Blisters During Corrosion Testing
Q. We are coating parts that are hot-dip galvanized, and we fail corrosion tests because we develop blisters. How can we investigate the root cause? Also, when we test parts that have been powder coated, they perform better than the e-coated parts. Is powder response to corrosion testing the same as e-coat? R.M.R.
A. Blisters are the result of some imperfection or anomaly in the paint layer or below the paint layer that shows during the corrosion testing. This corrosion testing can be neutral salt spray or any other automotive cycle that combines water and salt in any combination of concentrations or temperatures.
Three potential root causes of blisters are poor cleaning, poor phosphate (lack of phosphate or a phenomenon called nubbing), or because the part has previously been hot-dip galvanized as in this case, a metallurgical defect in the hot-dip galvanizing layer.
E-coaters typically control the cleaning, phosphate and e-coat processes but cannot control the formation of blisters during testing. In my experience, I’ve found that most blisters come from the hot-dip galvanizing layer.
Hot-dip galvanizing is applied to metal surfaces under many different specifications, thicknesses and quality, and with different physical characteristics.
Porosity is critical for the e-coating process, but it is not a variable that a hot-dip galvanizing facility would control.
Under microscopic evaluation, these hot-dip galvanized coatings exhibit many cavities, protrusions and cracks that trap contamination.
This contamination later bursts through the coating in the oven (a phenomenon also known as outgassing). The same cavities, protrusions and cracks also can show in corrosion testing as blisters, as these form around the irregularities of the substrates. It’s well-known that pre-baking a hot-dip galvanized part can reduce visual defects. This works because it eliminates the trapped gases or liquids prior to coating.
Ensuring clean parts will eliminate poor cleaning as a potential root cause of the blisters. Clean the test parts by hand, coat and then test.
Evaluate the phosphate layer by examining blistered parts under a scanning electron microscope coupled with a dispersive x-ray machine to perform elemental analysis. This can be done both before and after stripping the paint layer to determine if nubbing or other defects are present. Nubbing is an incorrect formation of phosphate crystals that sometimes occurs on zinc-based metals during zinc phosphate application. An excess of zinc forms powdery, zinc-rich crystals.
Corrosion performance of the e-coated parts depends on the quality of the hot-dip galvanizing. The final e-coat isn’t the only barrier to blisters, but one part of the entire coating system of steel, plus galvanizing, plus phosphate, plus e-coat.
In corrosion testing, powder would be expected to perform better not because of the technology, but because of the greater thickness of powder films. Typical automotive film thickness for e-coat is in the range of 18–25 microns. Powder coat is typically in the range of 40–60 microns. If you increase the e-coat film on your parts to 32–35 microns, you’ll see that corrosion performance increases dramatically. Of course, the cost does, too.
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