Tie down ring stripped

[PT20J]

21 minutes ago, Vance Harral said:

It's obvious from some reading that steel helicoils are inserted into aluminum structures all the time.  But isn't this a galvanic corrosion risk?

No. First, steel and aluminum are not that far apart on the galvanic chart. Also, while the aluminum is the anode and would be expected to corrode,  the area of the anode (aluminum) is so much greater than cathode (steel) that the effect is negligible. This is why you can use stainless steel screws to attach aluminum inspection panels. 

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[Guest]

5 hours ago, Vance Harral said:

It's obvious from some reading that steel helicoils are inserted into aluminum structures all the time.  But isn't this a galvanic corrosion risk?

Most Helicoils that I’ve encountered have been stainless steel.

Clarence

[Vance Harral]

26 minutes ago, M20Doc said:

Most Helicoils that I’ve encountered have been stainless steel.

I'm sure @PT20J is correct that the galvanic corrosion concerns are negligible.  But for what it's worth, stainless steel is a little further away from Aluminum on the galvanic corrosion chart than mild and chromium steel, and therefore technically more susceptible to the galvanic corrosion problem, not less: https://www.albanycountyfasteners.com/blog/2017/09/15/stainless-steel-and-aluminum/

My understanding is the "stainlessness" of stainless steel refers only to its resistance to iron oxide corrosion from water and/or most air.  It's not necessarily better protected than non-stainless steel from adjacent metals or other non-atmospheric elements.  That said, I'm not a materials expert and maybe I'm off base here.  Happy to be educated by people with a better understanding.

[Guest]

25 minutes ago, Vance Harral said:

I'm sure @PT20J is correct that the galvanic corrosion concerns are negligible.  But for what it's worth, stainless steel is a little further away from Aluminum on the galvanic corrosion chart than mild and chromium steel, and therefore technically more susceptible to the galvanic corrosion problem, not less: https://www.albanycountyfasteners.com/blog/2017/09/15/stainless-steel-and-aluminum/

My understanding is the "stainlessness" of stainless steel refers only to its resistance to iron oxide corrosion from water and/or most air.  It's not necessarily better protected than non-stainless steel from adjacent metals or other non-atmospheric elements.  That said, I'm not a materials expert and maybe I'm off base here.  Happy to be educated by people with a better understanding.

Like you I’m not a materials expert, all I can offer is experience based on more than 3 decades of GA maintenance and I’ve never seen corrosion between aluminum and Helicoils.

Clarence

[PT20J]

46 minutes ago, Vance Harral said:

I'm sure @PT20J is correct that the galvanic corrosion concerns are negligible.  But for what it's worth, stainless steel is a little further away from Aluminum on the galvanic corrosion chart than mild and chromium steel, and therefore technically more susceptible to the galvanic corrosion problem, not less: https://www.albanycountyfasteners.com/blog/2017/09/15/stainless-steel-and-aluminum/

My understanding is the "stainlessness" of stainless steel refers only to its resistance to iron oxide corrosion from water and/or most air.  It's not necessarily better protected than non-stainless steel from adjacent metals or other non-atmospheric elements.  That said, I'm not a materials expert and maybe I'm off base here.  Happy to be educated by people with a better understanding.

There are a couple of ways to make "stainless" steel": passivated and non-passivated. Passivated is preferred if you are trying to prevent corrosion of the stainless steel part (rust) because it makes the material surface less susceptible to corrosion. Non-passivated is preferred if you are trying to prevent galvanic corrosion from contact with another material -- in our case aluminum. From the Helicoil manufacturer's application note: http://www.olander.com/contentonly.aspx?file=images/corrosionhc68 1r5.pdf

Note: 18-8 Stainless (Active) is much closer to aluminum and magnesium than the Passive stainless. The conclusion is logically made that using passivated stainless steel will accelerate the corrosion of all metals above it. This is one reason why Stainless Steel Heli-Coil® Inserts are not passivated.

Bottom line: Materials engineers have figured all this stuff out so we don't have to. Helicoils are safe to use in aluminium.

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[PT20J]

26 minutes ago, M20Doc said:

Like you I’m not a materials expert, all I can offer is experience based on more than 3 decades of GA maintenance and I’ve never seen corrosion between aluminum and Helicoils.

Clarence

Only 3 decades? With all your knowledge, I'd have figured you for 5 decades -- at least. 

[carusoam]

Passivation is an interesting surface treatment process for SS...

Once the parts are made, the finished surface gets treated to remove the easily oxidized molecules near the surface...

in simplified cases, citric acid washes are used... get out the lemons...

An important pre qualification step before using SS tanks in the pharma industry...

PP thoughts only, not a chemist or material scientist...

Best regards,

-a-