What kind of corrosion is this and what caused it?

[Robert L. Stephens]

Metal Rust...Corrosion occurs when the protective coating or galvanization of the roof wears out and bare metal is exposed. In this case it looks like a material flaw from the manufacturer.

[Kenton H. Shepard, CMI]

It's one type of Crevice Corrosion.
This type begins during storage from moisture trapped between stacked panels.

[Kenton H. Shepard, CMI]

Originally Posted by dedwards

There is a type of rusting that occurs after a time due to the constant wearing away of the protection along edges from expanding and contractions of the metal, the wood beneath it. The rubbing away of the coating exposes the metal. The rusting begins at the edges, then works it way across the surface once the metal is exposed. Don't know the name of it and am too lazy to go look it up.

There's something similar to what you describe when metal roofing is laid over comp shingles with no underlayment between. With thermal cycling of the metal roof, the shingle granules sand the protective coating off the steel and corrosion begins.

[Brian E. Kelly]

Originally Posted by kshepard

I'll let this sit a while then post the answer.

I thought I could only see the corrsion along the seem where the fasteners are, and figured there was a metal frame of some sorts underneath.

Alas i am wrong again, great info Kenton.

[Marcel R. Cyr]

Crevice Corrosion;

http://www.corrosion-doctors.org/For...ce/Crevice.htm

[Kenton H. Shepard, CMI]

Good link Marcel, where do you find these things!?

[Larry D. Kage]

Originally Posted by kshepard

Good link Marcel, where do you find these things!?

I've watched him, over the last couple o' years, master Google. He always has good pertinent info. Nice Marcel...

[Chuck Evans]

Very interesting: Thanks for posting

[Steven C. Meyer]

Grat post!! much information. I did cheat, went to the end of the post first to get the answer!!!

Thanks, we can all learn from others!!!

[Kenton H. Shepard, CMI]

Thanks guys,

When it becomes available... take the Roof Defect Recognition course. All the steep-slope residential roof-covering materials will be covered in this kind of detail. No insurance agent or roofing contractor will be able to win an argument with you when it comes to these common residential roofing materials. You will know what you're talking about.

[Marcel R. Cyr]

Originally Posted by kshepard

Thanks guys,

When it becomes available... take the Roof Defect Recognition course. All the steep-slope residential roof-covering materials will be covered in this kind of detail. No insurance agent or roofing contractor will be able to win an argument with you when it comes to these common residential roofing materials. You will know what you're talking about.

Hi. Kenton; Cann't wait, you always put together good material.
Glad you like the link.
I am just getting back from my out of State job.
Bel glad when working local again.
Awwell, it is a job.