DFO: Errors caused by incorrect blasting

Fig. 1: SEM image, cross-section of the bubble-free area (Image: DFO)

SEM analysis — Fig. 2: SEM/EDX analysis, cross-section of the bubble-free area (Image: DFO)

Rapid test for oil contamination — Fig. 3: Rapid test for the detection of oil contamination (Image: DFO)

The causes of many defects that are visible in the coating can be found in the pre-treatment stage. Pre-treatment includes, among other things, the blasting process.

As DFO Service GmbH is frequently commissioned to investigate cases of damage that originate in the blasting process, the following defect patterns are intended to illustrate possible causes. In the first case, blast-cleaned components coated with powder coating showed severe blistering. For the investigation, several cross-sections were made in blister areas and in areas without blister formation using a microtome. Even viewing the cross-sections under an optical microscope revealed a heavily roughened substrate. The SEM images (see Figure 1) confirmed the highly roughened surface. When the cross-sections were examined using energy-dispersive X-ray spectroscopy (see Figure 2), accumulations of silicon and oxygen were detected in the heavily roughened area. Here, residues of the abrasive were incorporated into the substrate. In addition, cavities formed within the substrate due to the highly roughened substrate. During the curing of the powder coating in the curing oven, the trapped air expanded and led to the formation of bubbles in the coating. The residual blasting agent would lead to premature corrosion if the components were used for a longer period of time. In this case, reducing the blasting speed and blasting time led to defect-free components.

Additive eliminates oil contamination

Loss of adhesion can also often be attributed to the blasting process. As part of the second case of damage, DFO was commissioned to find out the cause of the strongly fluctuating results in the cross-cut test on coated components. First, the underside of the coating was examined using energy-dispersive X-ray spectroscopy. Large quantities of the conversion layer could be detected here. The examination of the corresponding substrate surface showed some areas in which no conversion layer could be detected. The cause of the adhesion disorder was therefore between the substrate and the conversion layer. The DFO suspected contamination by organic substances, which could be proven by means of infrared spectroscopy. A comparison of the spectra of the substrate surface with the database showed that the contamination was caused by an anti-corrosion oil. In this case, the blasting process was unable to completely remove the large quantities of anti-corrosion oil from the component. A simple test (see Figure 3) confirmed that the blast media had significant oil contamination. To do this, a small amount of abrasive is formed into a pile and a drop of water is added to this pile. If the water does not pass through the abrasive, this is an indication of organic contamination, as oils and greases reduce the free surface energy of the abrasive, making it water-repellent. In order to blast the components contaminated with high quantities of oil, the customer had to add an additive to the blasting agent. This powdered additive was used to continuously degrease the blasting medium and, following further pre-treatment steps, fault-free components could be produced.

Defect image of the month

German Research Association for Surface Treatment (DFO)