Corrosion Around Fasteners 2

[midsidenode]

I've been asked to help disposition some aircraft stucture that has some minor corrosion directly adjacent to some primary load carrying fasteners. The material that has the corrosion is aluminum 6061-T6 and the fastener holes are countersunk. I've read that corrosion adjacent to the edges of holes can sometimes create stress concentrations on the order of 3 to 4 which alarms me. The corroded section cannot be replaced without performing some very invasive work so the question has been asked whether it can be adequatley repaired. I am not a corrosion expert by any means but i need to evaluate this area with regards to strength. Can anyone offer some practical advice or point me in the right direction w/r to allowable corrosion or evaluating corrosion in these types of areas?

 

[bf109g]

No quick and dirty answer here.

If the corrosion emanates from under the fastener heads, the fasteners must be removed. You need to document all corrosion by location, size and depth and then compare to whatever the SRM allows you to clean up. If you can clean up the damage per the SRM and reinstall fasteners, you're good to go.

If your damage is beyond what the SRM allows, you need to go to the OEM to determine if your blendout will result in adequate margins. Failing that, you're into repair territory. OEM's/SRM's will not allow continued flight with active corrosion.

This is not considering that you could develop the acceptability of the cleanup if you knew enough about the structure (loads, primary/secondary, critical design condition, etc)

Good luck.

 

[WKTaylor]

midsidenode

First...

6061-T6 is a VERY odd material for aircraft structure [sheet?].

Also, 6061-T6 is pretty corrosion durable... so significant corrosion around fasteners is a major concern to me. almost sounds like a bare assy.

Second...

ARE YOU ABSOLUTELY SURE THE BASE MATERIAL IS 6061-T6... NOT 7075 or some other 7xxx series alloy?

Third... Bf109g is dead-on... follow his advice.

Forth... FYI...

As a rough rule-of thumb: for every 0.001" of penetration of corrosion into aluminum, there will be roughly 0.003" of corrosion product [aluminum oxide] released. This results in a surface "swell" of roughly 0.002" for every 0.001 of corrosion penetration. Obviously, for structure that is fastened together [IE: typical skin lap joints, etc] enormous compressive-pressure stresses will develop between tightly-fitted parts... and on adjacent individual fasteners (prying fastener heads or tails, etc).

NOTE: If steel and aluminum corrosion are involved [aluminum countersink, steel fastener], then even a thin layer of steel [red] rust will aggravate/accelerate the aluminum corrosion which traps more moisture in the aluminum oxide... which in-turn aggravates the steel rust... etc.... making the situation spiral out of repairable limits VERY quickly [especial sea-coast environments].

Regards, Wil Taylor

 

[tbuelna]

midsidenode,

The reason corrosion is a problem in load bearing structures that must be prevented, or as in your case repaired if present, is that the corrosion pitting provides the perfect nucleation points for fractures to form and propagate from.

Corrosion damage in load bearing structures should always be repaired properly. However, before designing the repair procedure, you should first establish the cause of the corrosion. And make sure that your rework instructions provide any necessary corrective actions to prevent a recurrence of the problem.

Good luck.
Terry

 

[RFSS]

Corrosion on skin surrounding fastener heads should be cleaned / reworked with the fastener removed. Then the amount of removed material should be evaluated per SRM. In quick checks where sufficient ground time to pull the fasteners / replacement fasteners are not available, corrosion removal per SRM chap 51 with the fasteners installed is allowed but repair will be temporary until the next aircrafts grounding time. Most likely, corrosion around and underneath fastener heads is galvanic, therefore, in seeking OEM approval / concession, it would help if after clean up, a 10mm x 10mm grid be prepared prior to UTI to verify remaining thickness / amount removed instead of measuring using depth gauge.

 

[midsidenode]

thought it might help to clarify that the corrosion is not on any skin, but rather on a beam extrusion made from 6061-T6511. The beam material is 3/16" thick in the area of the fastener. The fastener is SS, 5/16 dia and is countersunk. The corrosion was noticed adjacent to the fastener heads and when the fasteners were removed, corrosion was noticed underneath the fastener heads as well.

 

[Sparweb]

If it is primary structure made of 6061-T6, then are we talking about an ultralight / recreational aircraft? Some of those have laser-cut 6061 main wing attachment lugs (shudder). I hate to ask but is there even a budget available for a "correct" repair? The economic threshold for scrapping the part is also worth checking.

"...directly adjacent to some primary load carrying fasteners..."

Does that mean the corrosion is associated with the fasteners themselves? I assume you mean that the area of corrosion actually starts beside the fastener and grows out from there.

In that case you have an enlarged "hole". The amount of enlargement is proportional to the net lost cross-section and the orientation in the member.

Is there an individual fastener involved, or a series? Is one screw out of the row more affected than the rest?


Steven Fahey, CET

 

[tbuelna]

midsidenode,

I would agree with RFSS, the corrosion is likely galvanic in nature.

An interface of cres and 6061 alum has a fairly high galvanic potential (ie. >.25V). This joint will likely continue to suffer from corrosion if the faying surfaces are not properly protected and sealed.

After repairing the existing corrosion damage, you will also want to ensure that the new joint surfaces and fasteners are installed with any necessary surface coatings or sealants to prevent a recurrence of the galvanic corrosion.

 

[midsidenode]

clarifications: The aircraft is not recreational. The corrosion is not associated with the fasteners - just the aluminum. During routine inspections, the primer and paint is removed from the areas which includes the 100 degree countersink. Removing material from the countersink will reduce the bearing area and cause the countersunk fastener to sit below the surface.

 

[SAITAETGrad]

Cyclic load carrying extrusions and stainless steel fasteners...bad combination. There may be further exfoliation going on that is not all that visible in addition to the galvanic corrosion.