How to Measure Coating Thickness on Stainless Steel

Photo of a stainless steel structure

DeFelsko’s handheld coating thickness gages non-destructively measure the thickness of coatings applied to all metal substrates, including stainless steel. The varying magnetic properties of stainless steel make measuring paint thickness particularly challenging, so using the right equipment with the correct settings is critical.

Why Should Stainless Steel be Painted or Coated?

Stainless steel is commonly used for its resistance to corrosion, making it a long lasting and cost-effective option. The chromium present in stainless steel reacts with oxygen to form a thin, inert oxide layer over the entire surface that provides corrosion resistance, and regenerates itself when damaged. In addition to its inherent corrosion protection, stainless steel is often coated to provide additional corrosion resistance or for decorative/aesthetic purposes.

Stainless steel is used in a wide variety of industries including architecture, transportation, medical, energy, off-shore oil, pipes, aerospace and more. There are different families of stainless steel with various combinations of additive elements, and these families are further categorized into grades based on their composition.

Measuring the thickness of coatings applied to stainless steel substrates can be difficult for conventional handheld inspection instruments, depending on the magnetic properties of the specific family and grade.

The Challenge of Measuring Coating Thickness on Stainless Steel Substrates

Depending on its grade, stainless steel can be magnetic, non-magnetic or partially magnetic. These magnetic properties must be considered when selecting a suitable coating thickness gage. Using the incorrect gage or settings may result in inaccurate coating thickness measurements.

Photo of steel stock in many shapes and forms

Ferritic stainless steels are magnetic and include the 400-series grades, which offer excellent corrosion resistance and ductility. Martensitic stainless steels contain both 400- and 600-series grades and are also magnetic, with less corrosion resistance compared to other families.

Austenitic stainless steel is most commonly used and has the highest number of grades, specifically the 200-, 300- and 900-series grades. This family of steels is generally non-magnetic. However, austenitic stainless steels can become partially magnetic when cold worked. The specific material composition can also affect the degree of magnetism, as can the addition of nickel.

Duplex stainless steels contain a mixture of both austenitic and ferritic stainless steels, which provide a higher level of corrosion resistance and strength than the 300-series grades. This family is typically magnetic, but because duplex stainless steels contain a higher content of austenitic than ferritic steels, they may be less magnetic.

Taking accurate coating thickness measurements and getting a repeatable zero may be difficult on stainless steels that are slightly magnetic, or that have inconsistent levels of magnetism due to varying degrees of cold working. Electronic coating thickness gages using the magnetic or magnetic induction method will rarely measure accurately due to the weak and inconsistent magnetic field. Similarly, gages using the eddy current method are affected by magnetism in the substrate.

Coating Thickness Measurement Solutions from DeFelsko

Instruments designed for measuring coating thickness on metals work on either the magnetic or eddy current principle of operation, or both (combination).

Magnetic—measure the thickness of non-magnetic coatings on magnetic substrates

Eddy Current—measure the thickness of non-conductive coatings on non-magnetic substrates

Measuring Paint Thickness on Magnetic Grades of Stainless Steel

Measuring non-magnetic coatings applied to magnetic stainless steel substrates is straightforward using magnetic principle gages. Simply check zero on the uncoated part, and adjust if necessary.*  Available solutions include…

Photo of DeFelsko paint thickness gauges for magnetic grades of steel
DeFelsko paint thickness gauges for magnetic grades of steel

PosiTest — Magnetic pull-off thickness gage (banana gauge) for the non-destructive measurement of non-magnetic coatings (paint, enamel, galvanizing, metalizing, plating, etc.) on steel.

PosiTest DFT —  Dry Film Thickness gage measures paint and other coatings on metal substrates. It is the economical choice that retains the uncompromising quality of DeFelsko coating thickness and inspection instruments.

PosiTector 6000 F — The rugged, fully electronic PosiTector 6000 Coating Thickness Gage uses magnetic and eddy current principles to measure coating thickness on both ferrous and non-ferrous metals, accurately and quickly.

*For mechanical gages, instead of checking zero before measuring, measure a known thickness on the bare substrate (such as a shim), ensuring the results are within tolerance.

Measuring Paint Thickness on Non-Magnetic Grades of Stainless Steel

Eddy current gages such as the PosiTector 6000 N (Non-Ferrous) are ideal for measuring non-conductive coatings applied to non-magnetic stainless steel substrates. Simply check zero on the uncoated part and adjust if necessary.  

Measuring Paint Thickness on Partially Magnetic Grades of Stainless Steel

PosiTector 6000 FN with included N-Lock (Non-Ferrous Lock) Mode

PosiTector 6000 FN (Ferrous/Non-Ferrous) combination gages provide accurate coating thickness measurements on nearly all families and grades of stainless steel, including magnetic, non-magnetic and partially-magnetic stainless steels. PosiTector 6000 FN gages combine both magnetic and eddy current technologies to measure on magnetic and non-magnetic substrates. Combination gages first detect whether there is any magnetism present in the substrate, and if so, proceed to measure using the magnetic method. If no magnetism is detected, then the eddy current method is used. As with the other gages, it is recommended to check for a repeatable zero on the uncoated part and perform an adjustment if needed.

PosiTector 6000 FN gages are uniquely capable of overcoming the measurement challenge presented by partially magnetic stainless steels, provided that the applied coating is non-conductive.

PosiTector 6000 FN instruments feature N-Lock (Non-Ferrous lock) mode, which causes the gage to exclusively use the eddy current principle of measurement. The rare-earth magnet housed within the probe saturates the weak magnetic field that would otherwise interfere with the eddy current measurement, effectively turning a partially magnetic substrate into a temporarily non-magnetic substrate. The gage will then be capable of obtaining accurate and repeatable coating thickness readings over stainless steel, regardless of inconsistent or partial magnetism.  

Photo of a PosiTector 6000 FN measuring paint thickness on a red tank
PosiTector 6000 FN measuring paint thickness on a steel tank

Learn more about our PosiTector 6000 and complete line of coating thickness gages.

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