Belly Screws - '67 F

[Mooney-Shiner]

On 7/11/2023 at 5:10 PM, PT20J said:

Galvanic corrosion occurs when an less noble metal (anode) is in contact with a more noble metal (cathode) in the presence of an electrolyte (e.g., water with salt or other impurities). The corrosion removes material from the anode. So, in the case of stainless steel in contact with aluminum, the aluminum is the anode and would corrode. So, why is it that you can get away with stainless screws on an aluminum airplane without damage? There are two reasons:

1) If the airplane is mostly hangared and not subject to warm, moist, salty air then there is no electrolyte which is a necessary ingredient for corrosion. 

2) There is an area rule: If the area of the anode (aluminum skin) is much greater than the area of the cathode (screw heads) then the corrosion proceeds at such a slow rate as to be essentially unmeasurable.

Skip

Just bringing this thread back from the dead:

Since I’ve been based at a coastal Florida airport (beautiful spot, but hot and humid) with my Mooney since 2020, I made a habit of replacing corroded or worn fasteners with stainless steel ones. The line for hangars are decade long and I'm on the outside tiedown.

But the more I’ve read on this, the more I realize that cadmium-plated screws are actually less noble than stainless and closer to aluminum on the galvanic scale. Counterintuitive as it seems, that means cad-plated screws can promote less corrosion than stainless when installed in aluminum.

Sure enough, I started noticing filiform corrosion forming around my fuel tank access panel after swapping the original screws for stainless steel ones.

Does anyone have an experience with this? 

[DCarlton]

I'm no expert but if I remember correctly from my time with an Ocean Engineering group, stainless steel and aluminum were not great together.  I think you are correct in your assessment (yet we use them anyway).  Hopefully there's a materials expert on M/S that can offer an expert opinion.  I think Cadmium has fallen out of favor due to its toxicity and carcinogenic characteristics.  Cadmium corrosion (a white powdered dust) was a bad thing to inhale.  

[Ragsf15e]

One other thing my mechanic and I noticed was how the stainless screws were much more likely to gall and stick in the nuts (machine screws at least).  I haven’t had the same issue with cad screws.  After watching my mechanic drilling out a ss screw (and cringing) and then replacing the nutplate as well, I just use the cad plated ones and throw them away as soon as I see them deteriorate or strip at all.

[47U]

+1 @Ragsf15e

1.  Cad machine screws.  If the head shows any damage, replace it.

2.  Put a small smear of anti seize (use a tooth pick) on each screw before installing.  It’s time consuming, but once done, you don’t have to do it every year.

3.  Hand install screws.  Using a ratchet screwdriver helps.  Do not use a battery operated drill-driver.  Even the lowest torque setting is too tight.

4.  Use a new (quality) apex bit to remove the screws.  Worn bits strip out Phillips head screws.

You’ll spend much less time removing stuck/stripped head screws.

[DavePage]

On 7/9/2023 at 7:00 PM, MooneyMitch said:

Yes, always one my favorite tasks was removing  the 10,000 plus belly panel screws!! 
 

 

Better yet, is replacing the 10,000 plus belly panel screws !!!!    ;-) 

[Flyler]

Ex-machinist and shipyard guy here. Stainless fasteners just sorta stink to begin with, IMO. The galling or "snowballing" is a real problem. SS and aluminum are far apart on the galvanic scale which accelerates corrosion.  We used to use a product called "TefGel" when we couldn't isolate Carbon from SS very well.  Typically, though, we would bond a glass sleeve into the carbon fiber and use a glass washer, to keep the direct Carbon to SS contact low.  Of course with carbon fiber, Titanium just happens to be close on the galvanic scale.  A hassle to machine/fabricate and super expensive!

 

At any rate, that galvanic scale pictured is the gospel. If you have dissimilar materials, you want them to be next to eachother on that scale.  The further apart they are, the more they will corrode.

[DCarlton]

15 hours ago, Flyler said:

Ex-machinist and shipyard guy here. Stainless fasteners just sorta stink to begin with, IMO. The galling or "snowballing" is a real problem. SS and aluminum are far apart on the galvanic scale which accelerates corrosion.  We used to use a product called "TefGel" when we couldn't isolate Carbon from SS very well.  Typically, though, we would bond a glass sleeve into the carbon fiber and use a glass washer, to keep the direct Carbon to SS contact low.  Of course with carbon fiber, Titanium just happens to be close on the galvanic scale.  A hassle to machine/fabricate and super expensive!

 

At any rate, that galvanic scale pictured is the gospel. If you have dissimilar materials, you want them to be next to eachother on that scale.  The further apart they are, the more they will corrode.

Subs?  The folks in Portsmouth were the planning yard for the program I supported.  Thanks for the refresher.  

[PT20J]

I admit to using SS screws because I like the way they look and I've never had an issue. But there are disadvantages. Stainless is softer than carbon steel, and it it easier to cam out the head if you overtighten. Stainless will gall. Heat and friction cause this. There is also the corrosion potential. I have never had an issue with corrosion around the fasteners, but my airplane is in a hangar. If it was outside on the gulf coast, I would probably use cad plated steel screws. To avoid the camming and galling, I tighten and loosen screws with a ratchet screwdriver rather than a power driver to avoid turning them too fast (galling) and overtightening (camming). I also soak the screws in ACF-50 before reinstalling which provides lubrication for the threads and perhaps some corrosion protection.

[Flyler]

2 hours ago, DCarlton said:

Subs?  The folks in Portsmouth were the planning yard for the program I supported.  Thanks for the refresher.  

Private sector yard. We built mostly work boats like tugs and ferries, and large yachts.  One of the yards did build a few Coast Guard cutters with deck guns though!  My composites experience was outside of the shipyard and for a really interesting, but niche special project run through a high performance sailboat component manufacturing place.  I learned so much there, it was such a cool place.  Unfortunately they went under, and I hopped ship early as the writing was on the wall.