Will Pure 90-Degree Sheet Metal Bend Produce Invisible Cracks?

"If this is the problem, would a simple coating of primer be the solution?"

Sure, why not.

One thing not discovered on your other thread... how many days pass between the fabrication of these pieces and coating? Stress crack will reveal white metal, and will begin to corrode immediately. Should rust bloom be considered as the culprit?

A 180° bend is a seam used to reinforce an edge on thin sheet metal. For a 180° bend, I would form the edge over a minimum 1/4" dia wire.

Maybe, depends on the steel in question and it's properties. But you should also consider the way powder coating is applied. It is applied without a primer, and it is electrostatically sprayed on the part. Sharp edges dissipate static charges much more than gentler bends will, and since the electrostatic charge in that area is diminished, less powder will stick to that area.

"... sometimes the paint appears not to bond as well at the bend radius,...)

Well, If this is happening only sometimes, it means that the cleaning process or preparation is lacking sometimes! Also, when bending there will be some distortion and if the bending apparatus is properly set, there should be no cracks, otherwise re-set. What will happen is that the surface in that area will be smoother (more than the flats) and maybe a small amount of rubbing with a fine brush... before cleaning will help reduce the incident of bad adhesion.

Short answer is yes - plastic deformation is a certainty if the sheet metal stays in that shape after forming - cracking is unlikely. However, I agree with a previous poster than the coating failure is more likely caused by the edge effects of the static charge field and the natural tendency of coatings to pull back from edges and thin out (regardless of the coating technique - this is true of brush and roller, too). You might try priming the edge or stripe coating it prior to coating the bulk of the surfaces, or you might be able to deal with this by improving your process control on the powder coating when working near the edges.

I can answer this but R4 blocks my answers

Former steel research engineer

PEbobimm

You might talk to the finishing trade folks. www.pfonline.com will get you to the right discussion groups and experts if you don't find an answer here.