ASTM D7091 Standard Practice for Nondestructive Measurement of Dry Film Thickness of Nonmagnetic Coatings on Ferrous and Non-Ferrous Metals

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ASTM D7091 testing allows coating manufacturers, applicators, inspectors, engineers, technicians, and quality-control professionals to determine if dry coating thickness complies with specified requirements. The minimum and maximum thickness of individual layers and the total coating system are specified in many commercial and industrial coating specifications. An experienced technician checks and calibrates the coating-thickness gauge, then takes readings and observes a suitable measurement frequency throughout the area to be coated. The resulting measurements enable identification of insufficient or overcoating areas and offer information for quality control and specification evaluation. 

• Ferrous Metals: Measurement of nonmagnetic coatings applied to ferrous metal substrates using magnetic gages. 
• Non-Ferrous Metals: Measurement of nonmagnetic, electrically nonconductive coatings on nonferrous metal substrates using eddy-current gages. 
• Gage Verification: Verification of gage accuracy before and during use. 
• Gage Adjustment: Adjustment or optimization of the instrument for the substrate and coating being measured. 
• Dry Film Thickness: Measurement of the cumulative or applicable coating thickness after drying. 
• Measurement Frequency: Guidance for obtaining representative measurements over coated areas rather than relying on isolated readings. 
• Reporting: Recommendations for documenting coating thickness measurements. 
• Limitations: The method is unsuitable for coatings that can be readily deformed under the probe load. Metal-filled coatings can also produce erroneous readings depending on their composition. 

What are the uses of ASTM D7091 testing?

ASTM D7091 is a nondestructive testing method to achieve a thickness measurement of the coating on the coated object without destroying the object. This makes this practice useful for production inspection, field inspection, application control of coating, and evaluation of complete coating systems. Typical uses include:

• Coating Thickness Verification: Check Dry Film Thickness to see if it adheres to required specifications. 
• Quality Control: Ensure uniformity of coating application in production or field application. 
• Specification Evaluation: Measure thickness and compare it with minimum and maximum. 
• Coating Application Control: Determine areas that may need further coating or corrections. 
• Corrosion Protection: Ensure correct corrosion protection has been applied at the correct thickness. 
• Multi-Layer Coating Evaluation: Measure the thickness of coatings that are applied in layers. 
• Warranty and Documentation: Provide objective thickness records for coating projects. 

Which Materials Can Be Tested According to ASTM D7091?

ASTM D7091 applies to suitable coating systems on ferrous and nonferrous metallic substrates. The measurement technique depends on both the substrate and the electrical or magnetic characteristics of the coating. Examples include:

• Steel Components: Nonmagnetic protective or decorative coatings applied over ferrous steel. 
• Iron and Ferrous Alloys: Suitable nonmagnetic coating systems on ferrous substrates. 
• Aluminum: Nonmagnetic, electrically nonconductive coatings measured with eddy-current gauges. 
• Copper and Other Nonferrous Metals: Suitable insulating or nonconductive coating systems. 
• Paint Coatings: Dry organic coatings applied to metallic substrates. 
• Protective Coatings: Coatings used for corrosion and environmental protection. 
• Industrial Coating Systems: Multi-layer coating systems where cumulative thickness is required. 

Why is ASTM D7091 Testing Important?

The significance of ASTM D7091 lies in its impact on coating thickness, which is a critical factor in the performance and durability of numerous industrial coating systems. A thick coating may result in application and performance difficulties; a thin coating may not afford protection. It is the practice of a structured approach to calibration, verification, adjustment, and measurement, thus providing technicians with more accurate thickness data. The practice provides a structured approach to calibration, verification, adjustment, and measurement, helping technicians obtain more reliable thickness data.  ASTM D7091 notes that single-point measurements may not adequately represent the coating system thickness, and measurement frequency should be established according to the applicable requirements. 

ASTM D7091 Testing Process and Data Collection

The type of gage, magnetic or eddy-current, used will depend on the characteristics of the substrate and coating. The technician puts the probe right on the dry coating surface and takes readings at representative areas. The pattern and frequency of the measurements should be in keeping with the applicable project specification or measurement plan, as the coating thickness may vary throughout the surface. The technician writes specimen/component identification, type of substrate, description of coating, locations of measurements, type of gage used, information about reference standards, and individual measurements of thickness. The analyst/inspector compares readings with the appropriate minimum and maximum requirements.

Common Challenges and Troubleshooting

A common cause for inaccurate coating thickness measurement is gage adjustment error. Technicians should check the instrument against appropriate standards and set the instrument for the substrate, coating, geometry, and measurement conditions prior to taking any measurements. ASTM D7091 specifically identifies calibration, verification, and adjustment as important operational steps.  Metal-filled coatings can cause erroneous readings depending on the type and amount of metal present. Soft or tacky coatings may also deform under the probe and should not be measured using the standard procedure unless an appropriate provision applies. 

• The measured coating thickness represents the distance between the probe and the underlying metallic substrate as determined by the applicable magnetic or eddy-current measurement principle. Depending on the gage and application, the measurement may represent the cumulative thickness of multiple coating layers rather than the thickness of one individual layer. 
• The technician records specimen identification, substrate material, coating system, measurement locations, gage type, verification information, individual thickness readings, and coated-area information. The analyst or inspector documents the representative measurements, calculated or summarized thickness values, and comparison with the applicable specification.
• The report contains the component or specimen identification, substrate description, coating description, measurement method, gage information, reference-standard information, measurement locations, individual readings, average or other required statistical values, and applicable acceptance requirements.
• Dry film thickness is generally reported in µm (micrometers), while mm may be used when appropriate. The governing specification determines the required minimum and maximum coating thickness and the method for evaluating the measurement results.
• Standardized gage verification, adjustment, probe placement, measurement frequency, surface evaluation, equipment control, and data reporting allow technicians and inspectors to provide consistent, reliable, and comparable ASTM D7091 results.

Link to ASTM D7091