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From The Field - Edge/Corner Failures

September - 2008
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By: Travis Stirewalt of Tiger Drylac, USA

On many occasions, powder coating failures can be traced to improper pretreatment and cure. The improper use of pretreatment chemicals, or lack of pretreatment in general, is often linked to coating failures. On the other hand, if the substrate is perfectly pretreated and the powder is improperly cured, this may certainly lead to color and gloss failures as well as an overall breakdown of the powder coating. Poor quality powder may also be blamed for coating breakdowns due to inferior materials and a great deal of filler material .

I have, however, been involved with testing on parts that developed failure on edges and sharp corners. The parts, shown in the photos, were used as brackets to secure an electrical panel in place. These brackets were failing from the sharp edged corners and over a short time were beginning to creep backwards, as is always the case.

The powder coating was tested for cure, as well as backed up with oven temperature documentation. In addition, the pretreatment was tested and the part exhibited a perfect surface preparation. From the field notes, we were able to find out that the powder would not peel due to the proper cure and treated surface, but over a short period of time, the powder would continue to fail further from the original point of corrosion.

After examining the parts further, we were able to obtain some reasons for the failure. The powder and pretreatment were performing well together, as determined by both the other parts in the field without bent or sharp edges and the testing on the pretreatment and powder. As we looked closer at the substrate, we were able to determine that the edges of the metal in the corners had not been deburred or ground (smoothed) down properly. The microscopic raised metal peaks were evidently not being adequately covered by the powder coating. When the powder gelled and flowed out, it left small tips of the steel protruding (or way too close) to the surface of the powdered finish. This resulted in a fairly quick failure in the field.

Powder coverages for standard smooth coatings are most commonly rated at 2.5 - 3.5 cured mils. These mil requirements are there for a number of reasons; adequate substrate protection, hiding, and color accuracy to name a few. When you powder coat a surface that has varying surface profiles, such as blasted metal or sharp metal edges, you must either smooth out these surfaces or apply an adequate thickness to maintain corrosion resistance. Many times it is not possible to powder coat edges well enough to prevent corrosion issues in the field - with any amount of material. It is ultimately important to properly debar all edges if possible to insure coating integrity.

As just previously mentioned, one aspect of powder coating that can be particularly problematic are media blasted parts. A talented blaster is always careful of the blasting media ratios between virgin and used media. A good mixture of media, virgin and used, is very important. Too much virgin material will dig too deeply into the substrate and can cause failures when the powder flows and exposes the high tips of the metal substrate. Very small percentages of the virgin material should always be used and mixed somewhat thoroughly with the used media.
Without becoming too complicated, surface profiles can often be measured in micro inches, commonly known as RMS. (root mean square) There are approximately 27 standard RMS averages for processes and substrates that range from flame cut surfaces to die cast substrates. These surfaces can be measured from a fraction of a micro inch to 2000 micro inches. It is ultimately not important to always be capable of measuring surface profiles, but it is important to understand the relative relationship between final surface profile and adequate powder coverage.

As a further clarification, we wanted to point out that another concern is substrate preparation, or more precisely, pretreatment rinse. When the part is hung on the rack or hook and pretreated, the rinse stage can leave sediment (oils, dirt, metal shavings, chemical, water salts) at the lowest point of the part. When the part is dried, the sediment dries in place and becomes a relatively certain future failing point. In this particular case, that situation was ruled out do to the way the parts were pretreated.

We hope this helps you in the field. If you have any questions, please do not hesitate to call or email.

Travis Stirewalt, CSI
615-400-1220 cell