place when water from the atmosphere converts the film from a liquid to a solid. This is one of the processes by which the polyurethane coatings are formed.
Corrosion Prevention by Protective Coatings, Charles G. Munger

Standard Guide for Painting Inspectors (Metal Substrates) – ASTM D3276
Inspection Equipment – Section 10.2.2
Wet-Film Thickness Gages—This type of instrument is used to measure the thickness of a wet film of paint immediately after it is applied to a surface. Note that erroneous readings may result when using the gage on fast-drying paints such as inorganic zinc or vinyl. If a wet-film gage is used to determine the thickness of coats subsequent to the first, great care must be taken that partially hardened undercoats are not indented by the gage, thus giving high readings. If the coat being measured has an appreciable softening effect on the previous coat, a wet-film thickness gage cannot be used with accuracy. It is very important to record and retain test results.
Interchemical Gage—This instrument is rolled over the newly applied wet film on a smooth flat portion of the surface and the thickness read directly in mils (micrometers). Complete details are given in Method A of Test Methods D 1212.
Notched Gage—This device has a series of calibrated steps for measuring thin to heavy coats. This gage with the proper face is placed squarely on the fresh, wet film. It is then withdrawn perpendicularly without a sliding movement. The true wet-film thickness lies between the highest step coated and the next highest, which was not coated. The procedure is described in Practice D 4414.
Coating Storage and Handling – Section 6.3.3.2
To estimate the wet-film thickness of the thinned coating required to obtain the specified dried-film thickness, the percent volume of the nonvolatile (solids) in the original coating must be known. This figure is readily obtained from the manufacturer. With this information the calculation may be made as follows:
W = D x (1.0 + T) / S         (1)
where:
W = wet-film thickness,
D = desired dry-film thickness,
S = percent by volume (expressed as a fraction) of coating solids, and
T = percent by volume (expressed as a fraction) of thinner added.

Procedure for Determining Conformance to Dry Coating Thickness Requirements – SSPC-PA 2
Description of Gages – Section 4
Gage Types: The gage type is determined by the operating principle employed in measuring the thickness and is not determined by the mode of data readout, i.e. digital or analog.
Type 1 – Magnetic Pull-Off Gages: For magnetic pull-off gages, a permanent magnet is brought into direct contact with the coated surface. The force necessary to pull the magnet from the surface is measured and interpreted as the coating thickness value on an analog dial (scale) on the gage. Less force is required to remove the magnet from a thick coating. The gage scale is non-linear.
Type 2 – Electronic Gages: An electronic gage uses electronic circuitry to convert a reference signal into coating thickness.
Calibration, Verification of Accuracy and Adjustment – Section 5
ASTM D 7091 describes three operational steps necessary to ensure accurate coating thickness measurement: calibration, verification of accuracy and adjustment of coating thickness measuring gages, as well as proper methods for obtaining coating thickness measurements on both ferrous and non-ferrous metal substrates.
Conformance to Specified Thickness – Section 9
A minimum and a maximum thickness are normally specified for each layer of coating. If a single thickness value is specified and the coating manufacturer does not provide a recommended range of thickness, then the minimum and maximum thickness for each coating layer shall be ± 20% of the stated value.
Table 1 provides five thickness restriction levels. Level 1 is the most restrictive and does not allow for any deviation of spot or area measurements from the specified minimum and maximum thickness, while Level 5 is the least restrictive. Depending on the coating type and the prevailing service environment, the specifier selects the dry film thickness restriction level for a given project. If no restriction level is specified, then Level 3 is the default. It is possible to specify a maximum thickness threshold for Level 5 Spot or Area measurements for some generic product types and service environments.
Coating Thickness Restriction Levels – Table 1

Appendix 1 – Numerical Example of Average Thickness Measurement and Illustration of the Procedure for Determining the Magnitude of a Nonconforming Area
Appendix 2 – Methods for Measuring Dry Film Thickness on Steel Beams (Girders)
Appendix 3 – Methods for Measuring Dry Film Thickness for a Laydown of Beams, Structural Steel, and Miscellaneous Parts after Shop Coating
Appendix 4 – Method for Measuring Dry Film Thickness on Coated Steel Test Panels
Appendix 5 – Method for Measuring Dry Film Thickness of Thin Coatings on Coated Steel Test Panels that Have Been Abrasive Blast Cleaned
Appendix 6 – Method for Measuring the Dry Film Thickness of Coatings on Edges
Appendix 7 – Method for Measuring Dry Film Thickness on Coated Steel Pipe Exterior
Appendix 8 – Examples of the Adjustment of Type 2 Gages Using Shims
Appendix 9 – Precaution Regarding Use of the Standard for Coating Failure Investigations

Discontinuity (Holiday) Testing of New Protective Coatings on Conductive Substrates – NACE SP0188
Low-Voltage Wet Sponge Testing – Section 3
A low-voltage wet sponge tester is an electronic device powered by a self-contained battery with voltages ranging from 5 to 90 V direct current .(DC), depending on the manufacturer’s circuit design. It is used to locate discontinuities in a nonconductive coating applied to a conductive substrate. Operation
includes the use of an open-cell sponge electrode saturated with a solution for exploring the coating surface, a ground connection, and an audible or visual indicator for signaling a point of coating discontinuity.
High-Voltage Spark Testing – Section 4
High-voltage spark testers are suitable for determining the existence of discontinuities in coatings of all thicknesses. The coating manufacturer should be consulted for proper test equipment and inspection voltage. Suggested starting voltages are provided in Table 1.
CAUTION: Coatings that are applied at thicknesses of less than 500 urn (20 mil) may be susceptible to damage if tested with a high-voltage spark tester.
A high-voltage spark tester may be used to determine discontinuities in coatings on conductive concrete substrates. The conductivity of concrete varies depending on moisture content, type, density, and location of rebars. Conductivity shall be tested by attaching a ground wire to the rebar or another metallic ground permanently installed in the concrete and touching the electrode to the bare concrete. If the metallic ground is not visible, the ground wire shall be placed directly against the bare concrete surface and weighted with a damp cloth or wet-sand-filled paper bag. If the test indicates that the concrete is not conductive, determining discontinuities with a high-voltage spark tester will be ineffective.

Hardness Testers
Adhesion Testers
Cathodic Disbondment Tests
Laboratory Analysis
Coating Failure Modes