Powder Coating Thickness Testing

Ultrasonic Thickness Test
What Is Powder Coating?

Every industry wants its equipment to keep working as long as possible – no one has the time or money to constantly replace failing equipment. Ferrous metals such as carbon steel are especially susceptible over time to environmental wear caused by moisture, salinity, and other concerns. To combat these problems, you protect your outdoor equipment with a paint coating. Taking various aspects into account, such as affordability and scope of the project, many industrial companies choose powder coating over liquid paint.

Powder coating is applied differently than liquid paint. To use a wet paint method, first apply a primer to the substrate so that the paint will stick to it. The coating (made up of pigments for color and solvents for consistency) then dries over a long period of time – it bonds to the substrate and solidifies into a thin, protective layer. Powder coating works in a different way; the applicator applies a positive electrostatic charge to the powder, which is shot at the grounded substrate. The powder is magnetically attracted to the substrate and sticks to it until baked for a short period of time to solidify its bond into a thick, even layer.

Making the Case for Powder Coating Over Wet Paint

Each coating method has pros and cons make one better suited over the other depending on the project. However, powder coating is the preferred painting solution for industrial use for various reasons which are covered below.

Firstly, powder coating is more durable than wet paint. Wet paint dries due to the evaporation of the solvents that make it a liquid, and then hardens as the remaining contents bond and adhere to the substrate. This process leaves a brittle result that lacks the flexibility to painlessly adjust to metal’s expansion and contraction in varying temperatures. Brittleness in this case leads to cracks in the coating, which allow corroding and rusting agents access to the substrate underneath. Powder Coating dries due to a heat reaction as it is baked onto its substrate. Crosslinking, the chemical formation of bonds between two polymers, occurs when the powder is heated to a certain temperature in a specialized oven. This process yields a more flexible coating that is less prone to cracking. It also produces a thicker and more even coat than does wet paint, the latter of which requires multiple applications to get an even coat. Powder coating’s overall resistance to wear and tear is far superior to that of wet paint, which requires more refinishes and touch-ups over the course of its lifetime use.

Wet paint is more dangerous to use than powder coating as well. Wet paint is highly flammable before and during application due to the chemical characteristics of its organic solvents, while powders are far less flammable because of their lack of that same ingredient. Most solvents (which evaporate into the air as paint dries) also contain volatile organic compounds (VOCs) that are harmful to human health if inhaled. Finally, VOCs are an environmental hazard known to help create smog, and thus are regulated by the EPA. None of these safety concerns apply when using powder coating methods.

Furthermore, powder coating is easier to use and more efficient in its application than wet paint. Both a steady hand and a specific method are required to get a visually appealing, even coat of liquid paint, as it is prone to dripping and other defects if applied incorrectly. Powder coating, on the other hand, goes on even and smooth, due to the magnetic interaction between the charged powder and the grounded substrate. Overspray represents an efficiency concern in the paint world, both causing visual defects in the product and wasting costly paint on unintended surfaces. While it is a common trouble with inappropriately handled wet paint, overspray accounts for a much smaller factor in powder coating: the latter method has a better adhesion rate to its substrates, and excess powder used can be collected and reprocessed to decimate waste.

This abundance of benefits that powder coating offers over wet paint – increased performance, more durable, less toxic, more environmentally friendly, easier to use, and more efficient to apply – have led to its increasing popularity among various industries.

Testing Thickness

Ensuring that equipment coatings have a certain thickness is important to maintaining their effectiveness at protecting their substrates. Whether initially checking before the powder cures or re-examining the thickness after the coating has been in use for some time, a good thickness reading extends the livelihood of outdoor equipment.

Both types of coating thickness are measured in mils, which indicates one one-thousandth (1/1000) of an inch. Wet paint tends to yield thinner coatings per application than do powder coatings, due to the evaporation of solvents in the former’s drying process.

Powder coating thickness can be measured in many ways, but they all fall under two general categories:

  • Testing Before Curing
  • Testing After Curing

Testing before curing, used in quality control during production, is measuring the thickness of the powder itself before it crosslinks into the resulting, even coating. Two of the methods, involving a comb and a probe respectively, measure the height of the powder – the thickness is determined by factoring the reduction factor against the height measurement. Both methods displace powder to take a measurement, which may lead to an imperfection in the coating after curing. The third method uses an ultrasonic reader, which is completely non–destructive, to predict the thickness of the cured coating. These methods help establish that the coating is sufficiently thick during production but do nothing to help after the equipment has been cured.

Testing after curing, more commonly used in quality control inspections performed after the equipment is already in use, can be achieved in multiple ways:

  • Mechanical Magnetic Gauges: stick a magnet onto the test material and then pull it off – amount of pressure needed to separate gauge from coating determines thickness
    • Used on ferrous substrates only
    • More force needed = lower coating thickness
      • Increased distance between gauge and substrate (see: coating thickness) lessens magnetic attraction
    • Quick
    • Analog
      • Detach magnet through mechanical forces (applying tension to a coil)
      • Analog display
    • ASTM Standard D7091
  • Electronic Magnetic Gauges: stick a charged probe onto the test material and get a reading
    • Can adjust to either ferrous or non-ferrous metals
      • Applies magnetic principle above for ferrous substrate
      • Applies an alternating current to generate a magnetic field for measurement for non-ferrous substrate
      • Has to be a metal substrate, though
    • Only measures total coating thickness
      • Cannot differentiate between multiple coatings
    • Versatile
      • Versions that come with wireless probes and internal memory
    • ASTM Standard D7091
  • Ultrasonic Gauge: stick a probe onto the test material – it sends out an ultrasonic pulse that travels through the coating, hits the substrate, and bounces back. Converts the time lapse to a thickness reading
    • For use on non–metal substrates
    • Has difficulty if substrate and coating are of similar makeup
      • Can struggle when multiple coats are involved, as well
    • ASTM Standard D6132
  • Micrometer: measure thickness of substrate with coating vs substrate without coating – difference is coating thickness
    • Can measure any combination of substrate and coating regardless of chemical compositions
    • Has to have physical access to bare substrate
      • Cut down to substrate (destructive test)
    • View measurements with a microscope
      • Cuts are as small as possible
    • ASTM Standard D4138

ATS and Thickness Testing

Applied Technical Services, Inc. supplies all manner of thickness testing for equipment coating after it has cured. Our certified and talented engineers are able to perform these tests either in the field or in the lab. As one feature of a larger overall inspection provided, we often implement Ultrasonic Thickness Testing to ensure that equipment coatings remain at the manufacturer-standard thickness – or alert clients if our measurements indicate an insufficient thickness. For clients that need to send a part in to the lab for a more intensive test, such as a chemical or environmental analysis, we will also take a coating thickness measurement in the interest of offering the most comprehensive service possible.

For over 50 years, Applied Technical Services has provided a long list of engineering services to satisfied clients from a variety of industries. Allow us to ensure that your critical equipment is properly protected by supplying accurate thickness testing at a competitive price.

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