Measurement of coating thickness on stainless steel

Products covered in the article:

PosiTector 6000,  PosiTest DFT,  PosiTest,  PosiPen

Coating Thickness Gauges from mtv messtechnik non-destructively measure the thickness of coatings applied to all metal substrates, including stainless steel. The varying magnetic properties of stainless steel make measuring coating thickness particularly challenging, so using the right instrument with the correct settings is critical to the success of the measurement.

Why should stainless steel be painted or coated?

Stainless steel is typically used for its resistance to corrosion, making it a durable and cost-effective option. The chromium in stainless steel reacts with oxygen to form a thin, inert oxide layer over the entire surface, providing corrosion resistance and self-healing when damaged. In addition to its inherent protection against corrosion, stainless steel is often coated to provide additional corrosion resistance or for aesthetic reasons.

Stainless steel is used in a variety of industries including architecture, transportation, medical, energy, offshore oil exploration, tubing, aerospace and many more. There are different families of stainless steel with different combinations of additional elements, and these families are further divided into grades based on their composition.

Measuring the thickness of coatings on stainless steel substrates can be difficult for traditional gauges because it depends on the magnetic properties of each family and grade.

The challenge of measuring coating thickness on stainless steel substrates

Depending on the grade, stainless steel can be magnetic, nonmagnetic, or partially magnetic. These magnetic properties must be taken into account when selecting a suitable coating thickness gauge. Using an incorrect gauge or settings can result in inaccurate coating thickness measurements.

Ferritic stainless steels are magnetic and include the 400 series grades which have excellent corrosion resistance and ductility. Martensitic stainless steels include both the 400 and 600 grades and are also magnetic but less corrosion resistant than the other families.

Austenitic stainless steel is the most commonly used and has the most grades, particularly the 200, 300, and 900 series grades. This family of steels is generally nonmagnetic. However, austenitic stainless steels can become partially magnetic during cold working. The specific material composition can also affect the degree of magnetism, as can the addition of nickel.

Duplex stainless steels contain a mixture of austenitic and ferritic stainless steels that have higher corrosion resistance and strength than the 300 grades. This family is typically magnetic, but because duplex stainless steels contain a higher proportion of austenitic than ferritic steels, they can be less magnetic.

Accurately measuring coating thickness and achieving a repeatable zero point can be difficult with stainless steels that are slightly magnetic or have non-uniform magnetism due to differential cold working. Electronic coating thickness gauges that use the magnetic or magnetic induction method rarely measure accurately due to the weak and unstable magnetic field. Likewise, measuring devices that work with the eddy current method are influenced by the magnetism of the substrate.

Solutions for coating thickness measurement from mtv messtechnik

Devices for measuring the layer thickness on metals work either according to the magnetic or the eddy current principle or according to both functional principles (combination).

Magnetic - Measurement of the thickness of non-magnetic layers on magnetic substrates

Eddy current measurement of the thickness of non-conductive layers on non-magnetic substrates

Measurement of paint layer thickness on magnetic types of stainless steel

Measuring non-magnetic coatings on magnetic stainless steel substrates is easy with probes that work on the magnetic principle. Simply check the zero point on the uncoated part and adjust if necessary.

Some of the best available solutions include the following devices: