This is a question that has come up repeatedly over the years and continues to be asked.
If you look around a bit on the web it isn't too hard to find out where the truth lies. In a nutshell, microfracturing isn't an issue with carabiners and it is a pretty good bet that a carabiner that looks okay with a close visual inspection and has a smooth gate action is safe.
Here is an excerpt from an article written by Clyde Soles in a rock climbing magazine from the late 1990s:
Unlike nylon products, carabiners do not lose
strength with age -- they will pretty much last
indefinitely. Which is not to say that carabiners
don't wear out... or that you don't have to take
care of them.
The primary reasons you will need to retire a
carabiner are grooves worn by abrasive ropes,
gouges caused by steel pitons and bolt hangers,
and corrosion (white powdery substance) caused
by salt water or even the sweat from your hands.
Inspect your biners carefully for these signs, as
well as any evidence of tiny cracks -- particularly
around gate pins and hinge pins.
Some climbers are concerned about the poten-
tial for strength-robbing micro-cracks that are not
visible to the naked eye. Indeed, conventional
wisdom says that a carabiner which has been
dropped must be retired, even when there are no
signs of damage. Perhaps not.
In a recent test by Steve Nagode, an engineer
at the REI quality assurance laboratory, 30 cara-
biner bodies (half ovals, half D's) were each
dropped six times onto a concrete floor from a
height of 33 feet. Following the drops, their open-
gate strength was measured and compared to 30
control samples from the same production batch
and that had not been dropped. The statistical
result was no loss of strength.
According the Chris Harmston, the quality
assurance manager at Black Diamond, "I have
broken hundreds of used, abused and dropped
biners (even some that fell 3,000 feet from the top
of the Salathe). Never have I noticed any problem
with these unless there is obvious visual damage
to the biner." While somewhat reassuring, this
does not give you carte blanche to use carabiners
that have been dropped a significant distance.
Dropping them, even from great heights, may not
harm them unless they are in any way deformed
(because metal doesn't fatigue by impact but by
bending or stretching -- which can happen when
loaded almost to failure or over an edge).
Immediately retire any carabiner that is crooked,
has deep indentations or has a gate that doesn't
operate smoothly -- three problems more liely
to occur in ultralight carabiners.
If a carabiner gate is sticky because it's
tweaked rather than oxidized or gritty, you
should retire the biner. If you use your biners
near saltwater, be sure to rinse them thoroughly
in fresh water, allow to dry in a warm place and
then lubricate them before storage. Remember,
it's the water inside the biner, where you can't see
it, that is of greatest concern. WD-40 is a good
water dispersant and penetrating lubricant.
However, such a petroleum product will work
well at first but can attract grit, possibly causing
worse problems than what you tried to solve. A
better solution for sticking gates is a dry silicone
lubricant, such as Finish Line or Pedro's, both
used for bike chains.
(Clyde Soles, Rock & Ice #81, September/October 1997, pp 117-118)
And here is the latest advice from Black Diamond Equipment:
Q. Is it okay to use carabiners that have been dropped?
A. Unfortunately, the only way to know if “dropped” carabiners are fit for use is to test them to their breaking point. This doesn’t do you much good, now does it? It's best to inspect dropped gear for dings and significant trauma. If only light scratching is visible and gate action is still good, there is a good chance it is fit for usage. Remember, only you know what your gear has been through and if there is any doubt, it's best to retire the gear rather than take a risk.
Q. When should I retire my carabiners?
A. Here are our suggestions on what to look for when retiring aluminum carabiners:
1) Check for good gate action: The open-gate strength of carabiners is roughly 1/3 of the closed-gate strength. If a biner has a gate that rubs or sticks open, it should be cleaned and lubed. If this does not improve gate action, the biner should be retired. The same holds true for any gate locking mechanism.
2) Check for excessive wear: If you can feel that the rope-bearing surfaces of the biner are significantly worn (wearing off the anodization is normal after a few uses) the biner should be retired.
3) Check for deformation: If a biner has been loaded such that the body or nose has deformed—or the carabiner gate rivets have been bent (this usually results in poor gate action)—the biner should be retired.
4) Check for nicks or deep scratches: If a biner has nicks or deep gouges beyond the normal light scratching that occurs in use, it should be retired. Carabiners are more susceptible to surface damage near the nose hook or within an inch of the bending radii of the body.
5) Has the carabiner been exposed to extreme heat? If a biner has been exposed to "extreme heat" (i.e. a fire) it should be retired and destroyed due to possible negative affects to the heat treatment the carabiner underwent when it was made.
6) Has the carabiner been exposed to harsh chemicals or excessive corrosion? If your carabiner has been exposed to aggressive chemicals (like battery acid, petroleum-based fuel, ect.) its a good idea to retire the biner. Likewise, any corrosion beyond the normal thin gray/white oxidation layer that forms on exposed aluminum should be grounds for retirement, especially if it starts to affect gate action (see #1).
In closing: Keep in mind that only YOU know what your gear has been through. If your instincts tell you that the gear is dubious, retiring it is a good idea. Confidence in your equipment is not only key to climbing at your limit but helps you stay relaxed and having fun.
(Black Diamond Equipment FAQ --
http://www.blackdiamondequipment.com/en ... rvice/faqs)
Some research into the failure modes of carabiners, including cyclic load testing:
http://stuff.mit.edu/afs/athena/course/ ... ne_622.pdfor if you prefer, here is the slideshow presentation for this research:
http://web.mit.edu/sp255/www/reference_ ... tation.pdfCarabiners are cheap, almost disposible.