Q. What’s the best way to measure thermal spray aluminum thickness on a non-magnetic (austenitic) stainless steel?
A. Accurate measurement of thermal sprayed aluminum (TSA) coating thicknesses is important for both the applicator and the end user. The applicator wants to apply the correct thickness and minimize material costs. Additionally, an overly thick application can induce stress cracking in the coating and cause adhesion problems. The end user will spot check critical areas to be sure they achieve the specified corrosion preventive qualities required.
Applicators and end users have had problems measuring the true thickness of the TSA coatings as applied to non-magnetic (austenitic) stainless steel alloys. Standard DFT gages, using magnetic methods, can be used for TSA coatings on carbon steel alloys. However, these gages cannot measure TSA coatings when TSA is applied to non-magnetic (austenitic) stainless steel alloys, such as Type 316.
Current industry standards have the TSA applicator apply a sample coat of the TSA to a carbon steel test coupon and measure that thickness using a Type II DFT gage or a mechanical test method (micrometer). The assumption is that the coating applied to the structure has the same thickness as was applied to the coupon. However, the applied coating thickness may vary enough to create problems.
One test method that can be used to measure TSA coatings, as applied to nitic stainless steel, is phase-sensitive eddy current (PSEC). The PSEC test method enables both end users and applicators to measure the TSA coating as applied to non-magnetic stainless steel base metals. The applicator must still create a test coupon to calibrate the gage, but this test coupon can be of the same alloy as the structure. The newly calibrated gage can then be used to test the applied thickness anywhere on the structure; no guesses are made. Then, the structure may also be tested in accordance with industry-specific test specifications such as SSPC-PA2. Additional technical benefits of the PSEC method help inspectors by reducing errors inherent in other test methods:
- Surface roughness of the base material has a minimal impact on the measured thickness
- Reduced surface roughness influence of TSA coating
- Reduced impact of part curvature
- Impacts of the base material
The goal of the TSA coatings is to offer a long service life of critical structures used in harsh environments. Therefore, accurate measurement of the coating thickness, as applied to the actual structure, is critical in achieving the correct corrosion preventative requirements. Test methods designed specifically for TSA coatings on non-magnetic steels allow the applicator to confirm the correct TSA thickness has been applied and inspectors can then confirm the coating thickness anywhere on a structure.
Rob Weber is an applications and product support engineer at Fischer Technology. For more information, visit fischer-technology.com.
Originally published in the February 2017 issue.