I've got a clamp-on rear rack for my mountain bike (a few years old), and recently noticed the black coating on the aluminum (powdercoat?) cracking. When weight is applied to the rack, the area with the cracking is under tension. I'm not so worried about the black coating, but I'm wondering if the cracking indicates excessive stresses on the aluminum tubing -- should I worry about it failing?
I've attached a photo.

The tube is starting to bend at that point. Once it bends enough to kink on the bottom, it will have lost it's structural integrity, so it is in the process of 'failing'. I don't think it's going to break off any time soon, but aluminum does tend to be brittle.

NO- it won't break in your lifetime- unless you are really really overloading it.
Load it as close as possible to the seat- not waaaay out on the end.
Do you exceed its rated load? Which is?

It's rated at 25 lbs. (Delta Mega Rack Post Porter seat post rack with pannier support; here's a link: https://www.amazon.com/Delta-Post-Por.../dp/B000ACAMEW ) My guess is that the panniers and lock are about 5 lbs day-to-day load, and once every week or two I add some groceries for a few-mile ride.

Really hard to say. And load ratings and warranties aren`t a good indication either because they`re often just used as a marketing tool. Brand name and reputation are better indicators of durability.

Granted you have an unsupported rear charge on that rack, but I `ve used similar racks by Axiom, Voyager and Topeak and never seen an issue like that myself.

Whats the brand and model and warranty? Is that area next to the weld actually losing the powder coating completely? Or is it just the photo that makes it look that way?

Maybe somebody sat on it when you weren't looking?

If there were a single crack near the weld I'd be more worried. But as it is with multiple cracks spread over a distance, I think you're 100% OK. The spread of the cracks shows that it's flexing normally, and not getting ready to buckle at a weak spot.

It's likely that the paint or powder coat is less flexible than the aluminum, so as it flexes the paint gets stress cracks. It's similar to what happens on badly painted plastic auto bumpers.

Your only real concern is the potential for corrosion happening in bands which will eventually cause stress risers and lead to failure. This could be years out, but you may want to take some spray paint to it.

FB, that's what I figured. I can try some auto paint with flex in it. Burton, the brand/model and link to item are in my post above; it has a limited lifetime warranty against original defects, whatever that means.

Thanks - somehow I missed that when writing up my post. Checked out the rack on the manufacturers site and a couple places where its being retailed and no reviews mention similar problems.

This is a heavy duty rack thats marketed at a MSRP almost three times what most entry level racks go for. I`d suggest you at least email the company with an inquiry. A lifetime limited warranty may not cover all cercumstances, buts its definately useless if you refuse to use it.

Give it a shot. They may be as interested as you are. After all - its their reputation on the line.